ABSTRACT
Toxoplasma exists in nature in three forms, the trophozoite, the cyst and the oocyst. The most important mode of transmission of infection to man is through the ingestion of poorly cooked meat containing encysted organisms. While the trophozoitic form of the protozoon is particularly sensitive to enzymatic digestion, the encysted form can survive trypsin or acidpepsin for prolonged periods. The presence of viable organisms within striated muscle has incriminated meat as the prime mode of dissemination among carnivores but failed to explain the widespread prevalence in herbivores. The oocysts are the probable mechanism for dissemination of infection to sheep, horses and cattle. Carnivores like cats will excrete oocysts in their feces for two to three weeks following acute infection (Figure 55.1). Once excreted, the oocysts undergo further maturation for three to four days after which they are infectious and in warm moist soil may remain infectious for more than one year. Human or animal contact with soil, grass or other objects contaminated with oocysts and subsequent gastrointestinal processing results in an additional mode of dissemination. Flies can contaminate food with viable oocysts for up to 48 hours after contact with cat feces.
