We’ve all heard people complain about cats that leave their droppings in flower beds, or defecate in sandboxes, and dog droppings left on sidewalks and lawns. These concerns are about more than aesthetics: household pets can spread parasites such as Toxocara spp. in the environment. People who have contact with the soil, such as children and gardeners may accidentally swallow infectious parasitic organisms.
What is Toxocara?
Toxocara spp. are parasitic roundworms of animals, with T. canis being common in dogs, and T. cati in cats. The adult worms live in the intestine and produce many eggs which are passed in the animal’s feces. After a short period of time in warm moist soil (as little as a week in the right conditions) the eggs are infective and will hatch if ingested by either their definitive host or another animal.
If a dog that has been previously exposed ingests T. canis eggs, the larvae migrate into the tissues and become dormant, remaining there instead of returning to the intestine and maturing to the adult stage. In other animals that ingest the eggs, including humans, the larvae likewise move into and through the tissues, and don’t complete their life cycle. This has several implications for the spread of the parasite, and for the course of human infections:
- In pregnant bitches, dormant larvae resume their travels and migrate to the developing fetuses. Thus, puppies are born already infected with the parasite, even when the mother has no worms in the intestine.
- In small animals that may be eaten by dogs, dormant larvae can help complete the life cycle, but in humans that doesn’t typically happen. The wanderings of the larvae result in symptoms of varying range and severity.
- Ingestion of raw or undercooked meat that contains dormant larvae is another possible source of human infection (Garcia and Bruckner).
Ingestion of larvae in the tissues of prey is a significant source of T. cati infection in cats. And, although kittens are not literally born with the parasite as puppies are, they frequently are infected immediately after birth by larvae transmitted through the mother’s milk.
Visceral larva migrans
The disease caused by the migration of Toxocara larvae through the tissues is called visceral larva migrans or toxocariasis. It is most often attributed to T. canis, with T. cati being involved considerably less often.
Larvae may migrate through virtually any organ (the liver is often involved) and symptoms are often associated with the immune reaction to the larvae, rather than the movements of the larvae. Many infections probably cause only minor symptoms or go unnoticed.
Unfortunately, however, Toxocara larvae have a disconcerting tendency to end their travels in the brain or the retina of the eye, where they can do considerable damage. If many larvae are present in the brain, the infection can be fatal, while even a single larva in the eye can cause eventual loss of vision. Roberts and Janovy suggest that, especially where damage to the eye is concerned, a light infection may be more dangerous than a heavy one, because the immune response may fail to eliminate rare larvae before they arrive in the eye and wreak havoc there.
Signs of wandering Toxocara larvae may include:
- pneumonia-like symptoms
- fever
- enlargement of the liver
- eosinophilia (an increase in eosinophils - white cells that are involved in the immune response to parasitic infection)
- lesions in organs and tissues where larvae are present, including brain damage and inflammation of the retina or other tissues of the eye
Treatment and prevention
Toxocariasis can be treated with antiparasitic drugs, though such treatment will only kill the larvae and is thus most effective before tissues are seriously damaged. The best approach to this infection is prevention:
- provide good veterinary care to dogs and cats, and treat for roundworm infection appropriately
- clean up after household pets
- keep sandboxes clean by covering them when they are not in use
- deter roaming animals from defecating in flower beds and vegetable gardens
- prevent pets from eating small prey that might be infected
- prevent children from ingesting soil
- wash hands and garden produce thoroughly
Sources
- Garcia, Lynn S. and David A. Bruckner. 1997 Diagnostic Medical Parasitology 3rd ed. Washington: ASM Press
- Roberts, Larry S. and John Janovy Jr. Foundations of Parasitology 6th Ed. Boston: McGraw Hill, 2000.
Rosemary Drisdelle - Rosemary Drisdelle has been published many times as a nonfiction writer and several times as a poet. Her first book, Parasites: Tales of ...
