Heartworm disease is caused by a parasite called Dirofilaria immitis, which lives in the right side of the heart and the adjacent blood vessels. Its presence in these blood vessels causes cardiovascular weakness, compromised lung capacity, and eventual death in some patients. Heartworm disease occurs primarily in dogs but can occur in cats and other animals on rare occasion.
Heartworm is transmitted from dog to dog (and cat to cat) by mosquitoes. Over 70 species of mosquitoes have already been implicated. Transmission of the parasite occurs as follows: when a mosquito draws blood from a dog or cat infected with heartworm, it takes with it a number of small immature worms called microfilariae. Later, when the mosquito bites a new victim, the disease-causing larvae are injected, and that dog or cat becomes infected.
It takes about six and a half to seven months for the larvae to mature and start producing thousands of new microfilariae inside the circulatory system. The adult worms end up occupying the right chamber of the heart and the pulmonary arteries, while the microscopic microfilariae circulate throughout the bloodstream.
All these worms within the blood vessels produce an increased workload on the heart, along with restricted blood flow to the lungs, kidneys, and liver, eventually causing multiple organ failure if a progressive severe infestation is left untreated. At first, dogs may exhibit a chronic cough and reduced exercise tolerance, and this may or may not be followed by sudden collapse and death. Cats do not tend to show signs—rarely vomiting and perhaps sudden death can occur. They may show breathing problems rather than coughing. Cats tend to have only on e or a few heartworms and the lifecycle is incomplete in this species.
Once infected, one dog can easily become a "carrier" or reservoir of infection for an entire neighbourhood. By the time symptoms do occur, the disease is well advanced in dogs.
Prevention is preferred to treatment. While there are effective treatments available, most veterinarians prefer to promote prevention of heartworm disease. Oral and topical medications that are administered monthly, and have shown to be highly effective in preventing heartworm disease are available from your veterinarian.
In Canada, cats have a very low probability of getting heartworm so talk to your veterinarian. If cats become heartworm infested, they are not treated since treatment does not improve outcome. This is why if you plan to take your cat south into the lower states, make sure you talk to your vet before travel to prevent infection.
(Heartworm and Mosquitoes _Cat)
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