FELINE HEARTWORM INFECTION

FELINE HEARTWORM DISEASE 

CAN CATS BECOME INFECTED WITH HEARTWORMS?

The answer to this question is an unequivocal "yes" but feline heartworm disease is a lung disease rather than a heart disease as it is in the dog. The parasite is the same but because the heartworm's natural host is not a cat, the interaction between worm and host creates a very different condition. It isn't good for either the cat nor is it good for the worm. WHEN HEARTWORM MEETS CAT: THE BIOLOGY The story begins with a mosquito, just as in the canine situation. The mosquito feeds on a heartworm infected dog, the heartworm microfilaria (the youngest larvae) are sipped up into the mosquito during feeding, and develop in the mosquito's body for the next couple of weeks. This is all according to plan for the heartworm's life cycle until the mosquito bites a cat instead of another dog.

(Photocredit: Morguefile.com)

The third stage larval heartworms enter the cat's body and develop in the tissues. The feline body is an inhospitable host and worm development is fraught with immunological attack. By the time the larva has reached its 5th stage, it is on its way to the pulmonary arteries to complete its maturation but most infections will end here as the feline immune system is nearly relentless in its assault. Only about 25% at best of the original infecting larvae will survive to adulthood which means most infections are aborted in this last stage of larval development.

Most cats with adult heartworms only have a few worms (1-3 on average) and development to the adult stage takes an extra couple of months in the feline body compared to the canine body. Chances are that there will be a single sex worm population rendering reproduction impossible. Only about 20% of feline infections produce microfilaria (youngest worm larvae); further, the feline immune system is so aggressive that the larvae only live a matter of weeks whereas they can live for up to 2 years in a dog. When the adult worm dies, a huge amount of inflammation is generated and many cats do not survive this stage. If the cat does survive, there is likely long term damage to the lung tissue.

• It is unclear what percentage of an area's feline population will be infected. The standard statistic is that a region's feline incidence will be approximately 10% of the canine incidence but this appears to be a low estimation since feline infection cannot be defined by the presence of adult worms.
  
• Cats living in heartworm areas should be given heartworm prevention medications just as dogs should.
  In one study, 25-30% of heartworm infected cats were described as being "indoor cats." Many mosquito species are not shy about entering homes.

Heartworm disease in the cat can produce an assortment of clinical pictures:

• Coughing, wheezing, difficulty breathing.
• Vomiting
• Disease related to embolism or abnormal clots.
• Extreme nosebleed.
• Neurologic signs (probably associated with larvae accidentally migrating to the brain.)
• 10-20% of cats experience sudden death. (This is probably associated with death of adult worms.)
• Many cats never show noticeable symptoms and most of these cats (80% approximately) clear the infection on their own. How many cats are infected without symptoms? The answer is not clear because it is the symptomatic cats that get the most medical scrutiny.

HEARTWORM-ASSOCIATED RESPIRATORY DISEASE, abbreviated "HARD"
Also known as "Pulmonary Larval Dirofilariasis"

As stated, the cat is not a natural host for the heartworm which means the migrating larval heartworms are not likely to complete their life cycle. To recap from above, baby heartworms called "microfilaria" are slurped up by the mosquito feeding on an infected dog. The slurped up young heartworms must spend enough time (several weeks) in the mosquito's body to develop into an infective stage at which point they are ready to infect a new host. The infective stage heartworm larva is deposited in a drop of mosquito spit adjacent to a mosquito bite, the larvae crawl into the skin puncture made by the mosquito, gain access to their new host, and continue to develop in the soft tissues of the new host, eventually making their way into the circulation and to the host's pulmonary arteries.

The migrating young worm uses molecular signposts to tell it how to get to its host’s pulmonary arteries where it wants to finish growing up, mate and live out its life. The worm is prepared to read CANINE signposts and does not always migrate correctly trying to understand feline protein signals. The worm may get lost and end up who knows where in the body. If the young worms get to the pulmonary arteries at all most of them are killed by the especially reactive feline immune response against them. It is this immune reaction that causes heartworm disease in the cat and it can start as soon as 75-90 days after the infecting mosquito bite.

When young heartworms die in the pulmonary arteries, the immune system breaks them into fragments and attempts to remove them. The resulting inflammation leads to lung disease which manifests as coughing, respiratory effort, and vomiting. The inflammation associated with the death of fifth stage heartworm larvae is vastly compounded should a coexisting adult heartworm die. In this situation yet more inflammation results and even if the cat survives, all this inflammation creates permanent damage in the delicate lung tissues (as shown in the illustrations at the top of this page.)

Heartworm-Associated Respiratory Disease (HARD) mimics feline asthma and the two diseases look identical on radiographs. Cats with HARD will cough, wheeze ( a musical respiratory sign similar to a sigh), and vomit (though it may be hard to tell aggressive productive coughing from vomiting). Breathing may be shallow and rapid and may progress to actual respiratory distress. Heartworm testing is the only way to distinguish these conditions.

VASCULAR DISEASE SEPARATE FROM "HARD"

While in most situations feline heartworm disease is a lung disease and not a vascular disease as it is in the dog, sometimes cats do get adult worms in their pulmonary arteries just as dogs do. These adult worms do not live as long as they do in the canine body and they do not achieve the same length/size. If they do find a mate and give birth to microfilariae, the microfilariae are promptly killed by the feline immune system within the first month. In short, in the feline pulmonary artery, heartworms are much smaller. One would think this would make for milder vascular disease but because cats are so small, even one adult worm takes up a great deal of space in the vasculature. The more usual lung disease is all the worse if the reaction against the immature worms is complicated by the presence of a surviving adult worm in the vasculature. When this parasite finally dies, the subsequent blood clots and inflammation is frequently fatal to the cat. Death can be sudden and unexpected.

Most of heartworm disease in the cat is caused by the inflammatory reaction
generated by the worm’s presence.

In the dog, heartworm disease is mostly about
the obstruction of blood flow from the physical size of the worms.

DIAGNOSTIC TESTING

ANTIGEN TESTING?
In the dog, diagnosis is usually not complicated. A blood sample is tested for proteins that can only be found in the uterus and eggs of the adult female heartworm. Most infected dogs have a population of worms in their arteries, so if even one female worm is present an antigen test to show positive. In the cat, disease is caused by immature worms, not adult worms female or otherwise, so this kind of testing has limited applications. There may be only a couple of male worms or no adult worms present at all to generate a positive antigen test, yet the cat is infected.

MICROFILARIA TESTING?
In the dog, testing for microfilariae (offspring of adult heartworms born in the host’s body) is also commonly performed. Unfortunately, in the cat microfilaria testing is virtually worthless.
 First of all, infected cats usually do not have enough adult worms for the production of offspring. There may be only a few adult worms present; single sex infection is common. Further, microfilariae, if any, are simply cleared too quickly by the host's immune system and are not available for detection. As mentioned, in cats heartworm disease in cats stems at least in part from migrating immature larvae and their is no simple test for these.

ANTIBODY TESTING?
Cats produce antibodies to heartworms as soon as they are infected. The idea with antibody testing is that if antibodies are present, the young heartworm larvae must be present, too. Antibody testing may be more sensitive but is not adequate alone. A negative antibody test is good evidence that the cat is probably not infected; however, a positive antibody test may indicate several things. It could indicate a mature infection with one or more adult worms. It could indicate the presence of immature worms in the body. It could also indicate a past infection. (Antibody levels will remain somewhat elevated after the heartworms have long since died.)

So if no single test is reliable, what are we supposed to do for testing? First of all, unlike dogs where annual screening is the norm, healthy cat screening is probably not necessary. Instead, testing is best done if a cat is sick and heartworm disease is suspected. There is still some controversy about testing should be accomplished in a symptomatic cat. Both antibody and antigen tests in combination are recommended by some experts while others feel the antibody test alone is probably adequate. Of course, a cat with respiratory disease probably should have chest radiographs and cardiac echocardiography to further define the condition at hand.

TREATMENT

Since the major signs of disease in the cat are due to inflammation and immune stimulation, a medication such as prednisolone can be used to control symptoms. A bacteria called Wolbachia commonly lives within the heartworm and enhances its ability to generate inflammation. A course of doxycycline is often recommended to address these bacteria. The doxycycline course is short but the prednisolone will be long term. If the cat does not appear sick, the American Heartworm Society recommends attempting to wait out the adult worm's 2-3 year life span and simply monitor chest radiographs every 6 months or so. Median survival time is 1.5 years for heartworm infection in cats. Radiographs are monitored to check progress.

One might wonder why one cannot use the same treatment as in the dog to kill any adult heartworms the cat might have. Actually, the same heartworm adulticide therapy used in dogs is best not used in cats as it is extremely dangerous to do so and is considered a last resort. There may not be a choice, however, depending on the degree of illness from the heartworm disease. Approximately one third of cats receiving heartworm adulticide therapy will experience life-threatening embolic complications when the worms die suddenly (generally an unacceptable statistic). One month of cage confinement is typically recommended to control circulatory effort after adulticide treatment and adulticide therapy should be consider the last resort for an infected cat where symptoms of the disease cannot be controlled with prednisone.

PREVENTION

The good news is that feline heartworm infection is preventable and there are currently four medications on the market that are reliably effective.

IVERMECTIN
The dose of ivermectin (active ingredient of Heartgard^®) needed to prevent heartworm infection in the cat is about 4 times higher than that in the dog. Heartgard was the first FDA approved heartworm prevention medication available for cats. It is a monthly flavored chewable available by prescription.

MILBEMYCIN OXIME
Interceptor^® also makes a monthly chewable for cats with the same active ingredient (milbemycin oxime) as Interceptor for dogs. Interceptor for cats also protects against hookworms and roundworms. Milbemax is similar but adds praziquantel for regular tapeworm removal.

SELAMECTIN
Revolution^® entered the anti-parasite scene in 1999. This product covers fleas, roundworms, hookworms, and ear mites in addition to preventing heartworm in cats. Revolution Plus is a more recent update with enhanced flea and tick control. Either form of Revolution is an effective heartworm preventive. These products are applied topically rather than orally.

MOXIDECTIN
Advantage Multi® is a topical product from Bayer which combines imidocloprid for flea control and moxidectin for heartworm preventive all in one product. It covers roundworms, hookworms and ear mites as well as heartworm. This product is also applied topically. Similarly Bravecto Plus^® combines fluralaner for fleas and ticks with moxidectin for worm prevention. This product also covers the same parasites as Advantage Multi plus ticks.

(Photo Credit: CDC Public Health Image Library)

In studies of infected cats, 25% of infected cats were considered INDOOR ONLY CATS. Because of this and the disastrous effect of even one heartworm to a cat, the American Heartworm Society recommends monthly prevention for ALL CATS living in heartworm endemic areas. To view their feline guidelines visit:

https://heartwormsociety.org/images/pdf/Feline-Guidelines-Summary.pdf

Other helpful sites include:

www.petsandparasites.org/cat-owners/heartworms.html 

(Graphic Courtesy of the American Heartworm Society)

Page last updated: 10/18/2021