Heartworm Disease in Cats


Overview of Feline Heartworm Disease

Heartworm disease is a serious and fatal disease of the heart and lungs caused by a parasite, Dirofilaria immitis, that is transmitted by mosquitoes. Historically, heartworm disease in cats has been given less consideration than in dogs because the incidence is so much lower compared to dogs and the diagnosis is more difficult.

Below is an overview of Heartworm Disease in Cats followed by in-depth detailed information about the heartworm life cycle as well as diagnosis and treatment of this serious disease. 

Age is not a risk factor. Adult cats of any age can be affected, with cats as young as 1 and as old as 17 having been diagnosed. Indoor and outdoor cats can get infected, although outdoor cats have a higher prevalence. However, up to 33 percent of reported cases are in cats who are described by their owners as “strictly indoors.” Males are a bit more likely than females to be affected.

Transmission occurs when a mosquito bites an infected dog or cat and ingests heartworm larvae (baby heartworms), which live in the bloodstream. When the insect bites another dog or cat, some of the larvae are injected under the skin. The larvae grow for 3 to 4 months and eventually make their way into the heart where they develop into adults, and the process is ready to repeat itself.

What to Watch For

Symptoms don’t usually occur until damage has already occurred to the heart. Cats can have a wide range of symptoms, with some cats being completely asymptomatic (no symptoms at all). Others may show vague, generalized clinical symptoms. A small percentage, however, may show severe life threatening symptoms. Symptoms usually occur because of heart failure. These include:

  • Difficulty breathing
  • Rapid breathing rate
  • Coughing
  • Hemoptysis (coughing up blood)
  • Vomiting
  • Fainting
  • Lethargy
  • Anorexia
  • Weight loss
  • Decreased ability to exercise
  • Neurologic signs such as blindness, seizures, circling, incoordination
  • Diagnosis of Heartworm Disease in Cats

    Making the diagnosis of heartworm disease in the cat is tricky. There are several tests that your veterinarian might want to run, such as:

  • Complete blood count and chemistry panel
  • Chest X-rays (radiographs)
  • Serum heartworm antibody test
  • Serum heartworm antigen test
  • Microfilaria test (looking for larva in the blood)
  • Echocardiogram
  • Angiography
  • Transtracheal wash
  • Treatment of Heartworm Disease in Cats

    Treatment of heartworm disease in cats is complicated. There are two options, basically:

  • Adulticide treatment (killing the worms in the heart and lungs)
  • Conservative treatment (i.e., intermittent corticosteroid therapy).
  • Home Care and Prevention

    There is no specific home care other than to administer medications as prescribed.

    Milbemycin (Interceptor for Cats®),Ivermectin (Heartgard for Cats®)or Selamectin (Revolution® for Cats) are the most common drugs used to prevent heartworm disease in cats.

    In-depth Information on Heartworm Disease in Cats

    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.


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