Heartworm Disease In Dogs and Cats – Part 3 of 3

This is Part 3 of 3 of articles about Heartworm disease, originally posted at ThePetCenter.com by Diana Beam, DVM.

Heartworm disease in dogs and cats: In many areas of North America, April is the time of the year when veterinarians begin to check dogs and cats for exposure to heartworm organisms that may have occurred during the previous mosquito season. If your pet was infected last mosquito season, evidence of the disease may now begin to be detected. And like any other pathogenic situation, the earlier a diagnosis is made and treatment is begun, the better the chances are that the patient will recover properly. Give your veterinarian a call early in Spring about testing your pet for Heartworm.

NOTE! New Heartworm Disease therapies, preventatives and guidelines may be currently in use. Always check with your veterinarian for updates on any issues presented in this article.

Part 3 – HEARTWORM – In The Cat

THE CAT AND HEARTWORM DISEASE

It may be helpful to read the Life Cycle in the Dog before reading this section.

DIFFERENCES IN THE HEARTWORM LIFE CYCLE IN THE CAT

The cat is not the normal host for the Heartworm parasite. Since they are not the normal host, it is less common for cats to become infested. When Heartworm in the cat is seen, aberrant migration of the larvae is more common than in the dog and a lower percentage of the worms actually end up in the heart. It is known that more of the larval worms die before they can mature in the cat than in the dog. Those worms that do reach adulthood live a shorter time as adults, 2-3 years in the cat versus 7 years in the dog. Also, for some reason, male cats seem to be more susceptible to getting Heartworm disease than female cats.

Those worms that do complete a normal migration to the heart can cause many problems since the cat’s heart and blood vessels are much smaller than comparative structures in the dog. The adult worms reach a shorter adult length of 5-8 inches versus approximately 14 inches in the dog. The other feature not unusual in cats is single sex Heartworm infections. This means no microfilaria can be produced which makes identification of Heartworm disease more difficult. These single sex infections are commonly male worms only. In those cats that do have microfilaria present in the circulation the numbers of microfilaria are fewer and the production much shorter in duration than in the dog. Cats often will have 1-2 worms when infected. Larger numbers have been seen and sometimes cats have similar signs as the dog, but this is not common.

Any geographic area where dogs contract Heartworm disease can also be a hazardous area for cats. The current rate of identification of Heartworm disease in cats is estimated to be from 5-20% that of dogs in the same geographical area. This number will probably become more consistent and possibly increase as we develop more effective methods of Heartworm detection in the cat.

SIGNS OF HEARTWORM DISEASE IN THE CAT

The signs of Heartworm in the cat are usually much different than in the dog. A cat’s immune system is readily activated against Heartworms. So the signs of a cat with a heartworm infestation display more immune system responses than a similar infestation in the dog.

Cat owners must be aware that even a small number of Heartworms can cause sudden, serious, and even fatal disease in the cat. The main effect in cats is inflammation of the arteries (called endarteritis) and the surrounding tissues in the lungs. The amount of inflammation is much more exaggerated than what commonly occurs in dogs. The inflammation alone can result in partial to complete blocking of blood vessels. In cats, when worms die, complete blood vessel blockage often occurs. Depending upon where these blockages are located and the amount of inflammation, large portions of the lungs can be affected which results in a significant, often sudden breathing difficulty for the cat. The severity is also affected by the magnitude of the cat’s immune system response. A single blocked blood vessel with a large inflammatory response can result in an emergency situation for the cat.

The cat’s highly reactive immune response to heartworms may be the reason we see some of the following signs in Feline Heartworm Disease…It is rare to find circulating microfilaria in the cat.

Cats acquire Heartworms at a lower rate than dogs.

Fewer worms cause much more severe reactions.

Often the preferred treatment is controlling the symptoms and allowing the worms to die naturally rather than risking treatment. Severe reactions may still occur when the adult worms die.

Worms live a much shorter time than in the dog… 2 to 3 years in the cat, 7 years in the dog.

One worm can cause a severe medical emergency.

Common signs associated with a Feline Heartworm Disease include lung disease associated with respiratory stress (difficulty breathing or rapid shallow breathing), gagging or vomiting, lethargy, loss of appetite, and weight loss. Some of the signs are difficult to distinguish from feline asthma. Sudden death may occur in cats, as well as it does in the dog, however only a few worms in the lungs of a cat can trigger a fatal outcome. In the average size dog a large number are generally required to cause a sudden fatality. Other signs indicating lung obstruction are a loss of coordination, seizures, blood from the nose, or collapse. If collapse does occur the cat may die despite treatment even if you are able to visit your veterinarian for treatment.

DIAGNOSING FELINE HEARTWORM

Diagnosing Heartworm in the cat is a bit more challenging than making the diagnosis in the dog. Since many infestations in cats are single sex (usually male worms) and therefore sterile with only a few adult worms, the typical antigen Heartworm test is not always successful in diagnosing the presence of adult Heartworms. The antibody Heartworm test does not distinguish between an old, resolved infestation and a currently active Heartworm case. Since blood tests are very simple and are test is positive, you do have your answer because that indicates live worms are present. Usually these tests are only done when a Heartworm infection needs to be ruled out as a cause of the cat’s abnormal signs. Since circulating microfilaria are so rare in the cat, the Knotts test and filter tests are not very useful. Once Heartworm disease in the cat is positively identified, the Knotts test may be done to see if microfilaria are present, but this test method is not performed on a routine basis to make a diagnosis. Most Feline Heartworm infections are diagnosed by presenting signs and history and by using ultrasound instrumentation, x-rays, non-specific angiograms, and other blood work to positively identify the presence of Dirofilaria immitis.

TREATMENT OF FELINE HEARTWORM DISEASE

In the cat, the treatment of choice may be no treatment at all! Certainly supportive therapy is utilized if a patient requires help. Supportive therapy is using medication to reduce the side effects of inflammation in the lungs. The usual choice is prednisone since aspirin can be toxic to cats. The need for supportive therapy, length of therapy, and use of other medications varies widely from cat to cat. Since the need for supportive treatment can be intermittent, and existing parasites may die suddenly creating an emergency, your understanding of what is going on and working closely with your veterinarian is very important to your cat’s survival and long-term health.

Usually adulticide treatment is only used on those cats in stable condition that do not respond well to the supportive care. Of the few cats that are treated, at least one third will have life-threatening complications due to the effects of dying worms. Currently, the adulticide used in cats is thiacetarsamide (Caparsolate). The other adulticide, Melarsomine dihydrochloride (Immiticide), is very toxic to cats. Occasionally worms that can be seen via ultrasound or some other reliable method may be surgically extracted. This is not a common procedure in the cat, though.

In summary

If the cat does not appear sick and responds well to supportive care, usually no adulticide therapy is given and the worms are left to die on their own. Supportive therapy is used as needed, and ultrasound and chest X-rays are re-evaluated on a regular basis to monitor the Heartworm Disease. Re-evaluation once every 6 months is not unusual. In addition, usually a decision is made to start a monthly Heartworm preventative to preclude another infestation.

PREVENTION OF FELINE HEARTWORM INFECTIONS

There are two Heartworm preventatives approved for use in cats by the FDA. One is Interceptor ® Flavor Tabs® and the other is Feline HeartGard. These preventatives are given orally once a month and are quite effective. In the future, there will be more Heartworm preventatives approved for use in the cat.

Is the use of Feline Heartworm preventative justified? The answer to this question may not be determined yet. It is generally thought that in an area with a high incidence of Canine Heartworm Disease, feline Heartworm prevention is justified. These areas include most of the southeastern United States and Gulf States along with many other areas where Canine Heartworm infections are common. In geographical areas where Heartworm occurs only occasionally, it is still under much discussion within the veterinary community as to when it is justifiable to suggest the use of feline Heartworm prevention.

Veterinarians are continuing to learn better ways to diagnose Feline Heartworm Disease. As more cats are tested, and improved diagnostic techniques are perfected for cats, testing guidelines will be updated and recommendation more consistent. Until then, check with your veterinarian for current local recommendations regarding Feline Heartworm Disease.

SOURCE: Heartworm Disease In Dogs… Diagnosis, Prevention and Treatment in Dogs and Cats at ThePetCenter.com

Part 1 – HEARTWORM (Dirofilaria immitis)
Part 2 – HEARTWORM – In The Dog
Part 3 – HEARTWORM – In The Cat

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