Testing for Heartworm Disease in Dogs

What is heartworm disease? How does a dog become infected?

Heartworm disease is caused by a parasite called Dirofilaria immitis, better known as heartworm. The parasites are long, hair-like worms that live in the right ventricle of the heart and the pulmonary artery, which is the large blood vessel that carries blood from the heart to the lungs. Dogs become infected when they are bitten by an infected mosquito that is carrying immature heartworms called stage 3 larvae (L3). The larvae pass from the mosquito into the dog, and move through the tissues of the body, eventually entering the bloodstream and migrating to the right ventricle of the heart. Within the heart, the worms mature into adults, mate, and produce immature worms called microfilaria, which circulate in the bloodstream. When the infected dog is bitten by another mosquito, the microfilariae are taken up by the mosquito. Inside the mosquito, the microfilariae develop through three stages of maturation to become infective L3 larvae. The larvae move into the mouth parts of the mosquito where they wait until the next time the mosquito bites a dog. It takes about 5 - 6½ months for the heartworm life cycle to be completed.

Where is heartworm infection most common?

Heartworm disease is widespread in the United States and is particularly common along the southeastern and gulf coasts, and through the Mississippi River valley. In Canada, heartworm infection is more restricted and is localized to southern Ontario, southern Manitoba, and southern Quebec, with scattered occurrences elsewhere in the country.

The risk of infection is greatest when mosquitoes are actively feeding. This typically requires an average daily temperature of more than 64°F (18°C). In areas that get killing frosts, the risk of heartworm infection is highest in the warmer months (late spring to late fall). By comparison, in subtropical United States, heartworm infection is a year-round risk.

Can infection be spread directly from one dog to another, from dog to cat, or from a dog to a person? 

No. Dogs can only get heartworm from an infected mosquito. There is no spread of heartworm infection from dog to dog, dog to cat, or dog to people. However, both cats and people can be infected by heartworm if bitten by an infected mosquito.

What are the clinical signs of heartworm disease?

In the early stages of disease, dogs often have no clinical signs, especially if they are carrying only a small number of worms.

"In advanced disease, dogs develop
congestive heart failure."

As the disease progresses, clinical signs become more noticeable and include reluctance to exercise, rapid fatigue with exercise, coughing, and sometimes collapse. In advanced disease, dogs develop congestive heart failure. Dogs in congestive heart failure lose weight, have a poor body condition, breathe rapidly or with difficulty, and develop a build-up of fluid in the abdomen.

How is heartworm disease diagnosed in the dog?

Heartworm is usually diagnosed with a simple blood test.  In our area this testing is usually done in the spring in our area to test for any infections from last year

Do all dogs need to be tested?

No. Puppies less than 6-7 months of age do not need to be tested. Adult worms are not present at this age and both antigen tests and microfilariae tests will be negative.

Is there treatment for heartworm disease?  

Treatment usually involves two types of medication: one to kill the adult heartworms and one to kill the microfilariae. Treatment is usually effective although there are cases where small numbers adult heartworms remain after treatment.

There are frequent side effects associated with heartworm treatment due to dead worms being pumped out of the heart into the lungs. Side effects are more common in dogs with large numbers of adult heartworms but all infected dogs are at risk.  In order to minimize side effects, all dogs undergoing treatment, even if they are not showing signs of illness, must be kept very quiet during the treatment period and for 4 weeks afterwards. Other drugs such as anti-inflammatory medication, anti-histamines, and antibiotics (Doxycyline) may be used to reduce side effects and improve response to treatment. If serious complications arise, hospitalization for further treatment may be needed.

How can I prevent heartworm disease in my dog?

Annual heartworm testing followed by preventive medication is recommended to keep your dog free of heartworm.  This medication is given once a month starting June 1st and continued  until November 1st.(for any of you who travel with you pets to the US this season becomes longer and in southern US such as Florida prevention is necessary year round).

 There are a variety of medications available to prevent heartworm.  Some of the medications are also effective against fleas and some internal parasites.

The staff at South Peel will be happy to discuss these with you to see which best suits your pets needs.