It’s summer, and it’s hot. Bugs are in their high season. We see lots of fleas and intestinal worms that bother our cats this time of year, but what about another parasite called heartworm? Is this something cat owners should be concerned about in Orange County?

The official name of the heartworm is Dirofilaria immitis, and it is found throughout the world. Infection with heartworms occurs when a cat or dog is bitten by a mosquito carrying heartworm larvae (microfilaria). The larvae are injected into the pet’s bloodstream, where they mature and cause illness. Humans are not susceptible to heartworms. Outdoor cats with greater exposure to mosquitoes are at a higher risk than indoor cats, but both types of cats can be infected.

Cats are naturally more resistant to heartworms than dogs, and if they do become infected, cats typically develop one or two adult worms about three or four months after they have been injected by a mosquito. The adult heartworms generally live in the pulmonary arteries and can live two or three years.

Unfortunately, there are no unique signs that indicate heartworm infection in a cat. Many cats tolerate heartworms with vague signs of illness. Coughing and vomiting are common signs of infection . Most infected cats exhibit symptoms consistent with asthma or allergic bronchitis.

To complicate diagnosis further, testing cats for heartworms isn’t easy. Infected dogs commonly have microfilaria present in their blood, and this simplifies testing for them. In cats, a diagnosis involves chest X-rays, blood testing and, in some cases, echocardiography. The blood testing can be very confusing and uses antibody and antigen tests. Positive antibody tests confirm the presence of heartworm larvae. Positive antigen tests confirm the presence of mature heartworms, but rarely detect immature or male-only worm infections.

Treatment for feline heartworms focuses on decreasing swelling in the airway with steroids and treating severely ill animals for shock. It is not easy to kill adult heartworms in a cat using medication, so it is considered a last-resort treatment. In some cases, the worms can be surgically removed.

All of this sounds pretty terrible, but for- tunately for those of us living in Orange County, the incidence of heartworm infection is very low in dogs and cats. This is because we don’t have a huge mosquito population. Nor is the pool of potential heartworm carriers significant. In 27 years of veterinary practice in Orange County, I have never seen a case of heartworms in any native cat.

Could I have missed a diagnosis? Of course that is possible, but I don’t think it is likely. Nationally, it is thought that for every 20 infected dogs, there is one feline infection. One of the large veterinary laboratories in the country, Idexx, reports that since 2007, 82 positive cases of canine heartworms have been identified in Orange County. If I extrapolate this number, which would be an estimate only, it would mean about 12 canine cases a year, and the risk of a cat being infected would be less than one a year. There are probably one or two cases of feline heartworms in our county each year, but the data do not indicate whether any of those involved animals that lived elsewhere prior to being diagnosed in Orange County.

Out of curiosity, I looked at some data reporting feline heartworm cases in Florida, where there are lots of mosquitoes. Since 2000, there have been 548 feline heartworm cases in that state.

I recommend heartworm-prevention medications when owners tell me that they are taking their cat to a part of the country where mosquitoes and canine heartworms are more prevalent. A few of the routinely used veterinary spot-on products that kill fleas also protect against heartworms, so this makes it easy.

I do not routinely test cats for heartworms, but I would if a patient had signs that could be consistent with the disease and I knew there was a risk of mosquito exposure.

At this time in Orange County, the risk of feline infection with heartworms is very low. I will continue to monitor reports and the literature on this disease and let you know if there are any changes.

Written by Dr. Wexler-Mitchell of The Cat Care Clinic in Orange, CA
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