Prolonged outbreak of giardiasis with two modes of transmission

(Accepted 16 November 2005, first published online 29 March 2006)SUMMARYLarge outbreaks of giardiasis caused by person-to-person transmission, or a combination oftransmission routes, have not previously been reported. A large, prolonged giardiasis outbreakaffected families belonging to a country club in a suburb of Boston, Massachusetts, duringJune–December 2003. We conducted a retrospective cohort study to determine the source ofthis outbreak. Giardiasis-compatible illness was experienced by 149 (25%) respondents to aquestionnaire, 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 (riskratio 3.3, 95% confidence interval 1.7–6.5). In addition, 105 secondary cases probably resultedfrom person-to-person spread; 14 cases did not report an onset date. This outbreak illustrates thepotential for Giardia to spread through multiple modes of transmission, with a common-sourceoutbreak caused by exposure to a contaminated water source resulting in subsequent prolongedpropagation through person-to-person transmission in the community. This capacity for acommon-source outbreak to continue propagation through secondary person-to-person spreadhas been reported with Shigella and Cryptosporidium and may also be a feature of other entericpathogens having low infectious doses.INTRODUCTIONOutbreaks of giardiasis linked to drinking water [1]and food handlers [2] have been documented. Large,prolonged outbreaks linked to recreational water,particularly chlorinated sources, are less common[3–5]. Large outbreaks with extensive secondaryspread have not been reported.Giardiasis is an intestinal infection caused by theprotozoan parasite Giardia lamblia (also known asG. intestinalis or G. duodenalis). Giardiasis is the mostcommonly reported protozoal infection in humans[6]. Giardia exists in two forms: the trophozoite isresponsible for clinical illness, and the cyst for trans-mission of infection. Transmission of G. lamblia is bythe faecal–oral route, and can be person-to-person,animal-to-person, foodborne or waterborne. Inges-tionofasfewas10–25cystscancauseillness[7]. Aftera typical incubation period of 1–2 weeks, symptomslasting 2–4 weeks can develop, including nausea,vomiting, malaise, flatulence, cramping, diarrhoea,steatorrhoea, and weight loss. However, over 60% ofinfected individuals may be asymptomatic or havenon-specific symptoms [8, 9].