Heartworm preventive efficacy study shows surprising results
Historically, heartworm disease has been effectively prevented in dogs by using available preventives. However, recent research indicates that some field isolates and laboratory strains of Dirofilaria immitis may not be fully susceptible to preventive medications.1-3 The number of heartworm positive dogs has increased in certain areas of the central United States, namely the Mississippi Valley and Delta regions, despite compliant use of preventives. And other variables, such as testing strategy changes and increased mosquito vectors, have been ruled out as potential explanations. The concern is that a subpopulation of heartworms in these areas has developed resistance to heartworm preventive medications.
Research conducted at Auburn University’s College of Veterinary Medicine and in collaboration with others suggests that both biological and genetic changes have occurred in heartworms in at least some infected dogs. Not only are there field isolates with reduced susceptibility to heartworm preventives, there are laboratory strains that appear to behave similarly. One such laboratory strain, the MP3 strain, was identified and documented in a recent article.3 This strain was isolated from a naturally infected dog in Northeastern Georgia in 2006. It was placed in a Beagle and passaged several times in dogs of this breed for use in heartworm studies.
In a presentation at NAVC on Tuesday, Jan. 18, Dr. Byron Blagburn presented study findings on the issue of decreased effectiveness of heartworm preventives. The study results were recently published online in the journal Veterinary Parasitology.4 Dr. Blagburn and his colleagues tested the comparative efficacy of four commercially available heartworm preventive products against the MP3 laboratory strain of D. immitis. In the controlled blinded study, 40 dogs were assigned based on weight to five treatment groups of eight dogs each. All dogs were infected with 100 MP3 infective larvae and treated once as follows 30 day later:
• Group 1: Heartgard Plus
All products were administered as directed on each product label to dogs that had been fasted for 12 hours. Group 5 dogs served as nontreated controls. All dogs were examined for adult heartworms approximately 150 days after infection.
The results of the study were as follows. A range of 34 to 70 adult D. immitis per dog were recovered from each of the eight dogs in the nontreated control group (geometric mean of 51.6 worms per dog). No adult heartworms were recovered from the eight dogs treated with Advantage Multi for Dogs (100% efficacy). One or more adult heartworm were recovered from seven dogs treated with Heartgard Plus, Interceptor Flavor Tabs, and Revolution (12.5% efficacy); more than six heartworms were recovered from one dog in each of these three groups. The geometric mean worm counts were 2.3, 2.4, and 2.3 worms per dog, resulting in efficacies compared to controls of 95.6%, 95.4%, and 95.5%, respectively.
In this laboratory study, Advantage Multi for Dogs was the only product tested that was 100% effective against the MP3 isolate of D. immitis. To what extent the data obtained in this laboratory study might apply to field isolates of D. immitis remains to be determined; more study is needed to determine the prevalence and geographic distribution of MP3-like strains and other heartworm-related issues to allow practitioners to make the best recommendations possible for their canine patients. In the meantime, Dr. Blagburn stresses the importance of testing dogs annually for heartworm infection and using heartworm preventives religiously year-round.
1. Blagburn B, Dillon R, Prichard R, et al. Characterisation of heartworm prevention failures in the central United States, in Proceedings. 13th Triennial Heartworm Symp 2010;27.
2. Bourguinat C, Keller K, Schenker R, et al. Investigation of genetic changes in Dirofilaria immitis after the use of macrocyclic lactone heartworm preventives, in Proceedings. 13th Triennial Heartworm Symp 2010;28.
3. Snyder DE, Wiseman S, Cruthers LR, et al. Ivermectin and Milbemycin Oxime in experimental adult heartworm (Dirofilaria immitis) infection of dogs. J Vet Intern Med [serial online] 2010. Available at: acvim.org.
4. Blagburn BL, Dillon AR, Arther R, et al. Comparative efficacy of four commercially available heartworm preventive products against the MP3 laboratory strain of Dirofilaria immitis. Vet Parasitol 2011;175:in press.
This article was reprinted from DVM Newsmagazine.