There are many causes of central nervous system disease in the horse, including viral, bacterial, parasitic, developmental or traumatic disease. Overall, central nervous system disease is uncommon. The viral causes of brain or spinal cord disease affecting horses include: the arboviruses, herpesviruses and rabies.

The Arbovirus diseases are the viral diseases that are spread by various arthropods, such as mosquitoes, biting flies and ticks. In general, the signs of arboviral diseases include fever, depressed demeanour, altered behaviour, incoordination, paralysis and death. The viruses are transmitted from insects, e.g., mosquitoes, to horses and occur during the seasons when the insects are active (summer and fall). Birds are the natural hosts for these viruses. The two groups of Arboviruses that affect horses are the Alphaviruses and Flaviviruses.

The Alphaviruses belong to the family Togaviridae and include Eastern, Western and Venezuelan Equine Encephalitis (EEE, WEE, VEE). In Ontario, Eastern Equine Encephalitis has been identified in horses that reside in the province or have become infected while traveling through other areas of North America. The EEE virus is spread by the mosquito Culiseta melanura that normally feeds on birds. The virus is carried from one area to another in migrating birds. C. melanura has a narrow habitat range and is restricted to specific swampy areas. People, horses, pigs and birds may become infected during periods of high mosquito populations. The EEE virus has a range from southeastern Canada to the southeastern United States as well as the Caribbean and South and Central America. Eighty to ninety percent of infected horses develop acute and lethal disease with survivors developing neurologic signs. There are vaccines available to prevent these diseases.

Eastern Equine Encephalitis Cases 1938 - 2004

Year	Number of EEE Cases Confirmed or Probable	Location/Comments
1938	?	Horses from St. George and St. Catherines
1939 - Fall 1992	0
1992	2	Woodstock - 1 horse died; 1 was subclinically infected.
1994	3	Bracebridge - Of 7 horses, 2 confirmed, 5 others died. Cambridge - 1 clinically affected with significant antibody titre (acute and convalescent) survived.
1997	1	Orillia
2001	2	1 draft horse travelled extensively to shows around southern Ontario
2002	1
2003	11 (plus 1 confirmed case in an emu)	1 - Dundas County 1 - Northumberland County 4 - Lanark County 1 - Ottawa-Carleton 1 - Manitoulin Island 3 - Simcoe County
2004	2	1 - Lennox-Addington 1 - Middlesex

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The Flaviviruses include Japanese B, St. Louis (SLE), West Nile (WNV), Powassan, Murray Valley and others. St. Louis encephalitis was isolated in humans in 1975 in southern Ontario. No natural-occurring equine cases of St. Louis or Powassan encephalitis have been reported. St. Louis is spread by Culex spp. mosquitoes, while Powassan is a tick-borne disease.

West Nile Equine Encephalitis was identified in New York State in the late summer and fall of 1999. This was the first reported outbreak of the virus in the western hemisphere. The disease was first identified in Uganda in 1937. It is theorized that the virus gained entry into the United States by way of an imported bird(s). The virus has over-wintered in the wild bird and mosquito population in Canada. The life cycle of the virus starts with a mosquito taking a blood meal from birds infected with WNV. Birds amplify the virus. Infected mosquitoes then transmit WNV to humans and animals when taking a blood meal. Vertical transmission in the mosquito (adult to eggs to larvae to adult) of the WNV has been demonstrated. Culex spp. mosquitoes are the primary vector of WNV. The incubation period of a WNV infection in horses is usually five to fifteen days. The virus is present in mosquito pools and birds in every county of the province of Ontario.

West Nile Virus Cases in Ontario Horses 2001 - 2005

Year	Number of WNV Cases Confirmed or Probable	Region
2001	0
2002	107	4 - Brant 3 - Chatham-Kent 9 - Haldimand-Norfolk 4 - Halton 3 - Hamilton-Wentworth 6 - Lambton 1 - Middlesex 1 - Muskoka-Parry Sound 12 - Niagara 2 - Northwestern 1 - Ottawa 3 - Oxford 1 - Perth 6 - Toronto 5 - Waterloo 1 - Wellington 41 - Windsor-Essex 4 - York
2003	10	1 - Durham 1 - Eastern Ontario 2 - Essex 1 - Halton 2 - Niagara 1 - Peel 1 - Perth 1 - Peterborough
2004	9	1 - Brant 1 - Durham 1 - Elgin 1 - Lambton 1 - Muskoka 3 - Nipissing 1 - Sudbury District
2005 to Nov. 21	5	1 - Essex 2 - Lambton 1 - Niagara 1 - Waterloo

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Equine Herpesvirus is also known as equine viral rhinopneumonitis. There are numerous strains of Equine Herpesvirus (EHV) that are passed between horses by body fluids, including nasal secretions. Horses commonly have antibody titres to EHV subtypes 1, 2 and 4. This indicates that they have been exposed to the viruses at some time in their life. Subtypes 1 and 4 can cause central nervous system disease, respiratory disease and abortion. Neurologic disease may occur after a herpesvirus respiratory disease infection. Signs typically involve the hind end of the horse and may include paralysis of the limbs, bladder and rectum. The brain and brainstem may also be affected. The virus can remain latent in the majority of horses without showing clinical disease. If recrudescence of the virus occurs, shedding of the virus ensues. This explains why horses in closed populations may experience an outbreak of herpesvirus. Horses are not protected against the abortion or neurological forms of the disease even after repeated exposure. Modified live-virus vaccines and killed-virus vaccines are available. These vaccines are labelled for the prevention of respiratory disease and abortion. The manufacturers do not make claims about the prevention of neurologic disease.

Rabies Virus is a well-known cause of fatal disease in many mammals. The virus is transmitted through bite wounds by affected animals such as foxes, skunks and bats, though other mammals may transmit the virus. Skunks remain as a reservoir of the virus. The wildlife vaccination program of the Ontario Ministry of Natural Resources has virtually eliminated rabies in foxes. The newest threat comes from the emergence of raccoon rabies in Eastern Ontario. Rabies is a rapidly progressing disease that is preventable by vaccination. Vaccinations must be administered by a licensed veterinarian. An extensive control effort, by Ontario Ministry of Natural Resources, has been launched to prevent the spread of raccoon rabies.

Equine Rabies Cases in Canada by Province and Ontario by County, 1997 - September 30, 2004

Year	Equine Rabies Cases in Ontario/County	Equine Rabies Cases in the Rest of Canada
1997	0
1998	0	2 in Saskatchewan, 1 in Manitoba
1999	0	2 in Saskatchewan, 3 in Manitoba
2000	3 Bruce, Grey, Huron	1 in Saskatchewan, 4 in Manitoba
2001	5 Bruce, Dufferin, Simcoe,    Thunder Bay, Waterloo	3 in Saskatchewan, 2 in Manitoba
2002	1 Algoma	1 in Saskatchewan, 2 in Manitoba
2003	1 Grey	1 in Manitoba
2004	2 Bruce of 114 total Ontario cases    (all species)	2 in Saskatchewan, 1 in Manitoba, 1 in New Brunswick
2005 to September 30	0 equine    of 81 total Ontario cases(all species)	2 in Saskatchewan

Summary

Viral Encephalitis (WNV and EEE) in horses has been diagnosed sporadically in Ontario. Prevention is based on a preventative health program of vaccinations and control of arthropod vectors, such as mosquitoes and biting flies. Fly-sprays, approved for use on horses, may have some repellent effect for mosquitoes on horses. You can reduce the number of mosquitoes around your home and neighbourhood by reducing the amount of standing water available for mosquito breeding. Contact your veterinarian for advice regarding vaccines as part of a preventative health program to reduce the potential risk of your horses becoming infected with one of these viruses.

Further information can be obtained on:
West Nile Virus:
Centers for Disease Control and Prevention (CDC)
Pennsylvania's West Nile Virus Surveillance Program
U.S. Department of Agriculture (USDA), Animal and Plant Health Inspection Service (APHIS)

Rabies:
Dr. Rick Rosatte, Rabies Research Scientist, 705-755-2280
Ontario Ministry of Natural Resources
Positive Rabies in Canada