Prolonged outbreak of giardiasis with two modes of transmission.
Large outbreaks of giardiasis caused by person-to-person transmission, or a combination of transmission routes, have not previously been reported. A large, prolonged giardiasis outbreak affected families belonging to a country club in a suburb of Boston, Massachusetts, during June-December 2003. We conducted a retrospective cohort study to determine the source of this outbreak. Giardiasis-compatible illness was experienced by 149 (25%) respondents to a questionnaire, and was laboratory confirmed in 97 (65%) of these cases. Of the 30 primary cases, exposure to the children's pool at the country club was significantly associated with illness (risk ratio 3.3, 95% confidence interval 1.7-6.5). In addition, 105 secondary cases probably resulted from person-to-person spread; 14 cases did not report an onset date. This outbreak illustrates the potential for Giardia to spread through multiple modes of transmission, with a common-source outbreak caused by exposure to a contaminated water source resulting in subsequent prolonged propagation through person-to-person transmission in the community. This capacity for a common-source outbreak to continue propagation through secondary person-to-person spread has been reported with Shigella and Cryptosporidium and may also be a feature of other enteric pathogens having low infectious doses.
Epidemic Intelligence Service, Office of Workforce and Career Development, Centers for Disease Control and Prevention, Atlanta, GA, USA. davidkatzl@yahoo.com
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Matyas
D000293:Adolescent; D000328:Adult; D000368:Aged; D002648:Child; D002675:Child, Preschool; D004196:Disease Outbreaks; D005873:Giardiasis / Q000453:epidemiology* / Q000635:transmission*; D006801:Humans; D007223:Infant; D008404:Massachusetts / Q000453:epidemiology; D008875:Middle Aged; D011159:Population Surveillance; D012189:Retrospective Studies; D012307:Risk Factors; D011795:Surveys and Questionnaires; D013551:Swimming Pools; D014871:Water Microbiology