Beware of heartworm!

Heartworm (Dirofilaria immitis) is a blood parasite that lives in the dog’s heart. Dogs and other canines are its definitive hosts, but the parasite also infects cats. Adult heartworms are up to 30 cm in length and look like spaghetti. In the heart, parasites can live and reproduce for about 5 years. They produce microscopic larvae called microfilariae. They are so small that they enter through the dog’s bloodstream.


How are heartworms transmitted from one dog to another? They are mosquito-borne, when mosquitos feed on the dog. You see, when a mosquito bites a carrier they then take some microfilariae found in the carrier’s blood. Once in the mosquito, the microfilariae develop into infectious larvae. Once in the mosquito, the larvae become infectious within about fourteen days, when the exterior temperature is around 21 ˚C. If it is colder, the process can take a little longer.

As it becomes infectious, larvae can infect other dogs that the mosquito bites. As a matter of fact, when the mosquito feeds on blood, larvae migrate toward the dog and enter through the puncture hole, which is the first step of several months of travelling to reach the dog’s lungs. Six to seven months later, the larvae will reach the dog’s heart. Once it gets there, it becomes an adult that can reproduce, and a new cycle begins. If your neighbour’s dog is a carrier, they are a possible infectious source for all dogs in the area.

Prevention is always the way to go!

In the middle of the 1980s, Quebec veterinarians began to implement preventive measures. We shouldn’t hesitate to take measures to prevent the spread of this disease, which can be very difficult to treat. Heartworm parasites can cause all sorts of often-serious conditions, including, of course, heart problems. Once the worms are located in the dog’s heart, treatment to take them out is complex and dangerous for the animal.

It is definitely simpler and cheaper to prevent heartworm infection before larvae become adult worms. This is possible by using a monthly preventative treatment during, and immediately following, mosquito season. Such preventive treatments are easy to apply and kill larvae before they mature into worms. What’s more, today the same drug can prevent and treat fleas as well as all the usual summertime intestinal parasites.

Annual vaccines

It is important to prevent the disease every year. Also, depending on the area, we usually recommend a test before you administer the medication. You should also note that it takes at least six months before we can really see if a dog’s blood is infected with the microfilaria or not. Since the at-risk season ends in late November, it’s a good idea to wait until April or May before screening; then administering preventive medication June 1. During winter, it is still too early to detect the infection with a test.

Beside the neighbourhood dogs and cats, as well as many feral canines, coyotes are the main source of heartworms. Since they are not treated, worms remain active and mosquitoes easily ingest parasites and spread them on. This how the disease spreads.

Unfortunately, dogs that don’t get the preventative medication may become carriers of dirofilariosis and a possible source of heartworms. It is a vicious disease as the infected dogs don’t usually show symptoms right from the start. It can take a long time before the disease is detected in a dog, and by the time we know about it it’s usually too late. Once the animal develops heart issues, the most common symptoms are coughing, unable to exercise, some weight loss, scattered breathing, etc. It is sad to say that most dogs die because the worms have simply been in their hearts for too long.

The treatment for infected animals

If your animal becomes infected with heartworm, treatment is carried out over several stages and is slightly risky. Beside needing a powerful medication, the animal is also exposed to some risk due to the heartworms in their system, which can cause blood vessels to be blocked.

The message is clear: it is so much better to prevent heartworms than to treat them!

Each year your veterinary team will provide you with the latest information about the best types of preventive treatments, how to apply them, and the targeted parasites, in order to keep your loved one safe.

Have more questions?

Please do not hesitate to contact the Passionimo veterinary clinic near you.