As we approach the late summer and early fall, the clinical signs of West Nile Virus (WNV) will peak in number. Governments, private citizens and animal owners have all quickly taken steps to manage the threat of the disease since North America’s first reported case in New York City in 1999. However, since then, concerns over the disease have taken on mythical proportions. Experts predict that the number of cases of WNV will peak in mid August. So, with still a significant mosquito season ahead of us, and the Canadian Food Inspection Agency’s declaration that West Nile Virus is an agent indigenous to Canada, we all must remain vigilant in the battle against West Nile.
The disease was first discovered in a woman in the West Nile district of Uganda in 1937. An outbreak in Israel in 1957 was when the virus was first recognized as a cause of severe encephalitis in man. The disease soon migrated into Egypt and France where it was diagnosed in the equine population in the early 1960s. Since then, it has been detected in birds, humans and horses in Africa, Europe, Asia and the Middle East. According to Health Canada reports, as of mid-July 2003, birds testing positive for WNV have been detected in the provinces of Quebec, Ontario, Manitoba, Saskatchewan and Alberta.
It is well known that WNV is deadly to the horse and bird population. But, what of our cats and dogs?
According to Dr. Grant Maxie, Manager of the Animal Health Laboratory at the University of Guelph, “Yes, West Nile virus can infect dogs and cats, but they rarely appear to develop disease.”
Research indicates that dogs and cats are capable of becoming infected with the virus that causes West Nile, but evidence so far shows that for the most part, they simply don’t show signs of the clinical disease. A survey conducted by 15 veterinary clinics in Louisiana between August 25 and November 2 last year tested 442 dogs and 138 cats for West Nile Virus. Of those, 9.4% of cats showed a seroprevalence with 26.2% of dogs testing positive. The incidence of positive cases was twice as high in strays than family dogs, and 19 times higher in outdoor vs. indoor dogs. No deaths were noted. It’s noteworthy that Louisiana had the 4th highest number of human cases in the U.S. in 2002.
Because this disease is so new to North America, the medical community continues to make new discoveries relative to WNV. However, the Centers for Disease Control in the U.S. indicates that in addition to birds (138 species infected), horses and humans, WNV has been known to infect camels, cattle, sheep, mountain goats, cats, bats, chipmunks, skunks, squirrels, domestic rabbits, and dogs. The puzzling aspect with these species, however is that aside from horses, wild birds, and a fraction of the humans affected by WNV, it does not appear to cause extensive illness.
West Nile virus falls into a category of viruses called flavivirus, a member of a larger group of viruses called arboviruses, which are transmitted by blood-sucking vectors such as mosquitoes. Using birds as the host, the mosquito becomes a transmitter by biting the infected bird (or other species), incubating the virus for 5 to 15 days, and then transmitting the virus to other birds, humans or animals. Horses in Canada have had access to a vaccine for West Nile virus for the past two seasons. No vaccine for humans is presently available, and researchers find it difficult to predict when a vaccine may eventually come to market. There is little evidence of research into a vaccine for pets, as they do not seem to be adversely affected by the disease.
An Ounce of Prevention is Worth a Pound of Cure
There are countless resources available with recommendations on protecting ourselves from potentially infecting mosquitoes. Probably the easiest one is to apply an insect repellent containing DEET. However, it is critical that pet owners not apply human products to pets.
Too many pets have suffered serious side effects by ingesting human-labelled products applied by well-meaning pet owners. If you’re going to the cottage, or a location where you’re really concerned about a high mosquito population, bring along products such as pyrethrin-based insect repellents specifically designed, tested and labeled for safe use on companion animals. Owners must also recognize that mosquitoes are most active between dusk and dawn, and keep pets indoors during these periods.
Pet owners must also weigh the risk of their pet contracting WNV versus against the potential long-term effects of using insect repellents. It is up to the owner, to assess their level of risk aversion.
Perception Does Not Necessarily Equal Risk
Although the media continue s to bombard the public with messages about West Nile virus, the risk needs to be properly quantified:
- It is estimated that less than 1% of mosquitoes in regions affected by WNV actually carry the virus.
- The vast majority (nearly 99%) of people who are infected don’t feel any symptoms at all.
- Unless the disease progresses to cause encephalitis, it’s usually over in less than a week. (Severe illness, including encephalitis usually occurs in the very young, elderly or those with weakened immune systems. Of those who develop encephalitis, the mortality rate is estimated to be between 3-15%).
What is extremely necessary is for veterinarians and pet owners to properly assess the actual risk of WNV to themselves and their pets. The Centers for Disease Control in the U.S. reported only one death in an 8-year-old dog, which happened to be concurrently immunocompromised, in the Chicago area last year. No cases of WNV have been reported in Canadian pets, although it is a good idea to limit their exposure to mosquitoes for other reasons – such as heartworm.
A Symptom of What?
Ultimately, if there is still concern that a pet has WNV, there are certain clinical signs that may indicate disease, including incoordination, depression, decreased appetite, difficulty walking, tremors, abnormal head posture, circling and convulsions. However, these signs may indicate other very serious neurological conditions, and a thorough veterinary examination is the only means of ensuring an accurate diagnosis.
Confirmation of a WNV diagnosis is usually accomplished with a blood test (VecTest capture ELISA and PCR) or post-mortem examination. WNV is an immediately notifiable disease under the Health of Animals Act, which means that all laboratories are required to contact the Canadian Food Inspection Agency upon suspicion or diagnosis of the disease. District veterinarians of the CFIA are also excellent local sources of information on WNV.
Fundamentally, veterinarians should take reasonable precautions to prevent West Nile virus infection in themselves, their pets, and patients. It’s important that pets and people enjoy the season ….. mosquitoes and all.
West Nile Virus is relatively new to North America although our level of understanding of the disease increases each year. The article below was written in the late summer of 2003. Canadian Animal Health Institute
Other sources for information on West Nile Virus can be found at:
Public Health Agency of Canada
Canadian Food Inspection Agency
Health Canada
Centers for Disease Control
