Study type: Blood collected in a red-top tube or CSF collected in a sterile plastic container for ELISA; blood collected in a lavender-(EDTA) or pink-top (K₂EDTA) or CSF collected in a sterile plastic container for PCR; related body system: Immune system.

The WNV, a single-stranded RNA virus, is classified as a flavivirus. Prior to 1999, the virus was primarily found in Africa; since then, it is found worldwide throughout Australia, North America, Europe, the Middle East, South America, and West Asia. WNV is one of three types of arbovirus capable of transmitting encephalitis and other febrile-type diseases in North America via an arthropod vector: Togaviridae, Flaviviridae, and Bunyaviridae. The primary mode of this arboviral transmission to humans is mosquito bite. Many mosquito species (notably Culex) can serve as the disease vector; birds serve as the reservoir hosts (mainly crows, sparrows, and blue jays). WNV is not transmitted directly from human to human, by handling infected birds (dead or alive), from exposure to an infected equine, or by eating meat from infected birds or animals. Horses and humans are classified as “dead” hosts because, when infected, they do not produce a sufficient viral load to transmit virus when bitten by an uninfected mosquito vector. WNV can cause severe illness or death in horses; a vaccine has been developed for horses. There are rare reports of transmission by an infected person via blood product, tissue, or solid organ transplant; for that reason, all human products are tested for WNV prior to transfusion or transplantation. Although the majority of babies born to infected mothers do not contract the disease or suffer any associated congenital abnormalities, there are rare cases of known transmission from mother to baby during pregnancy and breastfeeding.

The incubation period for WNV disease is from 2 to 14 days but can extend beyond that range in immunocompromised patients. About 80% of WNV infections are subclinical or asymptomatic, but of those who develop symptoms, the onset is described as an acute systemic febrile illness that can rapidly escalate into life-threatening sequelae that could include viral meningitis, an acute flaccid paralysis that is indistinguishable from poliovirus-associated poliomyelitis, or a severe encephalitis. Conditions that present like radiculopathy and Guillain-Barré syndrome have also been associated with WNV infection and can be distinguished from WNV acute flaccid paralysis by clinical signs and symptoms and electrophysiologic studies. However, in most cases, the symptoms are non–life threatening and nonspecific: headache, lethargy, weakness, aching muscles and bones, gastrointestinal discomfort, and/or skin rash. Symptoms may last for a period of days or persist for weeks or months. Therapy is supportive, and there is no commercially available human vaccine at this time.

Assays for immunoglobulin G (IgG) and immunoglobulin M (IgM) antibodies to WNV assist in differentiating recent infection from prior exposure in the general population. Cerebrospinal fluid (CSF) specimens are used to document central nervous system (CNS) infection by WNV. WNV IgM antibodies appear the earliest and are soon followed by development of IgG antibodies. Presence of antibodies is detectable within 3 to 10 days after reported onset of symptoms and remain in circulation for approximately 30 to 90 days. If results from the IgM-specific or from both assays are positive, recent infection is suspected. Absence of detectable antibody within 10 days of onset does not rule out infection, and testing should be repeated within a short period of time. If the IgM-specific test results are negative and the IgG-specific antibody test results are positive, past infection is indicated. WNV testing is not widely available in routine clinical laboratories. Testing may be sent to a referral laboratory for enzyme-linked immunosorbent assay (ELISA) or to a public health testing facility for a more specific and quantitative confirmatory analysis known as plaque reduction neutralization testing (PRNT), depending on the situation. Aliquots of all specimens should be sequestered for a specified period of time after collection; protocols vary by facility. Positive results obtained by ELISA testing should be confirmed by PCR or neutralizing antibody testing on both acute and convalescent serum specimens that have been collected 2 to 3 wk apart. PRNTs can also confirm acute infection by demonstrating a fourfold or greater increase in PRNT antibody titer between acute and convalescent serum samples. Other testing methods for WNV disease include viral cultures on blood or body fluid (to detect viral RNA [ribonucleic acid]), immunohistochemistry using formalin-fixed tissue (to detect viral antigen), and reverse transcriptase polymerase chain reaction on serum, CSF, and tissue specimens (to detect viral RNA).