April 11, 2018

Heartworm: Where Are We Today?

Experts share their insight on why this disease is on the rise, whether resistance is a concern, and fresh approaches to prevention.
By Amanda Carrozza

Preventive Resistance

Although instances are not widespread, Dr. Little noted that veterinarians should be aware that resistant biotypes of D immitis have been confirmed. “We don’t have a feel yet for what percent of heartworm cycling in nature is resistant to the preventives,” she said, “and I don’t think anybody has been able to determine that...but resistance does happen.”

A veterinarian might suspect resistance if a dog whose owner has been compliant with giving a monthly preventive becomes infected. However, Dr. Little warned that it’s important to keep in mind that in such instances the dog may have vomited or spit out the tablet, or a topical product may not have been absorbed entirely. Resistance isn’t always the chief reason.

The best indication of a resistant isolate is when a veterinarian treating a dog for heartworm is unable to clear microfilariae with high doses of macrocyclic lactones. In these cases, Dr. Little recommends seeking assistance from a veterinary parasitologist at a local teaching hospital or calling a national specialist. “There are other strategies to clear the microfilariae,” she said, “but we definitely want to document resistance too.”

Globally, preventives are still largely effective. “The most common reason a dog gets infected with heartworm despite the fact that a veterinarian prescribed a preventive is that the dog wasn’t given the preventive,” Dr. Little said.

The official AHS statement on resistance echoes Dr. Little’s explanation.6 “Every compound currently marketed in every form of administration (oral, topical, and parenteral) has been shown to be less than perfect in at least 1 study. However, while the evidence indicates that resistance affects all macrocyclic lactones, differences in active ingredients, doses, and product formulation among the available preventives can result in varying rates of failures.” Perhaps most important, AHS reiterates that research findings do not demonstrate any widespread ineffectiveness of available heartworm preventives. And macrocyclic lactones continue to be successful in the vast majority of cases.

If anything, Dr. Little said, the notion of possible resistance further emphasizes the importance of regular heartworm testing.

Testing By Pretreating the Sample

The standard strategies of testing for heartworm—both looking for microfilariae and antigen testing to detect the adult female worm—are considered highly sensitive and very effective in diagnosing infection. But these strategies are not without imperfections.

As Dr. Little explained, “We know that as many as 50% of dogs that are infected with heartworm won’t have that larval stage in their blood, that microfilariae.” Additionally, in antigen testing, sometimes the protein is present in the animal but is masked from detection. This block is believed to be attributed to immune complex formations that trap the antigen.7

To combat false-negative results, Dr. Little said she has returned to the lapsed method of pretreating the sample prior to testing, via heat or the addition of an acid or a pepsin. This works to disrupt the immune complexes and reveal the protein. “Pretreating the samples was used routinely in the 1980s until about 1990,” she explained. It was not used again until around 2014, “when we realized there were many dogs that were infected with heartworm but didn’t have the antigen—the protein—in their blood. So we started pretreating again on subsets of dogs to try to figure how common it was, and realized that in animal shelters in the South, as many as 5% to 10% of dogs will be false-negative on a heartworm antigen test; whereas if you pretreat, they’ll become positive.”

Dr. Little recalled a particular instance in which a local working dog had so many heartworms that a cardiologist was able to see them while performing an echocardiogram, but heartworm tests came back negative. Once a sample of the dog’s blood was pretreated prior to testing, however, it revealed a strong positive.

While effective at unmasking previously undetected antigens, Dr. Little said, pretreating samples is not necessary in all cases. For one, it is a very involved process that requires a larger blood sample. And because many clinics don’t have the equipment necessary to spin the sample at the required centrifugal force, samples usually need to be sent to a lab for proper testing.

Instead, Dr. Little said pretreating the sample is recommended when a veterinarian strongly suspects heartworm infection even though previous tests were negative. It may also be effective in animal shelters, where dogs should be tested for heartworm to the best degree possible prior to being adopted.

Drastically Reducing Incidence Rates

Even with improved research and additional testing, experts report that one of the major obstructions—if not the chief hurdle—standing in the way of eliminating heartworm infection is pet owners failing to abide by preventive guidelines. Simply put, “we have a compliance problem,” said Christopher J. Rehm, Sr, DVM, president of AHS. And one of the major thrusts of AHS is to increase the number of pets that are being protected with heartworm preventives on a year-round basis, he explained. “Right now, roughly two-thirds of pets are not on prevention,” he said.

Although veterinarians can’t physically oversee the administration of preventive treatments for every patient, they (and their teams) can help educate clients and engage in conversations about proper heartworm prevention.

“Sometimes I think we take for granted that our clients know more about it or have some more parasite common sense, if you will, than they actually do,” Dr. Rehm said. “And we don’t probe enough to find out where our clients are on the learning curve about parasite prevention.” To counteract this, he recommends that veterinarians ask open-ended questions, such as “When did you last give a parasite preventive?” rather than simply “Are you good on heartworm?” The latter question might elicit a knee-jerk, affirmative reaction.

“I think sometimes people have a false sense of security that they think their dog won’t get heartworm or, if it does, we’ll just treat it, and that’s just absolutely the wrong approach,” added Dr. Little. “It’s so much better to prevent the infection to start with, instead of trying to rescue the dog once it is infected.”

  1. History of discovery. Stanford University website. web.stanford.edu/group/parasites/ParaSites2006/Dirofilariasis/History.htm. Accessed February 14, 2018.
  2. Survey of U.S. veterinarians uncovers per-practice increases in heartworm-positive cases. heartwormsociety.org/newsroom/in-the-news/347-ahs-announces-findings-of-new-heartworm-incidence-survey. American Heartworm Society website. Published April 20, 2017. Accessed January 25, 2018.
  3. Mosquito-borne diseases. World Health Organization website. who.int/neglected_diseases/vector_ecology/mosquito-borne-diseases/en/. Accessed February 14, 2018.
  4. McCall J, Varloud M, Hodgkins E, et al. Shifting the paradigm in Dirofilaria immitis prevention: blocking transmission from mosquitoes to dogs using repellents/insecticides and macrocyclic lactone prevention as part of a multimodal approach. Parasit Vectors. 2017;10(Suppl 2):525. parasitesandvectors.biomedcentral.com/articles/10.1186/s13071-017-2438-4. Accessed March 15, 2018.
  5. Learn about heat islands. US Environmental Protection Agency website. epa.gov/heat-islands/learn-about-heat-islands. Accessed February 14, 2018.
  6. American Heartworm Society resistance statement. American Heartworm Society website. heartwormsociety.org/images/pdf/Resistance_statement_FINAL.pdf. Accessed February 14, 2018.
  7. Pre-treatment of samples to improve detection of heartworm antigen. Oklahoma State University website. cvhs.okstate.edu/sites/default/files/docs/pdf/Improved%20Heartworm%20Testing.pdf. Published June 21, 2016. Accessed January 30, 2018.

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