June 10 2011, Dr Steven Garner DVM, DABVP writes. There is growing concern that the class of heartworm preventions that have been used for the past 25 years are losing their effectiveness – if only slightly. Heartworm preventions have for over two decades claimed 100% effectiveness when administered to pets once a month. Now it appears that this number is dropping. The reason for this drop may be the development of resistant strains of heartworms.
Resistant Heartworms Suspected in Southern USA
There is growing evidence that heartworm resistance is developing in the Southern United States. The FDA is seeing increasing numbers of reports claiming lack of effectiveness of heartworm prevention. Veterinarians are seeing more pets on prevention testing positive for heartworm. In response to this possibility, many of the drug companies, including Bayer and ELANCO, are doing research to see if resistant heartworms exist.
If the drug companies are doing the heartworm resistance research can we trust the results?
Concerns about research bias exist. For example, if a study sponsored by a pharmaceutical company, shows their product to be less effective, the results may not get widely publicized. And, when a study shows an advantage for one product the results may be overemphasized. However, there is no doubt that possible heartworm resistance is a hot topic for the pharmaceutical companies.
What does current research show about effectiveness of heartworm prevention?
Studies have shown that a single dose of heartworm preventative is not 100% effective. After a single dose of heartworm prevention some dogs did become infected with heartworm. However, 3 consecutive monthly doses of heartworm prevention appear to achieve 100% effectiveness. It also appears that the newer heartworm preventions containing moxydectin such as Advantage Multi are more effective than the older medications. This makes sense because heartworms may not have had enough time to develop resistance to the newer Advantage Multi yet. The other issue is that there are resistant strains – at least in the laboratory. No one knows if these strains or other strains exist in the wild.
Information produced by Bayer suggests that Advantage Multi is better than all the other heartworm preventatives. The science behind this study is good but much more research will be needed to confirm these claims. For now however, it appears that Advantage Multi is indeed a better product especially if you live in the South.
For moreinformation this subject from recent veterinary publications click hear.
Correlation between loss of efficacy of macrocyclic lactone heartworm anthelmintics and P-glycoprotein genotype.
Vet Parasitol. March 2011;176(4):374-81.
Catherine Bourguinat1; Kathy Keller; Byron Blagburn; Rudolf Schenker; Timothy G Geary; Roger K Prichard
1Institute of Parasitology, McGill University, 21 111 Lakeshore Road, Sainte-Anne de Bellevue, H9X3V9, Quebec, Canada.
Copyright © 2011 Elsevier B.V. All rights reserved.
Macrocyclic lactone (ML) molecules have been used for heartworm control for more than 25 years. However, in recent years, there have been reports of loss of efficacy of ML heartworm preventatives against Dirofilaria immitis in some locations in the United States. Macrocyclic lactone resistance is a common problem in nematode parasites of livestock, and more recently, evidence of ivermectin resistance has been reported in the human filarial nematode Onchocerca volvulus. In this study, four D. immitis sample groups from the United States with different treatment histories were investigated for evidence of ML-driven genetic selection. DNA from individual adult worms and microfilariae was amplified by polymerase chain reaction to investigate a gene encoding a P-glycoprotein, a protein class known to be involved in ML pharmacology. A significant correlation of a GG-GG genotype with ivermectin response phenotype was found. Moreover, a significant loss of heterozygosity was found in a low responder group; loss of heterozygosity is commonly seen in loci when a population has been under selection. Further studies are required to confirm ML resistance in heartworm populations. However, the genetic changes observed in this study may be useful as a marker to monitor for ML resistance in D. immitis.