Dirofilaria immitis, commonly known as heartworm, is a parasitic roundworm causing a severe and potentially fatal disease in animals. This thread-like worm primarily affects dogs, residing within their heart and major blood vessels. The disease, dirofilariasis, can lead to significant health complications. While dogs are the primary host, other mammals like cats and ferrets can also become infected.
The Heartworm Lifecycle and Transmission
The heartworm lifecycle begins when a mosquito, acting as an intermediate host, bites an animal already infected with heartworms. During a blood meal, the mosquito ingests microscopic first-stage larvae (microfilariae) circulating in the infected animal’s bloodstream. These microfilariae develop within the mosquito, typically over 10 to 14 days, transforming into infective third-stage larvae (L3) under suitable temperatures. Temperatures below 14°C (57°F) halt this development, limiting transmission to warmer periods.
Once the L3 larvae have matured inside the mosquito, they migrate to the mosquito’s mouthparts. When this infected mosquito subsequently bites another susceptible animal, the infective L3 larvae are deposited onto the skin and actively enter the new host through the bite wound. Over several months, these larvae migrate through the host’s tissues, molting into the fourth larval stage (L4) and then immature adults (L5). After approximately 3 to 4 months post-infection, these immature adult worms reach the heart and pulmonary arteries, where they mature into adult worms, a process taking about 6 to 7 months from initial infection.
Symptoms and Health Complications of Heartworm Disease
Adult heartworms reside in the pulmonary arteries and the right side of the heart, growing to significant lengths (females up to 30 cm, males about half). Their presence and movement within these vessels cause irritation and inflammation, leading to thickening and narrowing of the pulmonary arteries and reduced blood flow to the lungs. This obstruction forces the heart to work harder, potentially leading to heart failure over time. Dogs with heartworm disease may exhibit a persistent, dry cough, reluctance to exercise, increased fatigue, decreased appetite, and noticeable weight loss. In advanced cases, fluid accumulation may cause a swollen abdomen or legs. Severely infected dogs might collapse or die suddenly, especially during physical exertion.
Heartworm disease presents differently in cats, often called Heartworm Associated Respiratory Disease (HARD). Cats are atypical hosts; most heartworm larvae do not survive to adulthood, with typical infections involving only a few worms. The disease in cats is primarily an inflammatory lung condition caused by immature worms arriving and dying in the pulmonary vessels and lung tissue. Symptoms in cats can mimic asthma, including coughing, wheezing, and rapid, shallow breathing. Cats may also experience vomiting, decreased appetite, or lethargy. Many infected cats show no symptoms, but some can experience sudden collapse or death, often triggered by the death of even a single worm.
Diagnosing and Treating the Infection
Diagnosing heartworm infection in dogs involves blood tests. The primary method is an antigen test, detecting specific proteins released by adult female heartworms into the bloodstream. This test identifies infections with one or more adult female worms, usually about five months after a mosquito bite. A microscopic test for microfilariae (first-stage larvae) can also be performed, indicating reproducing adult worms. Additional diagnostics like chest X-rays and echocardiography may assess the severity of heart and lung damage before treatment.
Treatment for heartworm disease in dogs follows a multi-step protocol. The process begins with a 30-day course of an antibiotic, such as doxycycline, to target Wolbachia bacteria within the heartworms, weakening them and reducing inflammation. During this period, a veterinarian also administers an approved heartworm preventative to eliminate newly acquired larvae and circulating microfilariae. Following this preparatory phase, a series of injections of an adulticide medication, melarsomine dihydrochloride, kills the adult worms. The most common protocol involves one injection, followed a month later by two more injections 24 hours apart. Throughout the treatment period, and for several weeks after the final injection, strict exercise restriction is necessary to minimize complications from dying worms.
For cats, there is currently no approved adulticide treatment available. Medications used to treat dogs can be toxic to cats or lead to severe, potentially fatal reactions. Management of heartworm in cats focuses on supportive care to manage clinical signs, often involving corticosteroids to reduce lung inflammation and bronchodilators to aid breathing. Some cats with severe disease may undergo surgical removal of worms, though this is a specialized and risky procedure.
Heartworm Prevention and At-Risk Animals
Prevention is a safer and simpler approach to managing heartworm disease than treating an established infection. Effective year-round preventatives are available through veterinarians. These options include monthly oral tablets (often flavored chewables) and topical liquids applied to the skin. Injectable medications providing six or twelve months of protection are also available, offering a convenient option. These preventatives eliminate heartworm larvae that have infected the pet within the preceding month or longer, before they mature into adult worms.
While dogs are the primary hosts, other mammals are also susceptible to heartworm infection and require protection. Cats, even those kept exclusively indoors, are at risk because mosquitoes can enter homes. Ferrets can also contract heartworm disease, and due to their smaller size, even a single worm can cause severe illness. Human infection with Dirofilaria immitis is rare, as humans are dead-end hosts, meaning larvae rarely fully mature and do not produce offspring.