The name Heartworm Disease is misleading. Although the circulatory system does play a part, adult worms live mainly in the pulmonary (those related to the lungs) arterial system causing damage to vessels and lung tissue. A high worm burden may cause migration into the right ventricle of the heart. On autopsy the worms are found congregating in the heart and since this is where they were initially discovered, that was the name given them.
The process of getting this parasite called Dirofilaria immitis doesn’t really differ between dogs and cats, but the disease itself plays out differently.
An infected animal that has circulating microfilaria (baby D. immitis) gets bitten by a hungry mosquito.
The mosquito is an unsuspecting vector (or carrier) for the heartworm (D. immitis). Sucking up too many microfilariae can cause the death of the mosquito. We don’t expect many tears shed over that fact.
The microfilariae migrate to the Malpighian tubules (mal-pig-hian) which function for the mosquito basically as excretory organs. They contain digested blood, urea, amino acids and nitrogenous waste among other things. It is here, in this mosquito gut soup that the microfilariae transform and molt from Stage L1 to finally Stage L3 larva. Depending on the ambient temperature (the environment the mosquito is exposed to), this can take eight to twenty-nine days. Warmer air temperatures quickening the process.
Once reaching the L3 stage, the larvae migrate to the mosquito’s salivary glands and fall out like good little soldiers near the wound the mosquito makes, when taking a bloodmeal. They quickly find the wound entrance and burrow into the subcutaneous tissue, just under the skin.
The L3 larvae will molt again in three to twelve days becoming L4 stage larvae. It will take an additional 45-70 days for the L4’s living in subcutaneous tissue to molt again, transforming into stage L5 or juvenile adults. These juveniles live in the pulmonary blood vessels and finish growing into adult heartworms where they reside in the pulmonary arteries of the lungs feeding on blood plasma.
It is while the larvae are living in the subcutaneous tissue just below the skin surface that they are susceptible to the preventative veterinarians provide. These medications easily kill L3 and L4 stage larvae. When the larvae migrate into the muscles to begin their search for pulmonary arteries and transform into L5 larvae or juvenile adults the medications are unable to kill them.
Heartworm burdens in cats are much different than in dogs. Cats usually have one to three adult worms in their system compared to possibly hundreds in a dog’s. The cats small size makes this small worm burden just as difficult to cope with and can make diagnosing difficult. The easiest test to run is one that checks for antigens. These are produced by adult female heartworm (D. immitis). A cat with only a few or one worm may have no female worms or a single sex burden providing no offspring. Thus, no antigens present nor any microfilariae in the bloodstream to see under a scope.
It is widely believed that Heartworm in cats is underreported because of difficulties with diagnosis. Symptoms of heartworm disease mimic other respiratory diseases and unless multiple diagnostic tools are used a heartworm diagnosis may be missed. Heartworm disease symptoms can be mistaken for asthma or bronchitis, more common in cats.
Cats have a naturally occurring resistance to heartworm. Most L3 larvae never make it to adulthood in a cat. An adult heartworm in a cat has a lowered life expectancy of only about 2-3 years compared to that of one living in a dog to be 5-7 years old.
Unfortunately for the cat they usually have a severe inflammatory response when the worms die, causing just as many problems for them. This inflammatory response and the resulting symptoms are mostly pulmonary in nature and have been assigned their own term; known as Heartworm Associated Respiratory Disease (HARD).
Those symptoms are:
Increased respiratory rate or effort
Weight loss- often due to nausea and lack of appetite
Reluctance to do normal activities
Coughing- this is rare in cats- pay attention if your cat is coughing or gagging
Abnormal posture- may sit with head and neck extended and elbows back to facilitate easier breathing
As you can see many of the symptoms do not point toward heartworm specifically but are vaguely attached to a plethora of diseases from which to choose.
Diagnosing heartworm in cats usually takes several testing approaches. Starting with the simplest and inexpensive. If these tests are negative it is prudent to continue testing due to the low worm burden of the cat providing lots of opportunity for false negatives. A male only worm burden will only provide false negatives until further testing is pursued. The antigen tests and looking for microfilaria under a microscope which is often all that is needed to prove or disprove heartworm in dogs, requires further inquiry if coming up negative for a cat.
Further tests may include CBC and a blood chemistry profile, serum antibody tests, chest x-rays and echocardiography. It is an owner truly invested in its cat’s well-being who will pursue a definitive diagnosis and completely rule out heartworm disease.
The best we can do medically, is provide therapeutic support, sustaining a cat while it is having an inflammatory response, until it outlives the worms. There is no treatment to safely kill the worms in the cat like we have for our dogs. Even for our dogs there is a risk when treating for heartworm, and precautions are taken prior to and during treatment. Even successfully surviving death of the worms and being worm-free, there is the legacy of permanent damage to organs and vessels to be concerned about for both cats and dogs.
To be succinct: there is NO approved treatment for cats. The drugs currently available are very hard on cats. They can cause liver and kidney problems, development of arterial blockages caused by dying worms and death.
Wouldn’t it be easier to have just used a heartworm preventative all year in the first place, especially when we are already concerned about intestinal parasites, fleas and ticks? Advancements in pet medications now provide a monthly application that take care of all four main concerns. Check with your current veterinarian if you need more information.
*would like to thank: Cornell University, FDA.gov and AHS.org for providing updated information for this article.