Epidemiology of HIV/AIDS

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Epidemiology_of_HIV/AIDS 

 

HIV/AIDS pandemic
AIDS and HIV prevalence 2009   No data   < 0.1%   0.1–0.5%   0.5–1%   1–5%   5–15%   15–50%
Disease	HIV/AIDS
Virus strain	HIV
Source	Non-human primate
Location	Worldwide
First outbreak	June 5, 1981
Date	1981 – present
Confirmed cases	55.9 million – 100 million
Deaths	36.3 million total deaths (2020)

Disability-adjusted life year for HIV and AIDS per 100,000 inhabitants

  no data

  ≤ 10

  10–25

  25–50

  50–100

  100–500

  500–1,000

  1,000–2,500

  2,500–5,000

  5,000–7,500

  7,500–10,000

  10,000–50,000

  ≥ 50,000

HIV/AIDS, or human immunodeficiency virus, is considered by some authors a global pandemic. However, the WHO currently uses the term 'global epidemic' to describe HIV. As of 2018, approximately 37.9 million people are infected with HIV globally. There were about 770,000 deaths from AIDS in 2018. The 2015 Global Burden of Disease Study, in a report published in The Lancet, estimated that the global incidence of HIV infection peaked in 1997 at 3.3 million per year. Global incidence fell rapidly from 1997 to 2005, to about 2.6 million per year, but remained stable from 2005 to 2015.

Sub-Saharan Africa is the region most affected. In 2018, an estimated 61% of new HIV infections occurred in this region. Prevalence ratios are "In western and central Europe and North America, low and declining incidence of HIV and mortality among people infected with HIV over the last 17 years has seen the incidence:prevalence ratio fall from 0.06 in 2000 to 0.03 in 2017. Strong and steady reductions in new HIV infections and mortality among people infected with HIV in eastern and southern Africa has pushed the ratio down from 0.11 in 2000 to 0.04 in 2017. Progress has been more gradual in Asia and the Pacific (0.05 in 2017), Latin America (0.06 in 2017), the Caribbean (0.05 in 2017) and western and central Africa (0.06 in 2017). The incidence:prevalence ratios of the Middle East and North Africa (0.08 in 2017) and eastern Europe and central Asia (0.09 in 2017)". South Africa has the largest population of people with HIV of any country in the world, at 7.06 million as of 2017. In Tanzania, HIV/AIDS was reported to have a prevalence of 4.5% among Tanzanian adults aged 15–49 in 2017.

South & South-East Asia (a region with about 2 billion people as of 2010, over 30% of the global population) has an estimated 4 million cases (12% of all people infected with HIV), with about 250,000 deaths in 2010. Approximately 2.5 million of these cases are in India, where however the prevalence is only about 0.3% (somewhat higher than that found in Western and Central Europe or Canada). Prevalence is lowest in East Asia at 0.1%.

In 2019, approximately 1.2 million people in the United States had HIV; 13% did not realize that they were infected.

In 2020, 106,890 people were living with HIV in the UK and 614 died (99 of these from Covid-19 comorbidity).

In Australia, as of 2020, there were about 29,090 cases. In Canada as of 2016, there were about 63,110 cases.

A reconstruction of its genetic history shows that the HIV pandemic almost certainly originated in Kinshasa, the capital of the Democratic Republic of the Congo, around 1920. AIDS was first recognized in 1981, in 1983 the HIV virus was discovered and identified as the cause of AIDS, and by 2009 AIDS had caused nearly 30 million deaths.

Global HIV data

Since the first case of HIV/AIDS reported in 1981, this virus, although rare, continues to be one of the most prevalent and deadliest pandemics worldwide. The Center for Disease Control mentions that the HIV disease continues to be a serious health issue for several parts of the world. Worldwide, there were about 1.7 million new cases of HIV reported in 2018. About 37.9 million people were living with HIV around the world in 2018, and 24.5 million of them were receiving medicines to treat HIV, called antiretroviral therapy (ART). In addition, roughly an estimated 770,000 people have died from AIDS-related illnesses in 2018.

Globally, individuals suffer from HIV/AIDS; yet, there has also been a common trend as far as prevalence in cases and regions most affected by the disease. The CDC reports that areas like the Sub-Saharan Africa region is the most affected by HIV and AIDS worldwide, and accounts for approximately 61% of all new HIV infections. Other regions significantly affected by HIV and AIDS include Asia and the Pacific, Latin America and the Caribbean, Eastern Europe, and Central Asia.

Worldwide there is a common stigma and discrimination surrounding HIV/AIDS. Respectively, infected patients are more subject to judgement, harassment, and acts of violence and come from marginalized areas where it is common to engage in illegal practices in exchange for money, drugs, or other exchangeable forms of currency.

AVERT, an international HIV and AIDS charity created in 1986, makes continuous efforts to prioritize, normalize, and provide the latest information and education programs on HIV and AIDS for individuals and areas most affected by this disease worldwide. AVERT suggested that, discrimination and other human rights violations may occur in health care settings, barring people from accessing health services or enjoying quality health care.

Accessibility to tests have also played a significant role in the response and speed to which nations take action. Approximately 81% of people with HIV globally knew their HIV status in 2019. The remaining 19% (about 7.1 million people) still need access to HIV testing services. HIV testing is an essential gateway to HIV prevention, treatment, care and support services. It is crucial to have HIV tests available for individuals worldwide since it can help individuals detect the status of their disease from an early onset, seek help, and prevent further spread through the practice of suggestive safety precautions.

There were approximately 38 million people across the globe with HIV/AIDS in 2019. Of these, 36.2 million were adults and 1.8 million were children under 15 years old.

Historical data for selected countries

HIV/AIDS in World from 2001 to 2014 – adult prevalence rate – data from CIA World Factbook

HIV in World in 2014

By region

See also: List of countries by HIV/AIDS adult prevalence rate

The pandemic is not homogeneous within regions, with some countries more afflicted than others. Even at the country level, there are wide variations in infection levels between different areas. The number of people infected with HIV continues to rise in most parts of the world, despite the implementation of prevention strategies, Sub-Saharan Africa being by far the worst-affected region, with an estimated 22.9 million at the end of 2010, 68% of the global total.

South and South East Asia have an estimated 12% of the global total. The rate of new infections has fallen slightly since 2005 after a more rapid decline between 1997 and 2005. Annual AIDS deaths have been continually declining since 2005 as antiretroviral therapy has become more widely available.

World region	Estimated prevalence of HIV infection (millions of adults and children)	Estimated adult and child deaths during 2010	Adult prevalence (%)
Worldwide	31.6–35.2	1.6–1.9 million	0.8%
Sub-Saharan Africa	21.6–24.1	1.2 million	5.0%
South and South-East Asia	3.6–4.5	250,000	0.3%
Eastern Europe and Central Asia	1.3–1.7	90,000	0.9%
Latin America	1.2–1.7	67,000	0.4%
North America	1–1.9	20,000	0.6%
East Asia	0.58–1.1	56,000	0.1%
Western and Central Europe	.77–.93	9,900	0.2%

Sub-Saharan Africa

Estimated HIV infection in Africa in 2011.

 

Graphs of life expectancy at birth for some sub-Saharan countries showing the fall in the 1990s primarily due to the AIDS pandemic.

Main article: HIV/AIDS in Africa

Sub-Saharan Africa remains the hardest-hit region. HIV infection is becoming endemic in sub-Saharan Africa, which is home to just over 12% of the world's population but two-thirds of all people infected with HIV. The adult HIV prevalence rate is 5.0% and between 21.6 million and 24.1 million total are affected. However, the actual prevalence varies between regions. Presently, Southern Africa is the hardest hit region, with adult prevalence rates exceeding 20% in most countries in the region, and 30% in Swaziland and Botswana. Analysis of prevalence across sub-Saharan Africa between 2000 and 2017 found high variation in prevalence at a subnational level, with some countries demonstrating a more than five-fold difference in prevalence between different districts.

Eastern Africa also experiences relatively high levels of prevalence with estimates above 10% in some countries, although there are signs that the pandemic is declining in this region. West Africa on the other hand has been much less affected by the pandemic. Several countries reportedly have prevalence rates around 2 to 3%, and no country has rates above 10%. In Nigeria and Côte d'Ivoire, two of the region's most populous countries, between 5 and 7% of adults are reported to carry the virus.

Across Sub-Saharan Africa, more women are infected with HIV than men, with 13 women infected for every 10 infected men. This gender gap continues to grow. Throughout the region, women are being infected with HIV at earlier ages than men. The differences in infection levels between women and men are most pronounced among young people (aged 15–24 years). In this age group, there are 36 women infected with HIV for every 10 men. The widespread prevalence of sexually transmitted diseases, the promiscuous culture, the practice of scarification, unsafe blood transfusions, and the poor state of hygiene and nutrition in some areas may all be facilitating factors in the transmission of HIV-1 (Bentwich et al., 1995).

Mother-to-child transmission is another contributing factor in the transmission of HIV-1 in developing nations. Due to a lack of testing, a shortage in antenatal therapies and through the feeding of contaminated breast milk, 590,000 infants born in developing countries are infected with HIV-1 per year. In 2000, the World Health Organization estimated that 25% of the units of blood transfused in Africa were not tested for HIV, and that 10% of HIV infections in Africa were transmitted via blood.

Poor economic conditions (leading to the use of dirty needles in healthcare clinics) and lack of sex education contribute to high rates of infection. In some African countries, 25% or more of the working adult population is HIV-positive. Poor economic conditions caused by slow onset-emergencies, such as drought, or rapid onset natural disasters and conflict can result in young women and girls being forced into using sex as a survival strategy. Worse still, research indicates that as emergencies, such as drought, take their toll and the number of potential 'clients' decreases, women are forced by clients to accept greater risks, such as not using contraceptives.

AIDS-denialist policies have impeded the creation of effective programs for distribution of antiretroviral drugs. Denialist policies by former South African President Thabo Mbeki's administration led to several hundred thousand unnecessary deaths. UNAIDS estimates that in 2005 there were 5.5 million people in South Africa infected with HIV — 12.4% of the population. This was an increase of 200,000 people since 2003.

Although HIV infection rates are much lower in Nigeria than in other African countries, the size of Nigeria's population meant that by the end of 2003, there were an estimated 3.6 million people infected. On the other hand, Uganda, Zambia, Senegal, and most recently Botswana have begun intervention and educational measures to slow the spread of HIV, and Uganda has succeeded in actually reducing its HIV infection rate.

Middle East and North Africa

HIV/AIDS prevalence among the adult population (15-49) in the Middle East and North Africa is estimated less than 0.1 between 1990 and 2018. This is the lowest prevalence rate compared to other regions in the world.

In the MENA, roughly 240,000 people are living with HIV as of 2018 and Iran accounted for approximately one-quarter (61,000) of the population with HIV followed by Sudan (59,000). As well as, Sudan (5,200), Iran (4,400) and Egypt (3,600) took up more than 60% of the number of new infections in the MENA (20,000). Roughly two-thirds of AIDS-related deaths in this region happened in these countries for the year 2018.

Although the prevalence is low, concerns remain in this region. First, unlike the global downward trend in new HIV infections and AIDS-related deaths, the numbers have continuously increased in the MENA. Second, compared to the global rate of antiretroviral therapy (62%), the MENA region's rate is far below (32%). The low participation of ART increases not only the number of AIDS-related deaths but the risk of mother-to-baby HIV infections, in which the MENA (24.7%) shows relatively high rates compared to other regions, for example, southern Africa (10%), Asia and the Pacific (17%).

Key population at high risk in this region is identified as injection drug users, female sex workers and men who have sex with men.

South and South-East Asia

Main article: HIV/AIDS in Asia

The geographical size and human diversity of South and South-East Asia have resulted in HIV epidemics differing across the region.

In South and Southeast Asia, the HIV epidemic remains largely concentrated in injecting drug users, men who have sex with men (MSM), sex workers, and clients of sex workers and their immediate sexual partners. In the Philippines, in particular, sexual contact between males comprise the majority of new infections. An HIV surveillance study conducted by Dr. Louie Mar Gangcuangco and colleagues from the University of the Philippines-Philippine General Hospital showed that out of 406 MSM tested for HIV in Metro Manila, HIV prevalence was 11.8% (95% confidence interval: 8.7- 15.0).

Migrants, in particular, are vulnerable and 67% of those infected in Bangladesh and 41% in Nepal are migrants returning from India. This is in part due to human trafficking and exploitation, but also because even those migrants who willingly go to India in search of work are often afraid to access state health services due to concerns over their immigration status.

East Asia

Main article: HIV/AIDS in Asia

The national HIV prevalence levels in East Asia is 0.1% in the adult (15–49) group. However, due to the large populations of many East Asian nations, this low national HIV prevalence still means that large numbers of people are infected with HIV. The picture in this region is dominated by China. Much of the current spread of HIV in China is through injecting drug use and paid sex. In China, the number was estimated at between 430,000 and 1.5 million by independent researchers, with some estimates going much higher.

In the rural areas of China, where large numbers of farmers, especially in Henan province, participated in unclean blood transfusions; estimates of those infected are in the tens of thousands. In Japan, just over half of HIV/AIDS cases are officially recorded as occurring amongst homosexual men, with the remainder occurring amongst heterosexuals and also via drug abuse, in the womb or unknown means.

In East Asia, men who have sex with men account for 18% of new HIV/AIDS cases and are therefore a key affected group along with sex workers and their clients who makeup 29% of new cases. This is also a noteworthy aspect because men who have sex with men had a prevalence of at least 5% or higher in countries in Asia and Pacific.

Americas

Caribbean

Main article: HIV/AIDS in the Caribbean

The Caribbean is the second-most affected region in the world. Among adults aged 15–44, AIDS has become the leading cause of death. The region's adult prevalence rate is 0.9%. with national rates ranging up to 2.7%. HIV transmission occurs largely through heterosexual intercourse. A greater number of people who get infected with HIV/AIDS are heterosexuals. with two-thirds of AIDS cases in this region attributed to this route. Sex between men is also a significant route of transmission, even though it is heavily stigmatised and illegal in many areas. HIV transmission through injecting drug use remains rare, except in Bermuda and Puerto Rico.

Within the Caribbean, the country with the highest prevalence of HIV/AIDS is the Bahamas with a rate of 3.2% of adults with the disease. However, when comparing rates from 2004 to 2013, the number of newly diagnosed cases of HIV decreased by 4% over those years. Increased education and treatment drugs will help to decrease incidence levels even more.

Central and South America

Main article: HIV/AIDS in Latin America

The populations of Central and South America have approximately 1.6 million people currently infected with HIV and this number has remained relatively unvarying with having a prevalence of approximately .4%. In Latin America, those infected with the disease have received help in the form of Antiretroviral treatment, with 75% of people with HIV receiving the treatment.

In these regions of the American continent, only Guatemala and Honduras have national HIV prevalence of over 1%. In these countries, HIV-infected men outnumber HIV-infected women by roughly 3:1.

With HIV/AIDS incidence levels rising in Central America, education is the most important step in controlling the spread of this disease. In Central America, many people do not have access to treatment drugs. This results in 8–14% of people dying from AIDS in Honduras. To reduce the incidence levels of HIV/AIDS, education and drug access needs to improve.

In a study of immigrants traveling to Europe, all asymptomatic persons were tested for a variety of infectious diseases. The prevalence of HIV among the 383 immigrants from Latin America was low, with only one person testing positive for a HIV infection. This data was collected from a group of immigrants with the majority from Bolivia, Ecuador and Colombia.

United States

Main article: HIV/AIDS in the United States

Since the epidemic began in the early 1980s, 1,216,917 people have been diagnosed with AIDS in the US. In 2016, 14% of the 1.1 million people over age 13 living with HIV were unaware of their infection. The most recent CDC HIV Surveillance Report estimates that 38,281 new cases of HIV were diagnosed in the United States in 2017, a rate of 11.8 per 100,000 population. Men who have sex with men accounted for approximately 8 out of 10 HIV diagnoses among males. Regionally, the population rates (per 100,000 people) of persons diagnosed with HIV infection in 2015 were highest in the South (16.8), followed by the Northeast (11.6), the West (9.8), and the Midwest (7.6).

The most frequent mode of transmission of HIV continues to be through male homosexual sexual relations. In general, recent studies have shown that 1 in 6 gay and bisexual men were infected with HIV. As of 2014, in the United States, 83% of new HIV diagnoses among all males aged 13 and older and 67% of the total estimated new diagnoses were among homosexual and bisexual men. Those aged 13 to 24 also accounted for an estimated 92% of new HIV diagnoses among all men in their age group.

A review of studies containing data regarding the prevalence of HIV in transgender women found that nearly 11.8% self-reported that they were infected with HIV. Along with these findings, recent studies have also shown that transgender women are 34 times more likely to have HIV than other women. A 2008 review of HIV studies among transgender women found that 28 percent tested positive for HIV. In the National Transgender Discrimination Survey, 20.23% of black respondents reported being HIV-positive, with an additional 10% reporting that they were unaware of their status.

AIDS is one of the top three causes of death for African American men aged 25–54 and for African American women aged 35–44 years in the United States of America. In the United States, African Americans make up about 48% of the total HIV-positive population and make up more than half of new HIV cases, despite making up only 12% of the population. The main route of transmission for women is through unprotected heterosexual sex. African American women are 19 times more likely to contract HIV than other women.

By 2008, there was increased awareness that young African-American women in particular were at high risk for HIV infection. In 2010, African Americans made up 10% of the population but about half of the HIV/AIDS cases nationwide. This disparity is attributed in part to a lack of information about AIDS and a perception that they are not vulnerable, as well as to limited access to health-care resources and a higher likelihood of sexual contact with at-risk male sexual partners.

Since 1985, the incidence of HIV infection among women had been steadily increasing. In 2005 it was estimated that at least 27% of new HIV infections were in women. There has been increasing concern for the concurrency of violence surrounding women infected with HIV. In 2012, a meta-analysis showed that the rates of psychological trauma, including Intimate Partner Violence and PTSD in HIV positive women were more than five times and twice the national averages, respectively. In 2013, the White House commissioned an Interagency Federal Working Group to address the intersection of violence and women infected with HIV.

There are also geographic disparities in AIDS prevalence in the United States, where it is most common in the large cities of California, esp. Los Angeles and San Francisco and the East Coast, ex. New York City and in urban cities of the Deep South. Rates are lower in Utah, Texas, and Northern Florida. Washington, D.C., the nation's capital, has the nation's highest rate of infection, at 3%. This rate is comparable to what is seen in west Africa, and is considered a severe epidemic.

In the United States in particular, a new wave of infection is being blamed on the use of methamphetamine, known as crystal meth. Research presented at the 12th Annual Retrovirus Conference in Boston in February 2005 concluded that using crystal meth or cocaine is the biggest single risk factor for becoming HIV+ among US gay men, contributing 29% of the overall risk of becoming positive and 28% of the overall risk of being the receptive partner in anal sex.

In addition, several renowned clinical psychologists now cite methamphetamine as the biggest problem facing gay men today, including Michael Majeski, who believes meth is the catalyst for at least 80% of seroconversions currently occurring across the United States, and Tony Zimbardi, who calls methamphetamine the number one cause of HIV transmission, and says that high rates of new HIV infection are not being found among non-crystal users. In addition, various HIV and STD clinics across the United States report anecdotal evidence that 75% of new HIV seroconversions they deal with are methamphetamine-related; indeed, in Los Angeles, methamphetamine usage is regarded as the main cause of HIV seroconversion among gay men in their late thirties. The chemical "methamphetamine", in and of itself, cannot infect someone with HIV.

Canada

Main article: HIV/AIDS in Canada

In 2016, there were approximately 63,100 people living with HIV/AIDS in Canada. It was estimated that 9090 persons were living with undiagnosed HIV at the end of 2016. Mortality has decreased due to medical advances against HIV/AIDS, especially highly active antiretroviral therapy (HAART). HIV/AIDS prevalence is increasing most rapidly amongst aboriginal Canadians, with 11.3% of new infections in 2016.

Eastern Europe and Central Asia

Main article: HIV/AIDS in Eastern Europe and Central Asia

There is growing concern about a rapidly growing epidemic in Eastern Europe and Central Asia, where an estimated 1.23–3.7 million people were infected as of December 2011, though the adult (15–49) prevalence rate is low (1.1%). The rate of HIV infections began to grow rapidly from the mid-1990s, due to social and economic collapse, increased levels of intravenous drug use and increased numbers of sex workers. By 2010 the number of reported cases in Russia was over 450,000 according to the World Health Organization, up from 15,000 in 1995 and 190,000 in 2002; some estimates claim the real number is up to eight times higher, well over 2 million.

Ukraine and Estonia also have growing numbers of infected people, with estimates of 240,000 and 7,400 respectively in 2018. Also, transmission of HIV is increasing through sexual contact and drug use among the young (<30 years). Indeed, over 84% of current AIDS cases in this region occur in non-drug-using heterosexuals less than 26 years of age.

Western Europe

Main article: HIV/AIDS in Western Europe

In most countries of Western Europe, AIDS cases have fallen to levels not seen since the original outbreak; many attribute this trend to aggressive educational campaigns, screening of blood transfusions and increased use of condoms. Also, the death rate from AIDS in Western Europe has fallen sharply, as new AIDS therapies have proven to be an effective (though expensive) means of suppressing HIV.

In this area, the routes of transmission of HIV is diverse, including paid sex, injecting drug use, mother to child, male with male sex and heterosexual sex. However, many new infections in this region occur through contact with HIV-infected individuals from other regions. The adult (15–49) prevalence in this region is 0.3% with between 570,000 and 890,000 people currently infected with HIV. Due to the availability of antiretroviral therapy, AIDS deaths have stayed low since the lows of the late 1990s. However, in some countries, a large share of HIV infections remain undiagnosed and there is worrying evidence of antiretroviral drug resistance among some newly HIV-infected individuals in this region.

Oceania

Main articles: HIV/AIDS in Australia, HIV/AIDS in New Zealand, and HIV/AIDS in Papua New Guinea

There is a very large range of national situations regarding AIDS and HIV in this region. This is due in part to the large distances between the islands of Oceania. The wide range of development in the region also plays an important role. The prevalence is estimated at between 0.2% and 0.7%, with between 45,000 and 120,000 adults and children currently infected with HIV.

Papua New Guinea has one of the most serious AIDS epidemics in the region. According to UNAIDS, HIV cases in the country have been increasing at a rate of 30 percent annually since 1997, and the country's HIV prevalence rate in late 2006 was 1.3%.

AIDS research and society

In June 2001, the United Nations held a Special General Assembly to intensify international action to fight the HIV/AIDS pandemic as a global health issue, and to mobilize the resources needed towards this aim, labelling the situation a "global crisis".

Regarding the social effects of the HIV/AIDS pandemic, some sociologists suggest that AIDS has caused a "profound re-medicalisation of sexuality".

There has been extensive research done with HIV since 2001 in the United States, The National Institutes of Health (NIH) which is an agency funded by the U.S department of Health and Human Services (HHS) has substantially improved the health, treatment, and lives of many individuals across the nation. The human immunodeficiency virus (HIV) is generally the precursor to AIDS. To this day there is no cure for this virus; However, treatment, education programs, proper medical care, and support have been made available.

NIH, is coordinated by the Office of AIDS Research (OAR) and this research carried out by nearly all the NIH Institutes and Centers, in both at NIH and at NIH-funded institutions worldwide. The NIH HIV/AIDS Research Program, represents the world's largest public investment in AIDS research. Other agencies like the National Institute of Allergy and Infectious Diseases have also made substantial efforts to provide the latest and newest research and treatment available.

The NIH found that in certain areas of the world, the correlation in risky behaviors and the acquisition of HIV/AIDS is causational. Consistent drug usage and related risk behaviors, such as the exchange of sex for drugs or money, are linked to an increased risk of HIV acquisition in marginalized areas. NIAID and other NIH institutes work to develop and optimize harm reduction interventions that decrease the risk of drug use-associated and sexual transmission of HIV among injecting and non-injecting drug users. Most organizations work collectively around the globe to understand, diagnose, treat, and battle the spread of this notorious disease, through the use of intervention and preventive programs the risk of acquiring HIV and the development of AIDS has dramatically dropped by 40% since its peak of cases back in 1998.

Despite the advancements in scientific research and treatment, to this day there's no available cure for HIV/AIDS. Yet major efforts to contain the disease and improve the lives of many individuals through modernized anti-viral therapy have resulted in positive and promising results that may one day lead to a cure. The U.S. President's Emergency Plan for AIDS Relief (PEPFAR) is one of the largest U.S. Government's response to the global HIV/AIDS epidemic and represents the largest commitment by any nation to address a single disease in history. PEPFAR provided HIV testing services for 79.6 million people in Fiscal Year 2019 and, as of September 30, 2019, supported lifesaving anti-retroviral therapy for nearly 15.7 million men, women, and children. As of the end of 2019, 25.4 million people with HIV (67%) were accessing antiretroviral therapy (ART) globally. That means 12.6 million people are still waiting. HIV treatment access is key to the global effort to end AIDS as a public health threat. Individuals who not only are aware of their condition but also are prescribed ART, are encouraged to remain consistent with their daily-dosage treatment so they can reduce the spread, viral load, and live happy and healthy lives.

Because HIV is more prevalent in urban areas of the United States, individuals living in rural areas generally don't participate or receive HIV diagnosis. The CDC found huge disparities in HIV cases between Northern and Southern regions of the Nation. At a rate of 15.9 the Southern regions account for a large number of reports of HIV; subsequently, regions like the North and Midwest account for general rates between 9 and 7.2 making it significantly lower in case prevalence.

According to the CDC, populations affected and with most reported cases of HIV are generally found in gay, bisexual, and other men who reported male-to-male sexual contact. In 2018, gay and bisexual men accounted for 69% of the 37,968 new HIV diagnoses and 86% of diagnoses among males. HIV doesn't only affect individuals in this category, heterosexuals tend to be affected by HIV as well. In 2018, heterosexuals accounted for 24% of the 37,968 new HIV diagnoses in the United States.

• Heterosexual men accounted for 8% of new HIV diagnoses.
• Heterosexual women accounted for 16% of new HIV diagnoses.

UNAIDS also suggested that the individuals who may also be at risk of acquiring this disease are generally:

• 26 times higher among men who have sex with men.
• 29 times higher among people who inject drugs.
• 30 times higher for sex workers.
• 13 times higher for transgender people.

Muller's ratchet

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Muller%27s_ratchet 

 

Illustration of chromosome crossover during genetic recombination

In evolutionary genetics, Muller's ratchet (named after Hermann Joseph Muller, by analogy with a ratchet effect) is a process through which, absence of recombination (especially in an asexual population), an accumulation of irreversible deleterious mutations results. This happens due to the fact that in the absence of recombination, and assuming reverse mutations are rare, offspring bear at least as much mutational load as their parents. Muller proposed this mechanism as one reason why sexual reproduction may be favored over asexual reproduction, as sexual organisms benefit from recombination and consequent elimination of deleterious mutations. The negative effect of accumulating irreversible deleterious mutations may not be prevalent in organisms which, while they reproduce asexually, also undergo other forms of recombination. This effect has also been observed in those regions of the genomes of sexual organisms that do not undergo recombination.

Etymology

Although Muller discussed the advantages of sexual reproduction in his 1932 talk, it does not contain the word "ratchet". Muller first introduced the term "ratchet" in his 1964 paper, and the phrase "Muller's ratchet" was coined by Joe Felsenstein in his 1974 paper, "The Evolutionary Advantage of Recombination".

Explanation

Asexual reproduction compels genomes to be inherited as indivisible blocks so that once the least mutated genomes in an asexual population begin to carry at least one deleterious mutation, no genomes with fewer such mutations can be expected to be found in future generations (except as a result of back mutation). This results in an eventual accumulation of mutations known as genetic load. In theory, the genetic load carried by asexual populations eventually becomes so great that the population goes extinct. Also, laboratory experiments have confirmed the existence of the ratchet and the consequent extinction of populations in many organisms (under intense drift and when recombinations are not allowed) including RNA viruses, bacteria, and eukaryotes. In sexual populations, the process of genetic recombination allows the genomes of the offspring to be different from the genomes of the parents. In particular, progeny (offspring) genomes with fewer mutations can be generated from more highly mutated parental genomes by putting together mutation-free portions of parental chromosomes. Also, purifying selection, to some extent, unburdens a loaded population when recombination results in different combinations of mutations.

Among protists and prokaryotes, a plethora of supposedly asexual organisms exists. More and more are being shown to exchange genetic information through a variety of mechanisms. In contrast, the genomes of mitochondria and chloroplasts do not recombine and would undergo Muller's ratchet were they not as small as they are (see Birdsell and Wills [pp. 93–95]). Indeed, the probability that the least mutated genomes in an asexual population end up carrying at least one (additional) mutation depends heavily on the genomic mutation rate and this increases more or less linearly with the size of the genome (more accurately, with the number of base pairs present in active genes). However, reductions in genome size, especially in parasites and symbionts, can also be caused by direct selection to get rid of genes that have become unnecessary. Therefore, a smaller genome is not a sure indication of the action of Muller's ratchet.

In sexually reproducing organisms, nonrecombining chromosomes or chromosomal regions such as the mammalian Y chromosome (with the exception of multicopy sequences which do engage intrachromosomal recombination and gene conversion) should also be subject to the effects of Muller's ratchet. Such nonrecombining sequences tend to shrink and evolve quickly. However, this fast evolution might also be due to these sequences' inability to repair DNA damage via template-assisted repair, which is equivalent to an increase in the mutation rate for these sequences. Ascribing cases of genome shrinkage or fast evolution to Muller's ratchet alone is not easy.

Muller's ratchet relies on genetic drift, and turns faster in smaller populations because in such populations deleterious mutations have a better chance of fixation. Therefore, it sets the limits to the maximum size of asexual genomes and to the long-term evolutionary continuity of asexual lineages. However, some asexual lineages are thought to be quite ancient; Bdelloid rotifers, for example, appear to have been asexual for nearly 40 million years. However, rotifers were found to possess a substantial number of foreign genes from possible horizontal gene transfer events. Furthermore, a vertebrate fish, Poecilia formosa, seems to defy the ratchet effect, having existed for 500,000 generations. This has been explained by maintenance of genomic diversity through parental introgression and a high level of heterozygosity resulting from the hybrid origin of this species.

Calculation of the fittest class

In 1978 John Haigh used a Wright-Fisher model to analyze the effect of Muller's ratchet in an asexual population. If the ratchet is operating the fittest class (least loaded individuals) is small and prone to extinction by the effect of genetic drift. In his paper Haigh derives the equation that calculates the frequency of individuals carrying ${\displaystyle k}$ mutations for the population with stationary distribution:

${\displaystyle n_{k}\ =\ {\frac {Ne^{-\theta }\theta ^{k}}{k!}}}$

${\displaystyle \theta =\ {\frac {\lambda }{s}}}$

where, ${\displaystyle n_{k}\ }$ is the number of individual carrying ${\displaystyle k}$ mutations, ${\displaystyle N}$ is the population size, ${\displaystyle \lambda }$ is the mutation rate and ${\displaystyle s}$ is the selection coefficient.

Thus, the frequency of the individuals of the fittest class (${\displaystyle k=0}$) is:

${\displaystyle {n_{0}}\ =N\ \ e^{-\ \theta }}$

In an asexual population which suffers from ratchet the frequency of fittest individuals would be small, and go extinct after few generations. This is called a click of the ratchet. Following each click, the rate of accumulation of deleterious mutation would increase, and ultimately results in the extinction of the population.

The antiquity of recombination and Muller's ratchet

It has been argued that recombination was an evolutionary development as ancient as life on Earth. Early RNA replicators capable of recombination may have been the ancestral sexual source from which asexual lineages could periodically emerge. Recombination in the early sexual lineages may have provided a means for coping with genome damage. Muller's ratchet under such ancient conditions would likely have impeded the evolutionary persistence of the asexual lineages that were unable to undergo recombination.

Muller's ratchet and mutational meltdown

Since deleterious mutations are harmful by definition, accumulation of them would result in loss of individuals and a smaller population size. Small populations are more susceptible to the ratchet effect and more deleterious mutations would be fixed as a result of genetic drift. This creates a positive feedback loop which accelerates extinction of small asexual populations. This phenomenon has been called mutational meltdown.

Gene flow

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Gene_flow 

 

Gene flow is the transfer of alleles from one population to another population through immigration of individuals.

In population genetics, gene flow (also known as gene migration or geneflow and allele flow) is the transfer of genetic material from one population to another. If the rate of gene flow is high enough, then two populations will have equivalent allele frequencies and therefore can be considered a single effective population. It has been shown that it takes only "one migrant per generation" to prevent populations from diverging due to drift. Populations can diverge due to selection even when they are exchanging alleles, if the selection pressure is strong enough. Gene flow is an important mechanism for transferring genetic diversity among populations. Migrants change the distribution of genetic diversity among populations, by modifying allele frequencies (the proportion of members carrying a particular variant of a gene). High rates of gene flow can reduce the genetic differentiation between the two groups, increasing homogeneity.^[4] For this reason, gene flow has been thought to constrain speciation and prevent range expansion by combining the gene pools of the groups, thus preventing the development of differences in genetic variation that would have led to differentiation and adaption. In some cases dispersal resulting in gene flow may also result in the addition of novel genetic variants under positive selection to the gene pool of a species or population (adaptive introgression.)

There are a number of factors that affect the rate of gene flow between different populations. Gene flow is expected to be lower in species that have low dispersal or mobility, that occur in fragmented habitats, where there is long distances between populations, and when there are small population sizes. Mobility plays an important role in dispersal rate, as highly mobile individuals tend to have greater movement prospects. Although animals are thought to be more mobile than plants, pollen and seeds may be carried great distances by animals, water or wind. When gene flow is impeded, there can be an increase in inbreeding, measured by the inbreeding coefficient (F) within a population. For example, many island populations have low rates of gene flow due to geographic isolation and small population sizes. The Black Footed Rock Wallaby has several inbred populations that live on various islands off the coast of Australia. The population is so strongly isolated that lack of gene flow has led to high rates of inbreeding.

Measuring gene flow

The level of gene flow among populations can be estimated by observing the dispersal of individuals and recording their reproductive success. This direct method is only suitable for some types of organisms, more often indirect methods are used that infer gene flow by comparing allele frequencies among population samples. The more genetically differentiated two populations are, the lower the estimate of gene flow, because gene flow has a homogenizing effect. Isolation of populations leads to divergence due to drift, while migration reduces divergence. Gene flow can be measured by using the effective population size (${\displaystyle N_{e}}$) and the net migration rate per generation (m). Using the approximation based on the Island model, the effect of migration can be calculated for a population in terms of the degree of genetic differentiation(${\displaystyle F_{ST}}$). This formula accounts for the proportion of total molecular marker variation among populations, averaged over loci. When there is one migrant per generation, the inbreeding coefficient (${\displaystyle F_{ST}}$) equals 0.2. However, when there is less than 1 migrant per generation (no migration), the inbreeding coefficient rises rapidly resulting in fixation and complete divergence (${\displaystyle F_{ST}}$ = 1). The most common ${\displaystyle F_{ST}}$ is < 0.25. This means there is some migration happening. Measures of population structure range from 0 to 1. When gene flow occurs via migration the deleterious effects of inbreeding can be ameliorated.

${\displaystyle F_{ST}=1/(4N_{e}m+1)}$

The formula can be modified to solve for the migration rate when ${\displaystyle F_{ST}}$ is known: ${\displaystyle Nm=((1/F_{ST})-1)/4={\tfrac {1-F_{ST}}{4*F_{ST}}}}$, Nm = number of migrants.

Barriers to gene flow

Allopatric speciation

Examples of speciation affecting gene flow.

When gene flow is blocked by physical barriers, this results in Allopatric speciation or a geographical isolation that does not allow populations of the same species to exchange genetic material. Physical barriers to gene flow are usually, but not always, natural. They may include impassable mountain ranges, oceans, or vast deserts. In some cases, they can be artificial, man-made barriers, such as the Great Wall of China, which has hindered the gene flow of native plant populations. One of these native plants, Ulmus pumila, demonstrated a lower prevalence of genetic differentiation than the plants Vitex negundo, Ziziphus jujuba, Heteropappus hispidus, and Prunus armeniaca whose habitat is located on the opposite side of the Great Wall of China where Ulmus pumila grows. This is because Ulmus pumila has wind-pollination as its primary means of propagation and the latter-plants carry out pollination through insects. Samples of the same species which grow on either side have been shown to have developed genetic differences, because there is little to no gene flow to provide recombination of the gene pools.

Sympatric speciation

Barriers to gene flow need not always be physical. Sympatric speciation happens when new species from the same ancestral species arise along the same range. This is often a result of a reproductive barrier. For example, two palm species of Howea found on Lord Howe Island were found to have substantially different flowering times correlated with soil preference, resulting in a reproductive barrier inhibiting gene flow. Species can live in the same environment, yet show very limited gene flow due to reproductive barriers, fragmentation, specialist pollinators, or limited hybridization or hybridization yielding unfit hybrids. A cryptic species is a species that humans cannot tell is different without the use of genetics. Moreover, gene flow between hybrid and wild populations can result in loss of genetic diversity via genetic pollution, assortative mating and outbreeding. In human populations, genetic differentiation can also result from endogamy, due to differences in caste, ethnicity, customs and religion.

Human assisted gene-flow

Genetic rescue

Gene flow can also be used to assist species which are threatened with extinction. When a species exist in small populations there is an increased risk of inbreeding and greater susceptibility to loss of diversity due to drift. These populations can benefit greatly from the introduction of unrelated individuals who can increase diversity and reduce the amount of inbreeding, and potentially increase population size. This was demonstrated in the lab with two bottleneck strains of Drosophila melanogaster, in which crosses between the two populations reversed the effects of inbreeding and led to greater chances of survival in not only one generation but two.

Genetic pollution

Main article: Genetic pollution

Human activities such as movement of species and modification of landscape can result in genetic pollution, hybridization, introgression and genetic swamping. These processes can lead to homogenization or replacement of local genotypes as a result of either a numerical and/or fitness advantage of introduced plant or animal. Nonnative species can threaten native plants and animals with extinction by hybridization and introgression either through purposeful introduction by humans or through habitat modification, bringing previously isolated species into contact. These phenomena can be especially detrimental for rare species coming into contact with more abundant ones which can occur between island and mainland species. Interbreeding between the species can cause a 'swamping' of the rarer species' gene pool, creating hybrids that supplant the native stock. This is a direct result of evolutionary forces such as natural selection, as well as genetic drift, which lead to the increasing prevalence of advantageous traits and homogenization. The extent of this phenomenon is not always apparent from outward appearance alone. While some degree of gene flow occurs in the course of normal evolution, hybridization with or without introgression may threaten a rare species' existence. For example, the Mallard is an abundant species of duck that interbreeds readily with a wide range of other ducks and poses a threat to the integrity of some species.

Urbanization

There are two main models for how urbanization affects gene flow of urban populations. The first is through habitat fragmentation, also called urban fragmentation, in which alterations to the landscape that disrupt or fragment the habitat decrease genetic diversity. The second is called the urban facilitation model, and suggests that in some populations, gene flow is enabled by anthropogenic changes to the landscape. Urban facilitation of gene flow connects populations, reduces isolation, and increases gene flow into an area which would otherwise not have this specific genome composition.

Urban facilitation can occur in many different ways, but most of the mechanisms include bringing previously separated species into contact, either directly or indirectly. Altering a habitat through urbanization will cause habitat fragmentation, but could also potentially disrupt barriers and create a pathway, or corridor, that can connect two formerly separated species. The effectiveness of this depends on individual species’ dispersal abilities and adaptiveness to different environments to use anthropogenic structures to travel. Human-driven climate change is another mechanism by which southern-dwelling animals might be forced northward towards cooler temperatures, where they could come into contact with other populations not previously in their range. More directly, humans are known to introduce non-native species into new environments, which could lead to hybridization of similar species.

This urban facilitation model was tested on a human health pest, the Western black widow spider (Latrodectus hesperus). A study by Miles et al. collected genome-wide single nucleotide polymorphism variation data in urban and rural spider populations and found evidence for increased gene flow in urban Western black widow spiders compared to rural populations. In addition, the genome of these spiders was more similar across rural populations than it was for urban populations, suggesting increased diversity, and therefore adaptation, in the urban populations of the Western black widow spider. Phenotypically, urban spiders are larger, darker, and more aggressive, which could lead to increased survival in urban environments. These findings demonstrate support for urban facilitation, as these spiders are actually able to spread and diversify faster across urban environments than they would in a rural one. However, it is also an example of how urban facilitation, despite increasing gene flow, is not necessarily beneficial to an environment, as Western black widow spiders have highly toxic venom and therefore pose risks for human health.

Another example of urban facilitation is that of migrating bobcats (Lynx rufus) in the northern US and southern Canada. A study by Marrote et al. sequenced fourteen different microsatellite loci in bobcats across the Great Lakes region, and found that longitude affected the interaction between anthropogenic landscape alterations and bobcat population gene flow. While rising global temperatures push bobcat populations into northern territory, increased human activity also enables bobcat migration northward. The increased human activity brings increased roads and traffic, but also increases road maintenance, plowing, and snow compaction, inadvertently clearing a path for bobcats to travel by. The anthropogenic influence on bobcat migration pathways is an example of urban facilitation via opening up a corridor for gene flow. However, in the bobcat's southern range, an increase in roads and traffic is correlated with a decrease in forest cover, which hinders bobcat population gene flow through these areas. Somewhat ironically, the movement of bobcats northward is caused by human-driven global warming, but is also enabled by increased anthropogenic activity in northern ranges that make these habitats more suitable to bobcats.

Consequences of urban facilitation vary from species to species. Positive effects of urban facilitation can occur when increased gene flow enables better adaptation and introduces beneficial alleles, and would ideally increase biodiversity. This has implications for conservation: for example, urban facilitation benefits an endangered species of tarantula and could help increase the population size. Negative effects would occur when increased gene flow is maladaptive and causes the loss of beneficial alleles. In the worst-case scenario, this would lead to genomic extinction through a hybrid swarm. It is also important to note that in the scheme of overall ecosystem health and biodiversity, urban facilitation is not necessarily beneficial, and generally applies to urban adapter pests. Examples of this include the previously mentioned Western black widow spider, and also the cane toad, which was able to use roads by which to travel and overpopulate Australia.

Gene flow between species

Horizontal gene transfer

Main article: Horizontal gene transfer

Horizontal gene transfer (HGT) refers to the transfer of genes between organisms in a manner other than traditional reproduction, either through transformation (direct uptake of genetic material by a cell from its surroundings), conjugation (transfer of genetic material between two bacterial cells in direct contact), transduction (injection of foreign DNA by a bacteriophage virus into the host cell) or GTA-mediated transduction (transfer by a virus-like element produced by a bacterium) .

Viruses can transfer genes between species. Bacteria can incorporate genes from dead bacteria, exchange genes with living bacteria, and can exchange plasmids across species boundaries. "Sequence comparisons suggest recent horizontal transfer of many genes among diverse species including across the boundaries of phylogenetic 'domains'. Thus determining the phylogenetic history of a species can not be done conclusively by determining evolutionary trees for single genes."

Biologist Gogarten suggests "the original metaphor of a tree no longer fits the data from recent genome research". Biologists [should] instead use the metaphor of a mosaic to describe the different histories combined in individual genomes and use the metaphor of an intertwined net to visualize the rich exchange and cooperative effects of horizontal gene transfer.

"Using single genes as phylogenetic markers, it is difficult to trace organismal phylogeny in the presence of HGT. Combining the simple coalescence model of cladogenesis with rare HGT events suggest there was no single last common ancestor that contained all of the genes ancestral to those shared among the three domains of life. Each contemporary molecule has its own history and traces back to an individual molecule cenancestor. However, these molecular ancestors were likely to be present in different organisms at different times."

Hybridization

In some instances, when a species has a sister species and breeding capabilities are possible due to the removal of previous barriers or through introduction due to human intervention, species can hybridize and exchange genes and corresponding traits. This exchange is not always clear-cut, for sometimes the hybrids may look identical to the original species phenotypically but upon testing the mtDNA it is apparent that hybridization has occurred. Differential hybridization also occurs because some traits and DNA are more readily exchanged than others, and this is a result of selective pressure or the absence thereof that allows for easier transaction. In instances in which the introduced species begins to replace the native species, the native species becomes threatened and the biodiversity is reduced, thus making this phenomenon negative rather than a positive case of gene flow that augments genetic diversity. Introgression is the replacement of one species' alleles with that of the invader species. It is important to note that hybrids are sometime less "fit" than their parental generation, and as a result is a closely monitored genetic issue as the ultimate goal in conservation genetics is to maintain the genetic integrity of a species and preserve biodiversity.

Examples

Marine iguana of the Galapagos Islands evolved via allopatric speciation, through limited gene flow and geographic isolation.

While gene flow can greatly enhance the fitness of a population, it can also have negative consequences depending on the population and the environment in which they reside. The effects of gene flow are context-dependent.

• Fragmented Population: fragmented landscapes such as the Galapagos Islands are an ideal place for adaptive radiation to occur as a result of differing geography. Darwin's finches likely experienced allopatric speciation in some part due to differing geography, but that doesn't explain why we see so many different kinds of finches on the same island. This is due to adaptive radiation, or the evolution of varying traits in light of competition for resources. Gene flow moves in the direction of what resources are abundant at a given time.
• Island Population: The marine iguana is an endemic species of the Galapagos Islands, but it evolved from a mainland ancestor of land iguana. Due to geographic isolation gene flow between the two species was limited and differing environments caused the marine iguana to evolve in order to adapt to the island environment. For instance, they are the only iguana that has evolved the ability to swim.
• Human Populations: In Europe Homo sapiens interbred with Neanderthals resulting in gene flow between these populations. This gene flow has resulted in Neanderthal alleles in modern European population. Two theories exist for the human evolution throughout the world. The first is known as the multiregional model in which modern human variation is seen as a product of radiation of Homo erectus out of Africa after which local differentiation led to the establishment of regional population as we see them now. Gene flow plays an important role in maintaining a grade of similarities and preventing speciation. In contrast the single origin theory assumes that there was a common ancestral population originating in Africa of Homo sapiens which already displayed the anatomical characteristics we see today. This theory minimizes the amount of parallel evolution that is needed.
• Butterflies: Comparisons between sympatric and allopatric populations of Heliconius melpomene, H. cydno, and H. timareta revealed a genome-wide trend of increased shared variation in sympatry, indicative of pervasive interspecific gene flow.
  
• Human-mediated gene flow: The captive genetic management of threatened species is the only way in which humans attempt to induce gene flow in ex situ situation. One example is the giant panda which is part of an international breeding program in which genetic materials are shared between zoological organizations in order to increase genetic diversity in the small populations. As a result of low reproductive success, artificial insemination with fresh/frozen-thawed sperm was developed which increased cub survival rate. A 2014 study found that high levels of genetic diversity and low levels of inbreeding were estimated in the breeding centers.
• Plants: Two populations of monkeyflowers were found to use different pollinators (bees and hummingbirds) that limited gene flow, resulting in genetic isolation, eventually producing two different species, Mimulus lewisii and Mimulus cardinalis .
• Sika deer: Sika deer were introduced into Western Europe, and they reproduce easily with the native red deer. This translocation of Sika deer has led to introgression and there are no longer "pure" red deer in the region, and all can be classified as hybrids.
• Bobwhite quail: Bobwhite quail were translocated from the southern part of the United States to Ontario in order to increase population numbers and game for hunting. The hybrids that resulted from this translocation was less fit than the native population and were not adapted to survive the Northern Winters.