Table 22.9.
Some Characteristics of Environmentally Transmitted Parasites
| Organism | Infective Form (Size) | Mechanism of Transmission | Distribution | Reservoirs |
|---|---|---|---|---|
| Giardia lamblia | Cyst (8–16 μm) | Person–person waterborne, foodborne | Worldwide | Humans, beavers, muskrats, voles |
| Cryptosporidium parvum | Oocyst (3–6 μm) | Person–person waterborne, foodborne | Worldwide | Many vertebrates, especially cattle |
| Entamoeba histolytica | Cyst (10–16 m) | Person–person waterborne, foodborne | Areas of poor sanitation | Usually humans (potentially pigs, primates, and dogs) |
| Naegleria fowleri | Trophozoite (10–15 μm) | Trophozoite swims up nasal cavity | Worldwide | None: free-living in aquatic or soil environment |
| Cylospora cayetanensis | Sporulated oocyst (8–10 μm) | Waterborne, foodborne | Asia, Caribbean, Mexico, and Peru | None known |
| Enterocytozoon bieneusi | Spore 1–1.6 μm | Fecal–oral | Not known | None known |
| Encephalitozoon hellem | Spore 1–2.5 μm | Urine–oral | Not known | None known |
| Encephalitozoon cuniculi | Spore 1–2.5 μm | Fecal/urine–oral | Not known | Laboratory rabbits, rodents, dogs |
| Encephalitozoon intestinalis | Spore 1–2 µm | Fecal/urine–oral | Not known | None known |
| Toxoplasma gondii | Sporulated (11–12 μm) | Oral ingestion from soil or litterbox (oocyst), undercooked meats (oocyst), undercooked meats (tissue cysts), Waterborne | Worldwide | Cats (definitive host), humans, sheep, goats, pigs, cattle, birds (intermediate hosts) |
| Ascaris lumbricoides | Embryonated egg | Oral from soil contact | Worldwide | Humans |
| Trichuris trichiura | Embryonated egg | Waterborne, foodborne | Worldwide especially tropics | Humans |
| Necator americanus | Filariform larva | Skin penetration | Tropical Africa, Asia, Central and South America and Caribbean | Humans |
| Ancylostoma duodenale | Filariform larva | Skin penetration, oral from soil contact | Tropical Africa, Asia, Central and South America and Caribbean | Humans |
| Taenia saginata | Cysticercus | Ingestion of undercooked beef containing cysticerci, waterborne | Worldwide | Cattle (intermediate host) |
| Schistosoma mansoni | Cercaria | Penetrate skin | Arabia, Africa, South America, and Caribbean | Humans, snail (intermediate host) |