West Nile Virus Overview

·      What is West Nile Virus?

o   West Nile Virus is a virus of the family Flaviviridae, specifically of the genus Flavivirus Japanese Encephalitis Antigenic Complex.  Flaviviruses are about 40-60nm in size, enveloped and contain positive-sense single stranded RNA.

·      Where does West Nile Virus occur?

o   West Nile virus exists in Europe, Africa, the Middle East, west and central Oceania.  West Nile Virus was first documented in the United States in 1999 and since then, cases of West Nile Virus have been documented in every state except Oregon, Hawaii, and Alaska.

 

West Nile virus (WNV) activity reported to ArboNET, by state, United States, 2012 (as of December 11, 2012)

http://www.cdc.gov/ncidod/dvbid/westnile/Mapsactivity/surv&control12MapsAnybyState.htm

·      How is West Nile Virus transmitted?

o   There are at least 64 different species of mosquitos that can carry West Nile Virus.  Birds serve as the reservoir host for West Nile Virus, which is then amplified by adult mosquitos following blood feeding.  Mosquitos serve as the primary vector for transmission of West Nile Virus and carry the virus particles in their salivary glands.  When a mosquito feeds on a bird, the virus is transmitted to the bird where it causes an infectious viremia for 1-4 days.  After the viremia subsides in the bird, it develops life-long immunity.  Among bird species, geese have been documented as the only natural host of West Nile Virus, but house sparrows are noted as the most important reservoir host given the high quantity of virus they carry once infected and since they do not die from the viral infection like some other bird species.

There are a very small number of reported cases of West Nile Virus transmission through blood transfusion, transplants, nursing, and pregnancies.

        http://www.cdc.gov/ncidod/dvbid/westnile/cycle.htm

·      Who gets West Nile Virus?

o   West Nile Virus has been detected in more than 326 species of birds.  Some birds, mainly crows and jays, will die from West Nile Virus, but most birds do survive. People, horses, and most other mammals are known to be dead-end hosts for West Nile Virus, meaning they can contract the disease if bitten by an infected mosquito.  Serious disease has not been reported in dogs and cats, but could be possible if the virus is transmitted from an infected mosquito.  Horses are one of the most highly publicized victims of West Nile Virus, with about 40% of infections in horses resulting in death.

Humans can also contract West Nile Virus from infectious mosquito bites.  In 2012, a total of 5,387 cases were reported with 283 deaths.  Individuals over the age of 50 are at increased risk for severe disease.

Infections in chipmunks, bats, squirrels, skunks, llamas, and rabbits have also been reported infrequently.

·      What are the clinical signs of West Nile Virus?

o   Clinical signs of West Nile Virus vary with species affected, but the virus can cause very serious illness and death.  Please see section on Clinical Presentation for more information.

·      How can people get West Nile Virus?

o   There is no evidence that humans can contract West Nile Virus from handling dead or infected birds, but precautions such as avoiding direct contact with bare hands should still be employed.  There is also no evidence of direct human to human transmission or animal to human transmission of West Nile Virus, though blood transfusion/organ transplants and mother to child transmissions have been reported rarely.  The mosquito vector is therefore required to obtain most active West Nile Virus infections.

·      How is West Nile Virus diagnosed?

o   Clinical signs of West Nile Virus allow for presumptive diagnosis.  Collection of whole blood of suspected animals during acute and convalescent stages of the disease and demonstration of a substantial increase in antibody titers is best for definitive diagnosis of the disease.  Virus isolation, serology, Real-Time PCR are all methods of diagnosis applicable to West Nile Virus.

·      How do you treat West Nile Virus?

o   Supportive care for clinical signs and fluid losses are the main treatments of choice.  Virus-specific treatments do not exist for West Nile Virus, so prevention is crucial.

·      How do you prevent West Nile Virus?

o   Mosquito control and the prevention of mosquito bites is the best means to prevent West Nile Virus infection.  Avoiding outdoor exposures during dawn and dusk when mosquitos are most active, using insect repellent/protective clothing, and eliminating common mosquito breeding areas involving standing water are all valid means of decreasing mosquito exposure.

Vaccination for West Nile Virus in horses in the United States is considered by the AAEP as a core viral vaccine and should be used in all horses.  A DNA vaccine is licensed for use in horses as well as a canary poxvirus recombinant vaccine. Regular vaccination of horses is recommended, but should still be employed with appropriate mosquito control protocols.

There are currently no West Nile Virus vaccines for humans, but a vaccine may become available in the future.

·      What is being done to monitor West Nile Virus in the United States?

o   Any horse that develops CNS signs with suspicion of West Nile Virus should be quarantined and appropriate state veterinary officials should be notified.

Federal and State government agencies as well as field veterinarians and physicians monitor and report any cases of West Nile Virus (in any species) to information databases.  The CDC alone has an active surveillance program that provides weekly updates on reported West Nile Virus cases in all species.

REFERENCES

Centers for Disease Control and Prevention, (2012). West nile virus. Retrieved from website: http://www.cdc.gov/ncidod/dvbid/westnile/wnv_factsheet.htm

Subbiah, E. VM 8124 Course Notes, Equine Viruses I. Spring 2013; Lecture 13.