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 such as asymptomatic meaning no symptoms, coughing, weight loss, exercise intolerance, lethargy and life-threatening signs of collapse, heart failure, or even death. Once heartworm disease is diagnosed with a blood test, the goal is to treat the adult parasitic infection. 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.
The life cycle in dogs begins with an adult heartworm producing offspring that circulate in the bloodstream of an infected animal. The offspring is known as microfilaria or the first larval stage. The microfilaria are ingested by the mosquito while it is feeding on a heartworm positive animal. The microfilaria undergoes two molts to the infective third larval stage inside the mosquito. The mosquito deposits the infective third larval stage onto the animal around the bite wound where the larvae migrates or moves into the animal’s body. The infective third larval stage matures to the fourth larval stage in the subcutaneous tissue of the animal over one to three 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 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 mature adult stage. The mature female and male heartworms mate and produce the first larval stage or microfilaria as early as 6 months post infection. The life cycle continues when a mosquito takes a blood meal ingesting the first larval stage or microfilaria of an infected animal.1
Illustration of the Heartworm Life Cycle in the Dog courtesy the American Heartworm Society.
Heartworm disease causes damage to the lining of vessels throughout the body including the pulmonary arteries and to the heart. Although heartworm disease may be asymptomatic, damage is occurring to the pulmonary arteries and lining of the vessels as long as heartworms are present. Damage to the heart and lungs correlate with the number of heartworms present and the longer 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 a fatal condition caused by a cluster of heartworms residing in the Vena Cava restricting blood flow to the heart from the various organs leading to extensive damage of the kidneys, liver, and heart. Early detection is crucial for any chance of recovery.
Diagnosis involves a physical examination, diagnostic testing such as a commonly used ELISA heartworm test which detects antigen or a special protein found in the reproductive tract of female heartworms, microfilaria filtration testing, and the animal’s history. All dogs (and cats in endemic or 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 diagnosis the disease.
Heartworm disease is treatable in dogs; however, there is not an approved treatment for cats. The goal of heartworm treatment is to address the clinical signs and condition of the animal, eliminate the adult heartworms, and eliminate any circulating microfilaria to prevent spread of infection to other dogs or back to the treated dog. Additional consideration for heartworm treatment protocol involves using an antibiotic to reduce lung damage associated with infection and to eliminate the bacteria heartworms are known to harbor. Treatment protocols can be found at CAPCvet.org or American Heartworm Society.1,3
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 is by using an approved topical repellent and by avoiding the outdoors at dawn and dusk when mosquitoes tend to be more active.
The American Heartworm Society, CAPCvet.org and your veterinarian all serve as excellent resources regarding heartworm disease.
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)