Apr 14 2012

Heartworm Disease – What is the Risk?

April Showers Bring May…. Bugs?

As the temperatures rise, the tree buds swell, and the grass starts to grow the insects start planning for a fun-filled summer of reproduction and feasting! Unfortunately, this is often at the expense of us and our pets (and my poor roses!). Mostly these insects are just an itchy nuisance but sometimes they can have life-threatening consequences. Such is the case for Heartworm disease in our dogs.

Here is a little science review. Heartworm disease is spread by mosquitoes. The mosquito draws up microscopic worm larvae when they bite an infected animal. The microscopic worm larvae is then injected into the muscle of the dog during a mosquito feed; then just as their name implies, the larvae makes its way into the bloodstream and sets up home in the pulmonary blood vessels as they enter the heart. Here they mature into worms nearly a foot long! The mature worm infestation gradually causes obstruction of blood flow through the heart resulting in congestive heart failure. Animals infected with heartworms will appear completely healthy until the eventual situation of congestive heart failure develops.

Why don’t people contract heartworm disease? Well, we do – sort of. We are bitten by the same mosquitoes carrying the larvae, but when the larvae enter our body, our immune system instantly recognizes them as a problem and eradicates them. Similarly, cats are much less likely to develop heartworm disease then dogs. But for reasons unknown the canine immune system permits the larvae to travel through their body and live. Other canine relatives such as foxes, coyotes, raccoons and wolves are equally at risk. Depending on the number of larvae injected, the time from infection to heart failure can range from months to years. So wild animals, or untreated dogs, may serve as a reservoir of infection to others for years before they succumb to the disease.

Unfortunately, 70% of all of Canada’s Heartworm cases are in southwestern Ontario. Common use of Heartworm preventions have managed to keep the prevalence of positive cases reasonably low, but the last 10 years has seen a 50% increase in the number of cases in this region. Partly the increased cases are due to the import of adoption dogs from the southeastern USA where the disease is extremely prevalent. Many of these dogs arrive infected by various strains of Heartworm. Some of these new strains are proving harder to prevent and treat then what we historically see in this region.

So how much risk is your dog at? Really it comes down to – “who is your neighbour?”. Heartworm cases appear to cluster, suggesting the likelihood of an animal infected with heartworm, living unnoticed within the cluster. This individual is the source of infection for the local mosquito population which then travel within a 5km radius, spreading the disease to others. So if the dogs and wildlife within a 5km radius of you are heartworm free, your dog is at a pretty low risk, no matter how many times a mosquito bites them. But if there is a dog or, say, a raccoon with heartworm disease living nearby, your dog is at a very high risk, just one mosquito bite may be all it takes.

Is Heartworm disease treatable? Yes – most of the time. Dogs that are symptom-free and are diagnosed with heartworm disease on routine screening tests can usually be successfully treated but treatment typically cost between $800-$1500. Dogs that are not diagnosed with the disease until they are showing symptoms (due to the heart failure) can sometimes be successfully treated, but complications are common and costs even higher. For these reasons most people prefer to use routine screening and prevention medications.

Although prevention medications have high efficacy rates, failures do occur; especially with the new strains of Heartworm up from the USA. So routine regular blood screening is still very important to ensure the safety of our dogs. There are numerous heartworm prevention medications available on the market, oral and topical, most are administered monthly by pet owners at home. They range in costs depending on the size of your dog and what other parasites a product may also prevent (ie. fleas, mites, ticks). All products have very impressive safety records. Call your veterinary office for specific details on products available, and don’t let the mosquito’s summer party plans ruin your pet’s summer!

Article Prepared by: Dr. Claire Todd (Veterinarian) Fonthill Animal Hospital

LifeLearn Admin | Hospital News