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?

a black and white border collie dog face
Texas A&M Veterinary Medical Diagnostic Laboratory officials are reminding pet owners that with rising mosquito populations comes the need for testing and prevention against heartworms. (Michael Miller/Texas A&M AgriLife)

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 enzyme-linked 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.

TVMDL’s role in testing for heartworm disease

Most private veterinary clinics can perform heartworm antigen/ELISA testing in-house, Campbell said. TVMDL primarily conducts confirmatory heartworm testing with a different antigen test after these clinics have gotten an initial positive result.

Confirmatory testing is performed for several reasons. The first is to confirm a dog truly is positive for heartworms prior to undergoing treatment, because the treatment drug used for heartworms can have significant side effects and should not be used unless adult heartworms are present.

The second reason is to document heartworm prevention drug failure. Several manufacturers guarantee coverage of treatment for a dog that has been on their product but developed heartworms despite well-documented testing and dispensing of medication by a licensed veterinarian. There is evidence of a preventative-resistant heartworm variant, primarily in the Mississippi delta region of the U.S.

Lastly, sometimes test results can be unclear. TVMDL can offer a differing testing option to help support or confirm findings.

Although TVMDL serves a confirmatory role for most antigen heartworm tests, the agency typically conducts initial testing for the heartworm antibody test in felines. Due to the infrequency with which this test is needed in private practice on cats, many veterinary clinics do not perform this test in-house and defer testing to TVMDL.

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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