Cats are infected with heartworms in the same fashion as dogs. The bite of an infected mosquito spreads the parasite and can be responsible for infecting your cat.
Heartworms themselves are parasitic worms that live in the arteries of the lungs and in the heart of an infected animal. When a mosquito feeds on a heartworm-infected animal, the mosquito can become infected with microfilaria, a larval form of the heartworm that is found in the bloodstream of infected animals. Once introduced, the microfilaria continue to mature within the mosquito until they reach an infective stage. At that point, if the mosquito feeds on another susceptible animal, the microfilaria can be passed to that animal. The microfilaria will then finish their maturation cycle and grow into adult heartworms inside of that animal.
Cats are somewhat more resistant to heartworm infection than dogs but, according to the American Heartworm Society, 61-90% of cats exposed to infected larva will become infected (as opposed to 100% of dogs.) Outdoor cats are most likely to be infected but indoor cats are also at risk. Mosquitoes often find their way inside our homes where they can then threaten our indoor pets.
There are similarities between heartworm infection in cats and dogs as well as differences. Cats tend to be infected with fewer adult worms than dogs. However, this lower number does not necessarily translate to less severe disease. Cats also are less likely to have microfilaria circulating in their bloodstream than dogs, making cats less likely to be able to spread the infection to mosquitoes when bitten. Untreated dogs are commonly microfilaremic, having microfilaria in their bloodstream, and can often spread the disease via mosquitoes.
Perhaps the most important difference between heartworm-infected dogs and cats is the fact that canine heartworm disease affects both the heart and the lungs. In cats, the lungs typically suffer the most damage.
Symptoms of heartworm disease in cats are often non-specific and may include intermittent vomiting, depression, lack of appetite, weight loss, coughing, difficulty breathing, panting, and abnormal breathing patterns such rapid breathing or open mouth breathing. Heartworm disease in cats is often mistaken for feline asthma as symptoms are remarkably similar. Other symptoms associated with heartworm disease in cats are collapse, syncope (fainting episodes), convulsions, blindness, and sudden death.
Diagnosis of heartworm infection in dogs is usually relatively straight forward. Blood testing for heartworm antigens, the standard test used for detecting heartworm infection, is reasonably accurate in dogs. However, in cats, diagnosis of heartworm infection and heartworm disease is much more complicated. Heartworm disease in cats can mimic many other diseases and testing is not always terribly reliable with most feline tests having serious limitations.
Treatment of heartworm disease in cats is problematic as well. There are no medications that are safe and/or effective in curing heartworm disease in cats. In most cases, treatment is symptomatic. In severe cases, surgical removal of adult heartworms can be attempted but this type of procedure is obviously risky.
Fortunately, heartworm infection is preventable for both dogs and cats. Heartworm preventive medications for cats include monthly oral medications as well as monthly topical preparations. Your veterinarian can help you determine whether your cat is at risk and, if necessary, which preventive medication is best suited to your cat.
Dr. Lorie Huston