Heartworm Disease in Cats

Most people are aware that heartworm disease is a common, potentially life-threatening disease in dogs that can be easily prevented with routine heartworm testing and heartworm prevention medication. If a dog should contract heartworm disease, there is an effective treatment available to kill the adult heartworms and eliminate the infection.

In contrast, most people are unaware of how heartworm disease relates to cats. Many people think that heartworm disease in cats is too uncommon to worry about or causes only mild problems. This is not true. Recent studies indicate that heartworm disease in cats is a common, potentially life-threatening disease, just as in dogs. It is also entirely preventable with monthly heartworm preventive administration. However unlike dogs, there is no reliable test or good treatment option for cats with adult heartworm infections.

So how does heartworm infection occur? Heartworms are spread to cats and dogs through mosquito bites. Wherever there are mosquitoes (pretty much everywhere), cats can potentially get heartworms. Just as in dogs, indoor cats are not safe. Mosquitoes are very good at getting indoors. 25% of heartworm positive cats are indoors only. If humans in the household do not have mosquito bites, this does not mean there are not mosquitoes biting the cats. Mosquitoes actually prefer cats to humans due to their higher body temperature. Even during the winter months, small numbers of mosquitoes are still active. They survive in warm microenvironments created in urban areas. For this reason, heartworm prevention should be given year-round.

First, the mosquito bites the cat and deposits heartworm larvae on the skin. These larvae then enter the skin through the mosquito bite and travel through the cat’s fat and soft tissues for 2 months (yuck!). The larvae eventually grow to juvenile worms that enter the blood stream and travel to the blood vessels in the lungs where they can grow up to 2 inches long. Cats are not the natural host for heartworms (dog have that dubious honor), so the cat’s immune system is triggered by the juvenile worms and destroys most of them. This causes massive inflammation within the lung tissue. Some juvenile worms may survive and grow to adult worms. Adult worms are not as common in cats as they are in dogs. In dogs, 75% of infective larvae will mature to adult worms. In cats, only 0-3% of infective larvae will mature to adults. Adult worms can live for up to 4 years in cats, causing inflammation and damage to blood vessels in the lungs all the while.

Disease processes associated with heartworms in cats can be divided into two categories: disease caused by adult worms and disease caused by destruction of juvenile worms (Heartworm Associated Respiratory Disease or H.A.R.D.).

Adult heartworm infection is less common in cats than dogs. Adult heartworms in cats usually cause relatively mild signs, such as coughing, lethargy, weight loss, and chronic vomiting. However, sudden death may rarely occur due to thrombosis. Some cats can have a severe respiratory crisis when an adult heartworm dies. In dogs with adult heartworm infections, there is a series of injections that can be used to kill the adult worms and treat the disease. However, this medication is toxic to cats. Currently, the only way to “cure” or reduce adult heartworm burdens in cats is through surgical extraction of the worms. This is a highly specialized procedure that is generally only available at referral centers. Overall, 10-20% of cats with adult heartworms die as a result of the infection.

Heartworm Associated Respiratory Disease, or H.A.R.D., is defined as lung lesions (inflammation and scarring) caused by the death of juvenile heartworms. When the cat’s immune system kills a juvenile worm, the resulting lung inflammation can last up to 8 months. Symptoms of H.A.R.D. include coughing and/or difficulty breathing. Signs may appear seasonally following months when mosquito populations are high. If many juvenile worms die at once, severe inflammation leading to acute respiratory crisis may occur. The lung damage caused by H.A.R.D. may not show up until later in life when chronic lung disease or COPD have developed. Cats affected by H.A.R.D. have similar symptoms and similar x-ray findings as cats with asthma or allergic bronchitis. Cats with H.A.R.D. are also usually negative on heartworm tests as adult heartworms are not present. Both H.A.R.D. and asthma respond to antiinflammtory steroids. Therefore, we often cannot distinguish cats with H.A.R.D. from cats with asthma. This raises the question: how many cats diagnosed with asthma and allergic bronchitis actually have respiratory disease caused by heartworms? Probably quite a few. For these cats, their respiratory disease could actually be prevented with heartworm preventive medication.

So how common is heartworm disease in cats? We used to think that the incidence of feline heartworm disease was 10% that of canine heartworm disease in a given geographic region. Recent studies are finding differently. Only 10% of heartworm infected cats have adult heartworms and will show up positive on a heartworm test. The remaining 90% of heartworm infected cats suffer from varying degrees of H.A.R.D. and will likely have a negative heartworm test. Therefore, the actual incidence of feline heartworm disease is estimated to be the same as dogs. Feline heartworm disease is at least 3 times more prevalent than kidney disease, diabetes, feline leukemia virus, and FIV infection.

Can we test for heartworm disease in cats like we do in dogs? Yes and no. Testing is not as straightforward in cats compared to dogs. Only 75% of cats with adult heartworms will be positive on an antigen test. This means that the antigen test will miss 1 out of 4 cats with adult worms. Cats with H.A.R.D. will be negative on an antigen test. Tests looking for heartworm antibodies will generally be negative about 4 months after infection. Overall, testing is a good idea because a positive result does indicate heartworm infection. Testing is especially recommended in cats with chronic vomiting, coughing, or gradual weight loss and otherwise normal lab work. A negative result does not mean the cat is truly heartworm negative. This also means that, unlike dogs, cats can start heartworm prevention without first having a negative heartworm test.

There is no good treatment for heartworm disease in cats. The injection used to kill adult worms in dogs is not recommended due to its toxicity to cats. Surgical extraction of adult worms is usually unavailable or cost prohibitive. Medications, like steroids, can be used to reduce lung inflammation in chronic disease or during acute respiratory crises but do not treat the heartworms directly.

So, what do we do for a potentially life-threatening disease that can be difficult to diagnose and does not have a good treatment? Prevention! It is estimated that less than 5% of cats are on heartworm prevention. This is dramatically lower than the number of protected dogs. Since the incidence of heartworm disease is the same in cats and dogs, shouldn’t cats have an equal level of protection? ALL cats should receive a dose of heartworm prevention every month all year long, starting at 6-8 weeks of age. Older cats do not require a heartworm test to start prevention. Remember, indoor cats need prevention too (25% of heartworm positive cats are indoor only). Only a few products are available to prevent heartworm disease in cats. Examples include Heartgard chewables and Revolution topical doses.

The take home points about heartworms in cats are:

  1. Heartworm disease affects cats and dogs equally
  2. Heartworms can cause chronic respiratory disease or acute life-threatening respiratory crisis in cats
  3. Heartworms are very difficult to diagnose in cats
  4. There is no good treatment for feline heartworms
  5. Indoor cats are still susceptible to heartworms
  6. Testing is not required before starting prevention in cats
  7. Monthly prevention is recommended for all cats starting at 6-8 months of age

For more information on heartworm disease in cats, visit the American Heartworm Society at https://www.heartwormsociety.org/pet-owner-resources/heartworm-basics

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