September 2002

WEST NILE VIRUS IN DOMESTIC ANIMALS

West Nile Virus is a disease, new to North America, which is spreading rapidly westwards across the continent.  From a single case in New York State in 1999, thought to be from an imported African bird, the disease has spread northwards into Ontario and now westwards into Manitoba and Saskatchewan.  As of the end of August 2002, 10 horses have died in Manitoba and over 20 more have tested positive for the virus.  One dead crow tested positive in Saskatchewan, and it seems only a matter of time before cases are found in Alberta and BC.  In the USA, the disease spread rapidly southwards to Florida and has now been confirmed in Colorado, Wyoming and possibly Montana.

West Nile Virus is primarily a disease of birds affecting the Corvid family, which includes Crows, Ravens and Blue Jays.  It is spread by mosquitoes which bite the infected bird and then carry virus infected blood to other hosts, such as horses, dogs and cats, and humans.  These are incidental end-hosts that do not develop enough virus in their blood to pass on the infection.  Most cases of infection result in mild flu-like symptoms, but a few will progress to severe cases of encephalitis (inflammation of the brain).

In humans, it is estimated that only 1% of those infected with the virus will show flu-like symptoms and, of these, only 1% will progress to encephalitis (ie. 1/10,000).  However, among domestic animals, the horse seems to be the most susceptible.  No symptoms have been reported in dogs, cats or cattle and we do not know what percentage of infected horses will show symptoms.  But, of those horses showing symptoms of ataxia, poor coordination, stupor or sleeping sickness, up to 40% will die or be euthanized after going down and failing to regain their balance.  An ataxic horse is dangerous to be around and a horse unable to rise will soon develop severe pressure and friction sores to its head and legs, and have to be euthanized.

PREVENTION

A new vaccine was developed by Fort Dodge Laboratories and has been granted a "conditional release" in Canada and USA for use in horses.  There is no vaccine yet for humans or domestic pets.  Equine veterinarians recommend that horses traveling eastwards to locations such as Spruce Meadows in Alberta and the Toronto Winter Fair or southwards to the California or Florida winter racing circuits, start their vaccination course now.  Two doses of vaccine are required to be given four to six weeks apart, followed by an annual booster in the spring, before the mosquito season.

For horses in BC, veterinarians are recommending vaccination in Spring 2003, along with the regular vaccine against Tetanus, Flu, Rhino and East and West Encephalitis.  Horse owners who wish to start their vaccinations ahead of spring training can have their first and second doses of West Nile vaccine in Fall 2002 with a single dose booster in early Spring 2003.

Other measures to prevent infection include removal of stagnant water around horse pastures and barns to limit mosquito breeding areas, insecticide sprays and keeping a close watch next Spring for dead Crows or Blue Jays in the area.

SUMMARY

West Nile Virus is a disease new to North America, which is spreading steadily westwards.  As of Fall 2002, no cases have been reported west of the Rockies but we can expect further spread with the migration of birds and next year's mosquito season.

Vaccination is now available for horses only and is strongly recommended before the Summer 2003 mosquito season.

John Twidale, DVM

Chair, BCVMA Equine Committee