In healthy adults with no serologic evidence of past infection with C. parvum, a low dose of C. parvum oocysts is sufficient to cause infection.

Abstract. A 50% infectious dose (ID 50 ) of 132 Cryptosporidium parvum oocysts was previously determined in serologically negative individuals (ELISA). In this study, 17 healthy adults with pre-existing anti-C. parvum serum IgG were challenged with 500-50,000 oocysts. Infection and diarrhea were associated with the higher challenge doses. The ID 50 was 1,880 oocysts, Ͼ 20-fold higher than in seronegative volunteers. Fecal oocysts were detected in only seven (53.8%) of 13 individuals with clinical cryptosporidiosis, indicating that the host response may effectively decrease the number of oocysts produced. Subjects with the highest absorbances prior to challenge had little to no increase in IgG following challenge, whereas volunteers with lower reactivities showed significant postchallenge increases. This suggests that an upper limit of serum IgG was present in some subjects, while others were further stimulated by an additional exposure. These data indicate that prior exposure to C. parvum provides protection from infection and illness at low oocyst doses.Cryptosporidium parvum is a recognized cause of diarrheal illness in waterborne outbreaks [1][2][3] and in individuals with human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS). 4 The increased awareness of cryptosporidiosis in the general population and in specialized settings, such as day care centers, as well as the seriousness of the disease in immunosuppressed individuals have earned Cryptosporidium a place on the list of emerging diseases that are a threat to the public health. 5 Furthermore, no effective chemotherapeutic agent has been demonstrated to prevent or cure C. parvum infections. Thus, there is considerable interest in the development of immunotherapies designed to limit infection. To rationally design such interventions, it is important to understand the major immunologic mechanisms that are operative in self-limited disease. Animals 6-8 and humans 9 with intact immune systems are typically capable of clearing the parasite within 1-3 weeks after infection. Several studies with experimental animal models have demonstrated that most species are refractory to a second oocyst challenge. 7,8 Indeed, adult animals exposed to the parasite for the first time are relatively resistant to infection and often have only a transient period of oocyst shedding with few to no symptoms. In contrast, healthy adult volunteers can experience infections and diarrheal illnesses when oocyst challenges are at least one year apart. 10 Many studies in animals 11-15 and humans [16][17][18][19] have documented the presence of antibodies to C. parvum infection. However, these investigations in humans were carried out in serologic surveys or in select persons experiencing diarrhea and only after the infections were diagnosed. Thus, the observations were necessarily limited by the lack of information on the immune status and exposure history prior to the exposure. Also, differences among the studies in antigen preparations used in the testing and definiti...

The comparative occurrence of Cryptosporidium and Giardia was evaluated in 257 water samples from 17 states in the United States. Cryptosporidium oocysts were detected in 55% of the surface water samples at an average concentration of 43 oocysts/100 L, while Giardia cysts were found in 16% of the same samples at an average concentration of 3 cysts/100 L. Giardia and Cryptosporidium were more frequently detected in samples from waters receiving sewage and agricultural discharges as opposed to pristine waters. There was no correlation between the concentration of water quality indicator bacteria and either protozoa. Both protozoa were more frequently isolated in the fall than other seasons of the year. The concentrations of both organisms were significantly correlated in all waters. Cryptosporidium oocysts were detected in 17% of 36 drinking water samples (0.5-1.7 oocysts/100 L) while no Giardia cysts were detected. The widespread occurrence of cysts and oocysts in waters used as supplies of potable water suggests that there is a risk of waterborne transmission of Cryptosporidium and Giardia infections if the water is not adequately treated.

Healthy adults are susceptible to infection with small numbers ofCryptosporidium parvum oocysts, resulting in self-limited infection. We investigated if infection of humans withC. parvum is protective 1 year after primary exposure. At 1 year after a primary challenge with 30 to 106 oocysts, 19 healthy immunocompetent adults were rechallenged with 500 oocysts and monitored for the development of infection and/or illness. Oocyst excretion was quantitated by direct immunofluorescence with a C. parvum-specific monoclonal antibody, and anti-C. parvum antibodies in serum were detected by an enzyme-linked immunosorbent assay. Fewer subjects shed oocysts after the second exposure (3 of 19; 16%) than after the first exposure (12 of 19; 63%) (P < 0.005). Although the rates of diarrhea were comparable after each of the two exposures, the clinical severity as determined by the mean number of unformed stools passed was lower after reexposure (11.25 versus 8.62;P < 0.05). The number of anti-Cryptosporidium immunoglobulin G and A seroconversions increased after secondary exposure. However, the C. parvumserum antibody response did not correlate with the presence or absence of infection.

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