Fecal antibodies to Cryptosporidium parvum in healthy volunteers

This study examined the intestinal antibody response in 26 healthy voluntee rs challenged with Cryptosporidium parvum oocysts. Fecal extracts were assa yed for total secretory immunoglobulin A (IgA) and C. parvum-specific IgA r eactivity. Specific IgA reactivity was standardized to IgA concentration an d expressed as a reactivity index (RI). Anti-C. parvum fecal IgA (flgA) inc reased significantly in 17 of 26 (65.4%) following oocyst ingestion. Of tho se with detectable responses, 59, 76.5, and 94.1% were positive by days 7, 14, and 30, respectively. Volunteers receiving high challenge doses (>1,000 and 300 to 500 oocysts) had higher RIs (RI = 5.57 [P = 0.027] and RI = 1.6 8 [P = 0.039], respectively) than those ingesting low doses (30 to 100 oocy sts; RI = 0.146). Subjects shedding oocysts and experiencing a diarrheal il lness had the highest fIgA reactivity. When evaluated separately, oocyst ex cretion was associated with an increased fIgA response compared to nonshedd ers (RI = 1.679 versus 0.024, respectively; P = 0.003). However, in subject s experiencing diarrhea with or without oocyst shedding, a trend toward a h igher RT (P = 0.065) was seen. Extracts positive for fecal IgA. were furthe r examined for IgA subclass. The majority of stools contained both IgA1 and IgA2, and the relative proportions did not change following challenge, Als o, no C. parvum-specific IgM or IgG was detected in fecal extracts. Thus, f ecal IgA to C. parvum antigens was highly associated with infection in subj ects who had no evidence of previous exposure and may provide a useful tool in detecting recent infections.