Canine
Heartworm Disease


What is it and Where Does it Come From?

Canine heartworm disease is a serious and potentially fatal disease of dogs. Mosquitoes are the only natural agents of transmission. When a mosquito bites an infected dog it takes up blood containing immature heartworms called microfilariae, which are larvae that incubate in the mosquito for about 2 weeks. During this time they become infective larvae. When the mosquito bites another dog, the infective larvae are passed into the second dog, infecting it. The microfilariae migrate through the tissues of the body for about 3 months, and finally enter the heart where in another three months they reach adult size. Adult worms grow up to 14 inches. A dog can have several hundred worms in his heart and major arteries.

Clinical Signs of Heartworm Disease

Adult heartworms reproduce and accumulate in the dog's heart and lungs. If not removed,they can cause permanent heart and lung damage, possibly death. The important thing to remember is that a dog can have serious damage and not show clinical signs. Once the disease is advanced, the dog may cough, have difficulty breathing, tire easily, lose weight, and even collapse. Without proper treatment, the disease can cause congestive heart failure and even death.

Can Infected Dogs Be Treated?

With early detection, most dogs can be successfully treated for heart worms.This is one of the reasons that annual testing is so important. Adult worms are killed with a form of arsenic, which is given by a series of carefully administered injections. The worms die and are carried by the blood stream to the lungs where they lodge in small blood vessels. They decompose and are absorbed by the body. Heartworm treatment can pose a risk to the infected dog; However, fatalities resulting from treatment are rare. The patient should be given a thorough physical and laboratory examination prior to treatment. Any concurrent medical problem that might cause complications should be corrected before heartworm treatment begins. After treatment, complete rest is important to prevent lung damage from the decomposing worms. Excitement and exercise should be avoided for a least a month, followed by a gradual return to normal activity. After adult heartworms are eliminated, another drug is given to rid the bloodstream of microfilariae, the larval forms which are not affected by the drug used to kill the adult heartworms.

Testing For Heartworm Disease

The only way to detect heartworm disease in the early stages is with a blood test. Every dog should be tested annually to make sure he is free from heartworm disease. Testing should be done even if he is on preventative medication since infection can occur if for any reason a preventative tablet was not properly absorbed.

What Can You Do to Protect Your Dog?

Test your dog annually. If your dog is free of heartworms, your veterinarian can prescribe an appropriate preventative medication. Interceptor, a new heartworm preventative medication, not only protects against heartworm disease, but also against intestinal parasites - hookworms, roundworms, and whipworms. This added benefit is substantial, and for this reason our hospital is now recommending that dogs be given the preventative YEAR ROUND.


Berkshire Veterinary Hospital
730 1/2 Crane Avenue
Pittsfield, MA 01201
(413)499-2820