Rumors about dogs that have been on monthly heartworm prevention all year round but have still contracted heartworm disease have been circulating for years. These cases are very difficult to confirm, however, because it is next to impossible to determine whether or not a dog truly received and absorbed its heartworm prevention on an appropriate schedule, particularly since the lapse in question would have occurred quite a while before heartworm positive results. It takes five or six months for the juvenile heartworms passed through the bite of an infected mosquito to mature into the adults that are responsible for a positive heartworm test and most clinical signs of heartworm disease.
An Alarming Study
Now, scientific evidence that supports the presence of resistance to certain preventatives in some populations of heartworms in the United States is starting to accumulate. In one study, researchers infected forty dogs with heartworm larvae from a strain called MP3 that were originally collected in Georgia and are known to be somewhat resistant to some types of heartworm preventative. Infected dogs were divided into five groups and treated in the following manner 30 days later:
- Group 1: oral ivermectin/pyrantel pamoate (Heartgard Plus)
- Group 2: oral milbemycin oxime (Interceptor)
- Group 3: topical selamectin (Revolution)
- Group 4: topical imidacloprid/moxidectin (Advantage Multi)
- Group 5: untreated
All treated dogs were given the label dosage of their respective medications based on their weight.
When the dogs were examined for adult heartworm approximately five months after being infected with heartworms the researchers determined that 100% of the untreated dogs and 87.5% of dogs in groups 1, 2, and 3 were heartworm positive. Dogs in the untreated group had anywhere between 34 and 70 worms in their hearts and lungs. Heartgard Plus, Interceptor, and Revolution did prevent many of the larvae from developing into adult (dogs typically had only between 2 and 3 worms in their hearts and lungs) but only Advantage Multi was completely effective at killing the MP3 strain of heartworm larvae.
What This Means for Pet Owners
It is important to remember that the VAST majority of dogs and cats that develop heartworm disease do so because they did not receive a preventative heartworm medication every month all year round regardless of type. Nevertheless, owners should be aware that in some parts of the United States, certain populations of heartworms appear to be developing resistance to particular preventatives. If you live in the Southeast or Central regions of the United States, talk to your veterinarian about whether switching to Advantage Multi might be in your pet’s best interest.
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 Mar 10; 176(2-3):189-94. Epub 2011 Jan 1.