Rising mosquito activity heading into the summer months poses an increased risk of transmitting heartworm disease to dogs, which can lead to damage of the heart, lungs and arteries if left untreated.
Cathy Campbell, DVM, veterinary diagnostician at the Texas A&M Veterinary Medical Diagnostic Laboratory, TVMDL, in Bryan-College Station, explains the heartworm life cycle, the best time for testing, and tips for prevention.
What are heartworms and how are they transmitted to dogs?
Heartworm disease is caused by the parasitic worm Dirofilaria immitis and is transmitted to animals through mosquitoes, according to the American Heartworm Society. Mature heartworms can live within dogs for up to seven years.
“When a mosquito happens to bite an animal infected with heartworms, it ingests microfilariae, or baby heartworms, through a blood meal,” Campbell said.
Those microfilariae morph into larvae inside the mosquito in 10-14 days. That infected mosquito tends to fly around and bite a dog, transmitting the larvae to a new host. Once transmitted, the larvae enter the dog’s bloodstream and migrate to the heart.
“Larvae set up housekeeping in the heart and grow into sexual maturity in six to seven months,” Campbell said. “Mature worms produce microfilariae, which are released into the bloodstream where they await another mosquito to come and bite the dog, take up a blood sample and continue the cycle.” It takes six to seven months in the heartworm’s life cycle before they can be detected on a test. Therefore, most veterinarians recommend testing dogs for heartworms around 6 to 7 months of age.
Testing to detect heartworms in dogs
TVMDL offers two different approaches to testing.
The first approach is to detect the baby heartworms produced by adult heartworms in the heart. TVMDL’s clinical pathology section can detect microfilariae using either a filter method or through a modified Knott’s test.
However, not all heartworm infections produce baby heartworms, so it is best to include an “occult” heartworm test using an enzymelinked immunosorbent assay, ELISA — the second testing approach.
Technicians at TVMDL use the ELISA test to detect antigens associated with pregnant female heartworms. A positive result indicates adult female heartworms are present. This is the primary testing method to detect heartworms. Prevention recommendations
TVMDL encourages veterinarians to educate their clients on the benefits of giving year-round heartworm preventatives to both their canine and feline patients and supports annual testing of canines.
“Texas’ mild climate lends toward year-round mosquito activity,” Campbell said. “Missing a dose of heartworm preventative, or even administering a dose late, may allow a window of opportunity for mosquitoes to infect pets.”
For additional information on heartworm disease, visit the American Heartworm Society. For more information on testing, visit TVMDL’s testing services or call the TVMDL laboratory nearest to you.
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