Pet Threat: Yes | Human Threat: Rarely

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.

Learn more about Heartworms. View Hookworms


Pet Threat: Yes | Human Threat: Yes

Hookworms are dangerous parasites that live in a dog's small intestine. With remarkable efficiency, hookworms "graze" on the lining of the intestine, leaving multiple bloody holes in their wake. These can lead to anemia and may even cause a small puppy to bleed to death. In humans, hookworms migrate through tissue close to the skin, causing painful, itchy rashes.

Learn more about Heartworms. View Roundworms


Pet Threat: Yes | Human Threat: Yes

Roundworms may resemble earthworms, but they're a whole lot more dangerous, especially when they get inside a dog, or a person. The roundworm is a patient, persistent parasite that can lay up to 100,000 EGGS IN A SINGLE DAY.3 Once an egg is accidentally ingested by a dog, the roundworm hatches and makes its way through the body to an ideal feeding ground, the intestine.

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Pet Threat: Yes | Human Threat: Very Rarely

Whipworms get their name from their thin, whip-like shape. As far as health risks go, they are not generally considered to be in the same league as heartworms, hookworms and roundworms. Heartworms, hookworms, and roundworms can cause serious harm and even death in dogs, and in the case of hookworms and roundworms, may be passed on to humans. Whipworms are a less destructive nuisance.

Learn more about Heartworms. View Tapeworms


Pet Threat: Yes | Human Threat: No

As though fleas weren't bad enough themselves, they are also the primary carrier for another type of parasite: the tapeworm. Tapeworms are fairly innocuous; they are not nearly as dangerous, for example, as heartworms, hookworms or roundworms. Though they can be transmitted by other means, tapeworms primarily enter a dog directly through the ingestion of a flea, which serves as a parasitic Trojan Horse.

Learn more about Heartworms. View Heartworms

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

Welcome to Which Worms Why

Heartworms, hookworms, roundworms, whipworms, tapeworms, earthworms, gummy worms—there are so many different types of worms out there that it can be hard to keep up with which ones actually pose the most significant threats to dogs—and, in the case of hookworms and roundworms, to people, too.

Which Worms Why will give you a quick breakdown of the most common types of parasitic worms of dogs, what they do, how they are transmitted, and how to protect against them. With this reference, you can see which worms pose the greatest health risks to your dog, and then ask your veterinarian for the proper preventive.

You will also learn which parasitic worms may pose a threat to people.

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

3Center for Disease Control and Prevention, Division of Parasitic Diseases. Guidelines for veterinarians: prevention of zoonotic transmission of ascarids and hookworms of dogs and cats. Available at: www.cdc.gov/ncidod/dpd/parasites/ascaris/prevention.htm Accessed December 16, 2009.