Feline Heartworm Disease | Rochester MN

Although heartworm disease has routinely only been discussed or considered in dogs, there is growing evidence that cats are also affected by the heartworm parasite. There are many differences between heartworm disease in cats and dogs, which will be discussed in this handout.

Prevalence

1. 28% of cats diagnosed with heartworm disease were considered indoor-only cat
2. Necropsy surveys of shelter cats estimate ADULT heartworm infections at 5-15% of the rate in dogs (immature larvae only reach adulthood in cats 10% of the time)
3. It is difficult to estimate disease incidence. Disease in cats usually occurs with immature heartworm infections (HARD- heartworm associated respiratory disease) and does not require the heartworms to reach maturity to cause damage. Necropsy surveys only count adult heartworms and serologic blood tests are not always easy to interpret, further complicating the process of achieving an accurate disease estimate.

Signs of Disease - Vary greatly

1. Respiratory tract disease signs are most common (breathing difficulties, coughing) also known as HARD (heartworm associated respiratory disease) caused by immature heartworm larvae
2. Vomiting is the second most common sign
3. Sudden unexplained death with no clinical signs of disease
4. Heart disease is not usually seen in cats with heartworm disease
5. Any general signs of disease are possible: neurologic, appetite loss, weight loss, weakness, collapse, seizures, etc. (some worms can migrate in cats, which is not usually ever seen in dogs)

Diagnosis - Complicated and confusing

1. There is no “gold standard” for testing in cats like the antigen heartworm blood test in dogs
2. Blood tests - antigen (detects worm) and antibody (detects cats’ response to worm) are both available but not completely reliable
3. Antigen test-detects at least 7-8 months after infection
   1. positive result- is diagnostic
   2. negative result- inconclusive
   3. detects only the female adult worm reproductive tract (at least 7-8 months old)
   4. if a male only population of worms is present, will test negative
   5. cats usually have a very low worm burden (usually 1-4 worms)
   6. therefore this test does not detect immature heartworm infections, male only infections, and some low count female worm infections
4. Antibody test- detects at least 60 days after infection
   1. detects exposure to heartworms- immature or adult worms
   2. can detect past or current exposure
   3. positive test- does not confirm a diagnosis of heartworm disease (see b.)
   4. negative test- confirms absence of heartworms with only 50-90% accuracy
5. Physical exam of a cat with heartworm disease
   1. possibly harsh lung sounds are ausculted
   2. often no signs of disease found
   3. heart murmur or arrhythmia are not commonly found
6. X-ray, cardiac ultrasound, blood counts (CBCs)
   1. x-ray may show enlarge pulmonary arteries- not reliable nor diagnostic
   2. cardiac ultrasound may show heartworms in the vessels or heart- not reliable, but diagnostic if found
   3. CBCs may find eosinophilia, or basophilia- not reliable nor diagnostic

Prevention

Monthly heartworm prevention given once monthly either year round or seasonally (May 1st- Dec 1st) in all cats at least 8 weeks of age or older. Outdoor cats are obviously more at risk but 28% of diagnosed cats were considered to be indoor only cats so indoor cats are at risk as well.

Testing

Testing is not necessary before administering monthly heartworm preventative since the worm burden is low in cats and microfilaremia (the presence of “baby heartworms )in cats is uncommon. Since the adult heartworm burden is so low, there is usually no reproduction of the heartworm within the cat’s body. Therefore, pretesting before putting the patient on monthly prophylaxis is thought to be unnecessary.

The only reasons to test for heartworm disease in cats are:

1. when suspecting a case of heartworm disease
2. to monitor a case of heartworm disease
3. to establish a baseline reference prior to starting monthly propylaxis

Treatment

There is no known treatment for cat heartworm disease, except supportive care, depending on the status of the patient. Steroids are often utilized to suppress inflammation of the lung tissue caused by the heartworm parasite.