Heartworm Disease in Cats

Posted: Wed Feb 04, 2009 9:23 pm

Heartworm disease has been a major concern for dog owners for the last few decades. For cat owners, however, heartworm disease barely registers a blip on our radar screen, with most cat owners wrongly assuming that cats aren’t susceptible to the disease. In fact, feline heartworm disease (FHD), though relatively uncommon, is a serious and potentially fatal disease in cats.

The incidence of FHD is relatively low, and the clinical signs can be vague and non-specific, often mimicking other conditions, especially asthma. As a result, many veterinarians are not likely to suspect heartworms in cats, and the disorder goes undiagnosed or misdiagnosed.

Both cats and dogs can serve as definitive hosts for heartworm disease. Cats can acquire heartworm disease when heartworm larvae are transmitted to the cat via the bite of a heartworm-infected mosquito. The larvae develop into mature adult worms which travel to the heart and lungs. Cats are biologically less hospitable hosts for heartworms than dogs. Experimentally, if 100 heartworm larvae are injected into a dog, more than 70 will survive to adulthood. In cats, however, less than 10 make it to adulthood. Adult heartworms have a five year lifespan in the dog. In cats, however, adult worms live for only two years. Cats mount and intense immune and inflammatory response to the worms. The intense immune response may explain why adult heartworms have a shorter lifespan in cats. Although we tend to think of an intense immune response as a beneficial thing, in the case of heartworm disease, the exuberant immune response may cause more damage than the parasite itself.

Any age cat of either sex is susceptible to FHD, although middle-aged males have the highest incidence. Cats housed indoors as well as outdoors are vulnerable. According to Dr. Gary Norsworthy, owner of Alamo Feline Health Center in San Antonio and editor of “The Feline Patient: Essentials of Diagnosis and Treatment”, the most common findings in cats with FHD are sporadic coughing and labored or difficult breathing. Poor appetite, lethargy and weight loss are common non-specific complaints. “Interestingly, vomiting has been reported in a high percentage of cats with FHD and in some cases may be the only clinical sign”. Heartworms can cause a sudden obstruction of a vessel in the lungs, a condition called acute pulmonary thromboembolism. Cats with this condition can develop sudden respiratory distress and weakness followed by sudden death. Acute neurologic disturbances such as incoordination, blindness, and seizures are occasionally seen in heartworm-infected cats. Congestive heart failure occasionally develops in chronic cases.

Diagnosis of heartworm disease is much more difficult in cats than in dogs. Adult heartworms produce microfilariae – little “baby” heartworms. One method of diagnosis is to look for microfilariae circulating in the bloodstream. This method is effective in many dogs, but rarely for cats, because the intense immune response that the cat mounts quickly eliminates the microfilariae from the bloodstream. Finding these microfilariae indicates the cat is infected. A negative test is inconclusive. When the worms reach maturity (approximately 6 months of age), they shed proteins called antigens into the bloodstream. The cat’s immune system reacts to these proteins by making antibodies against them. One method of diagnosis is to look for antigen or antibody or both in the bloodstream. In dogs, the standard screening test is the antigen test. In cats, both antibody and antigen tests may be used. Antigen tests actually detect proteins shed by the adult female heartworms in the blood of the cat. Antibody tests detect the presence of antibodies that the cat has made in response to exposure to heartworm larvae and/or adult heartworms. Each test gives important information, but each test has its limitations. A positive antigen test reliably indicates that the cat is infected. A negative test, however, doesn’t necessarily mean the cat is free of disease. Because the test detects antigens given off by female worms, infection with male-only heartworms may be missed. While dogs may be infected with many heartworms, cats may be infected with only 1 or 2 worms. Low numbers of heartworms means little antigen given off, and though the tests are highly sensitive, such a small amount of antigen may be missed by the test. A positive antibody test indicates that the cat has been exposed to heartworms, but not necessarily the presence of adult worms in the heart. For example, 50 or 60 days after a mosquito has transmitted heartworm larvae to the cat, antibodies to these larvae may be produced. The cat’s immune system, however, might kill off the larvae before they develop into adults. Antibodies may also persist after heartworms have died. Cats that have survived heartworm disease will therefore test positive on an antibody test despite not having an active infection. The decision of which test to use first depends on the preference of the veterinarian. Some veterinarians prefer to antibody test all cats initially, with a negative test generally eliminating the need for further testing. A positive test indicates exposure and will require additional testing to see if there are adult worms in the heart. Additional tests that may be useful include chest x-rays and cardiac ultrasound.

Unlike dogs, treatment options for FHD are limited. In dogs, drugs are given, in stages, to kill the adult worms, and then the microfilariae. Cats don’t have circulating microfilariae, so that stage of treatment isn’t necessary. Melarsomine, the drug used to kill the adult worms is generally safe for dogs, but data on its use in cats is limited and contradictory. Most veterinary cardiologists do not advocate using drugs to kill the adult worms in cats. Instead, cats are treated symptomatically, with short-term use of anti-inflammatory drugs to reduce the intense allergic reaction that develops. “I do not use melarsomine”, says Dr. Norsworthy. “I treat with the anti-inflammatory drug prednisone”. Aspirin is often recommended to reduce the risk of blood clots in the lungs. Although there is still no proven benefit, giving aspirin is generally harmless and inexpensive. The best method of dealing with FHD is prevention. The disease is entirely and easily preventable. There are now three FDA-approved drugs that are marketed for monthly use in cats. Ivermectin as a chewable treat, milbemycin as a flavored tablet, and selamectin as a topical formulation. Because the consequences of FHD are potentially dire and treatment options are limited, monthly preventatives should be given to cats living in endemic areas. Dr. Norsworthy is a strong advocate of using heartworm preventatives, mainly because it can be so difficult to achieve a diagnosis.

Feline heartworm disease is a serious problem that affects cats worldwide. Although cats are not the natural hosts for heartworm, they are at more risk for severe disease once infected. Cat owners and veterinarians need to be aware of the incidence and risk of FHD. Your veterinarian can help advise cat owners as to the best prevention program for cats at high risk.

Reference: Arnold Plotnick MS, DVM, ACVIM, ABVP