Human sewage or contaminated manure may enter streams, rivers and reservoirs through direct discharge or surface water drainage following heavy rainfall and contaminate those surface waters with oocysts. USDA surveys indicate that newborn calves, lambs, goat kids, foals and piglets are at greatest risk of infection from Crypto . Livestock are most susceptible to contracting illness caused by the parasite from birth to about one month of age. Most species of wildlife may be infected by various species of Crytpo including mammals, birds, reptiles, and fish but not all these species cause human disease. Documented cases have been reported for deer, raccoons, rabbits, rats, mice and squirrels.
Infected animals pass oocysts in their feces.
The oocysts can be ingested by other animals as they drink contaminated water and graze on pasture grasses or they can be transported into surface water during periods of runoff associated with heavy storms. Human sewage may contain oocysts from infected individuals, so improperly functioning septic systems, raw sewage discharges and treated wastewater are other sources of exposure. Since oocysts are resistant to chlorine, normal wastewater treatment may not remove or kill all the oocysts. To become infected, the parasite must be ingested. Ingestion of as few as 30 oocysts may induce infection in normal individuals, but the number causing infection in individuals with weakened or ineffective immune systems may be much lower.