There are two species of roundworms affecting dogs and puppies. These worms can be seen if they are passed in the stool, but normally are diagnosed by looking at the stool under the microscope for the eggs of roundworms. The major species of roundworms infecting dogs is Toxocara canis. It is the most common roundworm of the domestic dog, but it is not able to infect cats. Its presence can go completely without symptoms, although more likely it is going to create some degree of diarrhea and possibly vomiting or general unthriftiness in its canine host.

An animal can become infected by consuming worm eggs from soil in the environment. An animal can also be infected by nursing from an infected mother. Another way of transmission is by consuming a prey animal that is carrying developing worms, usually rodents. During embryonic development, when an infected mother dog is pregnant, immature worms can be passed to the fetus in her uterus. Most puppies are infected this way.

Roundworms eggs are passed in the hosts feces where they can be detected when a stool sample is tested. Feces and any worms and eggs therein are deposited on the ground where they are rained on, dried by the sun, stepped on, etc. while the worms are developing during this time. Fresh feces are not infectious, because fresh feces do not contain the infectious stage of the roundworms. Worms require 30 days to become infectious, meaning it is contaminated dirt that is infectious to other dogs and sometimes to people.

Roundworms infection can have numerous negative effects. It is a common cause of diarrhea in young animals. Sometimes, the worms can be vomited up. This can be alarming as they can be large, reaching links of up to 7 inches. It should be noted that human infection by this parasite is especially serious. Fecal testing for worm eggs is a must for puppies and a good idea for adult dogs having their annual checkup.

Pet owners should always have their animal’s stool checked annually for roundworms. There are many effective pharmaceuticals that prevent roundworms infestations. Talk to your veterinarian about roundworms and medications used against this parasite.


Dr. Karsten Fostvedt is a veterinarian at St. Francis Pet Clinic in Ketchum.

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