If you aren’t giving a heartworm preventative 12 months a year, every month, you may want to re-think your position. This is from the DVM 360 magazine.

CAPC changes heartworm guidelines due to evidence of resistance
‘Slow kill’ method no longer recommended in cases of infection.
Jul 24, 2013 
The Companion Animal Parasite Council (CAPC) has altered its guidelines after evidence of preventive-resistant Dirofilaria immitis strains was presented at the American Association of Veterinary Parasitologists Conference at the end of July in Chicago. Researchers have now identified heartworm isolates from the Mississippi Delta region that develop in adult dogs receiving routine monthly heartworm preventives.
At the conference, a number of parasitologists—including Byron Blagburn, PhD, of Auburn University, Dwight Bowman, PhD, of Cornell, and others—presented the findings of investigations funded by Novartis Animal Health, which has contributed $1.6 million to date toward studying resistance. “Resistance has been demonstrated across the macrocyclic lactone (ML) product class,” writes Bowman in an introduction to a collection of the research abstracts. “All currently approved products have failed to prevent heartworm development in dogs when tested in experimentally induced infection models with Mississippi River Valley isolates.” These products include ivermectin, selamectin, milbemycin oxime and moxidectin.
This means treatment of heartworm-positive dogs should be immediate and aggressive, as noted in the newly revised CAPC guidelines. The “slow kill” therapy sometimes prescribed by veterinarians is never appropriate, as it has been demonstrated that using this modality—repeated macrocyclic lactone administration over a period of time—increases the proportion of circulating microfilariae that possess resistance markers.
These recommendations from the researchers can form a “first line of defense” against the spread of resistance, according to the conclusions presented in the abstract collection:
1. Macrocyclic lactones should never be used be used for slow-kill treatment of heartworm disease.
• Macrocyclic lactones are approved for prevention, not treatment of heartworm disease.
• Using a macrocyclic lactone, including ivermectin, moxidectin, selamectin or milbemycin oxime, for slow-kill treatment may incrase the risk of resistance developing.
2. Dogs should be tested for heartworms once a year. Existing infections should be aggressively treated with an approved adulticide and microfilariae should be eliminated.
3. The CAPC recommendations for year-round prevention with a broad-spectrum parasiticide should be followed.
4. Pet owners should be encouraged to reduce exposure to mosquitoes as much as possible.
Parasitology specialists emphasize that evidence for resistance does not mean abandoning current protocols but following them even more rigorously. “The new evidence confirming heartworm resistance underscores the importance of protecting pets year-round without gaps in prevention,” Bowman says in a statement from CAPC. “Veterinarians also should continue to emphasize annual heartworm testing. In areas where heartworm is more prevalent or breaks are appearing, testing every six months is recommended.”
While new strains may be preventive-resistant, CAPC says current products are still effective against many strains of heartworm and several control other parasites, including intestinal helminths, fleas and mites. “Preventives are still the best protection we have, and consistently administering them is key to maintaining pet health,” says CAPC board member Susan Little, DVM, PhD. Though the preventives cannot guarantee that infections will never occur, Little encourages veterinarians to “test dogs regularly to be sure they have not become infected, and when infections are identified in dogs, we have to treat whenever possible.”
The geographic spread of resistant isolates has not been determined, but CAPC says it will continue to monitor the evolving situation and modify recommendations to veterinarians as needed.