Yes, heartworms are primarily a problem in our dogs – but cats can also become infected. The same mosquito that carries the heartworm larva can, and does, bite cats, injecting them with heartworms as well.
Cats are not the natural host. Once injected into our cats, most of the larvae end up in the pulmonary arteries (the blood vessels to the lung). The cat’s immune system will mount a massive response to these developing worms and kill most while they are in the pulmonary arteries. In some cats, the worms will survive this assault and develop into adult worms.
We usually see this in two distinct stages. The first stage is three or four months after the cat has been infected. This is the time frame when the larva is killed in the lungs. Some of these cats will show signs of coughing, vomiting, open-mouth breathing, harsh lung sounds or increased respiratory rate. Some cats exhibit no symptoms at all, but may have permanent damage to their lungs. These signs many times mimic asthma or allergic bronchitis. This has recently been recognized as a separate disease, called Heartworm Associated Respiratory Disease.
In cats that go on to develop adult heartworms, these worms seem to cause little problem until they begin to die about two or three years after infection. Again, the cat’s immune system attacks these dying worms. These cats can have tremendous reactions in their lungs that can proceed to acute lung injury and sudden death.
Adult feline heartworms are quite different from the worms we see in dogs. Infection is much rarer – likely 10-15 percent of the rate in dogs. The worms are smaller and fewer in number. Yet, when we consider the small size of our kitties’ hearts and lungs, even a few worms can cause considerable disease.
How do you tell if your cat has heartworms? Unfortunately, diagnosis is very difficult. A common blood test called an antigen test will only show infection if many mature, adult female heartworms are present. This test misses infections with male worms, lower numbers of worms, or younger female worms. A second test, for antibodies (response to the worms), will test positive if the cat has ever been exposed to the larvae – but it cannot tell if the infection was eliminated or is still present.
X-rays can detect changes about half the time. Ultrasound is the most precise test; adult heartworms can be seen with an ultrasound exam in 90 percent of cases.
If testing is difficult, treatment is more so. Treatment is aimed at reducing the clinical signs caused by the inflammatory response. There is no safe treatment to kill adult heartworms in cats.
How common is feline heartworm infection? That varies greatly depending on the part of the country you live in. A recent study here in Georgia found that 30 percent of cats tested were positive for heartworm antigens – meaning they had been bitten by a mosquito carrying heartworms.
The problem is, we can’t tell which cats will fight them off, which ones will develop asthma-like signs, which ones will go on to develop adult heartworms and which ones will suddenly die.
So we have a parasite that is increasing in Georgia, is very difficult to accurately diagnose, and is impossible to eradicate after infection. The good news is that heartworm preventative is available for cats; it is safe and effective. It can be given alone or in combination products that control fleas and internal parasites.
Talk to your vet. I know I’m talking more to my clients about it.