Does My Cat Really Need Heartworm Prevention?
The short answer to this question is yes, even if they live strictly indoors. Though cats are an atypical host for heartworms and bites from infected mosquitoes are less likely to result in a mature infection like we would see in dogs, (studies estimate about 10% of the incidence rate of canines) even immature infections involving 1-2 juvenile heartworms can cause devastating disease in our feline friends. And unlike canine heartworm disease, we do not have a well tolerated treatment for cats to clear a heartworm infection once present so prevention is key.
Like dogs, cats become infected by the bite of a mosquito carrying infective L3 heartworm larva (baby heartworms). Once in the blood stream, the larva travel to take up residence in the pulmonary arteries surrounding the heart and lungs where they mature into adult worms (think spaghetti noodles) and reproduce. While dogs generally show no symptoms of the infection until the worms are large enough to cause vascular compromise or a blockage of the blood vessels from the physical size of the adult worms in the arteries, the cat’s immune system mounts an acute inflammatory response to the worms immediately, way before they have fully matured and compromised blood vessels. The resulting inflammation causes significant lung damage and can lead to respiratory issues as early as 75-90 days following the mosquito bite. 
If the heartworms survive the initial inflammatory response to reach full maturity, they can still cause problems when they begin to die off naturally (usually after 1-2 years in the cat). As they die the worms are broken into small fragments which break loose and travel though bloodstream, running the risk of lodging (much like an embolus) and causing further inflammation and damage in other areas of the body. Clinical signs of heartworm disease in cats can range from vague symptoms like weight loss, lethargy, and periodic vomiting to more serious asthma-like episodes with coughing and labored breathing. Unfortunately due to the severity of the immune response caused be heartworm disease in cats, sudden death is often the only symptom seen.
Testing for heartworm disease is cats is also more difficult than in dogs.
While heartworm antigen tests (a protein produced by adult female worms) are very reliable to diagnose a canine infection, a similar test in cats often yields a false negative result due to the sparse amount of mature worms present in feline infections. An antibody test (checks for antibodies produced by the cat against the heartworms) indicates that exposure to heartworm larvae has occurred but does not always mean an infection is currently present. The American Heartworm Society currently recommends checking both an antigen and an antibody test in cats that are possible infected.
The silver lining to feline heartworm disease is that we have great options for heartworm prevention. There are several products available that when applied topically (to the skin) once a month not only protect against heartworm disease but also function as flea and intestinal parasite preventatives. These preventatives are dosed based on weight and should be started no later than 8 weeks of age as kittens are just as susceptible to infection as adult cats.
Heartworm disease has been diagnosed in all 50 states but is especially prevalent in the Mid South region. In cats it is difficult to diagnose, even harder to successfully treat, and often fatal. Prevention is the only option to ensure your feline friends are protected so if you have not yet, speak to your veterinarian to determine which preventative is best for your pet.
More information on feline heartworm disease can be found on the American Heartworm Society website.
