Hunting, Hiking and Heartworm

By Stacey Oke, DVM, MSc

Although heartworm prevention techniques, including mosquito avoidance and drugs that kill heartworm larvae, have been recommended for years, the number of dogs diagnosed with heartworm continues to increase. One factor that contributes to the persistence of this preventable disease is “prophylactic failure,” which means that animals develop the disease even though they received a prophylactic drug to prevent heartworm for at least 1 year or season.

Other Insect-Borne Diseases to ConsiderAccording to the Canine Vector-Borne Diseases website (, insects are a growing concern. Other insects, such as fleas and ticks, spread diseases like tapeworm and Lyme disease, respectively. Discuss ways to “Fight the Bite” with your veterinarian and ensure your pet is adequately protected.

Hunting, Hiking and Heartworm
Heartworm is caused by the parasite Dirofilaria immitis. When a mosquito feeds on a dog that is infected with female heartworms, the “baby” worms, microfilariae, are taken in with the blood meal and grow to become infective in the mosquito’s mouthparts. The next time that mosquito bites a dog, the infective heartworm larvae enter its bloodstream and grow into adult heartworms. The adult female heartworms produce additional microfilariae, which are spread to more dogs and sometimes cats.

A recent study published in the May/June 2011 edition of the Journal of the American Animal Hospital Association (JAAHA) looked at the occurrence of prophylactic failure in hunting dogs.

“Of the 708 dog owners who participated in the study, 9% reported failure of prophylaxis. The dogs tested positive for heartworm even though a heartworm prophylaxis had been administered,” stated lead author Barton Rohrbach, VMD, MPH, Dipl. ACVPM (Epidemiology) from the University of Tennessee’s College of Veterinary Medicine.

According to Dr. Rohrbach, many of the dogs were not treated or tested for heartworm appropriately:

  • Dogs were treated using an estimated rather than actual body weight (and therefore could have been administered too little of the drug).
  • Owners did not record the date the drug was administered and relied on memory to treat their dog each month (resulting in missed doses).
  • 13% of owners observed their pet spit the pills out (many owners may not notice this, resulting in more missed doses).
  • Only 79% of owners tested their dog every year for heartworm (recommended by the American Heartworm Society). Testing was frequently performed at the incorrect time of the year; therefore, infections may have been missed.
  • Newly acquired dogs were frequently not tested prior to or at the time of acquisition.

Study participants also reported a failure to practice mosquito avoidance techniques, such as remaining indoors at dusk and dawn.

“Imperfect” Cats

For once, it is a benefit to be imperfect. Cats are susceptible but are imperfect hosts to the worm, making cats more resistant to infection with adult Dirofilaria immitis than dogs. Be aware, however, that many experts believe feline heartworm occurs more frequently than we think.

“This study suggests that simple steps, such as weighing the dog to determine the correct dose of preventive, watching to ensure oral medications are retained, and recording the actual date the medication is administered each month, are not being followed by many dog owners. These may be important factors leading to prophylaxis failure,” added Dr. Rohrbach.

A single missed dose of a heartworm preventive puts dogs and cats at increased risk to acquire heartworm, particularly animals that spend a lot of time outdoors.

For more information on preventing heartworm, visit the American Heartworm Society ( and the Companion Animal Parasite Council (