Secondary transmission of cryptosporidiosis associated with swimming pool use.

*Corresponding author: Mailing address: Division of Epidemiology, Chiba Prefectural Institute of Public Health, 662-2 Nitonacho, Chuo-ku, Chiba 260-8715, Japan. Tel: +81-43-266-7996, Fax: +81-43-265-5544, E-mail: [email protected] Cryptosporidiosis is a diarrheal disease initiated by ingestion of Cryptosporidium oocysts. The incubation period is 2 -10 days, and the chief symptom is watery diarrhea. The transmission to humans can occur by close association with infected animals, via person-to-person transmission, or from contaminated drinking water or less frequently a sprinkler fountain or a swimming pool (1,2). Here, we report secondary transmission of cryptosporidiosis through using a swimming pool. From August 20 24, 2004, school children in a sports center participated in a joint swimming training in Nagano Prefecture using a swimming pool in a hotel. Two hundred and twenty-two among 273 participant children developed diarrhea and other intestinal symptoms from August 20 to September 1 with a peak incidence at August 27. The outbreak was considered to be cryptosporidiosis caused by use of the contaminated swimming pool in the hotel. After returning from the joint training, the participants used 10 swimming pools belonging to the sports center in Chiba Prefecture. On September 4 these swimming pools were investigated for the presence of Cryptosporidium oocysts, and on September 8 it was recognized that two of them tested positive. The Chiba Prefecture public health authority suspended use of the two swimming pools and ordered that they be cleaned. The family members of patients were advised to consult doctors when they developed symptoms. The authority also alerted and called citizens’ attention to the infection. N and K public health centers that control the two swimming pools searched for possible secondary infection through the use of the two swimming pools. A search was made for people developing diarrhea, abdominal pain or fever later than August 25 from among the population who had a chance of contracting secondary infection, i.e., those who did not participate in the joint training in Nagano Prefecture but used either of the two swimming pools. Investigation by N public health center identified 41 symptomatic patients among 1,819 (2.2%), and investigation by K public health center identified 7 symptomatic patients among 1,004 (0.6%). Cryptosporidium was detected from one of the four symptomatic patients (first symptom on September 10) in N public health center and from one of the two symptomatic patients (first symptom on September 9) in K public health center (Fig. 1, Table 1). These cases were considered as secondary infections on the following grounds. First, the patients used the swimming pools from which Cryptosporidium oocysts were detected. Second, there was a cluster of patients with diarrhea and other symptoms in the 1st to 2nd week of September and the oocysts were