By Wendy C. Brooks, DVM, DipABVP
Educational Director, VeterinaryPartner.com
We normally do not think of human roundworm infections as 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. Antibodies testing shows approximately 14 percent of the U.S. population at large to have been infected by a roundworm species. Most infections occur in people under age 20, with children being at highest risk.
Most people have very mild or no symptoms, but it all depends on how many worm eggs one is exposed to. The worms of concern are Toxocara canis and Toxocara felis. These species of roundworms employ a full body migration through their host's body as part of their development, and symptoms in the human host result as a part of this migration. Three syndromes are described:
Variable symptoms occur as the worm larvae make their way through the human body: fever, abdominal discomfort, swollen lymph nodes, cough, headache, sleep disturbances. This is the mildest of the three syndromes. Visceral Larva Migrans
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 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 (age 1 to 4) as they are ones who are at 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 age 7 to 8 are typically infected.
Approximately 1,000 cases of ocular larva migrans are seen in the U.S. annually and about 700 result in permanent blindness in the affected eye.
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 soil to develop to an infective stage so fresh feces is not a risk but because it is feces that contaminates 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 culprit. Outdoor cats, feral cats in particular, 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, becoming 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, and 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.
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:
How to prevent transmission of intestinal roundworms from pets to people