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Feline Heartworm Disease

• Fact – 1 adult heartworm can result in the death of a cat.

• Fact – both indoor and outdoor cats are at risk. • Fact – clinical signs begin 3 months following an infection that is vectored by a mosquito.

• Fact - 28 percent of feline heartworm disease occurs without the presentation of symptoms.

• Fact – only 5% of cats are protected with heartworm preventative.

• Fact – there are 4 different products available for prevention of feline heartworm disease.

Surprisingly, cats are susceptible to heartworm disease contrary to common belief. Infection occurs across the continental United States and is highest in the southeastern US. Urban areas create “heat islands” that favor the development of mosquito populations and also extend the risk period in temperate climates such as ours. Mosquitoes are indiscriminant in terms of blood feeding and transmission. Also unexpected is that indoor cats can be infected as well; approximately one of every 4 infections occurs in an indoor only cat. A cat is an acceptable host animal for heartworm infection; however, the disease is different in cats in comparison to dogs. We now understand that equal numbers of cats are infected when both dog and cat infection rates are studied. Cats are able to mount an immune system response that limits both the numbers and longevity of adult heart worms in the feline body. The location of infection in the cat’s body also differs from dogs in that in a feline, heartworms will commonly localize in the pulmonary arteries (the large capacity blood vessels that lead away from the right side of the heart and enter the lung for aeration of blood). In the dog, infection with worms can occur on a much larger scale and the worms typically localize in the right ventricle or pumping chamber of the heart. Additionally, cats typically show clinical signs during two distinct times periods – as the immature worms are arriving in the blood vessels, causing an intense inflammatory reaction in the pulmonary arteries, and then again as the same worms are dying, which sets up (again) an intense inflammatory reaction in the lung tissues. This respiratory problem is frequently misdiagnosed as an asthmatic condition. The immune system of the cat is effective in responding to immature heartworm disease such that most heartworms die before they reach maturity. [In contrast, in the dog the worms live in the right side of the heart and clinical signs begin 2 to 4 years after infection. Clinical signs are related to dysfunction of the right side of the heart and include lethargy, exercise intolerance, fatigability, shortness of breath, and potentially collapse.] Felines show a more profound inflammatory reaction to the presence of the worm in comparison to dogs. In contrast, a dog shows signs related to right heart dysfunction; the cat never has the opportunity to show evidence of right sided heart embarrassment because the cat’s immune system halts worm development. Heartworms in general cause both dogs and cats to develop a problem analogous to a human disease – arteriosclerosis. The presence of the worm and the metabolic byproducts that the worm releases into the circulation induces the inside layer of the blood vessel to proliferate and form finger like projection into the lumen of the blood vessel. This change increases the work burden that is placed on the right side of the heart. This response is less notable in the cat but nonetheless occurs. To illustrate the susceptibility of dogs versus cats, studies have been done in which heartworm naive animals (animals that had never been exposed to heartworms) were selected for study. In dogs where 100 larval stages were injected into a group of dogs, 60 or more adult worms developed in 100 percent of the dogs studied. In a study group of cats injected with 100 larval stage worms, only 3 to 10 worms developed in 75 percent of the cats that were studied. This highlights the immune system response to immature worms that prevents large numbers of adult worms that could live in a cat’s lung blood vessels. That only 1 adult worm could cause profound disease is related to the relative size of the worm in comparison to a cat’s smaller body size and circulatory capacity. The mortality rate (of juvenile heartworms) is relatively high owing to the intensity of the felid’s immune response. Tragically, there is no medication available to treat the adult worm phase of this disease. The medication that is used to treat adult heartworms in dogs (Immiticide) is toxic and fatal when given to a cat. An established heartworm infection in a cat can only be managed by palliation of signs. Heartworm preventatives thus take on added significance as part of a dual effort to prevent heartworm disease as well as provide intestinal worm control. {Recall that this also helps prevent human infections with the common round-, hook-, and tapeworms that cats and dogs will transmit.] You may want to view the following links for additional information: https://www.heartwormsociety.org/pet-owner-resources/heartworm-basics and https://www.facebook.com/heartwormsociety Authored by Robert Z Berry, DVM Source material from the American Heartworm Society