Since the first horse was detected with West Nile virus in Canada in 2002, the prevalence of the disease has cycled annually through peaks and troughs.
The number of cases in horses initially peaked in 2003 following its introduction, with many fewer cases reported until 2007 when the incidence of the disease spiked once again. Since then, there were significantly fewer cases reported until this summer, when once again there has been a renewed spike in horses across the Prairie provinces.
The clinical prevalence of West Nile in horses is highly unpredictable due to the intimate relationship between the virus, its mosquito host, specific bird populations, and weather conditions. The West Nile virus is primarily maintained in nature by transmission cycles between mosquitoes and birds.
Wild birds within the magpie, crow, raven, grey and blue jay families play a crucial role in the spread of West Nile. These birds are highly susceptible to infection and act like feathered virus factories. The virus can reach tremendously high numbers in these birds. As the number of infected birds increases, so does the risk of infection to other types of animals including horses and humans.
Biting mosquitoes pick up the virus and transmit it to other hosts. Only one specific species of mosquito, the Culex tarsal, spreads West Nile and this species usually appears later in the summer season. As a result, the risk of West Nile infection for humans or horses tends to peak July through to early September, especially if standing water and other mosquito larva habitats exist and are coupled with extended periods of warm weather.
Both horses and humans act as ‘dead-end hosts,’ meaning they can be infected by the virus, but the virus does not replicate within horses or humans in high enough numbers to be infectious to others. There is no evidence that West Nile can spread directly from birds to humans or to horses without mosquitoes, nor can a horse infect a human or vice versa. Mules and donkeys can also be affected by the virus.
The majority of horses bitten by a mosquito infected with West Nile will develop an asymptomatic infection, clear the virus, and carry on without incident. In some animals, however, the virus breaches the blood-brain barrier and crosses into the central nervous system where it causes inflammation and damage to the brain and/or spinal cord. The nature, range and severity of clinical signs varies largely depending upon the areas damaged by the virus.
Signs of the disease in the sick horse can include fever, inappetence, teeth grinding, listlessness, aimless wandering, and/or neurological symptoms such as ataxia (stumbling, staggering, wobbly gait, unilateral leaning, circling, hindlimb weakness or inco-ordination), muscle twitching, lip droop/paralysis, head pressing, blindness, and in severe cases partial paralysis, inability to rise, convulsions and death.
It is highly recommended that all horses showing neurological signs be evaluated by a veterinarian. Clinical signs of West Nile infection in horses are similar to those caused by western equine encephalitis, eastern equine encephalitis, tetanus, and rabies. Specific diagnosis of West Nile is usually based on the nature of the clinical signs together with detection of antibodies to the virus in a blood sample or virus isolation from the tissues submitted upon post-mortem examination.
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.
Approximately one-third of the horses that develop clinical signs will die or are euthanized due to complications. Some horses that do recover may still have residual neurological deficits.
Since the prevalence of the West Nile virus in humans is associated with the prevalence of the virus in horses, this disease is under surveillance and is an Immediately Notifiable Disease under the Canada’s Health of Animals Act. This allows health agencies to increase their awareness in areas where horses test positive.
While the incidence of West Nile virus is likely to continue its oscillating trend into the future, the risk of West Nile disease in any individual horse is relatively low — some years lower than others. Most horses bitten by an infected mosquito do not develop clinical disease and go on to develop a natural immunity to the disease. Unfortunately for the small minority of horses that do develop clinical disease the consequences can be quite severe.
Effective vaccination against the West Nile virus in horses relies on strategic timing of vaccination in late May or June in order to position the maximum effects of the vaccine during the season of peak mosquito activity. Since this seasonal influence is often regionally specific, it is advisable to contact your local veterinarian for an appropriate vaccination protocol. It is not possible to eliminate the risk of mosquito exposure altogether for horses, however, it still remains a viable idea to practise good mosquito control.
NOTE: Readers were quick to notice that a photo of a bird accompanying this column in our Dec. 3 print edition (West Nile virus numbers spiked again this summer) “is most definitely not that of a baby crow.” They noted that baby crows are not born with down and said the photo accompanying the article was likely a corn crake, a bird that is not native to North America.