Yes, Cats Really Do Get Heartworm. The Story of Fergus

Until I met Fergus, I must admit that despite the fact that I knew feline heartworm existed I had little first -hand experience with the disease. Fergus presented one spring morning on the emergency service. The owners had woken to find him recumbent, now he was inches from dying. The staff and doctors that day jumped into action and over the next 24 hours he gradually stabilized. The owners reported that they had adopted him a few months ago from a rescue organization. He had been tested for the feline viral diseases and vaccinated. He was neutered and just beginning to adjust to his new home. Fergus was not allowed outdoors, outdoor cats are more likely than indoor cats to meet with various inexplicable catastrophic events, and until that morning he appeared to be doing very well.

Whenever a cat is presented in such critical condition, with multiple organ systems affected, it is common for us to check their viral status along with serum chemistries and a CBC. Feline viral diseases can be stealthy, but devastating, and cats that test negative 4 weeks earlier can be positive the next month. We tested Fergus’s blood for FELV(Feline Leukemia) and FIV(Feline Immunodeficiency Virus) and also for heartworm antigen. He tested positive for one, heartworm disease.

Feline Heartworm Disease is diagnosed less often in cats since the parasite does not survive as successfully in the cat as it does in the dog. Cats are also less likely to have circulating microfilaria which means that they are less likely to spread the disease. At this time we are unsure why this is the case. Feline heartworm disease can also be challenging to diagnose and can present in many different ways. Because the parasite does not thrive in the cat as it does in the dog, some cats may remain unaffected by a heartworm infection and never demonstrate any clinical signs.
However, when a cat does become ill, the infection can be devastating, even fatal, affecting their heart, lungs, liver and kidneys. Some cats may be mildly affected, demonstrating respiratory signs. Any cat that has evidence of lung disease (like Feline Asthma), coughing or other respiratory signs, should be tested for heartworm. Of course, heart murmurs and cardiac dysfunction, and liver failure can also occur as a results of the adult worms damaging the heart and occluding vessels. Also, gastrointestinal upsets and vomiting can be signs associated with the disease.

Unfortunately for Fergus , he basically had every conceivable clinical sign. He had vomiting, diarrhea, kidney dysfunction, anemia and eventually a heart murmur and heart failure along with an allergic pneumonitis (an inflammation of the lungs), all developing within 24 hours time. His infection had been present for months, but now he was experiencing the devastating consequences of a successful infection. It took Fergus almost a week of intensive care to recover. At one point some of the adult worms moved (we were actually able to see and document this on cardiac ultrasound, an echocardiogram) and lodged across his tricuspid valve, a heart valve between the atrium and ventricle on the right side of the heart. This led to cardiac dysfunction and failure. At that point things looked grim, Fergus was placed on cardiac medications and just when we thought the battle was lost the worm moved (again we documented this on ultrasound). Fergus began feeling better and eventually returned home.

Feline heartworm and its associated illnesses have not been a commonly diagnosed condition in veterinary medicine, so there is sparse data regarding survival rates and successful treatment plans. Published reports at this time usually include as few as 5 to 50 patients from which to draw conclusions. Small study groups may lead to poorly representative information regarding a disease. One study did find that cats like Fergus, if they survive the first 24 hours of their illness, have approximately a 60% chance of surviving that crisis and returning home. A study looking at 50 cats with heartworm disease, the median survival times as 4 years post diagnosis.

Over the next year Fergus will need to be closely monitored to continue to manage his disease as best as we possibly can. Unlike dogs, there is no treatment that has proven successful in eradicating an adult heartworm infection in the cat. Heartworm infected canine patients can be treated with medications that kill the adult worms. In most cases the medication, although strong, can be well tolerated. In the cat there is a high mortality associated with adulticide therapy and there appears to be little benefit from being treated. Surgery can be utilized to remove the adult worms, but again the risks and the mortality (death) rate is very high vs little apparent benefit. When cats have heartworm infections we have to carefully treat the consequences of the infection and place them on a monthly heartworm preventative. The adult heartworms will not live as long in the cats as they do in dogs and will eventually die off.
Feline heartworm preventative is available in pills, chewable tablets and topical solutions for your cat. Particularly if you cat goes outside, contact your veterinarian to discuss what product would be best suited for your cat.

Read more about feline and canine heartworm disease at www.heartwormsociety.org