Heartworm disease is caused by the parasite Dirofilaria immitis and is a serious threat to dogs, cats, and ferrets as well as other wild animals. It is found in all 50 states of the United States and is spread through the bite of an infected mosquito.
Heartworm disease can have a range of signs from asymptomatic (no symptoms) on up through coughing, weight loss, exercise intolerance, lethargy and life-threatening signs of collapse, heart failure, or even death. Once the heartworm disease is diagnosed, usually starting with a blood test, the goal is to treat the adult parasitic infection and eliminate infective larvae in the bloodstream. The prepatent period, or the time of exposure from a mosquito bite to the time when the infection can be detected, of heartworm disease is approximately 6 months, and the life span of the heartworm is 5 or more years in dogs.
The life cycle in dogs begins with an adult heartworm producing offspring that circulate in the bloodstream of an infected animal. The offspring are known as microfilaria or the first larval stage (L1 stage). The microfilaria are ingested by the mosquito while it is feeding on a heartworm positive animal. The microfilaria undergoes two molts (growth phases) to the infective third larval stage (L3) inside the mosquito. The mosquito deposits the infective third larval stage onto the animal around the bite wound where the larvae then move into the animal’s body through the bite wound. The infective third larval stage matures to the fourth larval stage (L4) in the subcutaneous tissue of the animal over three to four days. Over the next fifty to seventy days, the fourth larval stage makes its way to the larger blood vessels and matures to the fifth larval stage (L5) or immature adult stage. The immature adult heartworms continue to migrate to the heart and lungs of the animal ultimately arriving in the pulmonary artery where they mature into the final adult stage. The mature female and male heartworms mate, resulting in the females producing the first larval stage of microfilaria as early as 6 months post infection from the mosquito. The life cycle continues when a mosquito takes a blood meal ingesting the microfilaria of an infected animal.1
Illustration of the Heartworm Life Cycle in the Dog courtesy the American Heartworm Society.
Although heartworm disease may be asymptomatic, damage is occurring to the pulmonary arteries and smaller vessels of the lungs where the heartworms live. Damage to the lungs and heart correlate with the number of heartworms present and the length of time the worms remain alive in the dog. Additional organs of the body such as the kidneys may be affected secondary to infection. In extreme cases, caval syndrome may develop. Caval syndrome is an acute and extremely serious event caused by a large number of heartworms pushing backwards out of the pulmonary artery, through the tricuspid valve of the heart and into the vena cava. This causes a rapid decrease in blood circulation through the entire body. With rapid surgical intervention, dogs suffering from caval syndrome will die.
Diagnosis involves a physical examination and diagnostic testing such as commonly used blood ELISA heartworm test. Heartworm ELISA tests detect proteins (antibodies) from the dog’s immune system, specifically created in response to structures in the reproductive tract of female heartworms. Examination of the dog’s blood for the presence of microfilaria and a thorough health history are also key elements for heartworm disease diagnosis. All dogs (and cats in endemic or other areas of high exposure) should be tested annually for heartworm disease. In the southern United States, blood samples from as many as 5-10% of heartworm positive dogs may test false negative on an ELISA heartworm test due to formation of antigen-antibody complexes requiring heat treatment of the sample for an accurate result.2 Radiographs or ultrasound may be used to identify heartworm disease or further determine heartworm related damage to the heart and lungs prior to treatment. Radiographic findings are not specific for heartworm disease and should not be used alone to diagnose the disease.
Heartworm disease is usually treatable in dogs; however, there is not an approved treatment for cats. The goals of heartworm treatment are to address the clinical signs and condition of the animal, eliminate the adult heartworms, and eliminate any circulating microfilariae to prevent spread of infection. Antibiotic treatments, such as tetracycline compounds, are frequently used to help increase the effectiveness of medicines used to kill the adult heartworms and circulating microfilaria. Treatment protocols can be found at CAPCvet.org or American Heartworm Society.3
Using a heartworm preventative is key to preventing heartworm disease. These products work to eliminate the infective third and fourth larval stages present in the pet. Year round administration of heartworm prevention is recommended to effectively protect the pet from heartworm disease. Ways to further protect your dog from mosquito bites are by using an approved topical repellent and by avoiding the outdoors at dawn and dusk when mosquitoes tend to be more active.
As always, your pet’s veterinarian is your best resource for learning more about heartworm disease and how to prevent it.
1. CAPC Companion Animal Parasite Council. (Last updated 01 Jan 2016) Heartworm. Retrieved from https://www.capcvet.org/guidelines/heartworm/.
3. American Heartworm Society. Current Guidelines for the Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis) Infection in Dogs. 2014. (https://www.heartwormsociety.org/images/pdf/2014-AHS-Canine-Guidelines.pdf accessed 15Aug2018)