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West Nile Virus (WNV) is a mosquito-borne virus that can cause illness in humans, birds, and other animals. Although the virus was first isolated in 1937 in Africa, West Asia, and the Middle East, infections were first documented in the Western Hemisphere in 1999 when outbreaks of WNV encephalitis in humans were reported in the New York City metropolitan area, New Jersey, and Connecticut. Since those initial outbreaks in New York City, WNV has become well established in the United States, with cases now occurring in every state in the continental United States. In 2025, over 2,000 cases in 47 States were reported. Over 1,100 cases were also reported in Europe by the ECDC in 2025, including in several countries with USN/USMC presence. While WNV is globally distributed in regions harboring the mosquito vector, it is often underdiagnosed due to diagnostic challenges.
While cases occur year-round, in the Northern Hemisphere transmission is highest in the late summer and early fall, typically between July and September.
The most common mosquito vectors of WNV are Culex pipiens, Cx. tarsalis and Cx. quinquefasciatus, but WNV has been detected in many mosquito species. Most people infected with WNV (~ 80%) do not develop symptoms, but a small percentage (< 1%) develop severe, potentially fatal, neurological illness. Risk factors include age over 50 and those with weakened immune systems. Prevention is mainly mosquito-bite avoidance, which is best accomplished by careful use of the DoD Insect Repellent System and Insect Repellent Treatment of Military Uniforms. Mosquito control activities may also be employed to reduce vector exposure.
WNV is unlikely to pose significant risk to mission due to being largely asymptomatic. Risk to force is limited, although occasional severe or even fatal cases are possible during outbreaks. Personnel over 50 years old are at highest risk. In the event of identified WNV cases, early consultation with Preventive Medicine is critical to ensure that vector control and prevention activities are optimized. The Navy Entomology Center of Excellence can provide guidance on vector surveillance and control, while the cognizant Navy Environmental and Preventive Medicine Unit (NEPMU) can advise on and/or assist with case investigation activities and vector control measures.
WNV has an incubation period of 2-14 days.
Most healthy young adults and children do not develop symptoms, with approximately 80% of cases being asymptomatic.
Approximately 20% of infected people have mild symptoms, developing a flu-like illness that can include fever, headache, body aches, joint pains, vomiting, diarrhea, or rash.
Less than 1% of infected people, mostly over age 50, develop neuroinvasive disease, including meningitis and encephalitis. Symptoms may include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, and convulsions. Acute flaccid myelitis may occur concurrently with meningitis or encephalitis, and results in muscle weakness and paralysis similar to polio.
Less than 1 in 1000 infections result in death. In those who develop meningitis or encephalitis, overall fatality is approximately 10%, and long-term neurologic sequalae are common.
People over age 50 or with suppressed immune systems are more likely to develop severe symptoms. The risk of mortality due to WNV infection is extremely low in people under age 50.
No WNV vaccines are licensed for use in humans, although there are several licensed vaccines for horses. There are also no specific treatments for WNV. Treatment is supportive and may include hospitalization, respiratory support, IV fluids, and prevention of secondary infections.
WNV should be suspected in patients who develop symptoms of fever with meningitis or encephalitis in an area with reported cases of WNV or bird deaths, particularly in the setting of recent mosquito bites. Cases are most likely when mosquito activity is highest, typically in the summer months or rainy seasons.
Serum or cerebrospinal fluid (CSF) is typically tested for WNV-specific IgM antibodies, which are usually detectable 3-8 days after the onset of illness and persist from 1-3 months afterwards. Unfortunately, antibodies from other flaviviruses such as Dengue, Zika or even vaccination to Yellow Fever, Tick-Borne Encephalitis or Japanese Encephalitis may cause cross reactivity to the WNV serology and yield a false positive result. Paired acute and convalescent sera (2-3 weeks apart) with a 4-fold titer increase of WNV-specific antibodies is highly suggestive of WNV infection. Positive serologies should have a follow-up plaque reduction neutralization assay (PRNT), particularly when cross-reactivity is likely or there is a severe presentation/death.
Note: WNV is NOT part of the Biofire Global Fever Panel, and there is no rapid test for WNV.
Viremia in WNV is transient in immunocompetent individuals, making viral cultures and RT-PCRs challenging. They may be useful early in the disease, but negative results must be interpreted with extreme caution.
CDC WNV Diagnostic Algorithm
WNV is a nationally notifiable disease and must also be reported within DoD. BUMEDINST 6220.12D requires that all cases of WNV be reported to Preventive Medicine authorities in the Disease Reporting System internet (DRSi). Case classification details are fully described in the Armed Forces Medical Reportable Events Guide (note: WNV is listed under “Arboviral Disease, Neuroinvasive and Non-neuroinvasive”). DCPH-A has also put together a Flowchart for Case Definition. Detailed information on reporting in DRSi can be found on NMCFHPC’s Medical Surveillance and Reporting page.
Additionally, notify your Public Health Emergency Officer, cognizant Navy Environmental and Preventive Medicine Unit (NEPMU), and state or local health department so that measures can be taken to mitigate the risk of local transmission.
WNV is maintained in a cycle between mosquitoes and birds, with humans and other mammals being incidental or "dead-end" hosts. Mosquitoes become infected when they feed on birds infected with WNV. Infected mosquitoes can then transmit WNV when they feed on humans or other animals. Although corvids (crows and blue jays) are particularly susceptible to WNV infection, it has been detected in more than 130 bird species, 22 mosquito species, and a variety of mammals. Among domestic animals, horses are especially susceptible and often serve as an indication of viral activity in a given area.
Case Response: In the event of a WNV case, contact local Preventive Medicine resources for support. Public Health personnel should consult the Emergency Vector Control Plan (EVCP) and the installation WNV Surveillance and Control Plan, which monitor mosquito vector densities and high-risk areas. Note that they may be integrated into a single plan and may be part of the overarching Installation Pest Management Plan.
Isolation/ROM: WNV is transmitted to humans through the bite of an infected female mosquito. It is not transmitted from person to person, and unlike with dengue or chikungunya, it is not necessary to isolate patients in screened areas. As “dead-end” hosts, humans do not develop sufficient viremia to transmit virus to a biting mosquito and sustain a chain of transmission.
Risk Communication: Consider working with Public Affairs to implement a WNV public awareness campaign. CDC has a ready-made WNV Outbreak Communications Toolkit.
Dead birds: While there is no evidence that handling live or dead infected birds can cause human infection, the Center for Disease Control (CDC) warns that dead birds should not be handled with bare hands to avoid potential for WNV transmission. Note that dead birds may also be the result of H5N1 avian influenza A, which has been documented to be transmitted through handling dead birds.
Support: The cognizant Navy Environmental and Preventive Medicine Unit (NEPMU) can advise on and/or assist with case investigation activities and vector control measures. The Navy Entomology Center of Excellence (NECE) provides specialized operational entomology support and offers courses for pesticide applicator certification.
Some mosquito control resources with individual-level guidance for bite avoidance are:
Per OPNAVINST 6250.4C, installations are required to have a comprehensive Installation Pest Management Plan (IPMP). Detailed guidance for the IPMP is described in AFPMB Technical Guide No. 18. The Emergency Vector-borne Disease Control Plan (EVDCP), typically an appendix of the IPMP, is the response plan for vector-borne disease outbreaks, including WNV. It is highly recommended that an installation's EVDCP be developed in consultation with entomologists at the Navy Environmental and Preventive Medicine Units (NEPMU)'s, Naval Facilities Engineering Command (NAVFAC) and NECE.
The IPMP program must include:
Local, County, State or Host Nation Public Health/Mosquito Control Agencies may already have robust mosquito surveillance, and coordination with such agencies is encouraged.
Details of environmental surveillance and control measures for WNV are available in the following resources:
AFHSD MSMR Feb 2021 Vo1 28 No. 2 Surveillance for Vector Borne Diseases among AD, 2016-2020
AFHSD MSMR 2018 Vol 25 No. 2 Surveillance for Vector Borne Diseases among AD, 2010-2016
Fagre AC, Lyons S, Staples JE, Lindsey N. West Nile Virus and Other Nationally Notifiable Arboviral Diseases — United States, 2021. MMWR Morb Mortal Wkly Rep 2023;72:901–906. DOI: http://dx.doi.org/10.15585/mmwr.mm7234a1
Hoke Jr, Charles H. "History of US military contributions to the study of viral encephalitis." Mil Med. 2005; 170(4): 92-105. Doi:10.7205/milmed.170.4S.92
Nasci RS, Mutebi JP. Reducing West Nile Virus Risk Through Vector Management. J Med Entomol. 2019;56(6):1516-1521. doi:10.1093/jme/tjz083
Witt CJ, Brundage M, Cannon C, et al. Department of Defense West Nile virus surveillance in 2002. Mil Med. 2004;169(6):421-428. doi:10.7205/milmed.169.6.421
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