Heartworm Preventative Effectiveness Study

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.

The Results

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.


Protect Your Pets From Fleas, Ticks, and Mosquitoes

Ticks, fleas, mosquitoes and all other pests are out in full force and this can put your pet at an increased health risk.  The National Pest Management Association (NPMA) says pet owners should take special care to protect their pets from ticks, fleas and mosquitoes this season. You can find products to protect your pets at vetdepot.  Dogs, cats and other pets are especially susceptible to health risks posed by such pests.

Some health risks include the transmission of heartworms to pets via Mosquitoes which can result in heart disease, ticks can cause diseases such as Lyme disease and Rocky Mountain spotted fever by spreading bacteria to pets.  In addition, the saliva from fleas can cause anemia, dermatitis and the transfer of tapeworms to pets.  Fleas often jump onto pets when outdoors and enter into homes where they can multiply and quickly infest bedding, furniture and clothing.

It is important to always inspect pet coats thoroughly after spending time outdoors.  Here are a few recommended tips from the NPMA to keep your pet safe from pests:

  • Watch for excessive scratching or licking on your pet
  • Avoid walking pets in tall grass, pests love to gather here
  • Wash pet bedding, plush toys and vacuum frequently
  • Talk to your veterinarian about treatment options to protect your pet, and seek medical advice if ticks or fleas are found on your pet

Merial Immiticide, Heartgard Shortage

Merial Ltd has reported that it has limited supply of Immiticide (melarsomine dihydrochloride) and Heartgard (ivermectin) Tablets due to technical issues encountered during a planned manufacturing plant transfer. The shortage only relates to Merial’s hard, non-beef, Heartgard Tablets, not Heartgard Chewables or Heartgard Plus (ivermectin/pyrantel) Chewables.  Merial currently expects the Immiticide shortage through the first quarter of 2010 and does not expect Heartgard Tablets to be available until 2011.  In addition to Heartgard, other leading heartworm medications include Interceptor, Sentinel, and Tri-Heart