Which Worms Why - A quick guide to parasitic worms that pose the greatest threats to pets and people.

Pet Threat: Yes | Human Threat: Rarely

Parasite Profile

Heartworms are among the most dangerous parasitic worms that infect dogs. Hundreds of thousands of cases of canine heartworm disease are reported in the US every year.1 The disease is passed from infected dogs to other dogs by mosquitoes. That means all of these dogs are at risk for heartworm disease.

The disease is spread when a mosquito, previously infected by biting an infected ("reservoir") dog, bites a dog and deposits tiny immature heartworms, called larvae, near the bite wound. Then, the larvae enter the wound and migrate beneath the skin, eventually reaching the heart and lungs. These unwelcome intruders can grow up to 12 inches in length. Heartworm disease is debilitating, and may even prove fatal.

Targets

All dogs are potential targets for heartworm disease. In fact, canine heartworm disease has been detected in all 50 states.2

Clinical Signs

An infected dog may cough or wheeze occasionally, and may seem unusually tired and unwilling to play, but these early signs of heartworm disease can easily be missed, and may be mistaken for something else. The only way to tell if your dog has been infected is to have a veterinarian administer a heartworm test.

Treatment

Treatment for heartworm disease is difficult, expensive, lengthy and may be traumatic to dog and owner. Dogs receive a series of arsenic-based shots to kill the worms and must spend up to 6-8 weeks in an environment that will not tax the dog's heart and lungs any further. This may call for crating the animal to limit activity and prevent overexertion.

Prevention

A number of effective heartworm preventives have been developed to combat heartworm disease. It is important to consult with your veterinarian about the heartworm preventive that is appropriate for your dog. Before making a decision, be sure to understand that other parasites can threaten dogs and that some of them can even be transmitted to people.

For continuous protection, it's imperative that you give your dog the preventive dose monthly as directed. A single lapse in compliance could give mosquito-borne heartworm larvae the small window of exposure they need to infect your dog.

For more information, ask your veterinarian.

QUESTIONS FOR YOUR VETERINARIAN
When you visit your veterinary clinic—just as when you visit your own physician—it is often helpful to write down any questions you may have for the doctor. Here are some questions about the basics of dog and puppy care:

  • How often should I bring my dog to the clinic?
  • How soon should I begin giving my dog a heartworm preventive?
  • Will the recommended heartworm preventive also treat and control intestinal parasites?
  • How often must I give my dog this preventive?
  • Should I give my dog this preventive all year 'round?
  • Is my puppy likely to take this preventive willingly?

Testing
Many veterinarians recommend an annual heartworm test to ensure that a dog hasn't been infected with heartworm larvae during possible lapses in prevention. While a negative result is good news, it may not be definitive because heartworm tests detect the presence of adult females. Depending on how many doses of heartworm preventive may have been missed during the year and when, a dog could actually be infected with immature larvae (or, more rarely, only male worms) that are not yet perceptible.

1Heartworms in Dogs. Available at: www.petsandparasites.org/dog-owners/heartworms.html. Accessed on December 14, 2009.

2American Heartworm Society, Canine Guidelines. Available at: www.heartwormsociety.org/veterinary-resources/canine-guidelines.html. Accessed on December 16, 2009.