At the  of the Matter

Heartworm Disease

When someone says “I think my dog has worms.” they are probably not talking about worms in the heart. Heartworm disease is an important topic that every dog (and even cat) owner should be talking about as the result of heartworm infestation is far more serious than that of gastrointestinal worms. Today’s article will talk about Heartworm Disease or Dirofilariasis in dogs and the less common occurrence in cats.

Heartworm Disease is caused by an infestation of the roundworm Dirofilaria immitis.  D. immitis is acquired by the mosquito when it feeds on infested dogs, and the immature parasite is transmitted to new dogs when the mosquito feeds again. In the new host, the parasite matures through to the adult stage. The adult worms are primarily found in the blood vessel that takes blood from the heart to the lungs- The Pulmonary Artery. In heavy infestations, the worms may even extend back into the heart.

As outlined above, heartworm disease literally results in worms within the heart and its blood vessels. Infestation may go undetected with low parasite burden however as numbers increase the worms block blood flow as well as cause inflammation of the pulmonary artery. This increases the blood pressure going to the lungs and causes a dry cough, usually seen as the first sign of Heartworm disease. Right sided heart failure can also occur as the outgoing blood flow is reduced, possibly resulting in an enlarged abdomen and a distended, pulsing jugular vein. Other signs of heartworm disease include exercise intolerance and anorexia or in more severe cases rapid breathing, fainting and death.  In cats the disease may have similar signs, however, it is less common as cats are not the definitive host for the parasite.

Diagnosis of heartworm disease is based on the above signs, a history of lack of effective heartworm prevention and positive heartworm antigen blood test results.

The treatment for Heartworm disease spans several months and mainly involves Melarsomine, an adulticide aimed at killing the adult worms. This is used alongside dewormers that kill the immature form, antibiotics that kill bacteria within the worms and steroids to reduce inflammation. Another important aspect of treatment is exercise restriction to reduce the risk of dead worms dislodging and blocking blood flow to vital organs, potentially leading to death.

It is important to note not all dewormers and no herbal therapies are effective in preventing heartworm. Heartworm disease is most effectively prevented through regular deworming, particularly with macrocyclic lactone drugs such as Ivermectin. Common commercial preparations with the above include; Heartguard, Endolav Plus and Sentinel. Heartworm prophylaxis should begin around two (2) months of age and continue monthly to every three months throughout the animal’s life. Antigen testing is recommended for animals that have not been on heartworm prevention for over 6 months, to facilitate early detection and treatment.

In closing, heartworm is easily prevented by regular deworming. Prophylaxis is generally cheaper and less risky. If you are unsure of your animal’s heartworm status, please call the friendly staff at the Tobago Animal Hospital.

Todd M.J. Denoon