by Suzanne Morris, DVM
Since it is still February, there is still time for one more cat heart topic: Heartworms!
While heartworm infections and heartworm disease have been thought of primarily as a disease of dogs, the parasites can (and do) affect cats, albeit at 5-20% the rate of dog infection for any given geographic location. Heartworms tend to infect dogs at a greater rate simply because the dog is its definitive host, which means that the dog is the host of the parasite’s sexually active adult phase. However, it is possible for the heartworm to reach sexual maturity in the cat, although with apparently greater difficulty. Cats seem to have a greater resistance to infection than do dogs: less than one quarter of heartworm larvae entering the cat become adult heartworms and the life span of the adult worm in the cat is about half of the 5-7 years it can live inside of a dog. However, heartworm disease (HWD) can have dire consequences and prove more problematic to treat in the feline patient.
Heartworms are transmitted by mosquitoes, who carry the larval stage of the parasite, also known as microfilariae. The larvae must undergo two molts in the mosquito before they reach their infective stage. At this point, when the mosquito takes a blood meal from a cat, the larvae enter the cat’s skin and migrate into the cat’s bloodstream. Immature adult heartworms settle in the cat’s pulmonary arteries where they mature into a reproducing adult. During that period the heartworm can elicit an inflammatory response from the cat’s immune system, which can escalate as the adult worms reach the end of their life stage and begin to die. The death of the heartworm(s) is the most dangerous period for the host cat, as the dying worms can also lead to the host’s sudden death in an event similar to an anaphylactic reaction. If the cat survives the death of the heartworm(s), there can be permanent lung damage and/or chronic respiratory disease resembling other chronic diseases such as asthma.
For the majority of feline hosts in whom the heartworms never reach sexual maturity, it is still possible for the cats to develop pulmonary and vascular lesions from their exposure and can also have clinical (usually respiratory in nature) signs. In studies, the majority of cats exposed to heartworms are asymptomatic and a majority of those cats have been shown to clear their infection.
How can you find out whether your cat has heartworm disease? Testing for heartworms can be problematic: one blood test detects a protein (antigen) derived from the female heartworm’s uterus, another detects antibodies that the cat has generated from an exposure to heartworms. There are deficiencies with either test—the antigen can only detect mature female worms (the result would be negative if, for example, the worm happens to be male). The antibody test detects whether the cat has been exposed to heartworm (but may have since cleared the infection). The two tests are usually performed simultaneously to benefit from the information that each test provides. Other tests include an ultrasound (echocardiogram) of the heart, which enables the examiner to visualize inside of the patient’s pulmonary arteries. Chest x-rays can also reveal signs consistent with heartworm disease.
Treatment of dogs with HWD typically involves doses of doses of heartworm adulticide (designed to kill the adult worms). However, this treatment method is toxic to and can cause death in cats and is therefore not recommended. Continued, routine administration of heartworm preventative, though, is recommended. To decrease inflammatory responses to the heartworm, steroids are also often part of the treatment for cats. Monitoring with follow-up testing is then recommended every 4-6 months.
To protect your cat’s heart from these parasites, talk to your veterinarian about feline heartworm preventative.
Rothrock K, and Palermo V. VIN Associate Dirofilariasis (Heartworm Disease), https://www.vin.com/Members/Associate/Associate.plx?from=GetDzInfo&DiseaseId=835 (2017, accessed 12 Feb 2019).
Litster AL, Atwell RB. Feline Heartworm Disease: A clinical review. Journal of Feline Medicine and Surgery 2008; 10: 137-144.