Eliminating Heartworm

All dog in affected regions are seen as at-risk animals.

Should Dogs Be Given Medicine?

Studies show that all 50 US states have reported cases of heartworm infection, a condition that can affect all dogs regardless of sex, age or habitat. The highest incidence for dogs not taking preventive medicine rises to a high of 45%, including areas from the Gulf of Mexico to New Jersey, while some areas record rates of 5% and below for the canine heartworm disease.

The difference is due mainly to mosquito, environmental and dog population factors, although all dogs in affected regions are still seen as at-risk animals that need to be monitored regularly by veterinarians and covered by prevention programs.

Dogs become infected when they are bitten by mosquitoes with infective heartworm larvae. The infection is transmitted, with the larvae eventually growing into adult female and male worms that live not only in the heart, but also in the lungs and related blood vessels. The offspring, called microfilariae, are released by the female heartworm into the bloodstream. Practically all experimentally infected dogs were found to have adult worms, with up to 250 worms possible for one dog.

More adverse changes to the lungs and heart are observed for dogs with a higher number of the worms. The infections later lead to inflammation that affects the lungs and surrounding arteries. The heart is thus pressured by the increased workload, becoming enlarged and weakening and eventually, congestive heart failure kills the dog. The heartworms can also be found in the caudal vena cava - the main vein between the liver and the heart - where they can cause liver failure syndrome.

The main purpose of treating infected dogs is to kill both offspring and adult worms using a microfilaricide and adulticide respectively. A crucial condition of treatment: minimizing any adverse side effect due to the drugs used and a tolerable level of complications due to the dying heartworms.

Dogs without or with mild signs have shown significant success following treatment, while those exhibiting more severe signs also have successful treatment, but are more prone to complications and death.

The US Food and Drug Administration has already approved an organic arsenical compound, melarsomine dihydrochloride, to kill adult heartworms. Dogs that will use this therapy are required to undergo an extensive pretreatment evaluation and must remain in the hospital during treatment.

This FDA-approved drug, which has proven to be less toxic and more safe and effective than its predecessors, is given through intramuscular injection into a dog's lumber muscles. However, a key post-treatment concern is severe pulmonary thromboembolism, in which lesions in the lung arteries and capillaries and dead heartworms obstruct blood flowing through the pulmonary arteries.

More severe lesions and a higher number of dead worms cause greater obstruction, with cough, fever and hemoptysis appearing as symptoms. Dogs showing these clinical signs will have to be strictly limited in terms of treatment and exercise and given corticosteroids to reduce inflammation.

Although total elimination of adult heartworms may not be possible, dogs have shown clinical improvement following adulticide therapy. A common follow-up to the therapy is heartworm antigen testing: the antigen will not be detected four months after adulticide treatment if all or a very small number of the parasites survived.

Dogs found to be antigen-positive post-adulticide may have the treatment repeated, but only following an extensive case review. One option is to resume arsenical use with ivermectin or other preventive, a combination expected to eventually kill all surviving worms.

Meanwhile, selamectin, ivermectin, and moxidectin are available as treatments for microfilariae. These macrocyclic lactone anthelmintics are present in regular heartworm preventives as active ingredients, although they do not have FDA approval as microfilaricides.

Although these drugs do not have regulatory clearance, they are still popular as treatments because of the absence of approved drugs to eliminate microfilariae. Thus, dogs using these ML anthelmintics must remain in the hospital after treatment to be monitored for any potential side effect due to the rapid death of the offspring worms. Use of ML preventives is expected to eventually eliminate all microfilariae in six to nine months' time.

Thus, dog owners should thoroughly weigh the risk between heartworm treatment and the partial or full recovery of their pets. Whatever decision is made, the owners should always be pro-active in ensuring the best health care for their dogs.