Treat and Prevent Mosquito-Borne West Nile Virus in Horses

For Canadian horse owners, warm weather, sunshine and longer days cannot come soon enough. Winter does have some positive aspects, however: cold weather forces pesky insects such as mosquitos to go into hibernation, giving humans and animals alike a reprieve from their incessant biting. Mosquitos can also carry diseases, such as West Nile Virus, that can cause major health concerns for your horse.

There are three main equine diseases that are associated with mosquitos; West Nile Virus (WNV), Eastern Equine Encephalomyelitis (EEE) and Western Equine Encephalomyelitis (WEE). All three of these diseases cause neurologic symptoms and can very easily result in death. These mosquito-borne illnesses are also zoonotic, which means they can be transmitted from animals to people. Birds and reptiles are often hosts of these diseases, and while they do not get ill themselves, they carry enough viraemia [virus content] in the blood for mosquitos that feed on them to transmit to other species. Horses and humans are “dead-end hosts,” which means they can be infected and become sick, but do not carry enough of the virus in their blood to allow transmission to others.

WNV has been identified all over Canada, the United States, and Mexico. It is transmitted from avian (bird) reservoir hosts to mammals by mosquitos and is not directly transmitted from horse to horse or horse to human. The virus can cross the blood-brain barrier and gain entry to the brain and spinal cord, causing damage to both and serious neurologic signs that can range from fever and muscle twitching in the muzzle, face, neck, shoulders and pectorals, to more serious manifestations including incoordination of limbs, ataxia (lack of muscle coordination), paralysis, recumbency and death. Other clinical signs include changes in behaviour such as depression or overreaction to stimuli, leaning to one side, inability to swallow, stumbling or toe-dragging, seizures and aimless wandering.

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Diagnosing WNV can be tricky, as clinical signs of all the mosquito-borne diseases mentioned above are quite similar. Thankfully, there is a blood test that can be done to confirm WNV.

As it stands today, there is no specific antiviral treatment available for WNV, and only symptomatic treatment is available for infected horses. This includes intravenous fluid therapy, pain management, and anti-inflammatories. Sedation of the animal can also be important to keep it calm. Depending on length of treatment, nutritional support may also be necessary.

The mortality rate for horses infected with WNV is around 30%, and more than 25,000 cases of WNV have been reported since 1999 in the United States alone. In Canada in 2018, the Canadian Food Inspection Agency reported 123 horses with WNV infection from BC (1), Alberta (72), Saskatchewan (32), Manitoba (5), Ontario (11), and Québec (2). Thankfully, there is a commercial vaccine available that has significantly decreased prevalence of WNV. In Ontario, for example, in 2002 there were 101 confirmed cases, while in 2018, just 11 horses were diagnosed. This disease still very relevant, however, and an important threat for horse owners to be aware of. If your horse permanently resides in Canada, annual vaccinations should provide adequate coverage for WNV. If you are one of the lucky ones who get to travel south for the winter, bi-annual vaccines are recommended, as mosquitos are present year-round in the southern States.

EEE and WEE are also mosquito-transmitted illnesses, and similar to WNV in that they cannot be transmitted from horse to horse. They also cause severe inflammation of the brain and spinal cord, leading to neurological symptoms.

EEE is found in Eastern Canada, all US states east of the Mississippi River, Arkansas, Minnesota, Texas, and the Caribbean Islands. It is also transmitted from birds to dead-end hosts like horses. Clinical signs include altered mentation (mental activity), impaired vision, aimless wandering, circling, head-pressing, ataxia, inability to swallow, seizures, recumbency, coma, and death. Most infected horses progress to a recumbent state within 12-24 hours of onset of clinical signs and death occurs within two to three days. EEE, when compared to WEE and WNV, tends to show more behaviour changes and the mortality rate for EEE is the highest of the diseases, estimated to be between 75-90%.

WEE occurs mostly in Western Canada and west of the Mississippi River in the United States. Just like EEE and WNV, WEE carries similar clinical signs such as changes in mentation, fever, muscle weakness and incoordination. Case fatalities recorded for WEE are much lower at 20-40% and actually there hasn’t been a case reported in North America in over two decades.

Treatment for both EEE and WEE are again symptomatic and supportive. IV fluids, steroids, NSAIDs to control fever, and steps to prevent injuries from ataxia or neurologic deficits (stall rest, protective headgear) are among the main treatment choices for EEE and WEE. Thankfully, there is also a vaccine that is commercially available for Eastern and Western Equine Encephalomyelitis, and it is very effective at preventing severe disease. Similar to the vaccine available for WNV, it is recommended that annual boosters be carried out, unless your horse travels south for the winter in which case a bi-annual vaccination schedule should be in place.

Diseases that cause neurologic symptoms are very frightening to witness and can be extremely dangerous for the humans handling the infected animals. Thankfully, these diseases are very easily preventable and are considered to be a part of the core vaccines recommended by the American Association of Equine Practitioners. Vaccination is essential for prevention of illness, as it can be very difficult to save the affected animal once clinical signs appear. If you are interested in learning more about these diseases or vaccination protocols, please reach out to your veterinarian.