Susceptibility
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The cat is a resistant, but susceptible host. Thus they can get heartworms, but it takes a greater exposure than in dogs.
Anywhere heartworm disease has been found in the dog, it has been observed in cats.
What to Look For
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The most common historical complaints in cats with clinical signs are coughing, dyspnea,
vomiting, lethargy, anorexia, and weight loss. Vomiting and respiratory signs are the predominate complaints in chronic clinical cases, although
it is unusual for an infected cat to exhibit both symptoms concurrently.
Vomiting tends to be sporadic. The etiology of vomiting in heartworm cats is unknown although the release of inflammatory mediators from the lungs which stimulate the chemoreceptor
trigger zone has been hypothesized. The vomitus generally contains food or foam and is rarely bile stained. Retching and severe paroxysmal vomiting is a rare historical findings. Heartworm disease in endemic areas
should be included in the differential diagnosis of chronic emesis in the cat. The most common respiratory complaints are coughing and intermittent dyspnea. Hemoptysis is occasionally
noted. The coughing can be in severe paroxysmal attacks. Periods of normalcy (days to weeks ) is often seen between episodes. Based on historical data,
the coughing is usually temporarily corticosteroid responsive with exacerbation during therapy. The clinical presentation, radiographic pattern, and response
to therapy often lead to a tentative diagnosis of bronchial asthma. The dyspnea may be a result of acute emboli formation especially associated
with worm death. On occasion, occlusion of a pulmonary artery (right caudal being the most common) is accompanied by a radiographic appearance of lung lobe consolidation and the development of life-threatening acute
dyspnea. The non-specific clinical signs are consistent with many feline diseases. Anorexia and/or lethargy can be the only presenting signs in
heartworm cats. In these cases, heartworm disease is often an incidental finding on thoracic radiographs during diagnostic screening. Cats with
worms found in abnormal locations may have signs attributable to local pathology. Neurological signs are uncommon but can occur in infected cats
with or without worms in the CNS.
Clinical Signs
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The initial clinical signs associated with early infections occur most frequently
in the late Fall and early Winter months (4-7 months after the exposure).
At this time, because the worms are immature, antigen tests are usually
negative. After the initial host response, the signs may abate and become
subclinical for a period of time. However, the subsequent death of adult
heartworms causes additional severe signs. Infected cats may die acutely,
exhibit chronic signs, or be asymptomatic. Based on cardio-pulmonary
changes and experimental studies, most heartworm cats even with severe
heartworm disease are asymptomatic once the infection becomes established.
In the acute cases, death may be so rapid as to preclude diagnosis or treatment. Sudden death has been attributed to circulatory collapse and respiratory failure from acute pulmonary arterial and acute lung injury. Acute collapse may occur with or without previous clinical signs. Cats which die from heartworms can be clinically normal 1 hour before death. All cats with peracute death in heartworm endemic areas should be examined for heartworm disease. In acute cases as few as 1 worm has been found accompanied by severe pulmonary congestion, infarction and edema. The worms in the acute syndrome are not always found embolizing the main pulmonary arteries.
Clinical Management
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After the diagnosis of feline heartworm disease, the veterinarian
and client are in a lose - lose proposition. One can let the adult worms
die on their own over the next several years and run the risk of continued
problems and on occassion an acute crisis. Or one can use an adulticide
and eliminate the worms and run the risk of acute complications associated
with the worm(s) dying all at once with severe consequences.
Mechanical removal of heartworms through surgery or special forceps and brushes has been successful. Mechanical removal should only be attempted in cats where worms have been demonstrated in the rights ventricle or pulmonary arteries by echocardiogram. The nature of feline heartworm disease to cause chronic vomiting, intermittent respiratory signs, or to be asymptomatic often misleads the client into thinking the disease is not severe. Spontaneous acute complications and death in a small percentage of cats can occur. Therefore, the client must be warned that withholding therapy can be lethal in a minority of cases. In the asymptomatic cat, this risk appears to be small compared to the complications of adulticidal therapy. Because the adult heartworm has a shortened longevity in the cat compared to the dog, the possibility of spontaneous recovery should also be discussed. However, the natural death of the adult worms can be associated with severe respiratory signs. Cats which have been managed conservatively by intermittent corticosteroid therapy have developed peracute signs and died from heartworm disease. In the cat with recurrent dyspnea that is life-threatening or with clinical signs that are unacceptable to the owner, adulticidal therapy has been used safely and should be considered. In the symptomatic cat, clinical signs tend to improve after therapy. However, anorexic cats may require hyperalimentation. Although the presence of circulating microfilaria is uncommon, ivermectins have both been used successfully as microfilaricides. Imidicide at the dog dose should not be used in cats.
Preventive Medication
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In endemic areas with vector populations (dogs) providing the mosquito with a reservoir,
the incidence of heartworms in cats indicates that preventive medication are needed. Infection from D. immitis in cats can
be prevented with the newly released feline product Heartgard for Cats, (24 mcg/kg of ivermectin; Merial Limited, Iselin, NJ, 07065).
administered per os once a month. In endemic areas, it is suggested that preventative medication be administered as early as 6 weeks of age and continued
for the life of the cat.
Because current antigen testing is inconsistent in cats, especially those with a low worm burden, antigen testing before instituting preventative therapy in an asymptomatic adult cat would not seem to be cost effective. Although heartworm disease may be of low incidence in many areas, the high rate of complications associated with feline heartworm disease makes preventative medication an attractive alternative. Sub-clinical signs of heartworms may precede the more obvious clinical syndromes of allergic lung disease in cats.
Heartworm positive cats may be safely placed on preventative medication. A positive antibody test reflects that the cat has been successfully infected and that the parasite has lived several months and may or may not have developed to be an adult. A positive antibody test does not preclude administration of preventative medication.
Although heartworm disease can be self-limiting in many cats, the potential to initiate inflammatory lung disease and predispose to bronchial asthma may prove to be adequate indications for preventative medications for cats in endemic areas.
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