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Heartworm Prevention In Dogs

Heartworm disease in dogs is life changing and eventually deadly, yet easily prevented with monthly medication.

This information is provided to help temporary home providers understand why we treat our dogs the way we do.  It is not intended as medical advice.

Heartworm Resistance To Current Drugs

Resistance is growing to current drugs.  At Iowa Pet Adoptions, we attempt to prevent heartworm disease by treating all dogs with twice the level of ivermectin found in Heartgard, every 30 days.  This article will attempt to explain our rationale.  We are not veterinarians so do not consider this medical advice.  If you reach a different conclusion please let us know, perhaps we can improve how we treat our dogs.  Be aware that if you use Heartgard, or any of the other drugs, you may already be dosing at twice the dose suggested by the manufacturer, because the pills are made to treat a wide range of dog weights.  Please understand exactly how you are treating your pets.

Heartworms are spread to dogs through mosquitoes.  Iowa has historically had cold winters which would not allow heartworm transmission. With changing climate patterns, the risk is increasing. There are also strains of drug resistant heartworms. Thus, we treat dogs to prevent heartworm each month of the year.  

Following is an except from a July 2011 article in Whole Dog Journal, reprinted on DogAware.com

Useful information has since emerged from a small study conducted by researchers at Auburn University’s College of Veterinary Medicine in Alabama. Four commercial heartworm preventive medications (Heartgard Plus, Interceptor, Revolution, and Advantage Multi) were tested on the MP3 resistant strain of heartworm. Forty dogs were infected with heartworm larvae. Thirty days later, the dogs were treated with one of the four preventives.

Say what? How can the efficacy rate be more than 95 percent, when 7 of 8 treated dogs (87.5 percent) were heartworm-positive? One might guess that “95 percent effective” means that 95 percent of dogs remained heartworm-free, but it actually means the treated dogs had 95 percent fewer adult worms than untreated dogs – a whole different can of worms!Four months after treatment, an average of 2.3 adult heartworms were found in seven of eight dogs in each of the first three groups; only Advantage Multi was found to be 100 percent effective. There was no significant difference in efficacy against the MP3 strain between Heartgard Plus, Interceptor, and Revolution. Based on the number of adult worms found in the untreated control group, the efficacy rate for the other three products was determined to average 95.5 percent.

The article goes on to state the four preventive medicines were 100% effective after 3 consecutive doses.  This is a primary reason to give heartworm prevention all year, not just during the warmer months.

There are two reasons we use ivermectin, the active ingredient in Heartgard, although it may not be 100% effective against all current strains, at the recommended dose:

  1. generic ivermectin which we use is relatively inexpensive
  2. ivermectin can safely be given to dogs with current heartworm disease, which is important when we take in dogs with an unknown history

MDR1 and Ivermectin

The concern over whether ivermectin is 100% effective is why we dose at twice the amount found in Heartgard.  We feel this level is safe for all the dogs we intake.

There are some breeds of dogs with a sensitivity to certain drugs.  From Washington State University:

Drug sensitivities result from a mutation in the multi-drug resistance gene (MDR1). This gene encodes a protein, P-glycoprotein that is responsible for pumping many drugs and other toxins out of the brain. Dogs with the mutant gene cannot pump some drugs out of the brain as a normal dog would, which may result in abnormal neurologic signs. The result may be an illness requiring an extended hospital stay - or even death.

This fact has led many people to conclude that ivermectin is not safe in certain breeds, primarily herding breeds.  However, these people are wrong.  All four of the currently popular active ingredients in heartworm prevention pose similar risk:

Fortunately, the dose of ivermectin, selamectin, milbemycin and moxidectin in the commercial heartworm preparations are low enough to be used safely even in dogs with the MDR1 mutation. It is only when the drugs are used at high doses, such as those used to treat mange (50 times higher dose than the heartworm prevention dose), that dogs with the mutation will develop neurological toxicity. Attempting to use large animal formulations of these drugs is likely to cause neurological toxicity because it is difficult to accurately measure

Washington State University College of Veterinary Medicine states:

Ivermectin (antiparasitic agent). While the dose of ivermectin used to prevent heartworm infection is SAFE in dogs with the mutation (6 micrograms per kilogram), higher doses, such as those used for treating mange (300-600 micrograms per kilogram) will cause neurological toxicity in dogs that are homozygous for the MDR1 mutation (mutant/mutant) and can cause toxicity in dogs that are heterozygous for the mutation (mutant/normal).

For these reasons, Iowa Pet Adoptions currently provides monthly heartworm prevention of ivermectin to dogs, at twice the level found in Heartgard.

An alternative approach may be to administer ivermectin every 15 days instead of 30 days.  From a July 2011 article in Whole Dog Journal, reprinted on DogAware.com

Another option might be to give heartworm preventives twice a month rather than monthly, particularly if you live in an area with a high incidence of heartworm disease. There is a window of opportunity in which heartworm larvae are susceptible to the treatments used against them. Once the larvae reach a certain age, preventives will no longer affect them. By giving the preventives twice as often, more larvae may be killed. You could use the same product each time, or alternate between two different products when using this approach.

Please check with your veterinarian and let us know if you reach a different conclusion.

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