What About West Nile Virus in Dogs?

A reader sent me an email asking about the risk of West Nile Virus infection in dogs. There have been several confirmed cases of the disease in dogs living in areas thick with mosquitoes but most dogs show no symptoms at all. There have only been a few cases of fatal encephalitis (brain inflammation) in dogs due to the virus. In the Pacific Northwest infection in dogs is apparently rare. Dr. James Evermann of the Washington Animal Disease Diagnostic Laboratory (WADDL) at WSU told me they had no confirmed cases in dogs or cats in 2006. There were cases in horses, humans and birds.

The disease appears to be firmly established in the United States. According to the Center for Disease Control (CDC) in 2005 there were 3000 confirmed human cases, including 119 deaths. Most of these were in states with lots of mosquitoes. In Idaho there were 13 cases (no deaths,) in Oregon 7 cases (no deaths,) and in the state of Washington no record of any cases.

We are lucky to have very few mosquitoes and our pets are even luckier. Most of the modern mosquito repellents that work and are safe on people have "DEET" in them and are not recommended for dogs and cats. They are likely to swallow it after licking themselves. I found many recipes and products that were supposed to be natural mosquito repellents. Most of them were based on garlic. They may be safe - I don't know - but I don't think they would be very effective.

We have two things going for us in the Pacific Northwest that help keep mosquitoes out. Our summers are short and we don't have much standing water. A healthy mosquito population is critical to the spread of West Nile Virus because the virus multiplies most effectively as it is transmitted back and forth between mosquitoes and birds. Horses, humans, dogs and cats are all considered "dead end" hosts because they are very unlikely to spread the virus.

Mosquitoes like warm weather. They multiply most effectively when the temperature stays warm continuously day and night for a few months. We have some really hot days for a while in late summer, but even then it may get fairly cool at night, and it is usually not too long before a string of hot days is interrupted by a cloudy and rainy day or two.

A more important factor might be a lack of the standing water mosquito larvae require. The eggs are laid in standing water, or in damp earth that will likely be covered eventually with water. The larvae must go through multiple stages before they're ready to fly away. This can take as long as a month to as little as four days, depending on the species. It generally takes longer if the water is cool. If the mud puddle dries up or gets too cold before the larvae grow up, they die, which is probably what happens to most mosquito larva in the Northwest. We can help keep this safeguard by getting rid of places for standing water, like old tires, and by careful and regular cleaning and maintenance of swimming pools, and especially the back yard ponds and "water features" that have become more popular in recent years.

So the bad news is there is no vaccine available for West Nile Virus in dogs, and no specific treatment or "cure." The good news is that infection in dogs in the Pacific Northwest is extremely rare, and even if your dog gets West Nile Virus, he will probably not even get sick.