Why We Vaccinate - West Nile Virus

West Nile Virus was first reported in Canada in 2001, and first detected in a horses in Canada in 2002.  West Nile Virus in horses hit the peak of its prevalence in 2003 with 170 cases reported in Alberta alone.  Since then there have not been as many cases of the disease in Alberta, and so the question is often raised—do we still need to vaccinate our horses against West Nile Virus?

What is West Nile Virus?

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West Nile Virus is transmitted by mosquitos and causes swelling and inflammation in the brain and spinal cord.  It primarily cycles in wild birds, carried between birds by blood sucking mosquitos.  Most wild birds are not affected by the virus, but crows, ravens, grey and blue jays, and other members of the Corvidae family are very susceptible to infection.  Only specific species of mosquitos spread WNV, and the species of concern in Alberta is Culex tarsalis, which usually appears later in our summer insect season.  The peak of West Nile Infection in Alberta is July-September.  Horses and humans can act as “dead end hosts,” meaning they can be infected by the virus, but it does not replicate within horses or humans in high enough amounts to be infectious to others.  There is no evidence that WNV can spread directly from birds to humans or horses without mosquitos, nor can a horse infect a human or vice versa.  Mules and donkeys can also be severely affected by the virus.

Most horses infected with West Nile Virus will not develop any outward signs of disease and clear the infection without any clinical symptoms.  In those that do become sick, signs of the disease can include fever, inappetence, listlessness, or neurological symptoms such as muscle twitching, drooping lip, stumbling gait, incoordination, head pressing, and in severe cases, partial paralysis, inability to rise and convulsions.  To prevent handlers from being hurt, extreme caution must be exercised when handling horses affected with West Nile Virus.

Among the horses that show clinical signs of disease, the fatality rate is less than 40%, however some horses that do survive demonstrate long-term neurological impairment.

How are horses with West Nile Virus treated?

Since the level of virus present in the blood of infected horses is low and only occurs for a short period of time, and because there is no evidence that the virus can be transmitted from horse to horse, affected horses are not quarantined.  There is no specific treatment for the virus.  Supportive therapies such as intravenous fluids and good nursing care are used to address the effects of the virus and prevent secondary infections.

How common is the virus?

Interactions between the virus and its mosquito host result in regional changes in the prevalence and virulence of the virus from year to year.  This means that we cannot predict future trends in the local activity of the virus, and therefore cannot predict if clinical West Nile infections will be common in horses in any given year.  Weather conditions can play a large role in the yearly and local prevalence of West Nile Virus, by influencing the prevalence of the mosquito that transmits the virus, Culex tarsalis.

In 2003, as mentioned above, Alberta experienced 170 confirmed cases of West Nile Virus in horses.  The next few years showed much fewer cases, with between 3 and 9 cases in each year from 2004-2006.  In 2007, however, conditions were right for a higher level of infection, and 46 equine cases were detected in Alberta that year alone.  From 2008-2011, again the virus was “quiet” and there was only 1 reported case.  The prevalence increased again in 2012-2014, with 5 cases in 2012, 14 cases in 2013, and 6 confirmed cases (and one suspect case) in 2014.  We therefore have had 25 cases confirmed in horses in Alberta in the past three years, and this number represents confirmed cases—horses where suspicion of disease is high enough to merit testing for the disease.  As the majority of cases show no clinical symptoms, the number of horses actually infected by the virus is much higher.

What strategies other than vaccination can be used to lessen my horse’s risk of West Nile Virus?

Because the mosquito Culex tarsalis is crucial to the transmission of the virus, any strategy aimed at reducing a horse’s exposure to mosquitos can lessen the risk of disease.  Eliminating sources of standing water can decrease breeding sites for the mosquito. Avoiding outdoor activities and turnout at dawn and dusk, peak times for mosquito feeding, or providing screened housing can reduce exposure.  Smudges may be useful, and topical insect repellents give some protection, albeit usually for very short amounts of time.

What about natural exposure to the virus?

Natural exposure and therefore natural immunity to the virus is difficult to investigate.  Most horses bitten by a West Nile infected mosquito do not develop clinical disease, and could develop a natural immunity to the disease that could help protect them in future years.  However, certainly clinical cases of West Nile Virus are much less common in well-vaccinated horses. 

So should I continue to vaccinate my horse against West Nile?

The continued detection of West Nile cases in horses indicates that horses in Alberta are still at risk for West Nile disease.  Mosquito control methods can lessen exposure to the virus, but it is difficult to eliminate the risk altogether and the amount of West Nile virus transmission to horses can depend on many factors influencing mosquito and bird interactions.  Clinical signs of West Nile in horses can be devastating, and there is no cure.  The vaccines against West Nile offer good protection, and therefore we still recommend vaccination against the virus.