Health Stream Article - Issue 56 December 2009

Health Stream Article - Issue 56 - December 2009

Report on Colorado Outbreak

The Colorado Department of Public Health and Environment (CDPHE) has released a report detailing its investigation of the waterborne Salmonella outbreak in the town of Alamosa in March 2008 (1). The investigation concluded that the most likely cause of the outbreak was ingress of faecal contamination from small animals or birds through holes and cracks in a ground level water reservoir. Water from the affected reservoir was supplying about 75% of the town's water needs at the time of the outbreak, resulting in the rapid spread of contaminated water through most of the distribution system. The outbreak is believed to have caused as many as 1,300 cases of illness and one death among the population of 9,800 residents. It also resulted in significant economic disruption to the town and surrounding region over a five week period until a safe water supply could be restored.

The water supply for Alamosa is drawn from a deep confined artesian aquifer several hundred feet below the surface. The town has seven wells drawing from the aquifer, although not all are used simultaneously. At the time of the outbreak, three wells were operating. There are also two elevated storage reservoirs (capacity 500,000 and 150,000 gallons respectively), one ground level storage reservoir (capacity 320,000 gallons) and about 50 miles of distribution pipes. Prior to the outbreak, the water was distributed without chlorination as the town had been granted a waiver from disinfection by the State of Colorado in 1974. Bacteriological monitoring records showed few problems with microbial water quality in previous years and only one non-acute violation of the US EPA Total Coliform Rule in November 2002. However the water supply was non-compliant in terms of chemical quality as it contained levels of arsenic in excess of the current US maximum contaminant level of 10 micrograms /litre. A central water treatment plant was being built to reduce arsenic concentrations and this plant was scheduled for completion in late 2008.

The first signs of the outbreak were recognised on 12 March when three cases of salmonellosis were reported by local medical providers to the regional epidemiologist in the Alamosa County Nursing Service. These cases were reported to the CDPHE on 14 March, and by 17 March evidence from the preliminary epidemiological investigation had suggested the possibility of waterborne spread of the infection (2). Staff of the Safe Drinking Water (SDW) Program within CDPHE were then contacted. They verified that the last routine sampling of the water supply had been carried out on 5 March and no coliform positive samples were found. Widespread sampling of water from the distribution network was ordered on 17 March. By 19 March the results from 10 water samples were available: one was positive for coliform bacteria and two had elevated turbidity levels. Meanwhile the number of laboratory confirmed cases of salmonellosis had risen to 18, with a further 25 suspected cases reported. The epidemiological evidence for a waterborne source had strengthened, and at this point the Chief Medical Officer for Colorado and the SDW team decided to issue an order warning the public not to consume Alamosa tap water, but to use bottled water instead.

The decision to issue a 'bottled water' advisory on 19 March rather than 'boil water' advisory (as would be more usual in cases of microbial contamination) was based on several factors. Firstly it was anticipated that the entire distribution system would have to be disinfected and flushed, potentially dislodging accumulated biofilms, pipe deposits and sediment which might contain high levels of arsenic and other metals. Secondly, the source of the contamination was unknown, and there existed some possibility that non-microbial contaminants might also be present. Thirdly, boiling would tend to concentrate arsenic and perhaps other chemical contaminants in the water.

On the same day that the bottled water notice was issued, the CDPHE issued notices to the Colorado's Water/Wastewater Agency Response Network (CoWARN) initially alerting the network to the existence of the situation in Alamosa, and then requesting equipment and personnel needed for the emergency response. WARN networks have been formed in most US states as a result of Homeland Security Presidential Directives relating to emergency preparedness and response. The networks provide a means for public and private utilities to rapidly request and receive assistance in the form of equipment, materials, services and skilled personnel during emergency response and recovery. These activities are facilitated by a pre-existing template mutual aid agreement and protocols that cover legal, liability and reimbursement issues which would otherwise require time-consuming negotiations when emergency situations arose. The CDHPE report on the Alamosa incident repeatedly emphasises the key role of CoWARN in enabling an effective response to the waterborne outbreak by making additional human and material resources rapidly available to assist the local water utility.

On 20 March a plan to decontaminate the Alamosa water supply was developed which involved draining, cleaning and disinfecting the three storage tanks, then disinfecting and flushing the entire distribution system. The aim was to provide an initial chlorine level of 25 mg/L with a hold time of 24 hours and a residual chlorine level of at least 10mg/L after this time had elapsed. These targets are in excess of US standards for decontamination of water distribution systems, however the report notes that a conservative approach was deliberately taken as the system was previously unchlorinated, and there was uncertainty over the chlorine demand and how this would affect the stability of chlorine residuals. Five additional water samples were collected from the distribution system for Salmonella testing before the disinfection program began. All of these samples proved to be positive for Salmonella by PCR, confirming the widespread contamination.

The decontamination program began with the draining of the in-ground reservoir on 21 March. Chlorination equipment was then connected to the well which fed into the reservoir. An internal inspection of the reservoir structure revealed a number of holes in the upper corners and cracks in the roof and sides. The defects in the above-ground section were also visible from the outside of the reservoir. There was no evidence of birds having entered the structure (no nests or droppings observed inside the reservoir). About 12 to 18 inches of sediment was removed from the bottom of the reservoir, and the holes were patched. The reservoir walls were sprayed with a strong chlorine solution (50mg/L) to counteract the initial chlorine demand, and then it was filled with highly chlorinated water (25 mg/L). This water was left standing in the reservoir for about 39 hours, after which chlorine levels were checked and were found to still be around 25mg/L. The two elevated storage reservoirs were then filled with highly chlorinated water and left standing for 24 hours. Physical inspection of the elevated reservoirs was not performed until several days later, as these procedures could only be undertaken by a specialised company.

After disinfection of the three storage reservoirs, flushing of the distribution system with highly chlorinated water (25mg/L) was commenced. During the flushing program members of the public were notified to avoid contact with tap water when high chlorine levels were present in their area due to the possibility of eye and skin irritation. After each sector had been flushed, tests were carried out for total coliforms and Salmonella. Inadequate disinfection was encountered only in one area where lower flushing velocities had been used due to concerns over the physical condition of old pipes and valves. The disinfection and flushing procedure was carried out again in this area, and subsequent test results were negative.

The flushing operation was completed on 2 April, some 13 days after decontamination of the water supply began. The CDPHE notes this was much less time than the three to four weeks initially estimated. On 2 and 3 April the elevated tanks were taken out of service for inspection and cleaning. Both were found to contain sediment and one had holes in the top due to missing bolts that might have allowed faecal material from birds to enter the tank. Minor repair work was needed on both tanks. During the flushing process, monitoring of arsenic, lead, copper and disinfection byproducts in the distribution system showed a slight increase in arsenic levels (from 26 microgram/L to 32 microgram/L) but other parameters remained below regulatory limits. After flushing was completed, chlorination was maintained throughout the system at levels of 1-2mg/L. During the disinfection and flushing program precautions were taken to protect the town's sewage treatment plant from the effects of high chlorine levels that could have disrupted biological sewage treatment steps. Sewage inflows were monitored for chlorine levels and, when necessary, sewage was diverted to sludge ponds or treated by addition of sodium bisulphite to neutralise chlorine.

Based on the results of post-disinfection water quality tests, the CDPHE replaced the 'bottled water' advisory with a 'boil water' advisory on 3 April. They advised the City of Alamosa that further information would be needed before the boil water advisory was lifted, including a report on data collected, a summary of disinfection and flushing operations, certification that repairs to water storage tanks had been carried out, and confirmation that several high-hazard cross-connection risks discovered during the investigation had been addressed. Just when these final requirements for returning to normality had been fulfilled, an unexpected setback occurred when on 8 April the SDW team learned that some evidence of Cryptosporidium and Giardia contamination had been found in water samples taken on 20 March. These tests had been performed by the Centres for Disease Control and Prevention (CDC) who had communicated the results to the relevant agencies specified under the National Incident Management System, however the findings had not been passed on to the CDPHE.

The possible presence of protozoal contamination in the water supply system meant that the boil water alert could not be lifted as the decontamination procedures performed may not have been adequate to deal with these more chlorine resistant organisms, particularly Cryptosporidium. In order to rapidly assess whether such organisms were indeed present, the SDW team decided to test water samples by Microscopic Particulate Analysis. This required the collection of multiple 7-gallon water samples which were then transported overnight by road to the testing laboratory some 300 miles away. Tests of these samples on 10 and 11 April failed to detect Cryptosporidium or Giardia. A review of epidemiological data also failed to provide any evidence of increased rates of illness due to these pathogens during the Salmonella outbreak. As the CDC tests had been only weakly positive for protozoal genetic material, it was concluded that there was no definitive evidence of viable protozoa in the water supply, and a decision to lift the boil water notice on 11 April was confirmed. Among the conditions for lifting the notice were undertakings by the City of Alamosa to carry out additional repair work on the elevated water storage reservoirs, and permanent disconnection of the underground storage reservoir from the drinking water system.

The investigation uncovered a number of possible sources for the Salmonella contamination, however after considering all the evidence it was concluded that the most likely cause was ingress of faecal contamination into the ground level reservoir through holes and cracks in the structure. It was evident that the reservoir had been in a state of poor repair for some years and the inside had apparently had not been cleaned for at least 11 years. US federal regulations did not require inclusion of water storage tanks or distribution systems in sanitary surveys for groundwater supplies until December 2009, however Colorado state regulations did cover such facilities prior to the outbreak. Despite this requirement, the reservoir had not been included in the facility inventory for the most recent sanitary survey conducted by the SDW Program in 2007. The report states that due to personnel limitations the SDW sanitary inspections tended to focus on treatment issues for surface water supplies, implying that storage and distribution issues and groundwater systems generally may have been overlooked. The investigators also note that the attention of the local water utility was concentrated on the new water treatment plant which was due to come into service a few months after the outbreak. In addition to reducing arsenic levels in the water supply, the new plant would also provide disinfection. When this facility was commissioned, it was planned to remove the ground level reservoir from the water supply and use it for storage of irrigation water. This may have contributed to the failure to address the poor physical condition of the reservoir.

Serotyping and pulsed field gel electrophoretic analysis of Salmonella Typhimurium strains from water samples and human outbreak cases showed a single strain was involved. This suggests the contamination probably originated from one animal or a small number of animals. Calculation of the number of Salmonella bacteria required to contaminate the entire 1.6 million gallons of water in the storage tanks and distribution system suggested that only a few grams of animal faeces might be needed. A number of animal faecal specimens were collected from the vicinity of water storage facilities and analysed during the investigation but none were positive for Salmonella. Water utility staff reported that small animals such as rabbits were sometimes observed near the reservoir, and animal footprints were often seen in the snow around the tank during the winter months. The investigators speculated that the relative warmth of the tank (due to an incoming water temperature of about 75 degrees F from the deep aquifer) and venting of warm air through the holes might have attracted animals during the cold season. A significant snowfall event had occurred about a month prior to the outbreak, and this was followed by two relatively warm periods each lasting a few days where temperature rose above freezing. This may have allowed contaminated snowmelt to enter the reservoir.

Contamination was apparently present at least intermittently from late February or early March (estimated from symptom onset on 6 March in the first laboratory confirmed cases) until 20 March when the last pre-disinfection samples were taken. The daily outflow of water from the reservoir exceeded the storage capacity, so the duration of contamination may indicate either multiple episodes of faecal ingress, or persistence or perhaps growth of Salmonella bacteria due to the warm water temperature and stagnant areas within the reservoir where water did not mix thoroughly.

The detailed assessment of the water supply system, operational practices and monitoring records also revealed a number of shortcomings which did not play a role in the outbreak, but nevertheless constituted a risk to water quality and safety. For example, it was found that although the correct number of water samples was being regularly tested under the Total Coliform Rule (TCR), the sampling locations and dates of sampling were in violation of the TCR requirements. Samples were usually collected from points where each well entered the distribution system and at the outlet of the ground level reservoir, rather than from locations representing the whole distribution system. In addition samples were generally collected on the same day rather than being spread over the month. Three extreme hazard cross-connection risks were identified where there was potential for contaminated water from business premises (two mortuaries and a combined meat packing facility/restaurant) to flow back into the distribution system in the event of low-pressure events in the distribution system. More than 100 lower risk potential cross-connection sites were also detected.

Significant corrosion was found in the well casing for one of two wells where video inspection records were available from prior to the outbreak. Although all the town wells draw water from a deep confined aquifer (believed to be secure from microbial contamination), the well shafts pass through a shallow unconfined aquifer. Contamination from this shallow aquifer could potentially enter the water supply if the integrity of the well casing was breached within the shallow aquifer zone.

In summing up the lessons from the outbreak, the investigators note the outbreak was detected rapidly by the public health surveillance system and the probable link to the drinking water system was identified within one week of the first laboratory confirmed case being reported. The CoWARN system allowed a very rapid and effective response to be mounted, with equipment and personnel from other utilities being available within 24 hours of the initial call for assistance. The National Incident Management System Incident Command System was effective in coordinating communication with the media, distribution of bulk water supplies to businesses, managing volunteers and public communication activities. Local, state and federal laboratories cooperated effectively in providing rapid testing and assisting with logistical issues surrounding sample transport, however there was a failure in the communication chain regarding reporting of CDC test results to the CDPHE.

The outbreak response was very demanding on the personnel resources of the SDW Program as emergency response staff were not trained in drinking water system operations, and thus the SDW team had to provide review and input to communication and coordination activities and operational decisions. In addition to dealing with restoration of the public water supply, the SDW team also had to respond to numerous requests from businesses for technical advice and approval to establish individual water supplies to enable them to keep operating during the crisis. The report notes that under-resourcing of the Colorado SDW Program has been a long standing issue and that although staff numbers have increased significantly in recent times, they are still below the levels deemed adequate by the US EPA.

In the wake of the Alamosa outbreak the Colorado Safe Drinking Water Program has announced a number of actions to reduce the possibility of further waterborne outbreaks. These include:
• updating and modification of state regulations concerning groundwater disinfection and the granting of disinfection waivers.
• a review of all systems with current disinfection waivers to assess whether the waiver should be withdrawn and disinfection required.
• higher prioritisation of responses to regulatory violations and deficiencies in non-disinfected systems.
• inclusion of water storages and distribution systems in sanitary inspections, as well as improved oversight of total coliform monitoring practices.
• requiring timely correction of deficiencies identified during inspections.
• ensuring compliance with maintenance of chlorine residuals in disinfected systems.
• revision of regulations governing cross-connection risks.
• training initiatives to help public drinking water system operators improve operating and maintenance practices for storage tanks and distribution systems.
• development of strategies to enhance response capabilities for drinking water emergencies.

(1) This outbreak was initially reported in Health Stream Issue 49, March 2008. The CDPHE investigation report can be obtained from:
www.cdphe.state.co.us/wq/drinkingwater/AlamosaOutbreak.html
(2) Details of the epidemiological investigation which linked the Salmonella cases to the water supply have not yet been released, however the clues which may suggest a waterborne origin for outbreaks include a higher risk of illness in those reporting cold tap water consumption compared to those reporting no or little cold tap water consumption, the geographic pattern of cases relative to water distribution zones, and lack of association with food items, food venues, social events or other non-water exposure sources among cases.

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