Heartworm Disease in Cats

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In the past, heartworm disease has been considered a medical curiosity rather than the distinct, clinical disease that it is. Cats with heartworm infection can be found in all areas where cases of canine heartworm disease occur. The true incidence of the disease is unknown and is probably underestimated, because diagnosis is often difficult, and because some cats either spontaneously eliminate the parasite and never get diagnosed. Other cats may die acutely from the disease, again with no diagnosis ever being established.

Although there are some similarities, heartworm disease in cats is a very different disease than in dogs. The clinical importance of heartworm infection in cats has to do with the fact that even light infections are capable of producing severe disease with potentially life threatening consequences.

Cats of any age and any breed are susceptible.

Since the disease is spread by the bite of a mosquito, outdoor cats are more susceptible, although cat owners should be aware that up to 33 percent of reported cases are in cats described by their owners as being "strictly indoors." Mosquitoes can and do go indoors. Male cats tend to roam more than females and have a higher exposure; however, the incidence in males is not all that much higher than in females.

Cats are susceptible to heartworm disease, but they do not make good hosts for the worms, as do dogs. Dogs are considered "definitive hosts" because once inside the body the entire life cycle of the heartworm can be completed. Cats, however, are considered "resistant hosts." If you were to inoculate a dog with 100 infective heartworm larvae, 70 of them would survive and grow up to become adult worms. If you give 100 infective heartworm larvae to a cat, only 10 would develop into adult heartworms. Of cats and dogs that are infected naturally, a cat usually has between one and nine worms in their heart and lungs; dogs often have 20 or more. In dogs, adult heartworms live for approximately five to seven years, but in cats, they only live two years. In dogs, you don't usually see severe clinical signs unless you have at least 10 worms in the heart and lungs. In cats, it only takes one or two worms to cause severe disease, and possibly death.

The life cycle of the heartworm in the cat is as follows: a mosquito picks up microfilariae (baby heartworms) from an infected dog or, less likely, a cat.

The microfilaria is actually a larva, at larval stage L1. Inside the mosquito's body, the L1 babies molt into the next stage, L2, and again, to L3. The L3 larva is the infective stage, and at this point the mosquito bites an uninfected cat, and injects the L3 larvae into the cat's skin. The L3 soon molts into an L4 larva. Sometimes the L4 larva migrate to an unusual location and cause unusual clinical signs related to the organ system. Most of the L4s molt into L5s, which are juvenile worms. L5s find their way to the pulmonary arteries in the lungs, where they cause all sorts of problems for the cat.

Approximately eight months after infection, male and female heartworms mate and produce their own baby heartworms (microfilariae). Again, these microfilariae are L1 larvae. They do not develop into adult heartworms on their own. They have to be picked up by a mosquito, undergo development into L2 and L3 stages in the mosquito and then be injected into another cat where they continue the same process.
One or two diseases have similar clinical signs as heartworm disease. The most common are asthma and lung parasites.