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Dog Owners Encouraged To Be Aware Of Heartworm Risk As Temperatures Rise

Risk of heartworm transmission rises as mosquitoes become more active

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 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."

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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.

PHOTO: 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)

Protecting Your Pets From Heartworm

April brings more daylight, rising temperatures and people and pets spending more time outdoors. With more time outdoors comes an increased health risk to pets' health, making it an ideal time to proactively check-in on a pet's heartworm status and preventatives.

Best Friends Animal Society, a leading national animal welfare organization working to end the killing of cats and dogs in America's shelters in 2025, encourages pet owners and prospective adopters to educate themselves this Heartworm Awareness Month.

Heartworm is spread through mosquitoes, with many cases occurring near large bodies of water, but it is increasingly becoming present in every state.

"The most common signs of heartworm in dogs are coughing, exercise intolerance, collapsing or fainting episodes, decreased appetite and weight loss," said Dr. Erin Katribe, Director, National Veterinary Programs, Best Friends Animal Society. "Infection is less common in cats, but the disease can present as an asthma-like cough."

Katribe offered the following five facts to help educate pet owners on heartworm disease prevention:

• Keep dogs and cats on a chosen preventative year-round, regardless of location, because heartworm disease is becoming more rampant across the country.

• Even if a dog tests negative without being on preventatives, they could still test positive in six months as it takes that long to develop and be visible on standard screenings.

• If a dog does test positive for heartworm, it's important to understand that the majority of dogs go through treatment without any complications. The treatments used today are much safer than those used in previous generations, so heartworm should no longer carry a stigma.

• Many shelters that are struggling with overcrowding across the south are the same shelters with a high prevalence of heartworm disease. Sadly, heartworm positive dogs are the first to be killed when capacity becomes an issue, so they're an at-risk population in shelters. The great news is that heartworm isn't directly transferable to other pets in the household, so there is no reason not to foster or adopt a heartworm positive dog.

VETERINARY VIEWPOINTS: Prevention Is Key In Heartworm Disease

As we all anticipate warmer weather and enjoying the outdoors with our pets, it is important to remember to protect your dogs from parasites. One of the parasites that dogs need protection from is Heartworms.

Heartworms are parasitic worms that infect dogs and live in the pulmonary artery, the vessel that takes blood from the heart to the lungs, and sometimes in the heart itself. They are spread by mosquito bites and can be found in all 50 states.

Dogs infected with heartworms may not show any symptoms early on. As the disease progresses, they may develop symptoms which can include weight loss, lethargy, coughing, difficulty breathing, and heart failure. Chronic, untreated heartworm disease can be fatal.

All dogs are at risk of becoming infected with heartworms, but there are a few things that put your pet at increased risk. These include:

• Not being on heartworm prevention

• Living in or visiting the southern and southeastern united states

• Spending more time outside, especially in warmer months when mosquitoes are more active

• Being near areas where mosquito populations are high (e.G., near bodies of water or stagnant water, which is where mosquitoes reproduce)

While certain environmental factors can increase a dog's risk of exposure to mosquitoes and contraction of heartworms, it is important to note that avoiding these environmental factors is not enough to prevent heartworm infection. Mosquitoes do come indoors. And, even though Oklahoma has cold winters, we do have enough unseasonably warm days during the winter for mosquitoes to be active and spread heartworms.

The best thing you can do to protect your dog from heartworms is to keep them on year-round heartworm prevention. All heartworm prevention medications are in the same drug class, macrocyclic lactones. However, these medications come in a variety of formulations to meet your needs and your pet's lifestyle. They can be monthly oral products, monthly topical products, or injections given either every 6 or 12 months.

Heartworm infections can be treated, but even with treatment there can be lasting impacts from the infection. And treatment is difficult, time consuming, and often costly. It is certainly easier on you and your pet, and usually cheaper, to prevent heartworm infection than it is to treat it.

If you have questions about heartworm disease, prevention, or treatment, the veterinarians at OSU Veterinary Teaching Hospital are happy to help. We can be reached at 405-744-700.

Veterinary Viewpoints is produced by Oklahoma State University's College of Veterinary Medicine.

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