West Nile Virus (WNV) is a devastating disease that is carried by mosquitoes. Since the initial incursion of West Nile virus on Long Island in the late summer of 1999, the virus has recurred in every subsequent transmission season and has spread all the way to the west coast. The virus first appeared in Canada in Ontario, Quebec and Manitoba in 2001. It spread to Saskatchewan in ’02 and then Alberta in ’03. The virus is now widespread in the southern portions of all the Prairie Provinces, Ontario, Quebec and Nova Scotia. Significant illness and death rates have been observed in equine populations, particularly when naïve populations encounter WNV. There were 13 confirmed cases in Saskatchewan in 2013. As of August 13, 2014, there has been two confirmed cases.
West Nile is maintained by a natural amplification cycle in North America in which various species of birds and mosquitoes are involved. The virus is amplified by spreading the virus between birds and mosquitoes but also increasing the quantity within an infected host. The types of mosquitoes known to spread the WNV prefer to bite birds but will feed on horse and human blood if birds are not available. In birds, the infection can range from no clinical signs to severe disease. American crows, Ravens, Magpies, Gray Jays, and Blue Jays are the most common birds found to contain the virus. In Saskatchewan, the most common time frame for West Nile to spread has been mid-July to mid-September. This time period corresponds to the ideal environment for the mosquitoes. These mosquitoes thrive in warm temperatures and less rainfall.
Humans and horses are considered dead-end or incidental hosts. They do not amplify the virus enough to facilitate further spread. Unfortunately, they are capable of showing symptoms. Horses become infected with the WNV through a mosquito bite that contains the virus. The outcome of infection in a horse is variable. Many horses infected do not develop observable signs. When signs are evident, in about 10% of infected horses, they are associated with inflammation of the spinal cord or brain. The signs can vary from mild stumbling or walking wobbly and weakness to unable to stand, muscle tremors, inability to eat and potentially death. Of those that show signs, 25-40% will need to be euthanized or die. Some horses are capable of having a full recovery.
A suspected diagnosis of West Nile can be made by history and clinical findings. However, due to its overlap of signs with other diseases, laboratory testing is used to confirm the diagnosis through blood tests.
Treatment of horses infected with the West Nile Virus is aimed at supportive care. At this point, there is no antiviral agent available. Alleviation of pain and dehydration are routine treatments. Mosquito control and vaccination are key to control of WNV infection in horses. Removal of manure, standing water and containers that could be used as mosquito breeding sites will minimize the mosquito population. Vaccinating against West Nile is vital to prevention of infection and signs of West Nile. The vast majority of clinical cases to date have occurred in unvaccinated or incompletely vaccinated horses. All of the West Nile vaccines available at this time have been shown to be between 80 and 90% effective at preventing the disease, and the vaccines appear to be effective for up to 1 year.
If you have any questions regarding the above information or any questions/concerns in general, please contact Twin Valley VHS at 745-6642.
Dr. Justin Noble DVM
Twin Valley Veterinary Health Services