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Heartworm in cats is caused by infestation of the organism Dirofilaria immitis, a parasitic nematode (roundworm) commonly referred to as the heartworm. The severity of this disease is directly dependent upon the number of worms present in the body, the duration of the infestation, and the response of the host (the infested cat is the host).
The prevalence rate of heartworm disease in unprotected cats that have not received the proper preventative medication, or prophylaxis, is significantly lower than that of unprotected dogs -- approximately one-tenth the rate of dogs. Additionally, most cats have only a few heartworms present, and the worms infecting cats are physically smaller and have a shorter lifespan than those infecting dogs. Outdoor cats are at increased risk, and are twice as likely to contract heartworm disease as indoor cats.
Symptoms and Types
Signs of heartworm infestation in cats include coughing, labored raspy breathing (known as dyspnea), and vomiting. Respiratory problems and vomiting are the predominant symptoms in cases of chronic infestation. A physical examination may also reveal a heart murmur or otherwise irregular heart rhythm.
Heartworms are spread through mosquito bites carrying infective heartworm larvae. Because the lifecycle of heartworms affecting cats is shorter than the cycle of those infecting dogs, a look at the (more prevalent) infestation process in dogs is useful. In dogs, the larvae migrate from the bite wound through the body until they reach the heart and blood vessels of the lungs. Here, the larvae mature and reproduce, releasing immature heartworms, known as microfilaria into the animal’s blood. It is important to note that the presence of microfilaria in the blood is in fact uncommon in cats, and has been seen in less than 20 percent of infected cats.
It is also important to note that because heartworms in cats have a much shorter lifespan than those that infest dogs, a spontaneous cure without treatment is more likely to occur.
There are no specific tests that are able to diagnose heartworm disease in cats. A variety of tests that may be done to aid diagnosis include a urine analysis, heartworm antigen and antibody tests, x-rays which may reveal the enlargement of certain veins or arteries associated with heartworm disease, and an electrocardiograph (ECG), which may allow for identification of worms in the heart or pulmonaryartery. An ECG can also exclude or confirm other heart diseases that may exhibit similar symptoms.