West Nile Virus

About the Virus

Clinical Signs






Zoonotic Risk

Veterinarian's Role

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What are some differential diagnoses for the clinical signs seen with WNV in horses?

Not all horses with clinical signs of encephalitis have West Nile encephalitis. Encephalitis can be caused by both infectious and non-infectious processes.

Infectious causes of encephalitis include diseases such as rabies, equine protozoal myeloencephalitis (EPM), and other mosquito-borne viral encephalitic diseases of horses caused by Eastern, Western, and Venezuelan encephalitis viruses can cause a horse to have symptoms similar to WNV. Other less likely infectious causes include botulism and verminous meningoencephalomyelitis (caused by Halicephalobus gingivalis, Seteria, and Strongylus vulgaris).

Non-infectious causes of encephalitic signs that must also be considered include hypocalcemia, tremorigenic toxicities, hepatoencephalopathy, and leukoencephalomalacia.

How can WNV infection be diagnosed in horses?

Only laboratory tests can confirm the diagnosis of West Nile encephalitis. Laboratory tests can be performed on living and dead animals.

Antemortem Diagnosis (Living Animal)

Serology is the most useful test for antemortem diagnosis of WN encephalitis in horses. The available test to diagnose WNV infection in living horses are serum samples tested for antibodies to WNV by epitope-blocking ELISAs (Enzyme-linked immunosorbent assay). This blocking ELISA is performed using the WNV-specific monoclonal antibody (MAb) 3.1112G.

Another serological test used to identify WN infection in living horses is the IgM-capture enzyme-linked immunosorbent assay (MAC-ELISA). IgM rises sharply and falls during the first 6-8 weeks after exposure to the virus. The IgM capture ELISA is the test of choice for detection of recent exposure to the virus.

Post Mortem Diagnosis (Dead Animal)

The available tests to diagnose WNV infection in deceased horses all involve the previously mentioned tests as well as gross necropsy and histological and biochemical analysis of specific tissues.

Gross Necropsy Examination Findings

The gross necropsy of infected horses has findings that are rare and limited. In many animals there is usually some form of abnormality present within the central nervous system usually limited to small multifocal areas of discolorations and hemorrhage throughout the midbrain, brain stem and spinal cord.

These are sections of equine spinal cord demonstrating multifocal areas of hemorrhage.

Histological Examination and Findings

In many cases a non-suppurative, moderate to severe non-necrotizing polio-encephalomyelitis is observed. The lesions can be preferentially located in the lumbar-sacral region of the spinal cord and in the brain stem.

Inflammation may also be seen as slight to severe with multifocal perivascular cuffing composed of lymphocytes, plasmocytes and macrophages.

In some horses there is inflammation and edema present in the meninges of the spinal cord.

Viral antigens can also be viewed in neural cytoplasms in infected animals using immunohistochemistry (see picture below).


It appears that lesions induced by WNV are limited to the central nervous system and consist of moderate to severe meningo-encephalities associated with haemorrhages. These lesions are preferentially observed in the brain stem and ventral horns of the lumbar-sacral spinal cord, while the cerebrum and cerebellum show few changes.


Israel Veterinary Medical Association: West Nile Virus Encephalitis in Horses In Israel.

Blitvich BJ, Fernandez-Salas I, Contreras-Cordero JF, Marlenee NL, Gonzalez-Rojas JI, Komar N, et al. Serologic Evidence of West Nile virus infection in horses, Coahuila State, Mexico. Emerg Infect Dis [serial online] 2003 Jul [date cited]. Available from: URL:

Center for Disease Control: Equine West Nile Encephalitis, United States. http://www.cdc.gov/ncidod/eid/vol7no4/ostlund.htm

Center for Disease Control: Detection of North American West Nile Virus in Animal Tissue by a Reverse Transcription-Nested Polymerase Chain Reaction Assay

The Merck Veterinary Manual

Image References

Israel Veterinary Medical Association: West Nile Virus Encephalitis in Horses In Israel.

Maryland Department of Health and Mental Hygiene: http://edcp.org/html/wn.html