Oh, No! Heartworm Preventatives Aren't Perfect Anymore

Year Round Heartworm Prevention is Important                  by Dr. Michael Fagan

For twenty-plus years, we've enjoyed the privilege of telling our clients that our heartworm preventative medications were 100% effective in preventing heartworm disease if given monthly throughout the entire mosquito season.  Now we have to settle for telling you that they are Very Effective. 

Very Effective is still a good thing, but some aspects of our overall prevention strategy have become even more important.  Over the past several years, there have emerged a few strains of heartworms that are resistant to the entire class of drugs used as heartworm preventatives (known as macrocyclic lactones, or ML).  So far, it appears that none of the available products (Heartgard, Iverhart, Revolution, Interceptor, Advantage Multi), are superior to any other:  while every product is still very effective, each has been documented to be less than 100%. 

What this means for our patients:

• It is very important to give monthly heartworm preventive year-round.
• Annual heartworm testing is essential to identify any dog that have acquired a resistant heartworm infection. 

One of our patients, a year
after heartworm treatment

Dogs that do become infected with ML-resistant heartworms tend to be infected with lower numbers of worms than dogs that are not on preventatives at all.  They still need to be treated with injections of melarsomine to eliminate the adult heartworms for their own health, but also to eliminate the source of resistant worms that could be transmitted to other dogs.

Treating dogs that are infected is expensive and painful. The drug (melarsomine) is currently only available on a case-by-case basis from European production facilities. Melarsomine is injected deep into the muscle along the spine and it hurts! Dogs must be kept quiet for two months during treatment which means no running or playing. We are currently treating a really sweet black lab who moved here from the Southern U.S. where resistant heartworms have been identified.