Most people have heard the common wisdom that you can catch worms from eating dirt—it’s true that a number of parasitic worms infect humans through accidental ingestion of contaminated soil—but these are not the only parasites that may be present. Protozoan parasites, tiny single celled organisms, can remain infective for long periods in soil.
How do Protozoan Parasites Get Into Soil?
Intestinal protozoan parasites are passed in feces. When an infected human or animal defecates on the ground, millions of parasites contaminate soil. Many protozoan parasites have a tough cyst stage—a stage that isn’t actively growing or multiplying, but which can survive and remain infective in the environment until it can infect a new host. A few of the protozoan parasites that infect humans are actually environmental species—species that normally live in soil or water and only accidentally cause disease.
People Eat a Lot of Dirt
We don’t think we’re eating dirt but, actually, we all eat more than we realize. Not only do we often put dirty fingers and objects in our mouths (children are particularly likely to do this), but we eat particles of dirt on raw fruit and vegetables. Tiny protozoan cysts, 5 to 100 one-thousandths of a millimetre wide, are invisible to us—there might be hundreds of them on one dirty finger. While many protozoans are better known for spreading via dirty water, human excrement is sometimes used as fertilizer, and dirty water is used to water crops—soil can certainly be a source of infection.
Protozoan Parasites in Dirt
The parasitic protozoa that spread through soil and water are divided up into groups—amoebae, flagellates, ciliates, and coccidia:
- Amoebae: The most common of the intestinal amoebae that cause disease is Entamoeba histolytica, the agent of amoebic dysentery. This dangerous parasite not only causes severe diarrhea but can cause abscesses in the intestine, liver, and other organs. Best known for contaminating water supplies, E. histolytica can easily contaminate food crops as well. An environmental amoeba, Acanthamoeba spp., which is usually found in water, decaying organic matter, soil, and sewage, is of particular interest to contact lens wearers—unfortunately, it’s capable of multiplying in a human eye cornea and causing vision destroying infection.
- Flagellates: Found in surface waters all over the Earth and spread in the feces of both humans and animals, Giardia lamblia is the most infamous of the intestinal flagellates that cause disease.
- Ciliates: Only one ciliate parasitizes humans. It’s Balantidium coli, a large protozoan associated with pigs. Contact with pig feces or soil contaminated with pig feces spreads this parasite, which can cause severe diarrhea and intestinal abscesses.
- Coccidia: A number of coccidia cause parasitic disease in humans. Toxoplasma gondii contaminates the soil world wide and infects all warm blooded vertebrates. Cyclospora cayetanensis has been associated with contaminated fruits and vegetables, and Cryptosporidium parvum is found in surface waters everywhere.
Avoid Protozoan Parasites in Dirt
It’s impossible to completely protect yourself from microscopic intestinal parasites, but you can lessen your chances of ingesting them:
- Keep dirty hands out of your mouth.
- Wash your hands thoroughly before handling food or eating.
- In areas where sanitation is not good, avoid eating uncooked fruits and vegetables that you have not washed and peeled yourself.
- Do not drink untreated water.
Sources:
Diagnostic Medical Parasitology 3rd Ed. Garcia, Lynn S. and David A. Bruckner. Washington: ASM Press, 1997.
Foundations of Parasitology 6th Ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2000.
