Fielding D. O’Niell, DVM, MS
Heartworm disease in dogs is a mosquito-born blood parasite similar to malaria in humans. Like malaria, a single mosquito bite is all it takes to become infected. Amazingly, canine heartworm disease has been reported in all fifty states. Heartworm disease, like malaria, is preventable only if dogs take their heartworm preventive medication religiously. There are a variety of oral heartworm preventives given on a monthly basis. A lesser known heartworm preventive is given by injection every six months. In any case, dogs not given heartworm prevention on a regular basis are playing Russian Roulette.
Once a dog is infected, it takes six months for the disease to show up on a blood test. The worms have already migrated to the heart and large pulmonary vessels. At this point they are almost the size of earthworms and eventually result in congestive heart failure. The infected dog also serves as a reservoir for mosquitoes to transmit the disease to other dogs.
Heartworm disease is treatable but it is expensive and quite risky once congestive heart failure develops. For this reason, annual heartworm testing is recommended even for dogs on heartworm prevention since no drug is 100% effective. Also, heartworm treatment is safer, cheaper and more effective if the disease is diagnosed and treated before heart failure develops.
In early 2006, veterinarians in the Mississippi delta began to notice an increase in the number of dogs testing positive for heartworms in spite of taking their monthly medication. At first it was thought that these dogs were not being given their medication, but in 2009 a mutant resistant strain of heartworms was isolated. This new strain is known as MP3, named for "Miss Piggy", one of the Mississippi dogs harboring this resistant strain. Recently, MP3 has also been reported in Missouri. Laboratory studies indicate that the MP3 strain is resistant to three of the four different monthly heartworm medications; ivermectin, milbemycin and selamectin but MP3 is not resistant to moxidectin. The 6 month injectable heartworm preventive also just happens to contain moxidectin as the active ingredient.