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Heartworm Disease in Cats

Heartworm Disease is caused by a roundworm (nematode) parasite called Dirofilaria immitus, which is carried by mosquitoes. It is more common in warm, humid areas but can be seen anywhere, especially as animals are often moved around geographically.

Life Cycle

Animals cannot directly infect each other with heartworm (A mosquito is the go-between).


The life cycle is as follows:

  1. Female worms in an infected animal lay eggs which hatch into early larvae (microfilaria) in the bloodstream.
  2. A mosquito bites the animal and drinks the microfilaria with the blood.
  3. The microfilaria become mature larvae within the mosquito.
  4. When the mosquito bites the next animal, these larvae are injected into the body. The larvae mature further within the bloodstream, and as adults they settle into the heart and the pulmonary vessels (the arteries and veins that carry blood between the heart and lungs).
  5. The adult worms can then mate and lay eggs, continuing the cycle.

The entire process from an uninfected cat being bitten by an infected mosquito to being able to infect another mosquito with microfilaria takes about 7 to 9 months.

Clinical Signs

Cats have a lower risk of being infected than dogs, and if they are infected, tend to have only 1 to 3 worms (dogs can be infected with 1 to 250 worms). Regardless, they seem to develop severe illness even with the low number of worms. The symptoms include wheezing, vomiting, gagging, lethargy, and weight loss, and can be confused with other conditions such as asthma.


There are two main ways to diagnose heartworm infection.

One method is to look for actual microfilaria in bloodstream. However, if a cat is only infected with male worms, immature worms, or a single unfertilized female, there won't be any larvae.

The more common method is to look for a heartworm substance called an antigen in the bloodstream. This is not dependent on reproductive capabilities of the worms, but does require them to be mature, which can take an average of 7 months. This blood test can be sent to a laboratory or is often done as a quick (five minute) in-house test at the veterinary clinic.

Imaging studies, such as X-rays or echocardiography, can also find evidence of heartworm infection, but these are not routine screening tests.


Treatment options in cats are limited. While injections to kill mature heartworms are usually given to dogs, they cannot be given to cats. At this point, the only options are supportive care to minimize clinical signs and a heartworm preventative to clear any larvae in the bloodstream and prevent re-infection.


There are a number of preventatives available. These include daily or monthly tablets and monthly spot-on treatments. These preventatives are often part of a multipurpose product that also contains other dewormers, flea controls, and/or mosquito repellants, and your veterinarian can help you pick the most appropriate one for your cat. The goal of the preventatives is to stop any larvae in the bloodstream from maturing into adult worms. Cats do not need to be tested prior to starting preventatives but you should consult with your veterinarian before starting a heartworm preventative program.

Further Reading

For more information, check out The American Heartworm Society.

Note: There is no species-specificity of heartworm. In fact, it is most likely that a mosquito biting an infected dog is the source of transmission into cats, because there are so many fewer infected cats, and because cats often have a sterile infection, which means the worms infecting the cat don't produce the eggs that could be transmitted via a mosquito to another animal.

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