Some cats never show clinical signs of heartworm disease.
Most dog owners are aware that infected mosquitoes can transmit deadly heartworm disease to dogs, but fewer people know that the disease can affect cats as well.
According to Dr. Allan Paul, a veterinary parasitologist at the University of Illinois College of Veterinary Medicine in Urbana, “Cats are susceptible to heartworm disease, but are more resistant to infection than dogs are, and cats have a lower prevalence of heartworm disease.”
As in dogs, cats acquire heartworm disease through the bite of a mosquito infected with heartworm in an immature life stage. The dog is the definitive host for heartworms, which are only partially adapted to the cat host. In dogs these parasites live in the heart, lungs, and blood vessels. In cats heartworms frequently migrate to other places in the body, which makes heartworm disease more deadly in the cat than in the dog.
While infected dogs may host as many as 250 heartworms at a time, infected cats rarely host more than six heartworms. Although six worms may seem like a comparatively small parasitic load, even a few worms can have a very negative health impact given the small size of cats.
Heartworms live an average of 2 to 3 years in the cat, and although this is considerably shorter than their lifespan in the dog (up to 7 years), this plenty of time for severe disease to result.
According to Dr. Paul, some cats never show clinical signs of heartworm disease, but when signs are present, they usually develop during two stages of disease.
“The first stage of disease coincides with the immature adult worms reaching the pulmonary arteries 3 to 4 months after infection,” says Dr. Paul. “When these worms reach the lungs, they will be attacked by the cat’s immune system and die. The cat then acquires what is known as heartworm-associated respiratory disease.”
This initial phase is often misdiagnosed as asthma or allergic bronchitis. As the worms mature, the signs of the acute phase will subside.
“Worms that are degenerating in the lungs can cause the lungs to be inflamed and thromboembolism–an obstruction of a blood vessel by a blood clot that has become dislodged from elsewhere in circulation–can often result in fatal, acute lung injury. This is the second stage of the disease,” explains Dr. Paul. “This can occur even if there is only one worm in the lung.”
Owners may find it difficult to tell if their cat is showing signs of heartworm disease. Many cats can tolerate a moderate infection without showing any signs of illness, while other cats may show clinical signs for a brief time and then appear to be normal.
Clinical signs of heartworm disease in cats may present as a vague illness or can be predominantly respiratory, gastrointestinal, and occasionally manifests neurologically. The most common signs seen–rapid breathing, coughing, and increased respiratory effort–are consistent with chronic respiratory disease.
Vomiting with an increased frequency, which is not related to eating, is also a common sign of heartworm disease. It is not known why these parasites induce vomiting. Anorexia and weight loss are reported in some cats, and a combination of these signs in addition to sudden death may also occur.
To diagnose heartworm disease, a veterinarian may have to test the animal repeatedly (on different occasions) for this disease since diagnosis is more elusive in the cat than in the dog. Heartworm serology, x-rays of the cat’s chest, and echocardiography to access the size and shape of the heart will help the veterinarian make a diagnosis of heartworm disease.
There is no treatment for feline heartworms. The amount of medication required to treat the infection can be lethal to cats. Veterinarians often give steroids to cats that appear to have lung disease on x-rays to help with the inflammation, whether or not they show clinical signs of the disease. Once heartworm disease has been diagnosed, a veterinarian will want to monitor with serologic testing.
“Since there is no treatment, prevention of this disease is extremely important,” advises Dr. Paul. “Veterinarians can prescribe monthly preventive medications for a cat just as they do for dogs. Indoor cats are also at risk, so it is important to give preventive medication to both indoor and outdoor cats.”
For more information about feline heartworm disease, speak with your veterinarian.
By Sarah Netherton