Recommendations for diagnosing, treating, and preventing heartworm disease - Veterinary Medicine
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Recommendations for diagnosing, treating, and preventing heartworm disease
With so many different preventives, treatment protocols, and tests, it is difficult to know what is most appropriate for your patients. Here's some help.


Ivermectin is adulticidal at prophylactic doses,26 and monthly administration of ivermectin reduces the adult heartworm burden compared with control dogs.27 The ability of ivermectin to kill young adult worms is called reachback. In one study, initiating milbemycin five and a half months after infecting dogs with 100 infective larvae and continuing milbemycin oxime for one year resulted in no reduction in the worm burden compared with controls (both groups of dogs were necropsied one year after starting therapy). However, initiating ivermectin five and a half months after infestation and continuing ivermectin for one year decreased the worm burden by 56% and decreased it by 35% when ivermectin was started six and a half months after infestation.28 This effect is called trickle kill.

Sixteen continuous months of ivermectin targeted against an adult infection kills 56% of adult heartworms.27 The remaining worms exhibit abnormal motility and appearance. Milbemycin oxime is far less effective against adult infections.27

The American Heartworm Society recommends that adult dogs be evaluated before you initiate prophylaxis and that they be tested annually if they are not receiving year-round preventives.29 Adulticide therapy is the treatment of choice for adult heartworm infestations. The risk for canine heartworm infestation is correlated to signalment, lifestyle, and geography. Prophylaxis is recommended for all dogs at risk of infestation, and dogs should be tested before prophylaxis.29 The trickle kill and reachback capabilities of ivermectin have led to questions regarding the need for annual testing; however, because client compliance is often imperfect, annual testing is recommended. (For comprehensive guidelines on managing canine heartworm infection, see


Although cats are naturally resistant to D. immitis, they are still susceptible to heartworm infestation. The prevalence of heartworm infestation in cats is about 5% to 20% of the rate found in dogs located in the same geographical region.30 Feline heartworm prevalence has been studied in shelter cats naturally exposed to heartworms.31-33 Shelter cats with natural exposure have been necropsied, and in Michigan, the necropsy prevalence is less than 2.5%. In northern Florida, the serologic prevalence is less than 5%. This low prevalence has led to a low index of suspicion and a subsequent low index of diagnosing feline heartworm disease. Feline heartworm disease continues to be a challenge to diagnose. Once cats are infected with L3 larvae, five to eight months will pass before the larvae molt into the L5 stage or young adult worms.34

Clinical signs and diagnostic tests

Clinical signs of feline heartworm disease vary. Dyspnea and coughing are the most common signs, followed by vomiting, collapse, syncope, and sudden death. Murmurs are seldom present unless the cat also has coincidental cardiomyopathy.35 Feline heartworm disease and feline asthma have similar clinical signs. Both conditions can lead to coughing and dyspnea, and both usually respond to corticosteroids, cage rest, and oxygen therapy. Differentiating between the two can be difficult and depends on how aggressively further diagnostic testing is pursued to reach a definitive diagnosis.

Cats usually display clinical signs of heartworm disease when the larvae are in the L4 or L5 stages, making the results of antigen and antibody serology difficult to interpret. To complicate matters even more, antigen and antibody test results can be positive for months after the heartworm infestation has been cleared by the host's natural immune response.36

Antigen testing depends on proteins shed by the parasite. Specifically, antigen tests detect antigen from the sexually mature female reproductive tracts.31 So antigen test results depend on the sex of the worms, the size of the worm burden, and the larval stage (sexually mature females). Male worms, sexually immature worms, or single worm burdens can lead to false negative antigen test results. Positive antigen test results indicate heartworm infestation, but negative test results do not rule out heartworm infestation.35 In a large population of naturally infected cats, antigen tests detected 79% to 86% of the heartworm infections and were highly specific (true negative results).31 So antigen tests are used for confirming infection in antibody-positive cats, and although they are less sensitive, they are more specific.35


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