AVMA Collections: Heartworm disease summary

Single-topic compilations of the information shaping our profession

March 2010

Heartworm disease is a serious and potentially fatal condition caused by the nematode, Dirofilaria immitis, that is transmitted by at least 10 to 15 species of mosquitoes. Heartworm disease is most common in tropical and subtropical climates, but occurs throughout the United States, with an incidence of 45% along the Atlantic and Gulf coasts and the Mississippi River and its major tributaries. The number of adult heartworms found in affected animals can vary dramatically, with corresponding clinical signs including cough, dyspnea, syncope, weight loss, and lethargy.

First considered a disease primarily of dogs, heartworm disease has been increasingly recognized as a serious clinical concern in cats. Differences in manifestation of the disease in cats as compared to dogs, as well as a proposed new model for understanding the disease in cats, are presented within studies in this collection.

Please note that articles presented here are representative of scientific studies of heartworm disease published in AJVR and the JAVMA since 2000 and do not constitute a comprehensive overview of the subject. For a more complete overview of standard diagnostics, treatment options, and prevention, consult these resources available on the American Heartworm Society Web site:

This collection begins with studies that reveal the prevalence of heartworm disease in a variety of circumstances. Following the Gulf Coast hurricanes in 2005, researchers explored the question of whether abandoned animals might have a higher incidence of heartworm infection related to a lower level of veterinary care. The prevalence of heartworm disease among rescued animals was found to be similar to that of the general population in the region. In other studies, results show that nondomestic cats housed outdoors in endemic areas are at risk, that the geographic distribution of exposure in cats is similar to that of dogs which could be of value in making clinical recommendations, and that 2.5% of shelter cats in lower Michigan had evidence of infection.

The "Diagnosis" section begins with an exploration of clinical signs in 50 cats with heartworm disease. This is followed by 2 studies in which the performance of serologic tests were compared in dogs and cats. The usefulness of echocardiography in identifying heartworms in cats was studied by the authors of the next article, and they found that with echocardiography they were able to detect mature heartworms in 5 antigen-negative cats. The next 2 articles deal with pulmonary lesions in cats exposed to D immitis. The first contains findings of a study of 630 shelter cats in Florida in which pulmonary artery lesions were discovered in cats without adult heartworms. The second, "Reassessing the definition of heartworm infection in cats," is a commentary about the impact of such findings on current understanding of heartworm exposure in cats, in which the author suggests alternative classifications for heartworm exposure and infection in cats. The authors of the final article present findings from scanning electron microscopy and histology that provide a structural explanation for pulmonary signs of heartworm disease in dogs, including anastomoses between bronchial and pulmonary arteries.

In the first and second articles of the "Treatment" section, researchers present cases involving manual extraction of adult heartworms in dogs and cats, respectively. The third article consists of case reports in which neurologic complications developed after melarsomine dihydrochloride treatment in three dogs. The authors concluded that veterinarians should strictly follow manufacturer's recommendations for use of the drug.

The first article in the "Prevention" section, "Safety of moxidectin in avermectin-sensitive Collies," contains findings that demonstrate a wider margin of safety for moxidectin as compared to ivermectin or milbemycin in these dogs. The authors of the next 2 articles present findings about the safety and efficacy of sustained-release formulations of moxidectin in dogs. The authors concluded that these formulations can provide protection for 6 and 12 months, suggesting that their use could eliminate problems of poor compliance common with monthly dosing. Finally, the authors of "Impact of the owner-pet and client-veterinarian bond on the care that pets receive" present findings from a national survey of pet owners. Data obtained from the survey point to lack of communication between veterinarians and their clients as the primary reason for clients to refuse treatment recommendations, and they suggest that owner compliance with heartworm prophylaxis could be increased by 25% if communication were improved.

In the "On the horizon" section, authors of 2 articles explore alternative drug delivery methods with potential for broader use. The first article pertains to use of a covered-rod silicone implant for sustained release of ivermectin in dogs, with a finding that treated dogs were 100% protected for at least 12 months. The authors of the second article describe use of poly(D,L-lactic-co-glycolic)acid microparticles as a means of delivering ivermectin in dogs, and they found treated dogs free of heartworms 6 months after experimental inoculation with D immitis larvae.

These are just some of the highlights of the information to be found in the "Heartworm disease" collection. We hope you will find this resource useful as you address heartworm disease in your practice.