Health Stream Literature Summary - Issue 56 - December 2009

Outbreak of giardiasis associated with a community drinking-water source.
Daly ER, Roy SJ, Blaney DD et al. (2009) Epidemiology and Infection.
doi: 10.1017/S0950268809990744

Large outbreaks of giardiaisis associated with community water sources have been uncommon in the USA since the Safe Drinking Water Act of 1974 and subsequent amendments. This article reports on a large outbreak of giardiasis associated with a community drinking water system in 2007 in New Hampshire, USA. After routine water sampling detected the presence of total and faecal coliform bacteria within the distribution system, a boil water order was issued by the New Hampshire Department of Environmental Services (NHDES) on 9 September 2007 for a 205 home community served by a common water system. The community groundwater system consisted of a west system (system A) and an east system (system B), with a number of wells supplying each system. Water was usually distributed without filtration or disinfection. On 17 September NHDES received reports from two residents of system A alleging infection with Giardia. The New Hampshire Department of Health and Human Services (NHDHHS) was notified and an outbreak investigation began.

To identify risk factor for giardiasis, a cohort study was conducted. Questionnaires were distributed to water system customers asking about water consumption habits, use of water filtration devices and details of gastrointestinal illness experienced since 15 August. An environmental investigation was conducted to assess potential entry point(s) of contamination in the water supply and distribution system. Human stool specimens were examined by direct fluorescent antibody (FA) testing and polymerase chain reaction (PCR) testing and subtyping. There were 31 confirmed (n=17) and probable (n=14) giardiasis cases from 27 of the 43 responding system A households; four suspected cases were also identified. There were no confirmed cases identified in system B. A total of 200 surveys were distributed to system A and system B households, and 62 (31%) completed surveys were returned. As cases of illness were confined to system A, only the surveys returned by residents of this system were included in the analysis (43 households containing 100 individuals). Consuming tap water was significantly associated with illness. Of the 63 individuals who consumed tap water, 27 (43%) reported illness compared with three (9%) of the 33 who did not consume tap water [risk ratio (RR) 4.7, 95% CI 1.5-14.4]. The risk of infection was increased by drinking four or more cups of tap water a day (RR 5.0, 95% CI 2.5-10.0) and a significant trend was found between drinking increasing amounts of tap water and increased risk of infection (x2 test for trend = 28.9, P less than 0.001).

One of the two wells serving system A was found to be at moderate risk of surface water influence from a nearby brook. This well had been brought online by the previous owner of the water system without a permit from NHDES. A beaver dam was present on the brook but no significant human interaction with the brook was identified. Faecal coliforms were identified in this well on 10 September and the well was then hyperchlorinated. Repeat coliform testing of distribution samples were negative following hyperchlorination however chlorine residuals were high in the samples and this made the bacteria assay invalid. When chlorine residuals dropped, distribution samples were positive again for total and faecal coliforms signifying ongoing system contamination. There were no positive detections of Giardia cysts in samples of well water. On 21 September the well was disconnected from the system and thereafter no faecal coliform positive distribution samples were found. Human stool samples from 14 patients were found to be FA positive for G. intestinalis. Additional stool samples from three patients were found to be FA- and PCR-positive for G. intestinalis. The G. intestinalis was identified as assemblage B and three specimens from unrelated cases which were characterised by DNA sequencing of part of the triosephosphate isomerise (TPI) gene were found to exhibit a single subtype. The subtype found in human stool specimens in this outbreak has been previously found in beavers in Massachusetts and a Barbary macaque (monkey) in Italy. Giardia cysts were detected at concentrations of 40-50 cysts/litre in a water sample taken in early October from the brook near the contaminated well, although DNA sequencing of the TPI gene showed some differences from the human outbreak cases. The subtype found in the brook is commonly found in humans in the USA, Australia and Egypt and ringed seals in Canada.

This outbreak of human giardiasis is the largest such outbreak associated with a community drinking water system in the USA in 10 years. The epidemiological investigation showed that illness was significantly associated with consuming tap water, and evidence from laboratory and environmental investigations suggested that water from one of the two wells in system A had been contaminated by a nearby surface water body. The outbreak occurred in late summer when the water system experienced high summer usage. Despite the negative PCR result of samples from two beavers trapped during the investigation, the unique subtype previously found in beavers and the presence of these animals in the brook suggest that beavers could have been the source of contamination. The finding of different genetic subtypes in the human outbreak cases and in surface water one month after the outbreak does not exclude this explanation as the prevalence of Giardia cysts in surface water in the USA and Canada is high. This outbreak may have been prevented by adherence to state and federal drinking water regulations related to placement of wells and associated treatment.

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