Heartworm Prevention and Treatment

Only a few decades ago, it seems that no one had ever heard of heartworm, even though the parasite was known to veterinarians for over 100 years ago. Heartworms are members of the roundworm family, and cause health problems primarily in dogs.

Heartworm is dependent on its spread to female mosquitos (male mosquitos do not bite, they live on nectar), which are themselves dependent on warm weather for breeding. This is why, until recently, that heartworm was known only in warmer climatic zones in the United States. However, with elevated global temperatures and the mobility of the population which takes its dogs along on vacations, heartworm is now found in all of the 50 states. Female mosquitos need blood to produce eggs, which is why they bite.

Heartworms develop inside the body of the dog after a mosquito injects the larvae, and an adult female heartworm can grow to be nearly a foot long, with the male worm slightly smaller. Heartworm is not contagious, and one dog cannot spread it to another one - the only way to acquire heartworm is through a mosquito bite.

To begin the process of infection, the mosquito must pick up microfilaria, heartworm L1 larvae, from an infected dog. While in the mosquito, the larvae undergo several molts before they enter the mosquito's salivary glands. The next time the mosquito bites, it will inject some microfilaria along with its saliva.

For up to 7 months, the larvae will remain at the bite site, undergoing several molts and finally will have developed into a young adult heartworm. It will begin to migrate through your dog's body, often stopping off in the lungs as well as infecting the heart. The first sign that a dog may have heartworm will be the acquisition of a cough.

If there are heartworms of both sexes present, they will mate and the female will produce microfilariae, which are essentially microscopic roundworms. These microfilariae are what will be taken up by the next female mosquito to bite the dog, but they cannot complete their life cycle and become worms in the dog, their first molts must take place inside the mosquito.
This odd life cycle can actually benefit a dog that has been bitten by an infected mosquito - if no further microfilariae are introduced by subsequent bites, the original heartworms will simply die off in about 7 years. A fresh infusion of microfilariae that have been 'incubated' within a mosquito are needed, the new microfilariae produced by the female worm are incapable of causing harm to the dog. L1 microfilariae will simply die in 2 years within the dog's body unless they are taken up by a mosquito to undergo the next stages.

Because heartworm has spread so widely, no dog can really be considered safe in the United States or the southern part of Canada. If heartworm is a concern where you live, your veterinarian can perform a blood test to see if microfilariae are present in your dog's blood. If none are found, then most vets will recommend that a monthly medication be given to kill any microfilaria that may be introduced into the dog's body in the future. Those living in warmer regions will have to administer the medication year round, while dogs living in cooler areas will only need the preventative during the late spring, summer, and early autumn.

If a dog is found to be infested with heartworm, more aggressive treatment will be needed to kill the worms. There are two medications used to kill heartworms, Immiticide and Caparsolate, and both of them contain arsenic. These medicines are injected twice to kill the adult heartworms. Although these injections will result in the death of the heartworms, the dog is still in danger because the dead worms can clog arteries or cause pulmonary embolisms before the body is able to break them down and dispose of them.

Because of this hazard, the treated dog must be kept quiet for a month or more to be sure that the dead heartworms are gone. Activity can cause the pieces of heartworm to trigger a heart attack, especially if there were numerous worms in the heart region.

While avoidance will most likely consist of a monthly dose of preventative, dog owners can also take some actions to help prevent infection: keep the dog inside when mosquitos are most active, usually early in the morning and at twilight; pour out standing water that will provide a breeding place for mosquitos; and use a safe insect repellent on your dog.