Waterborne Outbreak of Salmonella I 4,[5],12:i:-

Introduction

A pproximately 1.4 million cases of salmonellosis occur in the United States annually (Voetsch et al., 2004). Salmonella Typhimurium is the most common serotype, representing 19% of confirmed cases (CDC MMWR, 2009). Salmonella serotype I 4,[5],12:i:- was described in the early 1990s, is similar to Salmonella Typhimurium (CDC MMWR, 2008; De la Torre et al., 2009), and is now among the 10 most common serotypes in human infections in the U.S. and worldwide (CDC, 2008; Switt et al., 2009). Infections have been attributed to contact with infected animals such as swine, poultry, and turtles, and to consumption of food containing pork, chicken, and turkey (CDC MMWR 2008; Switt et al., 2009; CDC MMWR, 2007; USDA FSIS).

Salmonella Typhimurium is the most commonly reported serotype in Tennessee. In 2009, 166 Salmonella Typhimurium cases were reported, and reports of Salmonella I 4,[5],12:i:- infection have gradually increased over the last 5 years (48 cases reported in 2005; 65 in 2008).

Waterborne disease outbreaks in Tennessee are infrequently identified. The Tennessee Department of Health (TDH) identified 39 waterborne disease outbreaks during 1972–2009. Sixteen (41%) were associated with drinking water, 13 (33%) with recreational water, 2 (5%) with water not intended for drinking, and 8 (21%) with source unidentified.

We describe a Salmonella I 4,[5],12:i:- outbreak investigation in a rural community. The public health response involved health department staff, water supply regulators, and the state public health laboratory, and identified an untreated water supply as the source of infection. This collaboration between agencies is part of an effort to improve surveillance of waterborne diseases in Tennessee.

Materials and Methods

For this investigation, routine laboratory-based surveillance for Salmonella infection was conducted as part of the Foodborne Diseases Active Surveillance Network (FoodNet) activities in Tennessee. FoodNet is the principal foodborne disease component of CDC's Emerging Infections Program.

Routinely, persons with laboratory-confirmed infections are interviewed to assess risk factors and potential links to other cases. All Salmonella isolates are serotyped and molecularly subtyped by pulsed-field gel electrophoresis. For this case, field investigation and water testing for coliforms, Escherichia coli, and Salmonella were also performed using standard methods (EPA, 2008).

Results

In August 2008, a child aged 4 months with bloody diarrhea was examined at an urgent care medical clinic in southeast Tennessee. Stool culture revealed Salmonella I 4,[5],12:i:-. The child had been fed only powdered infant formula mixed with tap water from the home. The water at the residence was supplied via a private spring-fed system. No other risk factors for salmonellosis were identified. Case interview and investigation identified four additional persons ill with diarrhea, the child's parents and two older siblings. No other community members reported illness.

To initiate interagency collaboration, TDH officials consulted experts with Tennessee Department of Environment and Conservation (TDEC) concerning water sampling and this system's regulatory requirements. TDH officials collected home tap water twice. The spring system supplied five other single-family dwellings and a 50-member church facility. Water samples were collected at the church for testing. Access to the spring was denied by the landowner until officials representing TDEC visited the spring's owner to discuss water safety and this investigation. Standard water samples were collected at the spring site and submitted for routine water testing.

Salmonella I 4,[5],12:i:- was isolated from water in the infant's home and the church (Table 1). Patient and environmental Salmonella I 4,[5],12:i:- isolates were indistinguishable based on pulsed-field gel electrophoresis. E. coli was detected in the water at the spring; Salmonella culture was not performed as part of this routine testing procedure by TDEC.

Officials noted that the area surrounding the spring consisted of porous limestone geology, referred to as karst, and was likely subject to the influence of surface water. Upon observation, the water emerging from the spring collected in an unprotected reservoir and then supplied a reservoir tank for the residences and church by gravity flow without treatment. In addition, newer homes using on-site waste water disposal have been built on a bluff above the spring.

Discussion

Surveillance and investigation of a case of salmonellosis by health department and regulatory officials identified an outbreak; a contaminated rural public water system was the likely cause. Health department officials informed homeowners of the contaminated spring water and provided education regarding safe drinking water options, including continuous water treatment, boiling, and purchasing drinking water.

Because water samples from multiple sources were contaminated with pathogens, it is likely that the water supply was contaminated at its source, representing evidence of environmental contamination and a possible environmental reservoir for Salmonella I 4,[5],12:i:-. Several risk factors for spring contamination were identified. TDEC recommends that owners of private water sources install their own filtration system and periodically test for bacteria. Karst geology, in some geographic regions, provides a link between surface water and ground water, allowing transfer of water contaminants. Because water from this spring collected in a small unprotected reservoir on a slope, it was subject to contamination from runoff and wildlife. As in other karst settings statewide and around the world, septic tanks are a potential source of contamination for ground water. Salmonella I 4,[5],12:i:- contamination of the water source could have resulted from any one or a combination of several risk factors.

Conclusion

Much of the United States is served by municipal water, yet some communities remain without municipal water. This investigation is an example of coordinated investigation and testing of a water system resulting in a public health intervention. Although environmental contamination and human infection by the emerging Salmonella I 4,[5],12:i:- were described, characterization of Salmonella I 4,[5],12:i:- and its potential risk to public health requires continued investigation.