Heartworm infection has been a recognized disease in the veterinary world for many, many years but the focus has always been on our canine companions. Spread through the bite of a mosquito, heartworm larvae will continue to mature while migrating to the heart and pulmonary arteries. In dogs, the juvenile worms mature and develop into adult heartworms. If left untreated, these adult worms will reproduce leading to larger worm burdens within the dog as well as circulating microfilaria ("babies") in the vasculature. Tests to diagnose heartworms detect either antigen produced by the heartworms, antibodies produced by the dog in response to the heartworms, or a drop of blood, where microfilaria may sometimes be observed.
Cats are an aberrant host for heartworms, so the larvae will migrate to the heart and pulmonary arteries but most often die and do not establish a mature adult infection. The problem for cats arises from the death of these immature worms which sets up a HUGE inflammatory response in the lungs ofan infected animal. Clinical signs may include coughing or dyspnea, vomiting, neurologic signs (from aberrant migration of a larva), or most worrisome - acute collapse and death. The inflammation is often transient, but may last for 6-8 months. There are some cats (approximately 30% in one study) that may remain asymptomatic and can self-cure.
Due to the atypical infection in cats: very low worm burden, single sex infections, and lack of microfilaria: diagnosis is challenging. The heartworm antigen test lacks sensitivity because of the typical low worm burden and possibility of an all male infection (only the female produces the detected antigen). False negative antigen tests are therefore common; however, false positives are rare, so if positive, an active infection is indicated. Antibody tests just indicate prior exposure, so these are generally used as screening tests in asymptomatic cats. In symptomatic cats, combining the antigen and antibody tests give us the best chance of diagnosing heartworm disease. Chest radiographs (x-rays) and echocardiography (ultrasound of the heart) may also be used to aid in diagnosis.
If heartworm disease is diagnosed in dogs, there is an approved drug to kill the adult heartworms (called "adulticide therapy"). Unfortunately, adulticide therapy is not used in cats due to the high risk of death associated with it. Due to this, we often are left with supportive treatment - most commonly with corticosteroids.
Due to difficulty in diagnosing, non-specific treatment, and potential for acute collapse and death as the first (and only) sign of heartworm disease in cats, what is a cat owner living in South Carolina (where the state bird is the mosquito) supposed to do? Prevent the disease from ever occurring is what we recommend! Heartworm preventatives are labeled for use in cats and the two that we recommend are Revolution and Heartgard Plus.
- Revolution is a topical flea/heartworm/intestinal parasite preventative, which is often easier to administer to cats.
- Heartgard Plus is a flavored heartworm/intestinal parasite chew.
Both of these products are delivered to the pet monthly, and in the southeast United States, should be done year round. So, why risk an untreatable and potentially fatal disease for your cat, when you can prevent it completely with regular use of a heartworm preventative?
Find out more on the Know Heartworms website.