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Heartworm in Cats
By Amy Hanson, DVM
September 19, 2022
When most people hear “heartworm disease” they think of a disease that affects dogs. Heartworm disease is a serious disease in dogs, but it is also a disease that can negatively affect cats. While cat heartworm disease is more common in outdoor cats, there have been cases of feline heartworm in indoor cats as well. One study showed 25-30% of heartworm infected cats were described by their owners as being indoor-only.
Cat Heartworm Symptoms
The disease process is vastly different between dogs and cats. Heartworm disease in dogs causes vascular disease leading to heart failure. Heartworm disease in cats impacts the lungs. Feline heartworm symptoms cause asthma-like symptoms such as the cat heartworm cough, wheezing, vomiting, and even sudden death.
Feline heartworm associated respiratory disease (HARD) has very similar clinical signs to asthma and cannot be easily distinguished on chest X-rays. Antigen testing is used in dogs, but the antigen tested for is found in the adult female heartworm, so it is unlikely that a cat would have an adult female heartworm to generate a positive antigen test. Likewise, microfilaria are killed quickly by the immune system, so would rarely be seen in a microfilaria test. An antibody test, sometimes in conjunction with an antigen test, is the best test to perform on a cat that is exhibiting clinical signs and is suspected of having heartworm disease. This method of heartworm test for cats in conjunction with chest X-rays can help distinguish between heartworm disease and asthma.
What Happens if a Cat Gets Heartworms?
The reason that heartworm in cats affects the lungs is due to the cat’s immune response to being infected by juvenile heartworms, called microfilaria, and adult heartworms. When a cat is bitten by a mosquito infected with microfilaria, those juvenile worms enter the bloodstream and begin searching for protein markers to lead them to their desired home, the pulmonary artery. Since cats are not a normal host of heartworms, the microfilaria cannot follow the protein markers as usual and can end up in any tissue in the cat’s body. Since the microfilaria are foreign to the cat, the cat’s immune system mounts an aggressive attack to kill the foreign invader. Most of the time, the immune system will kill microfilaria before they reach the stage where they migrate into the pulmonary artery. This aggressive immune response causes inflammation in the various tissues of the cat’s body.
Sometimes, however, an almost mature heartworm, or an adult, will reach the pulmonary arteries, where there is an even more aggressive immune system response that results in massive inflammation as the body attempts to break them into fragments and remove them. This inflammation causes an anaphylactic-type reaction which results in damage to the lungs. The damage manifests as coughing, wheezing, vomiting, and increased respiratory effort, and can even lead to sudden death in cats. A cat infected with even one adult heartworm can have lasting lung damage due to the inflammation associated with the immune response and can even affect other organs in the body.
Can Heartworm Be Passed from Cat to Cat?
Heartworm disease is not contagious between cats, dogs or humans. However, other cats in the house, dogs and even humans can be infected by the same mosquito carrying heartworms. Most often we see mosquitos biting a heartworm infected dog, taking a blood meal, then biting a cat and transmitting the heartworms to the cat. That’s why cats living in canine heartworm-prevalent areas should be on prevention.
Heartworm Treatment for Cats
Heartworm disease in cats can be treated with steroids to reduce the inflammation in the lungs. The treatment used for dogs is not recommended for cats due to the extreme danger of clots developing. There is also a bacterium that is synergistic with the heartworm, called Wolbachia, that increases inflammation. Treatment with doxycycline will kill the Wolbachia and help to decrease inflammation in the lungs.
Heartworm Prevention for Cats
Just like in dogs, heartworm disease can be prevented. Products such as Heartgard for cats, Revolution for cats, Revolution Plus for cats, and Advantage Multi for cats are some of the best heartworm prevention for cats and can be used monthly to protect cats from heartworms, as well as other parasites. Even indoor cats should be treated as mosquitos can enter houses very easily.
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Written by: Amy Hanson, DVM
Contributing Veterinarian at Revival Animal Health
Dr. Amy Hanson is an associate veterinarian at the Cat Clinic of Lawrence in Lawrence, Kansas. She is a 2010 graduate of Kansas State University College of Veterinary Medicine. Her special interests include felines, acupuncture and dentistry. Her hobbies include showing cats and she is a judge for the American Cat Fanciers Association (ACFA).