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Heartworm in Cats

Heartworm Disease in Cats

Heartworm disease is caused by Dirofilaria immitis, a parasitic worm that gains entry into cats via mosquito bites. There is widespread distribution of this parasite in Canada at the lower latitudes, and your local veterinary team can advise you regarding whether your region is affected. Heartworm disease in cats is very different from the disease in dogs, so if you have both dogs and cats, it is important to have your clinic give you recommendations relevant to each species.

Parasite Lifecycle

The disease is so-called “vector-borne” since a mosquito bite is essential to transmit the disease. The only time of year a new infection can occur is when mosquitoes are out. The mosquito season varies according to geographic region, but in the province of Ontario for example, it can be from about April to November. Note that early thaws and late Indian Summers can really affect the season. Any region where dogs can get heartworm, cats can too!

The mosquito vector becomes a carrier of the microscopic immature form of the parasite when it bites an infected animal (could be a dog or cat). This mosquito, when it finds the next victim, will inject a small amount of saliva into the bite wound, and this small volume of spit carries the parasite into the tiny wound. From here, they migrate into the body via the bloodstream, and set up shop in the heart preferentially. Once they are inside, they begin to mature and very soon, mature reproducing worms are in place. When only a small number of immature larvae are injected, it is possible that the reproductive cycle cannot go forward (when just a few male worms injected for example). This is called a sterile infestation because two sexes are not in the host. A big difference between dogs and cats is that one or two worms in a cat can be life-threatening, whereas in dogs, this level of worm burden will likely not cause such severe symptoms.

Once the worms mature, they release microfilariae (next generation babies) into the bloodstream, and they mature and begin their own life cycle. The worms do not just live in the heart, but often enter the lungs; filariae can actually enter many tissues. In cats, most infestations are classed as low worm burden; meaning that very few adults are found in the system.

How Does Heartworm Affect Cats?

The most common sign is a pulmonary (lung) condition that looks a lot like asthma, and/or chronic vomiting. Asthma can produce symptoms of difficult or fast breathing, coughing, reduced activity, open mouth breathing, and off-color gums (cyanosis, or bluish tinge). Another common scenario is that the infected cat shows no signs at all (termed “subclinical”). Note that many subclinical cases may go on to develop chronic asthma-like symptoms, vomiting, or can experience sudden death.
The third presentation we see is a sudden death. The cat will have been harbouring the parasite, but has been able to compensate for the inflammation in the lungs, so signs do not become obvious until late-stage disease. Another major health problem that your veterinarian will need to rule out is feline heart disease; this can produce many similar signs.

What Preventive Measures can be Taken?

Keeping a cat indoors goes a long way towards protecting the cat from the bites of mosquitoes. There have been cases of heartworm disease in indoor cats, so sometimes it seems, they (the skitters) sneak in through holes in screens, or when our doors are opened in the warm weather. Other times, the owners may take their cat out on the porch for some sunshine, and this provides the “skitter” a bite opportunity.

Is there a Test for it? A Treatment or Cure?

Tests are available that can help to identify the parasite infestation. Though we do not know the prevalence (occurrence) of heartworm in cats in Canada accurately, we do know that it is more common as you get to warmer climates, such as in the Southern USA. We thought we knew that for every ten dogs with heartworm disease, approximately one cat would have heartworm disease, but we have since learned that this ratio varies drastically between geographic areas. Because cats often have many fewer worms inside than dogs, we have come to prefer antibody tests in this species. If they have a positive on the heartworm antibody test, we know that they most likely have an infection in progress (2-3 months at least). The antigen test is more likely to have false-negatives, though false negatives can occur with the antibody test. If an antigen test is positive though (which is rare), it is a trusted result. X-rays are of limited use in cats for definitive diagnosis since the infestations are usually low burden ones, but ultrasound of the heart and lungs has a much greater diagnostic capacity. Ultrasonographers can often actually see the worms in the right ventricle chamber of the heart, or in the pulmonary artery. Note that X-rays are still very useful to assess the degree of inflammation in the lung fields, but they just do not tell us much more unless we get lucky and catch a worm in the radiograph view.

Interesting though that since the parasite much prefers dog hosts to cat hosts, not all infections in cats will progress to disease. In cats, as a result, some of them will never develop symptoms. In areas with year-round or long mosquito seasons in the USA, some clinics there are keeping cats on preventive therapy constantly. Preventive therapy may or may not be recommended, based on the incidence of cases in your area. As a very interesting note aside, humans can get heartworm disease, though these cases are VERY rare, and usually, the worm just produces a nodule in the human lung that mimics lung cancer.

A cat that presents with a respiratory distress situation can be treated in-hospital with antiinflammatories, oxygen, bronchodilators, and many can recover with this level of care. Definitive treatment to clear the parasite is almost never done in cats. Unlike dogs, the reaction to the treatment produces a fatality rate that equals the fatality rate if left untreated. This means that the cat does not benefit from current protocols. This does not mean a better regimen will not come along, but for now, treatment focuses on supporting the cat with anti-leukotrienes, antiinflammatories and perhaps bronchodilators since these cats will often experience low-level asthma-like symptoms until they can clear the parasite on their own, or the parasite ends its lifespan inside naturally.

Reprinted with permission from

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