West Nile virus spread across the eastern United States and Canada after it was introduced in New York in 1999. It eventually hit the Canadian Prairies in 2002.
It has now reached Mexico and the Caribbean and within the next decade, it will likely advance across South America as well.
As it establishes in new regions, there will be a large number of susceptible hosts with no immunity, leading to high numbers of cases. The virus is spread by mosquitoes and has become a mainstay of the summer.
Birds have a crucial role in the spread of West Nile virus. Some wild bird species, especially those in the corvid family, which includes crows, magpies, ravens and jays, are susceptible to infection and the virus multiplies rapidly in these birds. There are so many viruses in their blood that biting mosquitoes pick up the virus and can transmit it to other hosts. This is why these birds are called amplifier hosts, essentially functioning as feathered virus factories.
As the number of infected birds increases, so does the risk of infection to other types of animals including horses and humans.
These wild birds eventually die of their infection, so surveillance of dead birds has been used to indicate the circulation of the virus in specific areas. You can submit dead birds for testing to the Canadian Wildlife Health Cooperative at www.cwhc-rcsf.ca/report_submit.php.
Animals such as chickens, dogs and cats can become infected with the virus but usually do not develop illness. In terms of domestic animal health, horses are at greatest risk of this disease. From the mosquito bites, the injected virus multiplies and spreads to the lymph nodes.
Eventually, the virus makes its way into the spinal cord and brain. It is this stage that leads to the shocking and dangerous clinical signs. Infected horses can experience muscle tremors, clumsiness, weakness, lameness, colic and in severe cases, an inability to stand.
There is no specific treatment for West Nile, therefore clinical cases are given supportive care that includes fluids and anti-inflammatory medications. Roughly one third of horses that develop clinical signs will die or are euthanized.
Other diseases like equine herpesvirus 1, eastern and western encephalitis and rabies can also cause these types of neurological signs so it is important to establish a diagnosis.
It is unlikely you will pick up West Nile virus infection from a horse but you certainly can contract rabies. West Nile virus can be diagnosed based on the presence of antibodies or virus in the blood.
If horses are euthanized, an autopsy examination can determine the cause. Infected horses have damage to the spinal cord and there are tests available to show the presence of the virus in these damaged tissues and rule out other potential causes.
According to the Canadian Food Inspection Agency, 51 horses contracted West Nile virus in Canada, 23 of which occurred in the western provinces. Luckily for horses, effective vaccines were quickly developed and are readily available. The current recommendation for most horses is for an annual shot to protect against the disease.
Over the last few years, the rate of West Nile virus infections in people has dropped. According to the Public Health Agency of Canada, there were 2,215 human cases in 2007 but only 200 last year.
This could be related to the fact that many people have been exposed now and have developed immunity. In the early years of the outbreak, hardly anyone had ever encountered the virus so there was limited immunity in the population.
It is also possible that weather conditions over the last few years have kept the virus at bay.
West Nile virus is here to stay and will continue to circulate during peak mosquito months of June to September. Even though we expect fewer infections, it is a good idea to discuss the vaccination of horses against West Nile virus with your veterinarians and practise good mosquito control.