Feline Heartworm Disease
Heartworm infection takes place when a mosquito carrying infective, microscopic-size heartworm larvae, bites into a cat for a blood meal. The larvae then actively migrate into the new host and develop further as they travel through the subcutaneous tissue in the cat’s body. At about 3-4 months, they usually settle into the arteries and blood vessels of the lungs, where they continue to develop to sexual mature male and female worms (Dirofilaria immitis). The average time from when the microscopic parasites enter the host until the females develop into mature worms and produce offspring is approximately eight months and is referred to as the prepatent period. This is about one month longer than in dogs.
As adults, the heartworms can mate and the females can release offspring called microfilariae (pronounced: micro-fil-ar-ee-a) into the blood stream. The cycle begins again when a mosquito takes a blood meal from the newly infected cat and draws the microfilariae into its system.
Cats are resistant hosts of heartworms, and microfilaremia, (the presence of heartworm offspring in the blood of the host animal), is uncommon (usually less than 20% of cases). When present, microfilaremia is inconsistent and short-lived. Some cats appear to be able to rid themselves of the infection spontaneously. It is assumed that such cats may have developed a strong immune response to the heartworms, which causes the death of the parasites. These heartworms may die as a result of an inability to thrive within a given cat’s body.
Cats typically have fewer and smaller worms than dogs and the life span of worms is shorter, approximately two to three years, compared to five to seven years in dogs. In experimental infections of heartworm larvae in cats, the percentage of worms developing into the adult stage is low (0% to 25%) compared to dogs (40% to 90%).
However, heartworms do not need to develop into adults to cause significant pulmonary damage in cats, and consequences can still be very serious when cats are infected by mosquitoes carrying heartworm larvae. Newly arriving worms and the subsequent death of most of these same worms can result in acute pulmonary inflammation response and lung injury. This initial phase is often misdiagnosed as asthma or allergic bronchitis but in actuality is part of a syndrome now known as Heartworm Associated Respiratory Disease (HARD).
Which Cats Are Susceptible?
Although outdoor cats are at greater risk of being infected, a relatively high percentage of cats considered by their owners to be totally indoor pets also become infected. Overall, the distribution of feline heartworm infection in the United States seems to parallel that of dogs but with lower total numbers. There is no predictable age in cats for becoming infected with heartworms. Cases have been reported in cats from nine months to 17 years of age, the average being four years at diagnosis or death.
The clinical signs of heartworm infection in cats can be very non-specific, and may mimic many other feline diseases. Diagnosis by clinical signs alone is nearly impossible, but a cat may exhibit generic signs of illness, such as vomiting intermittently (food or foam, usually unrelated to eating), lethargy, anorexia (lack of appetite), weight loss, coughing, asthma-like signs (intermittent difficulty in breathing, panting, open-mouthed breathing), gagging, difficulty breathing (dyspnea) or rapid breathing (tachypnea).
Signs associated the first stage of heartworm disease, when the heartworms enter a blood vessel and are carried to the pulmonary arteries, are often misdiagnosed as asthma or allergic bronchitis, when in fact they are actually due to a syndrome newly defined as Heartworm Associated Respiratory Disease (HARD).
Some cats exhibit acute clinical signs, with disease often related to the organs where the adult heartworms are thriving. Occasionally such infected cats die quickly without allowing sufficient time to make a diagnosis or offer appropriate treatment.
Clinical Signs Associated with Feline Heartworm Disease
Heartworm infection in cats is harder to diagnose than it is in dogs and it is easy to overlook. Diagnostic tests have limitations, so negative test results do not necessarily rule out an infection. Antigen tests, for example, only detect adult female or dying male worms. Immature or male-only worm infections are rarely detected.
The diagnostic plan for heartworm disease in cats can include, but is not limited to, a physical examination, radiography (X-ray), echocardiography (ultrasound readings of the heart), angiocardiography (X-ray of the heart with injected contrast fluid), CBC (complete blood count), serologic testing (antigen and antibody study), microfilaria testing, and necropsy (after dealth).
The results of a physical examination may appear to be perfectly normal in cats infected with heartworms. Harsh lung sounds are a frequent abnormal finding and may be present in cats without any respiratory signs. The presence of a heart murmur or abnormal rhythm is uncommon. Only rarely, have there been reports of ascites (fluid in the abdomen), exercise intolerance and signs of right-sided heart failure. In cats, the primary response to the presence of heartworms occurs in the lungs.
In X-rays of cats, enlargement of the main trunk of the pulmonary artery, blunting and tortuosity (twists) of pulmonary arteries are not as common as in dogs. Typically, the pulmonary arteries are enlarged with ill-defined margins, most prominent in the lower lobes of the lungs. X-ray imaging is generally considered the single most useful test for confirming a diagnosis and for developing a prognosis (outlook for recovery).
Ultrasonography or echocardiography (noninvasive sound readings for visualizing areas within the body) can be useful for identifying heartworms in the heart, main pulmonary artery and close portions of its right and left branches. However, it is virtually impossible to visualize the extremities of the pulmonary arteries where heartworms, particularly young worms, are frequently found in cats. Visualization of heartworms by ultrasonography gives a definitive diagnosis of infection.
A non-specific angiocardiogram (X-ray after a contrasting fluid is introduced) is a relatively simple method of confirming a tentative diagnosis of heartworms. However, this catheterization method is not widely used due to its invasive nature.
The complete blood count (CBC) from cats with heartworm infection may demonstrate anemia, nucleated erythrocytes (immature red blood cells), eosinophilia (increase number of eosinophils, a type of white blood cell) and basophilia (increase number of basophils, a type of white blood cell). The absence of eosinophils does not rule out a diagnosis of feline heartworm infection, because eosinophils are usually present in circulation inconsistently, occurring four to seven months after infection and intermittently thereafter. The presence of basophilia increases the suspicion of heartworm disease.
Routine tests for microfilariae are not generally useful for identifying heartworm infection in cats. Microfilaremia is temporary in the cat because none are present when heartworms are in the immature adult stages. Microfilaria are also not present in a single sex infections or in obscure cases where the microfilariae are removed from circulation by antibodies produced by the host. In those instances, a negative test result does not rule out feline heartworm infection. However, if circulating microfilariae are found, it becomes an absolute confirmation that heartworm disease is present. No other types of circulating microfilariae, confusing the diagnosis, are found in cats in the United States.
Antigen tests detect specific antigens primarily found in adult female heartworms and are used routinely with much success to detect canine heartworm infection. Currently, tests are available in clinics, as well as at many veterinary reference laboratories. Several of these tests are approved for use in cats. Most commercial tests will accurately detect infections with one or more mature female heartworms that are at least seven or eight months old, but they generally do not detect infections of less than five months duration. Unfortunately, these highly specific tests lack the sensitivity to identify many infections in cats. They fail to detect infections of immature heartworms, infections with only male heartworms and some infections with only one or two adult female worms. However, when infection with adult female worms actually exists, antigen tests are more reliable than generally credited.
Antibody detection tests detect the body’s immune response to heartworms, and are currently available for routine screening of feline heartworm infection, either as in-clinic tests or at many reference laboratories. Antibody production occurs even if the heartworms do not complete their development. Antibody-positive cats are either currently infected or have been infected previously with Dirofilaria immitis. Such cats may or may not show clinical signs compatible with the disease, depending upon the age of the infection, the stage of the disease and whether or not the cat has spontaneously rid itself of the parasites. Therefore a positive antibody test may only indicate a previous infection with Dirofilaria immitis antigens , and not necessarily an existing heartworm infection. Antibody detection tests are only as specific as the antigen used to detect antibody. The use of a purified, recombinant (genetically engineered) heartworm antigen leads to improved test results.
Since both L5 larvae and adult worms are capable of causing clinical disease in the cat, both antibody and antigen test are useful tools and when used together increase the probability of making appropriate diagnostic decisions.
Comparison of Antigen and Antibody Tests for Cats
|Antigen tests||Antibody tests|
|Specificity||Virtually 100%||Virtually 100%|
|Age of infection when first detected||Approximately seven to eight months||As early as 50-60 days|
|Test detects||Adult female heartworm antigens||Antibodies to specific heartworm antigen|
|Detects only male heartworm infections||No||Yes|
|Useful as an initial screening test||Yes, when looking for adult infections||Yes, when looking for immature infections|
|Detects infections with small adult heartworm burdens||Only if older female heartworms are present||Yes|
|Useful to estimate infection prevalence in a region||No, it underestimates prevalence||Yes, a positive test indicates the cat was successfully infected at some point|
|‘Positive’ confirms presence of heartworms *||Yes||No|
|‘Negative’ confirms absence of heartworms **||No||Yes, with approximately 50-90% accuracy|
|* A positive antigen test is indicative of worms in the heart. It is possible that a cat could clear the infection and circulating antigen would remain detectable for weeks after clearance. A positive antibody test indicates the cat has been infected and does not mean an adult worm is always present in the heart. Antibody probably remains elevated for weeks or months after clearance. ** Antigen tests are not sensitive enough to consider a negative as indicative of absence of heartworms. Current research indicates antibody test have a 25% (50-90%) false negative rate.|
Currently, there are no products in the United States approved for the treatment of feline heartworm infection. Most cats with heartworm infection that are not demonstrating clinical signs are allowed the time for a spontaneous cure to occur. If there is evidence of disease in the lungs and their blood vessels consistent with feline heartworm infection, such cases (possibly in the early stage) can be monitored with chest X-rays every six to twelve months as needed. Supportive therapy with small, gradually decreasing doses of prednisone (a cortisone-like drug) is recommended for cats with radiographic or clinical evidence of lung disease.
Cats with severe manifestations of feline heartworm disease may require additional supportive therapy, and may benefit from intravenous fluids, oxygen therapy, cage confinement, bronchodilators (which expand the air passages of the lungs), cardiovascular drugs, antibiotics and nursing care.
Heartworm extraction with various surgical devices has been performed in cats in which the worms can be visualized with ultrasound at the tricuspid valve or in the right atrium (of the heart), and especially in those rare instances of caval syndrome (obstruction of blood flow affecting the heart and the liver.
It is generally recommended that all cats be tested for both antigens and antibodies (serology) prior to administration of a heartworm preventive. There are four heartworm disease preventive products approved by the FDA for use in cats, Heartgard® for Cats (Ivermectin, orally) from Merial, Interceptor® (Milbemycin oxime, orally) from Novartis, Revolution® (Selemectin, topically) from Pfizer and Advantage Multi™ for Cats (Moxidectin / imidacloprid, topically) from Bayer. All of these products are considered effective in preventing the development of adult heartworms when administered properly on a monthly basis relative to the period of transmission.