Before discussing the treatment options, it is very important to understand more about the heartworm disease, its life cycle and how it gets transmitted to cats. Contrary to common belief, indoor cats are also susceptible to getting heartworm disease; in fact nearly one-third cats suffering from the disease are indoor only cats. Let's start with learning about heartworm disease.
What is Heartworm disease?
Heartworms, in general, are blood-borne parasites (named Dirofilaria Immitis) that reside in the heart or adjoining large blood vessels of infested animals. The female worm is 6 to 14 inches long and 1/8 inches wide. The male is about half the size of the female.
Heartworm disorder is more common in dogs than cats. When compared to dogs, cats are more resistant to heartworm infestation with the rate of occurrence in cats reported to be 5-20% of the rate in dogs in same geographic area. Current research shows that in cats with heart and respiratory disorders, heartworm is found to be more common than previously thought. Generally, cats have less adult worms than dogs, often less than 6.
Transmission of Heartworms
Transmission of heartworm disease requires a carrier. However, it cannot transmit directly from one cat to another or from a dog to a cat. The life cycle of the heartworm is complicated. Heartworms require mosquitoes as an intermediate host and as many as 30 species of mosquitoes can act as a host that carries and can transmit heartworms. Mosquitoes consume immature heartworm larvae, referred to as Microfilariae, by feeding on an infested cat or, usually, an infested dog. Once consumed, the Microfilariae continues to grow further within the mosquito for 10 to 30 days and then passes into its mouthparts. It can then transmit to a cat through mosquito bite.
It takes months for the transmitted larvae to develop and migrate to the right side of the heart and the pulmonary arteries. Once there, the larvae grow into mature heartworms that are capable of reproduction in approximately six months from the time the larvae is transmitted into the cat. The next stage in life cycle of heartworms happens in about eight months’ time when they begin to produce new crop of Microfilaria that will live in cat’s blood for about a month. Cats are resistant hosts, and few circulating Microfilaria are generally found.
Diagnosing Heartworm in Cats
Diagnosing heartworms in cats is often difficult because none of the methods used for diagnosis are 100% reliable. The diagnostic order generally move on as follows:
Symptoms of heartworm disease often do not lead to precise diagnosis of the disease because these symptoms are not unique to heartworm and they might be caused by various other disorders. Not only that, different cats show different symptoms which further make the diagnosis very difficult. The most common symptoms are a sudden onset of coughing and fast & heavy breathing. Other symptoms include, but are not limited to - weight loss, vomiting, sudden collapse, dyspnea, convulsions, diarrhea, blindness, anorexia, lethargy and even sudden death.
Heartworm should be taken seriously because it can cause severe respiratory failure or even death in evidently healthy cats. Unexpected death can occur if dead or alive heartworms enter into the pulmonary arteries and become a barrier to the flow of blood to the lungs. It can also be caused by a reaction to immature heartworms within the lungs.
Heartworm antibody and heartworm antigen tests are the two most promising tests for diagnosing heartworm disorder in cats.
The heartworm antibody test reveals that the cat's immune system has been exposed to heartworms. An infected cat will have antibodies in its body for an indefinite time even if the heartworms in cat’s body have died. Based on recent studies, it is estimated that the antibodies can remain in cat’s body from two to four months after the heartworms have died. Also, cats with late stage larvae that has not matured yet or cats with heartworms in places other than the heart may also test positive with antibody test. The heartworm antibody test is very sensitive and while it confirms infection, it does not confirm active presence of heartworms in the cat. If this test comes out positive, the heartworm antigen test is then conducted.
The heartworm antigen test is a much more defined test for diagnosis of heartworms. This test is not as sensitive as the antibody test but it is more likely to confirm the presence of mature female heartworms in cat’s body. While a positive test confirms that heartworms are present in the cat’s body, a negative test does not mean that the cat is free of any heartworms. The cat must have at least two mature female worms present to make this test positive. A negative test may also mean that the cat either has male worms present or it has immature worms or a single mature female worm.
In short, feline heartworm infection diagnosis is certain when both the antibody and antigen tests are positive.
Other types of blood tests available are blood sample test and Eosinophil count test but these tests are not very conclusive.
Radiographs (X-Rays) can be taken to examine the size and shape of the heart as well as measure the diameter of the pulmonary arteries. Cats with heartworms can show an increase in size of the pulmonary arteries, or the arteries may shrink on their way to the lungs due to worms blocking them. However, radiographs are not reliable way to diagnose heartworms either since for many cats they reveal no findings, especially early in the infection.
Heart Ultrasounds can also be performed to examine the internal structures of the heart and surrounding vessels to evaluate the condition and function of the heart. In some cats, the adult worms can be spotted, confirming the presence of heartworms. However, most cats have a low number of worms so an ultrasound does not provide conclusive evidence often.
Treating Heartworm in Cats
When it comes to treating heartworm in cats, Prevention is the best cure for it. Unlike in dogs, there are no approved medications for curing heartworms in cats. The medications that are used to treat heartworms in dogs are known to have side effects when used with cats and sometimes these side effects can be severe.
Treating cats with the medication made for dogs can make the heartworms to pass through the pulmonary arteries into the lungs; where the reaction to the dead and dying worms can cause severe lung failure or even sudden death.
For infected cats, often treating the symptoms of the disorder is done in hope the cats can outlive the worms. Heartworms live in a cat for about two to three years so treatment for the symptoms needs to be done for several months. The danger of a severe failure or sudden death is always looming when treating the symptoms so it is necessary to keep a close watch on the cats for any emergency that may arise. Consult your Veterinarian for advice.
Another possible treatment option is removing the heartworms by surgery. It is not a popular option in North America yet. It is followed more in some parts of Europe and Japan.
Preventing Heartworm in Cats
As mentioned above, prevention is the best form of defense against fighting heartworms in cats. Lack of treatment options and difficulty in diagnosing heartworms with certainty has led many Veterinarians to strongly suggest that all cats should receive monthly heartworm preventatives.
Since mosquitoes act as a carrier of the disease, cats should receive monthly heartworm preventatives all year long in locations where mosquitoes are active all year round. In areas where mosquitoes are seasonal (like in colder areas), cats should be on monthly preventatives for at least 6 months of the year.
Many excellent heartworm preventatives are now available for cats, making prevention of heartworm disease simple and safe. Heartworm preventatives are available in the form of topical applications or oral medications. Cats that have been given heartworm prevention drugs have shown no signs of toxicity in further support for use of heartworm preventatives. Consult your Veterinarian about use of any drugs.