Heartworm Disease

Heartworm is one of the most lethal parasites that can affect our canine companions. If an infestation is undiagnosed and untreated it will prove fatal. Annually, heartworms infest millions of dogs in North America, most especially in the southern United States. Once concentrated in these warmer areas (Florida in particular) with their heavy mosquito populations, the disease has been spreading northward for decades and is well-established in Southern Ontario and other regions of Canada. In Ontario in 2010, 431 dogs tested positive for heartworm (and 3 cats, 2 coyotes and 2 foxes). Last year in the general area of Quinte West 7 dogs was diagnosed with heartworm. In 2009 we treated 5 dogs for heartworm at Hillcrest Animal Hospital alone! We actually detected 6 positive cases, but one owner refused to treat the dog.

Adult heartworms live primarily within the heart chambers and large vessels of the heart or lungs of the dog. Every day mature female worms release thousands of larvae called microfilariae into the bloodstream of an infected dog. Although these larvae can survive for years in circulation, they cannot develop to the adult form without an “intermediate host”. This intermediate host is the mosquito. A mosquito bites an infected dog and ingests the microfilariae along with her blood meal. These larval worms then undergo two phases of maturity within the mosquito (this can take as little as 72 hours). The same mosquito then bites another dog and infests it with a new phase of the parasite. This stage of larva can then develop into a mature heartworm in the infected dog. As these new larvae mature they circulate throughout the body within the blood vessels until they make their way to the vicinity of the heart, where they anchor to the wall of a heart chamber or a major vessel and grow.

This two-stage life cycle is why heartworm can spread so quickly within an area. A positive dog will be a source of infection for any dogs in its area until that dog is tested and treated. Even a dog visiting the area can be a source of infection! A recent pet study revealed that 69% of dog owners bring their pets with them when they travel. Ontario, especially the Quinte West region, is a traditional summer tourist destination, which makes it particularly susceptible to the importation of heartworm disease. In addition many wildlife species can harbour heartworm, including foxes, wolves, coyotes and raccoons. Once heartworm disease becomes established in local wildlife populations you have a permanent source of infection as these animals obviously cannot be diagnosed and treated. Cats can also contract heartworm disease although it is very rare in our climate and they will often recover from the infestation with no treatment at all.

For the first year or two that an animal has heartworm, there are absolutely no physical signs that they have a problem. However, once the worms become large enough to interfere in blood flow patterns (they grow up to about 12 inches in length) the animal will begin to show signs of illness. Affected dogs gradually lose weight, develop laboured breathing, often have a cough, and cannot exercise for as long as they used to (exercise intolerance). These are the classic signs of congestive heart failure in dogs. When a dog has begun to show signs of disease, the treatment for heartworm becomes substantially riskier. This is why prevention of heartworm disease is vastly preferable to treating the dog after symptoms have developed.

To diagnose heartworm disease before symptoms arise requires a small blood sample, generally taken in the spring of each year. This sample is tested and if the test is negative the dog is put on a monthly preventive that kills the larval forms of the worm. In our area of Quinte West (South-Eastern Ontario) the infective season is generally regarded to extend from mid-June until early or mid-October. This is why we recommend giving the preventive medications starting on June 1st and running them until November 1st. One thing to note about these medications is that they kill larvae that are already in the bloodstream, so they effectively “kill backwards” for a period up to 6 weeks.

Dogs who test positive for heartworm must undergo a couple of other tests before they can be treated. X-rays of their chests are taken to assess whether they have any serious changes to their heart or lungs as a result of the infestation. They also have bloodwork performed to assess whether any other organs have been affected, like the liver or kidneys. A dog must be hospitalized for treatment of heartworm disease. If signs are favourable then we treat the dog.

A potent, and the very irritating drug called Immiticide is injected deep into the muscles of the back to kill the worms. As the adult heartworms die, they lose their grip on the vessel or heart chamber wall and they are pumped to the lungs by the circulatory system, where they are eventually broken down and eliminated. The dog must be closely monitored while the adult worms are killed because if too many large worms die too quickly, the lungs may react and the dog could go into shock and die as the lungs become congested with fluid. If things have gone well, this injection is then repeated 24 hours later to ensure that all of the adult worms are dead.  In really severe infestations these injections may be broken up even further to ensure that the adult worms die in smaller numbers. In some cases, worms actually are removed surgically from the heart prior to treatment with the drug! After the adults have been eliminated, the dog is put on preventive medication to keep it from becoming re-infested.

Prevention is obviously vastly preferable to treating a dog who is in heart failure produced by this parasite. Preventive medications come in many forms, for further information as to what will best suit your pet please contact our office.

You can read more about heartworm at Veterinary Partner.