Virulence of Three DistinctCryptosporidium parvumIsolates for Healthy Adults

Abstract

The infectivity of three Cryptosporidium parvum isolates (Iowa [calf], UCP [calf], and TAMU [horse]) of the C genotype was investigated in healthy adults. After exposure, volunteers recorded the number and form of stools passed and symptoms experienced. Oocyst excretion was assessed by immunofluorescence. The ID₅₀ differed among isolates: Iowa, 87 (SE, 19; 95% confidence interval [CI], 48.67–126); UCP, 1042 (SE, 1000; 95% CI, 0–3004); and TAMU, 9 oocysts (SE, 2.34; 95% CI, 4.46–13.65); TAMU versus Iowa, P = .002 or UCP, P = .019. Isolates also differed significantly (P = .045) in attack rate between TAMU (86%) and Iowa (52%) or UCP (59%). A trend toward a longer duration of diarrhea was seen for the TAMU (94.5 h) versus UCP (81.6 h) and Iowa (64.2 h) isolates. C. parvum isolates of the C genotype differ in their infectivity for humans.