Heartworms in the Feline Patient

Filed Under: Cats, Parasites

Are there mosquitoes in your area? Have you ever been bitten by a mosquito? One mosquito bite and your cat can be affected by heartworm disease. It is just that easy for your cat to obtain this life-threatening infection. Fortunately, heartworm disease is completely preventable, yet less than 4% of the cats in the United States are given heartworm preventative on a regular basis. We are diligent in protecting our cats from Feline Leukemia (FeLV) and Feline Immunodeficiency Virus (FIV) through vaccination while we ignore heartworm disease, which is far more prevalent and easier to prevent.

Heartworm disease is a worldwide problem yet we are ignoring its significance in cats, preferring to think of it as a disease only in the dog. Veterinary researchers have found cats with adult heartworms, or Dirofilaria immitis, a spaghetti-type worm that inhabits the pulmonary artery of infected individuals, in all areas of the United States. Adult heartworms were found in 10% of the cat populations in Louisiana and Texas. Feline heartworms were found to be present at even higher levels of 14% in Kentucky and the number of infections is being detected in ever-increasing numbers. The antibody level to heartworms in this same population of cats, indicating exposure, is much higher and is reportedly at a 15.9% level nationwide.

Heartworm disease is transmitted by the bite of a mosquito and may occur in any environment mosquitoes inhabit. Domestic and exotic felines may all become infected. Inside cats are actually considered more susceptible to heartworm disease than are outside cats. This is due to limited mosquito exposure. Inside cats mount less of an active immune response to heartworm infection, thereby resulting in an increase in disease development following exposure. Are any exclusively inside cats truly restricted from any contact with the outside environment? Mosquitos may enter the home through an open window with a screen that is not totally secure, the cat may have access to a porch area, or mosquitoes may enter the home through an open door as family members are passing in and out of the home. A North Carolina study demonstrated that one-third of all cats diagnosed with heartworm disease were in fact termed as residing exclusively indoors.

In cats, heartworm disease should be considered a pulmonary or lung disease rather than a cardiac problem. Most infected cats are asymptomatic (do not show any overt clinical signs of disease). In cats, immature worms that never make it to adulthood may create significant disease and even result in the death of the infected individual. There are many causes of bronchial pathology in cats. Unfortunately, heartworms deserve to be near the top of a differential list (list of possible disease causes) when confronted with pulmonary disease in heartworm endemic areas.

Clinical signs of feline heartworm disease, when present, include: collapse, dyspnea (difficult breathing), coughing, vomiting, and sudden death. Vomiting, when it occurs, is usually unassociated with eating and in many cases may be the reason why the cat is brought to a veterinarian for treatment. Neurologic clinical signs such as ataxia (unbalanced), blindness, circling, dementia, and seizures may also occur, usually due to aberrant worm migration. Aberrant migration occurs when an adult worm migrates to another part of the body rather than the heart or lungs, often lodging alternatively in the brain or spinal cord.

In most cases of feline heartworm infection it is not the adult worm but rather the developing heartworm larvae that is responsible for the pathology associated with the infection. In the cat, most heartworm larvae die before they reach adulthood, causing severe pulmonary disease (inflammation of the lungs and bronchi) in the process. This syndrome, often mistaken for severe asthma, is now termed “HARD” for heartworm associated respiratory disease and is the result of the L5 stage of the heartworm larva. Affected cats are typically seen coughing and have difficulty breathing which may be so severe that it results in death of the affected individual. This inflammatory response typically occurs 70 to 80 days following infection by the mosquito. Clinical signs of asthma may be intermittent in occurrence. Those cats not showing signs clinically will still have significant arterial pathology.

Larva maturing into adulthood will develop the ability to suppress the severe lung inflammation that was occurring during their larval stages by inactivating or otherwise camouflaging themselves from the host cat’s immune system. The adult heartworms and the cat will coexist until such time that the heartworm dies, usually months to years later. An adult heartworm will typically have a life span of less than two years within the host cat. When the adult worm dies, an anaphylactic, or exaggerated allergic-type reaction, typically occurs to the adult worm. The dying heartworm creates some nasty byproducts which result in severe lung injury that is often fatal. The dying worm may also cause the physical obstruction of a blood vessel thereby causing a thromboembolism (the formation of blood clots) or verminous emboli (the release of a worm or segment of worm). Anaphylaxis is the most likely cause of sudden death in cats due to heartworm disease. These reactions are possible whether the adult worm dies of natural causes or is killed by heartworm treatment protocols, therefore there is not an effective treatment for the elimination of adult heartworms in the cat.

Cats are generally considered to be a dead-end host for heartworms. Since disease in the cat is associated with low adult worm numbers or the response to larva before they reach maturity and are arrested in development, the number of reproductively active worms is extremely low. Infections may be severe with only one adult worm in attendance. Microfilaria, or immature baby heartworms, are not produced or are produced in extremely small numbers. Because microfilaria are not being produced, the mosquito, also known as the intermediate host, cannot pick up a heartworm infection from an infected cat while feeding upon it. In other words, a heartworm infected cat will not result in additional numbers of mosquitoes becoming carriers and capable of infecting additional cats or dogs. Mosquitos capable of transmitting heartworms must have also fed on a dog prior to feeding on a cat for infection to occur.

Two types of blood tests are available to detect heartworms in the feline patient. A heartworm antigen test detects proteins shed from adult female worms. The antigen test rarely results in a false positive result. False negative results are, however, common with antigen testing. Since cats with heartworm disease tend to have low numbers of adult heartworms present, the infection may be the result of a single female worm or a male worm infection. Depending on the antigen test used, a false negative result may be obtained in over 50% of cats actually suffering from heartworm infection. Idexx Snap® Feline Heartworm Antigen Test is one such antigen test that is easily performed at most veterinary clinics. This particular antigen test boasts an increase in sensitivity of 15% over other conventional antigen tests but still results in almost 35% of feline heartworm cases going undetected.

Serum antibody tests used for the detection of feline heartworm disease may be used as early as 1 to 2 months following exposure to heartworms. Antibody tests will detect up to 70% of all heartworm infections but have a wide range of false positive results, of up to 19%. An in-clinic feline heartworm antibody test is now available by Heska and is marketed as the Solo Step®.

The American Heartworm Society currently recommends that an antigen and an antibody test both be done in combination. When these two tests are combined the sensitivity is improved tremendously for the detection of feline heartworm infection.

Additional helpful tests in the diagnosis of heartworm disease include: thoracic radiographs, echocardiography and a CBC or complete blood count. Radiographic changes are seen in approximately 53% of cats with feline heartworms. The most characteristic radiographic criteria is when the left caudal pulmonary artery is demonstrated to be 6 times the size of the ninth rib at the ninth intercostal space (space between the ribs). Echocardiography may be expensive but will demonstrate adult heartworms in 78% of infected individuals as well as documenting the resulting pathology on the heart itself. A CBC, or complete blood count, may demonstrate an increase in the number of eosinophils and basophils (types of white blood cells) which are typically seen in allergic and parasitic infections.

Treatment to kill adult heartworms can be hazardous even to the mildly infected feline. Heartworm treatment is therefore primarily supportive in the feline patient. Prednisone and bronchodilators may be helpful in decreasing lung inflammations and coughing. Supplemental oxygen may be necessary during severe bouts of respiratory (breathing) crisis. Tetracycline use, for complicating bacteria called Wolbachia spp., is advocated in the hope of decreasing inflammatory mediators. Antileukotriene agents such as montelukast sodium may decrease mediators of the lung inflammation. Aspirin use is controversial and should not be used concurrently with corticosteroids such as prednisone. Surgical retrieval of adult worms has been tried, but the complication rate is high. Surgery typically results in a failure to retrieve the entire worm population and mortality levels of 25% or more are not uncommon.

In heartworm endemic areas, prevention is the best way to avoid infection. Currently in the United States a mere 4% of cats are given heartworm preventative with only two-thirds to three-fourths of those doses actually being given. It is deplorable that a completely preventable disease of this prevalence and severity is treated with such ambivalence. Several easy to use heartworm preventatives are available for use in the cat. Some of the most popular products currently being used are available for topical administration and are applied directly to the skin by parting the hair, preferably at the back of the head where the cat may not lick the medication off. Topical products currently available include Feline Revolution®, manufactured by Pfizer Animal Health, and Advantage Multi®, marketed by Bayer. Both of these products boast additional activity against a multitude of additional parasites including fleas, ear mites, and mange mites depending on the particular product being used. An oral product is also available from Merial and is packaged as a monthly flavored semi-moist’ chewy tablet called Heartguard® for Cats.

Monthly heartworm preventatives should be used regardless of whether a cat lives indoors or out. Testing for heartworms before administering a preventative is unnecessary. The risk of adverse reaction to heartworm preventative use in a heartworm positive feline is minuscule because of the low microfilaria burdens seen in cats. Adverse reactions to heartworm preventatives in the dog are attributed to dying microfilaria. Since microfilaria production does not occur in cat infections or occurs at such low levels, anaphylaxis does not occur when heartworm preventative is administered to a heartworm positive cat. Cats with heartworm infections should also be prescribed a monthly preventative to prevent an increase in their larva and adult worm burdens.

The median survival rate for cats diagnosed with heartworm disease is 1.5 years. The median survival rate for all heartworm infected cats that live beyond the first day of diagnosis is 4 years. Survival through the first day of heartworm diagnosis is critical. Most feline patients are in severe cardiac and respiratory distress when the diagnosis of heartworm disease if first made. When a cat survives the initial period of distress the prognosis improves.

References:

“Add Feline Heartworms to your Priority List”. Veterinary Economics. April 2008. P. 26.

“Antech Diagnostics News.” March 2007.

Atkins, Clarke. “Understanding Feline Heartworm Disease.” Parasite Developments. Proceeding from the 2007 NAVC Conference and the 2007 Western Veterinary Conference. Supplement to Veterinary Forum Vol. 24. No. 3 (A). March 2007. Pp. 2-7.

Dillon, Ray. “The Nature of Heartworm Disease in Cats: Different from Dogs.” Western Veterinary Conference. Parasite Developments. Proceedings from the 2007 NAVC Conference and the 2007 Western Veterinary Conference. Supplement of Veterinary Forum. Vol. 24 No. 3 (A). March 2007. Pp. 16-18.

Nelson, Charles. “Heartworm-Associated Respiratory Disease in the Cat”. NAVC Clinician’s Brief. May 2007. P. 15.

Richards, James moderator. “Heartworm –Associated Respiratory Disease in Cats.” A roundtable discussion supported by an educational grant from Pfizer Animal Health. A supplement to the Compendium for Continuing Education for Veterinarian. Advanstar Communications 2007. Pp. 2-13.

Rosenthal, Marie. “New Paradigm in Thinking about Feline Heartworm Disease.” Veterinary Forum. May 2007. P. 13.

Topics: heartworms, mosquitoes

Symptoms: coughing, dementia, seizures, vomiting

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