Intestinal Parasite Prevention

Medications to Prevent Yucky Stuff on the Inside


Our dogs live in a different world than us- unless you are crawling through grasses, sniffing hind ends, and licking things with an interesting odor. Dogs are exposed to all sorts of insects and parasites that we clothed, shod, upright and comparatively clean adult humans are not.

Some canine parasites are species specific and only infect dogs and wild canids, while others can also infect humans and in particular small children who often practice questionable hygiene. (I am the mother of small children and can attest to this, despite proper parenting- our handwashing song is sung to the Row Row Row your Boat tune: “Wash wash wash your hands, bacteria can be mean- E. coli, typhoid, salmonella, get those hands all clean.”) Parasite prevention medications help reduce the risk of illness to both our furry friends and their human families.


All dogs should be on monthly parasite control medication to keep them and their human families healthy. “Dewormers” are found in most monthly heartworm prevention medication for dogs.


Milbemycin (in Sentinel and Interceptor) is effective in controlling roundworms, hookworms and whipworms; pyrantel (found in Heartgard) is effective against roundworms and hookworms. All these worms can cause diarrhea, abdominal cramping, and poor health in dogs, as well as anemia in the case of hookworms, which sink their 6 sharp teeth into the intestinal lining and drink blood. 

Humans are infected with hookworm by skin contact (like when walking barefoot) with contaminated soil or sand containing microscopic hookworm eggs, which survive for months in soil. The hookworm larva puncture and travel under the skin, causing itchy and unsightly worm migration tracks before eventually dying.

Humans are infected with roundworms through accidental ingestion of contaminated soil (fresh eggs are not infective)- the eggs can remain infective for years. The larva become confused in the “wrong” host (they wanted to be a dog or cat!) and travel around the body. In some people, especially children, the worms cause illness as they migrate through organs including the liver and lungs, and cause blindness if lodged in the human eye. According to the CDC, 14% of humans in the US have antibodies showing infection with roundworms.

Whipworms rarely infect humans. Picking up stool quickly, and washing hands before eating are good prevention strategies in addition to monthly parasite control medication for pets.

Heartworm is transmitted by mosquito bites. Mosquitoes feed on a dog or wild canid that is infected and suck up tiny larval worms. When the infected mosquitoes then bite another dog, the infective larval stage is transmitted.  The larvae mature under the skin of the dog, and migrate through the tissues of the dog into the circulation and end up as long, spaghetti-like worms in the blood vessels of the lungs and heart.

The heartworm disease incidence has been increasing in Massachusetts, along with the rest of the country, which is likely due to warmer weather, a huge number of bats dying from White-nose syndrome, and with the influx of the disease from rescue dogs brought up from the southern US. Heartworm infection in dogs is fatal over time if left untreated.

Death occurs by heart failure, bleeding into the lungs, worm embolism, or secondary kidney damage. Treatment is expensive and has risks; year-round prevention is much safer for your dog and better for the wallet. Rarely, heartworms infect humans but do not complete their life cycle- the dying larva will form a benign circular lesion in the lung which unfortunately looks like lung cancer or tuberculosis on scans, leading to invasive testing.

Humans are susceptible to infection with other related filarial worms, primarily found in Africa, that cause elephantiasis (lymphatic filariasis), river blindness, and eye worm (loa loa).

In sum: Ensure your dog is on a monthly year-round heartworm preventative that has deworming medication built in. Check with your rescue/shelter/breeder that your puppy has been dewormed every 2 weeks before bringing home the cute furry bundle, and follow up with appropriate deworming treatments at your vet.

Always pick up your dog’s stool (and promote “pick up after your pet” public health regulations). Wash your hands before eating or touching your mouth. And remember to wash your hands after touching humans or objects humans have touched, since fellow Homo sapiens carry many more germs that are dangerous to you than a well looked after dog!


Dr. Astrid Kruse graduated from Tufts University Cummings School of Veterinary Medicine in 2003. After completing a year long rotating small animal medicine and surgery internship, she joined the staff at a large general practice hospital on the North Shore, where she enjoyed forming long term connections with her patients and clients. She is excited to join the team at Domino Veterinary Hospital in Concord, MA! Her special interests include dog and cat internal medicine, preventative health care, rabbit and guinea pig medicine, soft tissue surgery, and dentistry in all species.