Photo courtesy of U.S. Fish and Wildlife Service.

Human roundworm infection isn't normally considered a problem in the modern world, but it is. Studies have surveyed playground soil samples across the U.S. and found up to 20 percent of samples to be contaminated with roundworm eggs. Antibody testing shows approximately 14 percent of the U.S. population has been infected by a roundworm species. Most infections occur in people under the age of 20, with children being at the highest risk.

Most people have very mild or no symptoms, depending on how many worm eggs someone is exposed to. The worms of concern are Toxocara canis and Toxocara cati. These species of roundworms employ a full-body migration through their host as part of their development, and symptoms in the human host result from this migration. 

Three syndromes are described.

Covert Toxocariasis
Variable symptoms occur as the worm larvae make their way through the human body: fever, abdominal discomfort, swollen lymph nodes, cough, headache, and sleep disturbances. This is the mildest of the three syndromes. 

Visceral Larva Migrans
This syndrome is basically a more intense version of the above and happens when large numbers of larvae are migrating (same symptoms but more severe since more larvae are involved). After larvae are finished with their migration or have gotten lost in the process, symptoms usually subside in a few weeks, but human death has occurred from visceral larva migrans in severe cases. Most visceral larva migrans patients are young children (ages one to four) as they are the ones who are at the highest risk from oral exposure to contaminated soil.

Ocular Larva Migrans
When roundworm larvae migrate in their natural host (dog, cat), they are able to recognize molecular signposts directing them on their route so that they end up in the GI tract at the end of their journey, having undergone the proper development along the way.  In the human body, however, the signposts are all wrong, and the larvae tend to get lost. They die or are killed by the host immune system during their journey. For unknown reasons, roundworm larvae are frequently misdirected to the human eye, where they die and generate tremendous inflammation. Usually, one eye and one single larva are involved. Vision loss occurs over days to weeks, and permanent blindness can result. Damage to the eye is permanent. Children aged 7 to 8 years old are typically infected.

Sleeping Cats

Photo Courtesy Dr. Teri Ann Oursler

How Does This Happen?

Human roundworm infection involves accidentally consuming dirt that has been contaminated with roundworm eggs. The eggs require at least a month in the soil to develop to an infective stage, so fresh feces is not a risk, but because it is feces that contaminate soil, most communities require dog owners to dispose of feces deposited in public parks and restrict dog access to many public areas.

Dogs are not the only culprits. Outdoor cats, particularly feral cats, will happily deposit their feces in children's sandboxes, so it is important that such play areas be covered when not in use to reduce contamination. That said, it is usually the canine roundworm that is involved in human infection.

It is possible to become infected by eating undercooked meats in the same way that dogs and cats can become infected through the consumption of prey.

It is possible to become infected by kissing a pet or by petting the fur and then touching your mouth. The fur would have to be contaminated with dirt; however, for such transmission to occur, fur contact is considered low risk for transmission. In a study of dog fur, only half the dogs were found to have roundworm eggs in their fur (no more than one egg per dog was found) and only 4 percent of the eggs found were infectious.

Prevention

Two Dogs

Photo Courtesy Dr. Teri Ann Oursler

The most significant thing you can do to prevent this infection is to wash your hands before cooking and eating and teach your children to do the same.

Prevent children from putting dirty objects in their mouths whenever possible.

Regularly deworm pets according to the roundworm recommendations of the Companion Animal Parasite Control.

For more information on this subject, see one of the Centers for Disease Control and Prevention fact sheets on visceral larva migrans:

Toxocariasis

How to prevent transmission of intestinal roundworms from pets to people