What's new in heartworm disease?

In the March edition of DVM Newsmagazine, I wrote an article titled "Heartworms: An Emerging Disease." The premise behind the article was that all the factors associated with the transmission of heartworm infections are constantly changing. Man-made changes to the landscape and environment have created suitable habitats for mosquitoes. That, coupled with the steady influx of heartworm positive dogs from endemic areas to traditionally non-endemic areas, has fueled the methodical march of this deadly parasite into the northern and western states. And despite the plethora of heartworm preventives available, there are a million new cases of heartworm infections every year in the canine population. Of course this isn't a new development — it has been going on for years. But we do have new knowledge concerning the biology of this parasite and how that can affect the prevention and treatment of heartworms. I remember a professor telling my graduating class from vet school that half of what we currently know will be obsolete in five years. That statement certainly rings true when I look back on what has been learned about Dirofilaria immitis over the past few years.

Before - standard treatment: A histopathology slide depicting pulmonary parenchyma with a standard melarsomine treatment.

In March, the American Heartworm Society hosted the Thirteenth Triennial Heartworm Symposium in Memphis, Tennessee. As in the past, this meeting brought together the leading heartworm researchers from the United States and abroad for several days of presentations and poster sessions revealing the latest information on heartworm disease. The first session was denoted as "Continuing Worldwide Spread" and several presentations documented the broadening of heartworm endemic territories in Europe. There was also a talk on the "Katrina Effect" as approximately 40,000 dogs from the highly heartworm endemic area of New Orleans were dispersed over the northern United States and Canada.

The next segment of the Symposium was "Emerging Issues in Heartworm and Associated Diseases." Attendants were given insight into Angiostrongylus vasorum, also known as the French Heartworm, and Dirofilaria repens, a subcutaneous filarial nematode of dogs and man in Europe. Angiostrongylus has been diagnosed in native dogs of Newfoundland, Canada for the past decade. Cases of D. repens have been diagnosed in dogs and people that have either traveled or originated in Europe. With the mobile nature of today's society, these two parasites have the potential to become established in the United States. The challenges in diagnosing, preventing, and treating these parasites was discussed.

The session that received a lot of attention from local practitioners was on the subject of heartworm preventive failures. Since 2003, there has been an increase in the number of lack of efficacy (LOE) reports from the Mississippi River Valley. "Is it Resistance? An Evidence-Based Inquiry" featured eight speakers who discussed the effect of vectors, compliance, and genetics on this issue. As heartworm transmission involves a reservoir of infection, a mosquito vector, and favorable climatic conditions, all of these factors have to be investigated whenever there is a change from the norm. When preventives fail, these same factors, as well as compliance of administration and biological variability of the parasite, have to be added into the mix.

After - combined treatment: Pulmonary paranchyma after pre-treatment with doxycycline before administering melarsomine.

The technological advances in genetic mapping have shown that genetic polymorphism does exist in heartworms. This means that there is more than one allele or an alternative form of a gene that is located at a specific position on a specific chromosome. This is not an unexpected finding as most organisms display varying degrees of polymorphism. Nor is it unique to the Mississippi River Valley, as polymorphism has been found in heartworms from around the world, thus the varying strains of the parasite. However, polymorphism can lead to biological variability in how susceptible a strain of heartworm is to macrocyclic lactone heartworm preventives and there are strains of heartworms that are more tolerant of the preventives, and warrant further study before any conclusions can be drawn.

What does this mean for veterinarians working in these areas of increased LOEs who are frustrated in having to explain to a client why their dog, who takes heartworm preventive, now tests positive for heartworms? What does it mean for veterinarians in other parts of the country and world? And more importantly, what does it mean for the animals whose health is dependent on parasite prevention? First let me try to assure you that the sky is not falling. While our heartworm preventives may not be 100 percent effective, by all accounts they are still over 99 percent effective and the overwhelming majority of LOE can be explained by failure of compliance. Resistance does not develop — it is selected for. For resistance to become a widespread problem, a strain of heartworm that is tolerant to the standard doses of heartworm preventive needs to become the predominant strain. This type of selection requires heavy pressure with a large majority of dogs receiving heartworm prevention. Current market surveys show only 44 percent of owned dogs as being on a heartworm preventive. When you add feral dogs and wild canids to the mix, that number is further diluted. The presence of refugia (wild type genes) and a mosquito vector that is constantly mixing the genetic pool by bringing in the wild type genes further decreases the likelihood of resistance becoming widespread. The heartworm preventives are still good products, but now more than ever, year-round use every 30 days is imperative.

The last session of the Heartworm Symposium was "Diagnosis and Treatment Strategies: Impacting Prognosis, Efficacy and Safety." There were several lectures on Wolbachia and its role in the pathogenesis of heartworm disease that added to our knowledge of this intriguing intracellular bacterium. Instead of extrapolating treatment data from the filarial nematodes that infect man, information was presented specific to heartworms. While research is ongoing and much more needs to be explored, eliminating Wolbachia from heartworms with doxycycline prior to administration of melarsomine has been shown to reduce the pathology associated with worm death. The histopathology slides shown illustrate the difference in pulmonary parenchyma between standard melarsomine treatment and pre-treating with doxycycline before administering melarsomine.

The papers presented at the Symposium will be published in a special edition of Veterinary Parasitology to be published at the end of the year. For more information, visit the American Heartworm Society's Web site at www.heartwormsociety.org.

Dr. Nelson practices at the Animal Medical Center in Anniston, Ala. He is past president of the American Heartworm Society and serves on the board of the Companion Animal Parasite Council.