Virulence of three distinct C. parvum isolates in healthy adults

The virulence, as measured by infectious dose (ID₅₀), diarrhea attack rate (AR), ratio of infected to uninfected (IR) volunteers, duration of diarrtiea (DD) and relapse rate (RR), produced by three distinct C. parvum isolates was investigated in healthy adults challenged wtth various oocyst doses. Oocysts used were originally isolated in Ames, Iowa from a calf (Iowa), Auburn from a calf (UCP) and Texas from a horse (TAMU). Various doses of oocysts of the three strains were given to volunteers to establish an ID₅₀. Depending upon infectivity results obtained in groups of challenged volunteers the dose range administered was from 10 to 10⁶ oocysts. Volunteers recorded the number and form of stools passed and symptoms experienced. All stools passed were collected daily for two weeks, and 24 hour samples were collected twice a week for a total of 45 days. Excretion of oocysts was assessed by direct immunofluorascence. Oocysts used were comparable in age, excystation rates and mouse infectivity. The ID₅₀ for humans differed between Isolates: TAMU=6 (projected from existing data), IOWA=92 and UCP=500 (also in progress), as did the AR (TAMU 86%, IOWA 44%, UCP 54%; TAMU vs. IOWA p=0.045) and the IR (TAMU 13:1, IOWA 3.6:1, UCP 4.5:1). A trend towards a longer mean OD was seen for the UCP (163 hr) compared to the TAMU (94.5 hr) and Iowa (71.95 hr) isolates. The RR was similar for all isolates. These data suggest that C. parvum isolates differ in their virulence 'or humans.