Feline Heartworm

Preventable, But Still a Puzzle

Felines are resistant but susceptible hosts for Dirofilaria immitis, the parasitical worm that causes heartworm disease. While rare in many areas, heartworms can have serious consequences – including sudden death – for your cat.

Location is key, says Mark Rishniw, BVSc, a cardiologist at the Cornell University Hospital for Animals. In heartworm-endemic  areas (Texas [mainly Gulf coast], Louisiana, Alabama, Mississippi, Georgia, North Carolina, South Carolina, Tennessee, and Florida), where 100 percent of exposed, untreated dogs are infected, up to 10 to 20 percent of the cat population may be infected. Heartworm is most common in warm, humid areas where the parasites necessary vector, the mosquito, flourishes.

Feline heartworm is relatively easy and inexpensive to prevent. Products containing ivermectin or selamectin are marketed for use in cats. All current preventatives are 100 percent effective when given correctly, says Ray Dillon, DVM, professor of small animal surgery and medicine at the College of Veterinary Medicine at Auburn University. Many veterinarians recommend that all cats in heartworm-endemic areas be on preventatives. In areas where heartworm is rare, a caregivers course is less clear. Ultimately, its a risk-benefit assessment, says Rishniw, who is board certified in cardiology by the College of Veterinary Internal Medicine. It comes down to both an economic and peace of mind decision. Ask your veterinarian for guidance.

Heartworm transmission
For a cat to get heartworms, there must be a reservoir (usually dogs) of the parasites larvae, and there must be mosquitoes in which these larvae can molt. More than 50 species of mosquitoes can transmit heartworm, but not all of them readily feed on both cats and dogs. Which mosquitoes are active in a region largely determines how badly local cats are affected. In heartworm-endemic areas, though, its prudent to consider every mosquito a threat.

Its harder to infect a cat than a dog, but even a single mosquito bite can infect a cat, notes Dillon. Thus, heartworm disease can strike both indoor and outdoor cats. A female mosquito bites an infected animal, usually a dog, with a patent infection (meaning there are heartworm larve called microfilaria in the bloodstream). With her blood meal, she gets a load of microfilariae. Over the next 8 to 28 days, these microfilariae molt into the L3 infective stage. When the mosquito bites another animal, 10 to 12 of the L3 larvae are injected into the new hosts skin.

Signs and diagnosis
If the new host is a cat, the larvae take seven to eight months to grow to adult size. But signs of disease can occur much earlier. Three to four months after the mosquito bite, the L3 larvae molt into the sub-adult L5 stage and reach the lungs, often causing intense inflammation. Thats when the cats first symptoms – coughing, gagging, rapid or difficult breathing, frequent vomiting, and lethargy – often appear. Or, an infected cat may exhibit no symptoms at all and still be at risk for sudden death. As Dillon notes, Cats that die from heartworms can be clinically normal one hour before death.

Heartworm signs mimic numerous respiratory and gastrointestinal diseases, including asthma and inflammatory bowel disease. If heartworm is rare in your area, your veterinarian may not consider it. If you have any reason to suspect your cat may be infected (youve moved from or visited a heartworm-endemic area, or live near a swamp, for example), be sure to tell your veterinarian.

A definitive diagnosis of heartworm often requires a combination of tests, including serum antibody and antigen tests, x-rays, and echocardiography (ultrasound). In heartworm-endemic areas, serum antibody tests are useful for screening. A negative result nearly always rules out infection. A positive result means the cat has been infected with early-stage heartworm larvae and calls for additional tests. A positive result on a serum antigen test means the cat has adult heartworms.

There are ways to kill adult heartworms, including injections of an arsenical compound (thiacetarsamide sodium), but most veterinarians prefer to avoid adulticide treatment in cats unless the caregiver insists, because when the worms die – either naturally or during treatment – an extremely rapid, intense inflammatory reaction can occur, causing massive swelling of the lung tissues, acute respiratory failure, and death. Much preferred is palliative treatment of symptoms with corticosteroids – and simply waiting out the worms. Still, the risk of sudden death remains.

Feline heartworm presents challenges to researchers, veterinarians, and caregivers. The actual prevalence of the disease is unclear. Diagnosis is tricky, and treatment, risky. Mild symptoms can lull caregivers into a false sense of security. You owe it to your cat to learn about and try to minimize this potential threat to his health – and life.