Speakers discuss complexities of heartworm disease

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When it comes to preventing heartworm disease in dogs, most veterinary professionals say their biggest concern is owner compliance with preventive administration. A smaller number say their biggest concern is the possibility that some heartworms might be resistant to currently available preventives.

That's according to an electronic audience poll conducted during the first of several well-attended sessions that the American Heartworm Society presented Jan. 15 at the North American Veterinary Conference in Orlando, Fla. Drs. Clarke E. Atkins, a professor at North Carolina State University, and Matthew W. Miller, a professor at Texas A&M University, spoke about diagnosis and treatment of heartworm disease as well as prevention.

Microfilaria testing is back, Dr. Atkins said. The AHS recommends testing for microfilariae as an adjunct to testing for antigens from adult female heartworms. Microfilaria tests can validate the results of antigen tests, identify patients that are reservoirs of infection, and predict adverse reactions to administration of heartworm preventive as a microfilaricide.

Dr. Atkins noted that antigen testing will not detect an infection until about six months afterward.

"We need to have realistic expectations here," Dr. Atkins said. "Turning the test positive does not mean that we have a life-threatening infection."

Even if an adulticide cannot clear the infection, he said, the adulticide will reduce the infection substantially. The AHS recommends antigen testing again six months after administration of an adulticide.

Dr. Atkins suggested that the pre-treatment assessment should include an antigen test, microfilaria test, thoracic radiography, complete blood count or platelet count, chemistry panel, and urinalysis.

Dr. Miller said some dog owners cannot afford the entire work-up. He said thoracic radiography is useful, but he cautioned that the radiographs cannot always predict the risk of complications with adulticide therapy.

In spite of all we know about heartworm—and our ability to diagnose and prevent it—we have not been successful in substantially decreasing incidence of heartworm disease.

Dr. Wallace Graham, president, American Heartworm Society

"We have to be really careful about overinterpreting radiographic evaluation," Dr. Miller said. "Whether we recognize it or not, I think we convey that information to our client, directly or indirectly."

The AHS recommends administering melarsomine as an adulticide, unless unavailable because of a shortage. Dr. Miller said heartworm preventives will kill adult heartworms, but exceedingly slowly.

The AHS also acknowledges doxycycline as having a role as an adjunct treatment, Dr. Atkins said. Doxycycline can reduce the number of Wolbachia organisms, a type of endosymbiotic bacteria inside heartworms, helping to kill the heartworms.

According to the AHS, macrocyclic lactones are nearly 100 percent effective at preventing heartworm disease with correct usage. An audience poll, however, found that almost a third of veterinary professionals believe they have observed lack of efficacy despite correct usage.

The Food and Drug Administration announced in 2005 it had been receiving an increasing number of reports of lack of efficacy of heartworm preventives, and pointed out that the increase could result from growing recognition of existing lack of efficacy. Several recent studies have found that the MP3 strain of heartworm is less susceptible to preventives.

Using Merial's "window of infection" tool at www.heartwormedu.com, Dr. Atkins analyzed a number of the lack-of-efficacy claims. For 77 percent of claims, dogs had a period during the window of infection when their owners did not purchase preventives. Only 3 percent of claims appeared to have no possible prevention gaps.

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American Heartworm Society launches educational initiative

The American Heartworm Society has developed a new educational initiative to help veterinary teams and dog and cat owners implement the society's recommendations for reducing the incidence of heartworm infection among pets.

The AHS launched the initiative in late January. Earlier in the month, during the North American Veterinary Conference, the society announced updates to its heartworm guidelines.

"In spite of all we know about heartworm—and our ability to diagnose and prevent it—we have not been successful in substantially decreasing incidence of heartworm disease," said Dr. Wallace Graham, AHS president. "We must do more to help veterinarians and their clients follow our guidelines for heartworm screening and prevention."

Through the educational campaign "Think 12 in 2012," the society will help veterinarians place information on year-round prevention in the hands of pet owners. The campaign features a resource center at www.heartwormsociety.org to provide veterinary professionals with heartworm information and updates as well as educational materials to share with pet owners. The goal is to keep annual heartworm testing and 12-month prevention top of mind.

Each month, the AHS will produce new program materials—including client handouts, case studies, and scientific articles. The society encourages veterinarians to pledge their support to the "Think 12" mission.