Heartworm Disease

Dogs often show no signs early. The first are a mild persistent cough and reluctance to exercise, progressing to exercise intolerance, reduced appetite, lethargy and breathing difficulty, and eventually right-sided heart failure. Caval syndrome causes an acute, life-threatening heart failure. Cats may vomit, become less active, lose appetite and weight, develop respiratory signs from heartworm-associated respiratory disease, or die suddenly without earlier signs. Antigen tests detect infection only from about seven months after infection.

Heartworm disease is caused by the filarial nematode Dirofilaria immitis. Mosquitoes take up microfilariae from an infected dog, the larvae become infective in the mosquito within 10-14 days, and they enter a new host when the mosquito feeds. Larvae migrate through the body and arrive in the pulmonary arteries as young adults about six months later; females reach 10-12 inches. The worms depend on the endosymbiont Wolbachia. Young adult and adult worms cause a vascular disease of the pulmonary arteries that impairs blood flow, leading to pulmonary hypertension, compensatory right heart enlargement and eventually right heart failure. Heavy burdens can move into the right atrium, ventricle and vena cava (caval syndrome). In cats, even dying immature worms provoke severe pulmonary inflammation.

Dogs are the definitive host and ferrets are similarly susceptible. Cats are less readily infected, usually harbour only one to three small worms, and rarely have microfilariae. Infection occurs in tropical and temperate regions worldwide. A 2025 meta-analysis for North Africa and the Middle East estimated D. immitis prevalence at 2.7% across the dogs and cats sampled, and Dirofilaria spp. at 2.6% in dogs; its combined estimate of 2.4% fell to 1.0% once outliers were removed. It drew mostly on data from Turkey, Egypt, Algeria and Morocco, with competent Aedes, Anopheles and Culex mosquitoes present in the region. Transmission is only by mosquito bite; people cannot catch heartworm from their pets, though humans bitten by infected mosquitoes can develop usually symptomless lung nodules.

In cats, the respiratory signs closely mimic feline asthma, yet treatment and prognosis differ, so heartworm testing matters. In dogs, circulating microfilariae must be told apart from those of Dirofilaria repens, which causes subcutaneous rather than cardiopulmonary disease, and Acanthocheilonema reconditum; multiplex PCR distinguishes D. immitis from D. repens.

Pulmonary arteries, lungs, right ventricle and right atrium, vena cava, heart

Year-round preventive medication from the macrocyclic lactone class, given monthly or by injection every 6-12 months, with regular testing. Resistant heartworms have been reported in the Mississippi River region.

Untreated disease progresses to right heart failure; caval syndrome is fatal without surgical worm removal. Dogs get staged adulticide treatment with strict exercise restriction to limit thromboembolism. No FDA-approved treatment exists for cats.