Roundworms (ascarids) are common parasites of many animal species. In dogs and cats, Toxocara canis and Toxocara cati are the main problems. In people, it’s Ascaris lumbricoides, which is often called the "human roundworm." It’s a very common parasite that is thought to infect about 25% of the world’s population. Rates are highest in tropical, developing areas with poor sanitation, poor hygiene, and (as one author stated) a tendency for "promiscuous defecation." Basically, the more human feces in the environment and the lower the degree of hygiene, the greater the risk of inadvertently ingesting parasite eggs.
Traditionally, it’s be thought that this parasite is specific to people, but occasionally, A. lumbricoides eggs have been found in the feces of dogs. The general assumption in these cases has been that the dogs just ingested eggs from the environment, and the eggs simply passed through the dog’s intestinal tract and out the other end. However, a recent paper has challenged that thought. The study (Shalaby et al. Parasitology Research, 2010) found adult A. lumbricoides worms in the small intestine of 8% of tested dogs in Egypt. The presence of adult worms means that the dogs ingested the eggs and that the eggs were able to develop to adults in the dog’s intestinal tract. The adult worms were producing eggs, suggesting that dogs could be a reservoir for A. lumbricoides, beyond just spreading around eggs that they ingested from human feces.
Overall, dogs probably play a minimal role in human infection with this parasite, but it’s an additional dynamic to consider. However, the best approach to reducing the risks associated with dogs and A. lumbricoides probably don’t have anything to do with dogs – rather, the keys are reducing environmental contamination with human feces and improving general hygiene practices.
Image: Adult female A. lumbricoides. (source: CDC Division of Parasitic Diseases (DPDx))