Chemokine

From Wikipedia, the free encyclopedia

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

 

Small cytokines (intecrine/chemokine), interleukin-8 like
Solution structure of interleukin-8, a chemokine of the CXC subfamily
Identifiers
Symbol	IL8
Pfam	PF00048
InterPro	IPR001811
PROSITE	PDOC00434
SCOPe	3il8 / SUPFAM

Chemokines (Greek -kinos, movement) are a family of small cytokines, or signaling proteins secreted by cells. Their name is derived from their ability to induce directed chemotaxis in nearby responsive cells; they are chemotactic cytokines.

Cytokine proteins are classified as chemokines according to behavior and structural characteristics. In addition to being known for mediating chemotaxis, chemokines are all approximately 8-10 kilodaltons in mass and have four cysteine residues in conserved locations that are key to forming their 3-dimensional shape.

These proteins have historically been known under several other names including the SIS family of cytokines, SIG family of cytokines, SCY family of cytokines, Platelet factor-4 superfamily or intercrines. Some chemokines are considered pro-inflammatory and can be induced during an immune response to recruit cells of the immune system to a site of infection, while others are considered homeostatic and are involved in controlling the migration of cells during normal processes of tissue maintenance or development. Chemokines are found in all vertebrates, some viruses and some bacteria, but none have been found in other invertebrates.

Chemokines have been classified into four main subfamilies: CXC, CC, CX3C and XC. All of these proteins exert their biological effects by interacting with G protein-linked transmembrane receptors called chemokine receptors, that are selectively found on the surfaces of their target cells.

Function

Chemokines released by infected or damaged cells form a concentration gradient. Attracted cells move through the gradient towards the higher concentration of chemokine.

The major role of chemokines is to act as a chemoattractant to guide the migration of cells. Cells that are attracted by chemokines follow a signal of increasing chemokine concentration towards the source of the chemokine. Some chemokines control cells of the immune system during processes of immune surveillance, such as directing lymphocytes to the lymph nodes so they can screen for invasion of pathogens by interacting with antigen-presenting cells residing in these tissues. These are known as homeostatic chemokines and are produced and secreted without any need to stimulate their source cell(s). Some chemokines have roles in development; they promote angiogenesis (the growth of new blood vessels), or guide cells to tissues that provide specific signals critical for cellular maturation. Other chemokines are inflammatory and are released from a wide variety of cells in response to bacterial infection, viruses and agents that cause physical damage such as silica or the urate crystals that occur in gout. Their release is often stimulated by pro-inflammatory cytokines such as interleukin 1. Inflammatory chemokines function mainly as chemoattractants for leukocytes, recruiting monocytes, neutrophils and other effector cells from the blood to sites of infection or tissue damage. Certain inflammatory chemokines activate cells to initiate an immune response or promote wound healing. They are released by many different cell types and serve to guide cells of both innate immune system and adaptive immune system.

Types by function

Chemokines are functionally divided into two groups:

• Homeostatic: are constitutively produced in certain tissues and are responsible for basal leukocyte migration. These include: CCL14, CCL19, CCL20, CCL21, CCL25, CCL27, CXCL12 and CXCL13. This classification is not strict; for example, CCL20 can act also as pro-inflammatory chemokine.
• Inflammatory: these are formed under pathological conditions (on pro-inflammatory stimuli, such as IL-1, TNF-alpha, LPS, or viruses) and actively participate in the inflammatory response attracting immune cells to the site of inflammation. Examples are: CXCL-8, CCL2, CCL3, CCL4, CCL5, CCL11, CXCL10.

Homing

The main function of chemokines is to manage the migration of leukocytes (homing) in the respective anatomical locations in inflammatory and homeostatic processes. 

Basal: homeostatic chemokines are basal produced in the thymus and lymphoid tissues. Their homeostatic function in homing is best exemplified by the chemokines CCL19 and CCL21 (expressed within lymph nodes and on lymphatic endothelial cells) and their receptor CCR7 (expressed on cells destined for homing in cells to these organs). Using these ligands is possible routing antigen-presenting cells (APC) to lymph nodes during the adaptive immune response. Among other homeostatic chemokine receptors include: CCR9, CCR10, and CXCR5, which are important as part of the cell addresses for tissue-specific homing of leukocytes. CCR9 supports the migration of leukocytes into the intestine, CCR10 to the skin and CXCR5 supports the migration of B-cell to follicles of lymph nodes. As well CXCL12 (SDF-1) constitutively produced in the bone marrow promotes proliferation of progenitor B cells in the bone marrow microenvironment.

Inflammatory: inflammatory chemokines are produced in high concentrations during infection or injury and determine the migration of inflammatory leukocytes into the damaged area. Typical inflammatory chemokines include: CCL2, CCL3 and CCL5, CXCL1, CXCL2 and CXCL8. A typical example is CXCL-8, which acts as a chemoattractant for neutrophils. In contrast to the homeostatic chemokine receptors, there is significant promiscuity (redundancy) associated with binding receptor and inflammatory chemokines. This often complicates research on receptor-specific therapeutics in this area.

Types by cell attracted

• Monocytes / macrophages: the key chemokines that attract these cells to the site of inflammation include: CCL2, CCL3, CCL5, CCL7, CCL8, CCL13, CCL17 and CCL22.
• T-lymphocytes: the four key chemokines that are involved in the recruitment of T lymphocytes to the site of inflammation are: CCL2, CCL1, CCL22 and CCL17. Furthermore, CXCR3 expression by T-cells is induced following T-cell activation and activated T-cells are attracted to sites of inflammation where the IFN-y inducible chemokines CXCL9, CXCL10 and CXCL11 are secreted.
• Mast cells: on their surface express several receptors for chemokines: CCR1, CCR2, CCR3, CCR4, CCR5, CXCR2, and CXCR4. Ligands of these receptors CCL2 and CCL5 play an important role in mast cell recruitment and activation in the lung.There is also evidence that CXCL8 might be inhibitory of mast cells.
• Eosinophils: the migration of eosinophils into various tissues involved several chemokines of CC family: CCL11, CCL24, CCL26, CCL5, CCL7, CCL13, and CCL3. Chemokines CCL11 (eotaxin) and CCL5 (RANTES) acts through a specific receptor CCR3 on the surface of eosinophils, and eotaxin plays an essential role in the initial recruitment of eosinophils into the lesion.
• Neutrophils: are regulated primarily by CXC chemokines. An example CXCL8 (IL-8) is chemoattractant for neutrophils and also activating their metabolic and degranulation.

Structural characteristics

All chemokines share a typical Greek key structure that is stabilised by disulfide bonds between conserved cysteine residues.

 

Proteins are classified into the chemokine family based on their structural characteristics, not just their ability to attract cells. All chemokines are small, with a molecular mass of between 8 and 10 kDa. They are approximately 20-50% identical to each other; that is, they share gene sequence and amino acid sequence homology. They all also possess conserved amino acids that are important for creating their 3-dimensional or tertiary structure, such as (in most cases) four cysteines that interact with each other in pairs to create a Greek key shape that is a characteristic of chemokines. Intramolecular disulfide bonds typically join the first to third, and the second to fourth cysteine residues, numbered as they appear in the protein sequence of the chemokine. Typical chemokine proteins are produced as pro-peptides, beginning with a signal peptide of approximately 20 amino acids that gets cleaved from the active (mature) portion of the molecule during the process of its secretion from the cell. The first two cysteines, in a chemokine, are situated close together near the N-terminal end of the mature protein, with the third cysteine residing in the centre of the molecule and the fourth close to the C-terminal end. A loop of approximately ten amino acids follows the first two cysteines and is known as the N-loop. This is followed by a single-turn helix, called a 3₁₀-helix, three β-strands and a C-terminal α-helix. These helices and strands are connected by turns called 30s, 40s and 50s loops; the third and fourth cysteines are located in the 30s and 50s loops.

Types by structure

CC chemokines
Name	Gene	Other name(s)	Receptor	Uniprot
CCL1	Scya1	I-309, TCA-3	CCR8
CCL2	Scya2	MCP-1	CCR2	P13500
CCL3	Scya3	MIP-1a	CCR1	P10147
CCL4	Scya4	MIP-1β	CCR1, CCR5	P13236
CCL5	Scya5	RANTES	CCR5	P13501
CCL6	Scya6	C10, MRP-2	CCR1	P27784
CCL7	Scya7	MARC, MCP-3	CCR2	P80098
CCL8	Scya8	MCP-2	CCR1, CCR2, CCR5	P80075
CCL9/CCL10	Scya9	MRP-2, CCF18, MIP-1?	CCR1	P51670
CCL11	Scya11	Eotaxin	CCR2, CCR3, CCR5	P51671
CCL12	Scya12	MCP-5		Q62401
CCL13	Scya13	MCP-4, NCC-1, Ckβ10	CCR2, CCR3, CCR5	Q99616
CCL14	Scya14	HCC-1, MCIF, Ckβ1, NCC-2, CCL	CCR1	Q16627
CCL15	Scya15	Leukotactin-1, MIP-5, HCC-2, NCC-3	CCR1, CCR3	Q16663
CCL16	Scya16	LEC, NCC-4, LMC, Ckβ12	CCR1, CCR2, CCR5, CCR8	O15467
CCL17	Scya17	TARC, dendrokine, ABCD-2	CCR4	Q92583
CCL18	Scya18	PARC, DC-CK1, AMAC-1, Ckβ7, MIP-4		P55774
CCL19	Scya19	ELC, Exodus-3, Ckβ11	CCR7	Q99731
CCL20	Scya20	LARC, Exodus-1, Ckβ4	CCR6	P78556
CCL21	Scya21	SLC, 6Ckine, Exodus-2, Ckβ9, TCA-4	CCR7	O00585
CCL22	Scya22	MDC, DC/β-CK	CCR4	O00626
CCL23	Scya23	MPIF-1, Ckβ8, MIP-3, MPIF-1	CCR1	P55773
CCL24	Scya24	Eotaxin-2, MPIF-2, Ckβ6	CCR3	O00175
CCL25	Scya25	TECK, Ckβ15	CCR9	O15444
CCL26	Scya26	Eotaxin-3, MIP-4a, IMAC, TSC-1	CCR3	Q9Y258
CCL27	Scya27	CTACK, ILC, Eskine, PESKY, skinkine	CCR10	Q9Y4X3
CCL28	Scya28	MEC	CCR3, CCR10	Q9NRJ3
CXC chemokines
Name	Gene	Other name(s)	Receptor	Uniprot
CXCL1	Scyb1	Gro-a, GRO1, NAP-3, KC	CXCR2	P09341
CXCL2	Scyb2	Gro-β, GRO2, MIP-2a	CXCR2	P19875
CXCL3	Scyb3	Gro-?, GRO3, MIP-2β	CXCR2	P19876
CXCL4	Scyb4	PF-4	CXCR3B	P02776
CXCL5	Scyb5	ENA-78	CXCR2	P42830
CXCL6	Scyb6	GCP-2	CXCR1, CXCR2	P80162
CXCL7	Scyb7	NAP-2, CTAPIII, β-Ta, PEP		P02775
CXCL8	Scyb8	IL-8, NAP-1, MDNCF, GCP-1	CXCR1, CXCR2	P10145
CXCL9	Scyb9	MIG, CRG-10	CXCR3	Q07325
CXCL10	Scyb10	IP-10, CRG-2	CXCR3	P02778
CXCL11	Scyb11	I-TAC, β-R1, IP-9	CXCR3, CXCR7	O14625
CXCL12	Scyb12	SDF-1, PBSF	CXCR4, CXCR7	P48061
CXCL13	Scyb13	BCA-1, BLC	CXCR5	O43927
CXCL14	Scyb14	BRAK, bolekine		O95715
CXCL15	Scyb15	Lungkine, WECHE		Q9WVL7
CXCL16	Scyb16	SRPSOX	CXCR6	Q9H2A7
CXCL17	VCC-1	DMC, VCC-1		Q6UXB2
C chemokines
Name	Gene	Other name(s)	Receptor	Uniprot
XCL1	Scyc1	Lymphotactin a, SCM-1a, ATAC	XCR1	P47992
XCL2	Scyc2	Lymphotactin β, SCM-1β	XCR1	Q9UBD3
CX3C chemokines
Name	Gene	Other name(s)	Receptor	Uniprot
CX3CL1	Scyd1	Fractalkine, Neurotactin, ABCD-3	CX3CR1	P78423

Members of the chemokine family are divided into four groups depending on the spacing of their first two cysteine residues. Thus the nomenclature for chemokines is, e.g.: CCL1 for the ligand 1 of the CC-family of chemokines, and CCR1 for its respective receptor.

CC chemokines

The CC chemokine (or β-chemokine) proteins have two adjacent cysteines (amino acids), near their amino terminus. There have been at least 27 distinct members of this subgroup reported for mammals, called CC chemokine ligands (CCL)-1 to -28; CCL10 is the same as CCL9. Chemokines of this subfamily usually contain four cysteines (C4-CC chemokines), but a small number of CC chemokines possess six cysteines (C6-CC chemokines). C6-CC chemokines include CCL1, CCL15, CCL21, CCL23 and CCL28. CC chemokines induce the migration of monocytes and other cell types such as NK cells and dendritic cells.

Examples of CC chemokine include monocyte chemoattractant protein-1 (MCP-1 or CCL2) which induces monocytes to leave the bloodstream and enter the surrounding tissue to become tissue macrophages.

CCL5 (or RANTES) attracts cells such as T cells, eosinophils and basophils that express the receptor CCR5.

Increased CCL11 levels in blood plasma are associated with aging (and reduced neurogenesis) in mice and humans.

CXC chemokines

The two N-terminal cysteines of CXC chemokines (or α-chemokines) are separated by one amino acid, represented in this name with an "X". There have been 17 different CXC chemokines described in mammals, that are subdivided into two categories, those with a specific amino acid sequence (or motif) of glutamic acid-leucine-arginine (or ELR for short) immediately before the first cysteine of the CXC motif (ELR-positive), and those without an ELR motif (ELR-negative). ELR-positive CXC chemokines specifically induce the migration of neutrophils, and interact with chemokine receptors CXCR1 and CXCR2. An example of an ELR-positive CXC chemokine is interleukin-8 (IL-8), which induces neutrophils to leave the bloodstream and enter into the surrounding tissue. Other CXC chemokines that lack the ELR motif, such as CXCL13, tend to be chemoattractant for lymphocytes. CXC chemokines bind to CXC chemokine receptors, of which seven have been discovered to date, designated CXCR1-7.

C chemokines

The third group of chemokines is known as the C chemokines (or γ chemokines), and is unlike all other chemokines in that it has only two cysteines; one N-terminal cysteine and one cysteine downstream. Two chemokines have been described for this subgroup and are called XCL1 (lymphotactin-α) and XCL2 (lymphotactin-β).

CX₃C chemokines

A fourth group has also been discovered and members have three amino acids between the two cysteines and is termed CX₃C chemokine (or d-chemokines). The only CX₃C chemokine discovered to date is called fractalkine (or CX₃CL1). It is both secreted and tethered to the surface of the cell that expresses it, thereby serving as both a chemoattractant and as an adhesion molecule.

Receptors

Chemokine receptors are G protein-coupled receptors containing 7 transmembrane domains that are found on the surface of leukocytes. Approximately 19 different chemokine receptors have been characterized to date, which are divided into four families depending on the type of chemokine they bind; CXCR that bind CXC chemokines, CCR that bind CC chemokines, CX3CR1 that binds the sole CX3C chemokine (CX3CL1), and XCR1 that binds the two XC chemokines (XCL1 and XCL2). They share many structural features; they are similar in size (with about 350 amino acids), have a short, acidic N-terminal end, seven helical transmembrane domains with three intracellular and three extracellular hydrophilic loops, and an intracellular C-terminus containing serine and threonine residues important for receptor regulation. The first two extracellular loops of chemokine receptors each has a conserved cysteine residue that allow formation of a disulfide bridge between these loops. G proteins are coupled to the C-terminal end of the chemokine receptor to allow intracellular signaling after receptor activation, while the N-terminal domain of the chemokine receptor determines ligand binding specificity.

Signal transduction

Chemokine receptors associate with G-proteins to transmit cell signals following ligand binding. Activation of G proteins, by chemokine receptors, causes the subsequent activation of an enzyme known as phospholipase C (PLC). PLC cleaves a molecule called phosphatidylinositol (4,5)-bisphosphate (PIP2) into two second messenger molecules known as Inositol triphosphate (IP3) and diacylglycerol (DAG) that trigger intracellular signaling events; DAG activates another enzyme called protein kinase C (PKC), and IP3 triggers the release of calcium from intracellular stores. These events promote many signaling cascades (such as the MAP kinase pathway) that generate responses like chemotaxis, degranulation, release of superoxide anions and changes in the avidity of cell adhesion molecules called integrins within the cell harbouring the chemokine receptor.

Infection control

The discovery that the β chemokines RANTES, MIP (macrophage inflammatory proteins) 1α and 1β (now known as CCL5, CCL3 and CCL4 respectively) suppress HIV-1 provided the initial connection and indicated that these molecules might control infection as part of immune responses in vivo, and that sustained delivery of such inhibitors have the capacity of long-term infection control. The association of chemokine production with antigen-induced proliferative responses, more favorable clinical status in HIV infection, as well as with an uninfected status in subjects at risk for infection suggests a positive role for these molecules in controlling the natural course of HIV infection.

West Nile virus

From Wikipedia, the free encyclopedia

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

West Nile virus
A micrograph of the West Nile Virus, appearing in yellow
Virus classification
(unranked):	Virus
Realm:	Riboviria
Kingdom:	Orthornavirae
Phylum:	Kitrinoviricota
Class:	Flasuviricetes
Order:	Amarillovirales
Family:	Flaviviridae
Genus:	Flavivirus
Species:	West Nile virus

Ribbon representation of the NS2B/NS3 protease of West Nile virus

West Nile virus (WNV) is a single-stranded RNA virus that causes West Nile fever. It is a member of the family Flaviviridae, specifically from the genus Flavivirus, which also contains the Zika virus, dengue virus, and yellow fever virus. West Nile virus is primarily transmitted by mosquitoes, mostly species of Culex. The primary hosts of WNV are birds, so that the virus remains within a "bird–mosquito–bird" transmission cycle.

Structure

Like most other flaviviruses, WNV is an enveloped virus with icosahedral symmetry.  Image reconstructions and cryoelectron microscopy reveal a 45–50 nm virion covered with a relatively smooth protein shell; this structure is similar to the dengue fever virus, another Flavivirus. The protein shell is made of two structural proteins: the glycoprotein E and the small membrane protein M. Protein E has numerous functions including receptor binding, viral attachment, and entry into the cell through membrane fusion.

The outer protein shell is covered by a host-derived lipid membrane, the viral envelope. The flavivirus lipid membrane has been found to contain cholesterol and phosphatidylserine, but other elements of the membrane have yet to be identified. The lipid membrane has many roles in viral infection, including acting as signaling molecules and enhancing entry into the cell. Cholesterol, in particular, plays an integral part in WNV entering a host cell. The two viral envelope proteins, E and M, are inserted into the membrane.

The RNA genome is bound to capsid (C) proteins, which are 105 amino-acid residues long, to form the nucleocapsid. The capsid proteins are one of the first proteins created in an infected cell; the capsid protein is a structural protein whose main purpose is to package RNA into the developing viruses.^[9] The capsid has been found to prevent apoptosis by affecting the Akt pathway.

Genome

The West Nile virus genome. Modified after Guzman et al. 2010.

 

WNV is a positive-sense, single-stranded RNA virus. Its genome is approximately 11,000 nucleotides long and is flanked by 5' and 3' non-coding stem loop structures. The coding region of the genome codes for three structural proteins and seven nonstructural (NS) proteins, proteins that are not incorporated into the structure of new viruses. The WNV genome is first translated into a polyprotein and later cleaved by virus and host proteases into separate proteins (i.e. NS1, C, E).

Structural proteins

Structural proteins (C, prM/M, E) are capsid, precursor membrane proteins, and envelope proteins, respectively. The structural proteins are located at the 5' end of the genome and are cleaved into mature proteins by both host and viral proteases.

Structural Protein	Function
C	Capsid protein; encloses the RNA genome, packages RNA into immature virions.
prM/M	Viruses with M protein are infectious: the presence of M protein allows for the activation of proteins involved in viral entry into the cell. prM (precursor membrane) protein is present on immature virions, by further cleavage by furin to M protein, the virions become infectious.
E	A glycoprotein that forms the viral envelope, binds to receptors on the host cell surface in order to enter the cell.

Nonstructural proteins

Nonstructural proteins consist of NS1, NS2A, NS2B, NS3, NS4A, NS4B, and NS5. These proteins mainly assist with viral replication or act as proteases. The nonstructural proteins are located near the 3' end of the genome. 

Nonstructural Protein	Function
NS1	NS1 is a cofactor for viral replication, specifically for regulation of the replication complex.
NS2A	NS2A has a variety of functions: it is involved in viral replication, virion assembly, and inducing host cell death.
NS2B	A cofactor for NS3 and together forms the NS2B-NS3 protease complex. Contains transmembrane domains which bind the protease to intracellular membranes.
NS3	A serine protease that is responsible for cleaving the polyprotein to produce mature proteins; it can also acts as a helicase.
NS4A	NS4A is a cofactor for viral replication, specifically regulates the activity of the NS3 helicase.
NS4B	Inhibits interferon signaling.
NS5	The largest and most conserved protein of WNV, NS5 acts as a methyltransferase and a RNA polymerase, though it lacks proofreading properties.

Life cycle

Once WNV has successfully entered the bloodstream of a host animal, the envelope protein, E, binds to attachment factors called glycosaminoglycans on the host cell. These attachment factors aid entry into the cell, however, binding to primary receptors is also necessary. Primary receptors include DC-SIGN, DC-SIGN-R, and the integrin α_vβ₃. By binding to these primary receptors, WNV enters the cell through clathrin-mediated endocytosis. As a result of endocytosis, WNV enters the cell within an endosome. 

The acidity of the endosome catalyzes the fusion of the endosomal and viral membranes, allowing the genome to be released into the cytoplasm. Translation of the positive-sense single-stranded RNA occurs at the endoplasmic reticulum; the RNA is translated into a polyprotein which is then cleaved by viral proteases NS2B-N23 to produce mature proteins.

In order to replicate its genome, NS5, a RNA polymerase, forms a replication complex with other nonstructural proteins to produce an intermediary negative-sense single-stranded RNA; the negative-sense strand serves as a template for synthesis of the final positive-sense RNA. Once the positive-sense RNA has been synthesized, the capsid protein, C, encloses the RNA strands into immature virions. The rest of the virus is assembled along the endoplasmic reticulum and through the Golgi apparatus, and results in non-infectious immature virions. The E protein is then glycosylated and prM is cleaved by furin, a host cell protease, into the M protein, thereby producing an infectious mature virion. The mature viruses are then secreted out of the cell.

Phylogeny

Phylogenetic tree of West Nile viruses based on sequencing of the envelope gene during complete genome sequencing of the virus

 

WNV is one of the Japanese encephalitis antigenic serocomplex of viruses, together with Japanese encephalitis virus, Murray Valley encephalitis virus, Saint Louis encephalitis virus and some other flaviruses. Studies of phylogenetic lineages have determined that WNV emerged as a distinct virus around 1000 years ago. This initial virus developed into two distinct lineages. Lineage 1 and its multiple profiles is the source of the epidemic transmission in Africa and throughout the world. Lineage 2 was considered an African zoonosis. However, in 2008, lineage 2, previously only seen in horses in sub-Saharan Africa and Madagascar, began to appear in horses in Europe, where the first known outbreak affected 18 animals in Hungary. Lineage 1 West Nile virus was detected in South Africa in 2010 in a mare and her aborted fetus; previously, only lineage 2 West Nile virus had been detected in horses and humans in South Africa. Kunjin virus is a subtype of West Nile virus endemic to Oceania. A 2007 fatal case in a killer whale in Texas broadened the known host range of West Nile virus to include cetaceans.

Since the first North American cases in 1999, the virus has been reported throughout the United States, Canada, Mexico, the Caribbean, and Central America. There have been human cases and equine cases, and many birds are infected. The Barbary macaque, Macaca sylvanus, was the first nonhuman primate to contract WNV. Both the American and Israeli strains are marked by high mortality rates in infected avian populations; the presence of dead birds—especially Corvidae—can be an early indicator of the arrival of the virus.

Host range and transmission

Culex pipiens mosquitoes are a vector for WNV.

The natural hosts for WNV are birds and mosquitoes. Over 300 different species of bird have been shown to be infected with the virus. Some birds, including the American crow (Corvus brachyrhynchos), blue jay (Cyanocitta cristata) and greater sage-grouse (Centrocercus urophasianus), are killed by the infection, but others survive. The American robin (Turdus migratorius) and house sparrow (Passer domesticus) are thought to be among the most important reservoir species in N. American and European cities. Brown thrashers (Toxostoma rufum), gray catbirds (Dumetella carolinensis), northern cardinals (Cardinalis cardinalis), northern mockingbirds (Mimus polyglottos), wood thrushes (Hylocichla mustelina) and the dove family are among the other common N. American birds in which high levels of antibodies against WNV have been found.

WNV has been demonstrated in a large number of mosquito species, but the most significant for viral transmission are Culex species that feed on birds, including Culex pipiens, C. restuans, C. salinarius, C. quinquefasciatus, C. nigripalpus, C. erraticus and C. tarsalis. Experimental infection has also been demonstrated with soft tick vectors, but is unlikely to be important in natural transmission.

WNV has a broad host range, and is also known to be able to infect at least 30 mammalian species, including humans, some non-human primates, horses, dogs and cats. Some infected humans and horses experience disease but dogs and cats rarely show symptoms. Reptiles and amphibians can also be infected, including some species of crocodiles, alligators, snakes, lizards and frogs. Mammals are considered incidental or dead-end hosts for the virus: they do not usually develop a high enough level of virus in the blood (viremia) to infect another mosquito feeding on them and carry on the transmission cycle; some birds are also dead-end hosts.

In the normal rural or enzootic transmission cycle, the virus alternates between the bird reservoir and the mosquito vector. It can also be transmitted between birds via direct contact, by eating an infected bird carcass or by drinking infected water. Vertical transmission between female and offspring is possible in mosquitoes, and might potentially be important in overwintering. In the urban or spillover cycle, infected mosquitoes that have fed on infected birds transmit the virus to humans. This requires mosquito species that bite both birds and humans, which are termed bridge vectors. The virus can also rarely be spread through blood transfusions, organ transplants, or from mother to baby during pregnancy, delivery, or breastfeeding. Unlike in birds, it does not otherwise spread directly between people.

Disease

Humans

In humans, West Nile virus can cause a disease known as West Nile fever. According to the U.S. Centers for Disease Control and Prevention, approximately 80% of infected people have few or no symptoms, around 20% of people develop mild symptoms, such as fever, headache, vomiting, or a rash, while less than 1% of people develop severe symptoms, such as encephalitis or meningitis with associated neck stiffness, confusion, or seizures. The causes of West Nile Virus mediated encephalitis have been explored by Dr. Robyn Klein at Washington University in St. Louis. She has found that West Nile infection increases cytokines and chemokines in the blood making the blood brain barrier more leaky and susceptible to infection. The risk of death among those in whom the nervous system is affected is about 10%. Recovery may take weeks to months. Risks for severe disease include age over 60 and other health problems. Historically, people in areas where the virus was endemic, such as the Nile Delta, usually experienced subclinical or mild disease. Diagnosis is typically based on symptoms and blood tests. While there is no specific treatment, pain medications may be useful.

Horses

Severe disease may also occur in horses. Several vaccines for these animals are now available. Before the availability of veterinary vaccines, around 40% of horses infected in North America died.

History

The virus was discovered in Uganda in 1937 and was first detected in North America in 1999.

West Nile Virus has been reported in Europe, Africa, Asia, Australia, and North America. In the United States thousands of cases are reported a year, with most occurring in August and September. It can occur in outbreaks of disease. A surveillance system in birds is useful for early detection of a potential human outbreak.

Epidemiology

According to the Center for Disease Control, infection with West Nile Virus is seasonal in temperate zones. Climates that are temperate, such as those in the United States and Europe, see peak season from July to October. Peak season changes depending on geographic region and warmer and humid climates can see longer peak seasons. All ages are equally likely to be infected but there is a higher amount of death and neuroinvasive West Nile Virus in people 60-89 years old. People of older age are more likely to have adverse effects of being infected.

There are several modes of transmission but the most common cause of infection in humans is by being bitten by an infected mosquito. Other modes of transmission include blood transfusion, organ transplantation, breast-feeding, transplacental transmission, and laboratory acquisition. These alternative modes of transmission are extremely rare.

Prevention

Prevention efforts against WNV mainly focus on preventing human contact with and being bitten by infected mosquitoes. This is twofold, first by personal protective actions and second by mosquito-control actions. When a person is in an area that has WNV, it is important to avoid outdoor activity, and if they go outside they should use a mosquito repellent with DEET. A person can also wear clothing that covers more skin, such as long sleeves and pants. Mosquito control can be done at the community level and include surveillance programs and control programs including pesticides and reducing mosquito habitats. This includes draining standing water. Surveillance systems in birds is particularly useful. If dead birds are found in a neighborhood it should report it to local authorities. This may help health departments do surveillance and determine if the birds are infected with West Nile Virus.

Despite the commercial availability of 4 veterinary vaccines for horses, no humans vaccine has progressed beyond phase II clinical trials. Efforts have been made to produce a vaccine for human use and several candidates have been produced but none are licensed to use. The best method to reduce the risk of infections is avoiding mosquito bites. This may be done by eliminating standing pools of water, such as in old tires, buckets, gutters, and swimming pools. Mosquito repellent, window screens, mosquito nets, and avoiding areas where mosquitoes occur may also be useful.

Climate change

Infectious disease transmission is sensitive to local, small-scale differences in weather, human modification of the landscape, the diversity of animal hosts, and human behavior affects vector-human contact, among other factors.

Climate change affects human health in various forms. The impacts of climate change are complex, vary in scale and timing and depend on environmental conditions and human vulnerability. Climate change influences the emergence of the vector-borne disease West Nile Virus. Climate change alters disease rates, ranges, seasonality and affects the distribution of WNV, Climate change is a major environmental factor that influences the epidemiology of the disease.

Projected changes in flood frequency and severity can bring new challenges in flood risk management. For urban areas in particular, flooding impacts critical infrastructure in ways that are difficult to foresee and can result in interconnected and cascading failures.

Weather conditions affected by climate change including temperature, precipitation and wind may affect the survival and reproduction rates of mosquitoes, suitable habitats, distribution, and abundance. Ambient temperatures drive mosquito replication rates and transmission of WNV by affecting the peak season of mosquitoes and geographic variations. For example, increased temperatures can affect the rate of virus replication, speed up the virus evolution rate, and viral transmission efficiency. Furthermore, higher winter temperatures and warmer spring may lead to larger summer mosquito populations, increasing the risk for WNV. Similarly, rainfall may also drive mosquito replication rates and affect the seasonality and geographic variations of the virus. Studies show an association between heavy precipitation and higher incidence of reported WNV. Likewise, wind is another environmental factor that serves as a dispersal mechanism for mosquitoes.

Global Distribution of West Nile Virus CDC

Mosquitoes have extremely wide environmental tolerances and a nearly ubiquitous geographical distribution, being present on all major land masses except Antarctica and Iceland. Nevertheless, changes in climate and land use on ecological timescales can variously expand or fragment their distribution patterns, raising consequent concerns for human health.