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Heartworm disease is caused by a roundworm called Dirofilaria immitis. Heartworms are most commonly found in dogs, although they can infect a wide variety of mammals, including cats. In dogs, adult heartworms live in the heart and major arteries of the lungs where they interfere with the function of the heart and lungs.
Cats that remain indoors are at lower risk of becoming infected than are cats that go outdoors because of the reduced exposure to mosquitoes. Despite this, heartworm infections have been reported in strictly indoor cats; these infections are caused by infected mosquitoes that entered the house.
Heartworms can only be transmitted from one animal to another by mosquitoes. Adult worms living in the heart and arteries of the lungs produce microfilariae (small immature heartworms) which are found circulating in the blood of the infected animal, usually a dog.
If a mosquito feeds on an infected dog with microfilariae in the blood, the mosquito will ingest some of these immature heartworms along with the blood meal. Inside the mosquito, the immature heartworms develop to a stage called the infective larval stage. When the mosquito feeds on another dog or a cat, some of these infective larvae will escape from the mosquito during the blood meal. The larvae pass through the animal's skin through the bite wound left by the mosquito. Once the infective larvae have entered an animal, they will begin migrating through the tissues. They eventually make their way to the heart and lungs where they will mature and begin producing microfilariae.
In cats, heartworms live for 1 to 5 years and it is uncommon for cats to have more than 2 or 3 adult heartworms. Small numbers of heartworms, however, may cause serious disease in cats.
Clinical signs of heartworm disease in cats vary considerably. Some cats do not show any significant clinical signs and may appear normal. Other cats develop chronic (long-lasting) disease. Vomiting or respiratory signs (coughing and difficult breathing) are commonly seen in chronic cases of heartworm disease in cats. Vomiting tends to be sporadic and may or may not be related to eating. Coughing may be intermittent or occur in severe, sudden attacks that may take place days apart. Cats may have severe, acute disease with signs of respiratory collapse and, in some cases, sudden death. In acute cases, death may be so rapid that there is insufficient time to make a diagnosis or offer treatment. Cats that die from heartworm disease can appear clinically normal 1 hour before death.
Many other diseases can cause similar clinical signs so it is almost impossible to diagnose feline heartworm disease based on clinical signs alone.
Testing for heartworms in cats can be very difficult due to the small number of worms usually present. The types of tests required to find out if heartworms are present include antigen and antibody testing with a blood sample, x-rays, ultrasound, and angiograms.
Unfortunately, treatment to remove heartworms from infected cats can be difficult and hazardous to your cat's health. Various medications can be used to help minimize the clinical signs (coughing, vomiting, difficulty in breathing) your cat may be exhibiting. Surgical removal of heartworms can sometimes be an option.
Preventives are available for feline heartworm infection, but they will not eliminate an existing infection. Preventives can be used on cats that test positive, to prevent further infections. Currently there are oral and topical methods of heartworm prevention.
Heartworms can infect cats, even if they live exclusively indoors. The syndrome consists of vague clinical signs and can include unexpected death. Treatment is difficult and often dangerous, but prevention is simple. Please ask us about Heartworm Prevention in cats.
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