Experts weigh in on the possiblity that mosquitoes are becoming resistant to some heartworm medications.

In years past, experts blamed the occurrence of heartworm disease in dogs on owner negligence — they failed to give preventive medication to their pets every month. However, a recent spike in the number of dogs on year-round preventives that are testing positive for heartworms is raising concern that these parasites might be becoming resistant to preventives.

Canine heartworm is a parasitic roundworm Dirofilaria immitis, which is spread by mosquitoes. The disease is serious and potentially fatal, but regular use of an approved heartworm preventive usually averts it. Although some veterinarians once believed dogs in the South were more susceptible to heartworm disease because of the high humidity levels, heartworm is now found in all 50 states. Today, the highest infection rates occur in the coastal regions from Texas to New Jersey, and along the Mississippi River and its tributaries.

“While there are parts of the country that are experiencing increased reports of heartworm, we don’t have definitive proof that resistance occurs,” says Wallace E. Graham, Jr., D.V.M., president of the American Heartworm Society. “And while we don’t deny that something is going on, we don’t know what it is yet.”

Lack of efficacy
The increasing number of apparent failures of heartworm preventive medication as reported by the Food and Drug Administration’s Center for Veterinary Medicine indicates a lack of efficacy. (The CVM defines lack of efficacy as a dog testing heartworm positive while consistently receiving heartworm prevention.) However, the exact cause of a reported lack of efficacy of a product is difficult to determine.

“There can be many reasons for these reports of lack of efficacy,” Graham says. “Resistance is one possibility, but there are many others, including inconsistent administration of approved preventive medications, vomiting or regurgitating by the dog after receiving oral medication, or encountering an infected mosquito while not on medication.”

However, not all cases have clear explanations. Researchers have identified one strain of heartworm that shows resistance to preventives in the laboratory.

For drug manufacturers to be able to state that their product effectively prevents heartworm, it must be evaluated using strict testing requirements, including the use of recent parasite strains. When a laboratory strain called MP3 was used to test a potential new parasitic drug, it showed resistance to existing heartworm preventives.  A recent study describes further testing that was done on this particular strain that was originally found in a dog from Georgia in 2006 (Blagburn, et al. “Comparative efficacy of four commercially available heartworm preventive products against the MP3 laboratory strain of Dirofilaria immitis.” Veterinary Parasitology. 2011: Vol. 176: 189-194).

“There are a lot of people looking at this right now, which is good,” says Dwight D. Bowman, M.S., D.V.M., Ph.D., professor of parasitology at the Cornell University’s College of Veterinary Medicine in Ithaca, New York. “It has caught their attention because of the larger overarching threat, but we just don’t know what the reality is at this point.”

Recommendations
Aggressive research into the possible resistance of heartworms to current preventives is ongoing at numerous universities throughout the country. Until the issue is fully understood, it’s important for owners to continue to give their dogs heartworm preventives.

Many veterinarians recommend year-round prevention, even in areas where mosquitoes become prevalent only seasonally. However, according to the American Heartworm Society, approximately 56% of dogs seen by veterinarians in the southern United States never receive any preventive medicine.

“People should keep their dogs on year-round prevention — from Puerto Rico to Alaska and from Maine to Hawaii, all the way over to Guam and Saipan,” Bowman says. “The medication is not just for treating heartworm. It also provides intestinal parasite control, including hookworms and roundworms.”

Graham reminds owners to administer the medicine regularly. “It’s incredibly important to give the medication on the same day every month,” he says. “If six weeks go by without any medicine, you’re running risks.” Graham adds that there are numerous tools available, including websites and mobile phone applications, that send emails or text messages to help you remember to give medications on time.

Dogs on preventives must also be test for heartworms annually. It’s an integral part in preventing the disease from occurring, as well as ensuring that the disease is diagnosed early if it present.

“Heartworm symptoms happen very late in the game,” Graham says. “We need to treat infected dogs before they ever show symptoms, otherwise the risk of the treatment goes way up.”

Additionally, if veterinarians start seeing heartworm cases in dogs that have been properly treated with a preventive medication, annual testing will enable them to collect information about whether resistant strains might be emerging in a specific region.

Preventives still a safe bet
You can additionally minimize the risk of heartworm infection by keeping their dogs indoors, especially at dawn and early evening during peak mosquito season, and by draining standing water to eliminate mosquito breeding sites. However, nothing replaces the need for heartworm preventive medications.

“I don’t know that you can ever definitely prove that resistance doesn’t exist,” Graham says. “We’re not near that yet, but for the vast majority of dogs in the United States, the administration of an FDA-approved heartworm preventive medication every 30 days, either oral or topical, or once every 180 days for an injectable, is going to be efficacious.”

Signs of Heartworm Infection
The symptoms of heartworm infection depend on the number of worms and the dog’s size. Dogs typically exhibit clinical signs in the later stages of the disease. The signs become more severe as the disease progresses. Because heartworm signs often mirror other diseases, early detection can mean the difference between life and death.

• Coughing
• Fatigue
• Labored breathing
• Exercise intolerance
• Vomiting and diarrhea
• Rapid weight loss
• Dry coat
• Rasping sounds coming from the lungs
• Fluid retention in the chest or abdominal cavity