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The infectivity of Cryptosporidium parvum in healthy volunteers
H L DuPont1, C L Chappell, C R Sterling
1University of Texas Medical School, Houston.
Background:
Small numbers of Cryptosporidium parvum oocysts can contaminate even treated drinking water, and ingestion of oocysts can cause diarrheal disease in normal as well as immunocompromised hosts. Since the number of organisms necessary to cause infection in humans is unknown, we performed a study to determine the infective dose of the parasite in healthy adults.
Methods:
After providing informed consent, 29 healthy volunteers without evidence of previous C. parvum infection, as determined by the absence of anti-cryptosporidium-specific antibodies, were given a single dose of 30 to 1 million C. parvum oocysts obtained from a calf. They were then monitored for oocyst excretion and clinical illness for eight weeks. Household contacts were monitored for secondary spread.
Results:
Of the 16 subjects who received an intended dose of 300 or more oocysts, 14 (88 percent) became infected. After a dose of 30 oocysts, one of five subjects (20 percent) became infected, whereas at a dose of 1000 or more oocysts, seven of seven became infected. The median infective dose, calculated by linear regression, was 132 oocysts. Of the 18 subjects who excreted oocysts after the challenge dose, 11 had enteric symptoms and 7 (39 percent) had clinical cryptosporidiosis, consisting of diarrhea plus at least one other enteric symptom. All recovered, and there were no secondary cases of diarrhea among household contacts.
Conclusions:
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.
Insights
A low dose of Cryptosporidium parvum oocysts, as few as 30, can infect healthy adults. This study determined the median infectious dose for Cryptosporidium parvum in humans was 132 oocysts.
Area of Science:
- Infectious Diseases
- Parasitology
- Public Health
Background:
- Cryptosporidium parvum oocysts can contaminate drinking water, posing a risk for diarrheal disease in healthy and immunocompromised individuals.
- The minimum number of oocysts required to cause human infection was previously unknown.
Purpose of the Study:
- To determine the infectious dose of Cryptosporidium parvum in healthy adult volunteers.
Main Methods:
- 29 healthy volunteers received a single dose of 30 to 1 million C. parvum oocysts.
- Participants were monitored for oocyst excretion and clinical illness for eight weeks.
- Household contacts were monitored for secondary spread.
Main Results:
- Infection occurred in 88% of subjects receiving 300+ oocysts and 20% receiving 30 oocysts.
- The median infectious dose was calculated to be 132 oocysts.
- 11 of 18 infected subjects experienced enteric symptoms, with 39% developing clinical cryptosporidiosis.
Conclusions:
- A low dose of Cryptosporidium parvum oocysts is sufficient to cause infection in healthy adults without prior evidence of infection.
- Findings highlight the importance of water treatment to prevent Cryptosporidium transmission.