West Nile Virus in Horses Application Paper

West Nile Virus in Horses

The objective of this work is to examine West Nile Virus in horses in terms of its’ origin, prevention and critical analysis for the reason of increase or decrease in statistical data related to West Nile Virus.

Both people and animals may become infected with West Nile Virus due to having been bitten by a mosquito that is infected with the virus. Infection is more likely in the late summer or early fall in the northeast and Mid Atlantic regions. Horses are a species that is stated to be “susceptible to infection with the virus” although infection does not always lead to signs of illness in human beings or animals.


West Nile Virus is a mosquito-borne virus, which can cause encephalitis. West Nile encephalitis was first diagnosed in the United States when outbreak occurred in 1999 in the metropolitan area of New York City in 62 people and seven of whom did not survive the disease. First, the infection was diagnosed in several horse breeds, various zoo animals and various native species of bird and most particularly in crows. There were twenty-five horses from Long Island, New York in 1999 that were confirmed to have symptoms of West Nile encephalitis resulting in the death or euthanization of nine of the twenty-five horses. The following illustration depicts the transmission cycle of the West Nile Virus.

Transmission Cycle of the West Nile Virus

Source: College of Agricultural Sciences – Agricultural Research and Cooperative Extension


For horses that do become clinically ill “the virus infects the central nervous system and may cause symptoms of encephalitis” and include “a general loss of appetite and depression” as well as the following symptoms:

fever weakness of hind limbs paralysis of hind limbs impaired vision ataxia (weakness) head pressing aimless wandering convulsions (seizures) inability to swallow walking in circles hyperexcitability coma (Pennsylvania’s West Nile Virus Surveillance Program, 2008)

It is reported that West Nile Virus (WNV) was first diagnosed in horses in 1999 in the United States” and that it is now an important consideration in the “differential diagnosis of horses presenting with signs of a neurological disease in all areas of North America.” (West Nile Virus Vaccination Guidelines, 2005) West Nile Virus is a ‘flavivirus’ and was identified first as a “cause of infection and fatal encephalomyelitis (inflammation of the spinal cord and brain) in horses and people in Egypt, Uganda, and France…” (West Nile Virus Vaccination Guidelines, 2005)


The incubation period for this virus is stated at between approximately three to fifteen days. The mortality rate for horses exhibiting clinical symptoms of West Nile Virus infection is stated at approximately 33%. Data indicates that approximately forty-percent of horses surviving this illness will “exhibit residual effects, such as gait and behavioral abnormalities that were attributed to the illness by owners, six months post diagnosis.” (West Nile Virus Vaccination Guidelines, 2005) Stated as variable clinical signs associated with WNV infection which necessitate inclusion of many neurological disorders in the differential diagnoses are those as follows:


equine protozola myeloencephalitis (EPM);

equine herpesvirus-1;


eastern, western and Venezuelan encephalomyelitis (EEE, WEE, VEE);

heat stress;


bacterial meningitis;

cervical vertebral myelopathy (wobbler syndrome); myloencephalopathy; and equine degenerative myelopathy. (West Nile Virus Vaccination Guidelines, 2005)


Serological tests used to diagnose WNV include the following:

plague reduction naturalization (PRNT);

virus neutralization;

hemaglutination inhibition;

complement fixation;

ELISA and antigen (IgM and IgG) capture ELISA (West Nile Virus Vaccination Guidelines, 2005)

Stated as the most reliable of these testing methods is the IgM-capture ELISA in a horse that is exhibiting clinical signs. Risk of exposure is stated to vary each year due to changes in the “distribution of insect vectors and reservoirs of virus.” (West Nile Virus Vaccination Guidelines, 2005)


Because of this variation in the nature of risk factors and the effects of the disease, the recommendation has been made that all horses in North American receive vaccination against the West Nile Virus. Minimization of the risk for transmission of the virus from infected mosquitoes includes preventive management practices including reduction and elimination of stagnant or standing water as well as removal of old tires and keeping horses in the barn from “dusk to dawn” since this is the prime time for mosquito feeding. Setting out mosquito traps is advised as well as keeping air moving with fans and removal of organic debris (muck) quickly. There are stated to be specific controls through use of chemical agents including “use of topical anti-mosquito repellant agents” which have been approved for topical use. Finally, mosquito dunks may be used in areas that hold standing water. However, stated as the primary method of reduction of infection risk from West Nile Virus is that of vaccination although it is not always completely effective in prevention.


While many vaccines are currently available, only one of these vaccines is “monovalent or multivalent inactivated and the other is a live canarypox vector vaccine.” (West Nile Virus Vaccination Guidelines, 2005) Instructions for these vaccines states that two doses are to be administered three to six weeks apart. Boosters are a requirement depending on local conditions relating to disease risk. Vaccinations should be given semi-annually or even more frequently, stated at approximately every four months depending on the risk with “annual revaccination” being “best completed in the spring, prior to the onset of peak insect vector season.” (West Nile Virus Vaccination Guidelines, 2005)

None of the vaccines presently licensed are labeled for administration to mare that are pregnant however, it is stated that mares “be ideally vaccinated before breeding when possible.” (West Nile Virus Vaccination Guidelines, 2005) it is however stated that “practitioners have vaccinated thousands of pregnant mares due to the risk associated with pregnant mares getting the disease from infected mosquitoes” and in fact, “booster vaccination of pregnant mares 4 to 6 weeks before foaling provides augmented passive colostral protection to their foals, lasting for 3-4 months.” (West Nile Virus Vaccination Guidelines, 2005)


Foals should be started with vaccinations between three and four months of age and foals born of non-vaccinated mares “may be vaccinated earlier than three months of age as they may not have colostral interference issues…” (West Nile Virus Vaccination Guidelines, 2005) Revaccination of foals should occur at one year of age, or “in the spring of the year following their birth to ensure adequate protection.” (West Nile Virus Vaccination Guidelines, 2005)

Veterinarians in the Southern U.S. states due to activity of mosquitoes year-round “prefer to vaccinate horses semi-annually or more frequently to help ensure uniform protection throughout the year, although this practice is not specifically recommended by manufacturers of vaccines.” (West Nile Virus Vaccination Guidelines, 2005) Horses that do become infected with WNV “should be vaccinated one year after the acute illness. Although the ability of horses to become re-infected with WNV is unknown at this time…” (West Nile Virus Vaccination Guidelines, 2005) the following table lists the West Nile Virus Vaccination Schedule for horses by age and type.

Source: (West Nile Virus Vaccination Guidelines, 2005)

There is no evidence to support that horses transmit West Nile Virus to other horses “birds or humans” and in fact, “evidence exists that the virus can be found in the horse’s bloodstream for only a few days during the entire course of the infection.” (Penn State University: College of Agricultural Scientists, nd) This makes it very important that one follows precautions handling the “blood, spinal fluid or nervous tissue from suspect animals, since these may contain the virus.” (Penn State University: College of Agricultural Scientists, nd)


West Nile Virus is stated to represent a “real threat to horses because they appear more susceptible to the disease than other livestock or companion animals.” Penn State University reports that the “killed virus vaccine seems highly efficacious when administered properly…” however, it is reported to have failed to “protect 100% of all horses studied.” (nd) it is reported that in August 2003 licensing was issues for an antiserum for use in horses which would aid in controlling “signs and progression of the disease.” (Penn State University, nd)


In 2008, it was reported by the Science Daily website that a new and improved test for West Nile Virus in Horses had been developed and states that West Nile Virus is “widely distributed in Africa, the Middle East and Europe.” (2008) U.S. researchers are stated to have discovered a new test “designed to detect antibodies produced by horses” and claims are that this test is “highly effective at diagnosing West Nile virus infections” and as compared to the former testing method, the new test “is much faster and gives accurate results. It was also useful in confirming past infections.” (Science Daily, 2008) the method, while developed specifically for testing for West Nile Virus in horses “can be easily modified to develop new antibody tests for humans and wildlife.” (Science Daily, 2008) it is related that these testing and diagnosis of West Nile in horses assist in the identification of where the disease is spreading and helps in the decision concerning whether to vaccinate horses in an area. Dr. Magnarelli states that the information used in this testing ” is useful in confirming the epidemiology of the virus, determining when it arrived in certain areas and how it spreads.” (Science Daily, 2008) the following illustration shows West Nile Virus (2008) in State with Equine cases with the total number of cases as of September 2, 2008 being 42.

West Nile Virus in 2008 (States with Equine Cases)

Total number of cases September 2, 2008

Source: United States Department of Agriculture: Animal and Plant Health Inspection Service (2008)


West Nile Virus in horses requires diligent preventive factors because horses are more prone to become infected with West Nile Virus than are other animal species. West Nile Virus is a mosquito born infection and there are precautions that can be taken to reduce the risk of infection such as putting horses in the barn from dusk to dawn, installing screening around barn stalls that are open to the outside, and by eliminating standing water on the property where horses are pastured. Because the horse is more prone to become infected with West Nile Virus than are other animal species, vaccination of horses has been deemed by veterinarians to be very important even to the point of vaccinating mares who are pregnant and vaccinating colts as early as three months of age. Optimum vaccination schedules are stated to be semi-annually for horses while some veterinarians are known to vaccine as often as every four months. There has been a new vaccine developed for horses infected with West Nile Virus and it is believed that this same vaccine could be modified for use in vaccinating humans against West Nile Virus. Horses that have contracted the West Nile Virus infection often have the same signs and symptoms of horses infected with other diseases therefore, it is very important that the horse be tested upon notice of the telltale symptoms related in this research study.


West Nile Virus Vaccination Guidelines (2005) American Association of Equine Practitioners. Supplement to the AAEP Guidelines for Vaccination of Horses, January 2001. Online available at http://www.aaep.org/pdfs/AAEP_WNV_Guidelines_2005.pdf

West Nile Encephalitis in Horses (nd) Penn State University. College of Agricultural sciences – Agricultural Research and Cooperative Extension. Online available at http://pubs.cas.psu.edu/FreePubs/pdfs/un008.pdf

Society for General Microbiology (2008, August 22). New and Improved Test for West Nile Virus in Horses. ScienceDaily. Retrieved October 13, 2008, at http://www.sciencedaily.com/releases/2008/08/080819213052.htm

Animal Health Monitoring & Surveillance (2008) West Nile Virus – States with Equine Cases, Total Cases as of September 2, 2008-42. United States Department of Agriculture – Animal and Plant Health Inspection Service.

West Nile Virus (2004) Veterinary Services. Aphis.usda.gov. 2004 October Online available at http://www.aphis.usda.gov/lpa/pubs/fsheet_faq_notice/fs_ahwnv.html

Minimizing the threat of West Nile Virus (2008) WSDA. Online available at http://agr.wa.gov/FoodAnimal/AnimalHealth/Diseases/WestNileVirus/default.htm

West Nile Virus in Horses

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