By Kelly R. Reynolds, MSPH, PhD

Salmonella is in the news again but this time due to an unlikely source- drinking water. The small town of Alamosa, Colorado realized in March, 2008 that even deep aquifers are subject to contamination with bacterial pathogens. How did this rare waterborne organism make it’s way to the community’s tap water and what can we expect next from this persistent pathogen?

An Unlikely Outbreak
Concern of the waterborne disease outbreak in Alamosa, Colorado spread across the nation as the affected population of approximately 10,000 were coping with a boil water notice and told to not even bathe in the tap water until the source and etiology of the outbreak could be better understood. After declaring a state of emergency, calling in the National Guard to distribute bottled water, and weeks of investigation the impact of this latest identified waterborne outbreak is just beginning to be realized.

At the time of this publication, the Alamosa outbreak was linked to 326 confirmed or suspected cases of Salmonella infection. At least 13 people were hospitalized but no deaths were reported. Salmonella are not uncommon pathogens and outbreaks have been documented around the globe but in the U.S. it is usually associated with food. Although not new (S. typhi and Vibrio cholera were the first recognized waterborne pathogens), the recognized routes of transmission for this bacteria may be expanding.

More than 2000 species of Salmonella have been identified and all of these are considered harmful to humans. The primary serovars associated with human enteric illness include S. enteritidis, S. typhi, and S. paratyphi. Since their identification in the 19th century, they have been responsible for tremendous morbidity and mortality worldwide. In the United States, the number of nontyphoidal salmonellosis (illnesses primarily caused by S. enteritidis) is between 2-5 million per year. The odds of contracting salmonellosis are 1:50 to 1:125 per year in the U.S., making it the second most diagnosed gastrointestinal illness. More than 15,000 cases each year result in hospitalization.

Following exposure, symptoms typically occur within 12-72 hours. The bacteria are capable of penetrating through the gut to the intestinal lining where inflammation occurs and enterotoxin is produced. Symptoms may last a week or more and include nausea, vomiting, abdominal cramps, diarrhea, fever, and headache (i.e., salmonellosis). More severe effects are associated with Salmonella serovars that cause typhoid and typhoid-like fevers. Here the bacteria may infect any of the internal organs with a mortality rate 10-times higher than those causing salmonellosis.

Most illnesses last less than a week and those infected usually recover completely. For some, it may be several months before the intestines and bowels function properly. About 2% of infected persons will develop a condition called Reiter’s syndrome, with pains in their joints, irritation of the eyes and painful urination. This syndrome can last for months or years and may lead to chronic arthritis.

Waterborne Transmission
Salmonella outbreaks in water are relatively rare. Readily found on raw poultry products and in human and animal wastes, they are spread by numerous transmission routes. About 95% of the millions of Salmonella cases that occur in the U.S. are due to foodborne exposures, mostly eggs. Water, however may be either directly or indirectly associated with Salmonella transmission.

According to the CDC, there have only been 5 reported Salmonella outbreaks in municipal water over the last two decades. Of these, one was due to contaminated groundwater, two to contamination in the distribution system and another two due to inadequate disinfection. In the Alamosa outbreak, the source water was a series of five deep wells that received no chlorination. Historically this has been accepted given the widely-held assumption of naturally pure groundwater sources. Recognizing that groundwater is also vulnerable to contamination, the USEPA (Environmental Protection Agency) recently promulgated the Groundwater Disinfection Rule (published in the Federal Register on November 8, 2006).1 The rule is aimed at reducing risks of microbial disease in drinking water by targeting all systems that utilize ground water sources (the source linked to 76% of the 183 waterborne disease outbreaks since 1991).

While the contamination source in the Alamosa outbreak has not been identified, Salmonella are commonly found in waters subject to fecal contamination and have been isolated from sewage-impacted groundwater where runoff from agricultural land, leaking domestic drains, and seepage from septic tanks occurs. In the water environment, Salmonella are able to survive for weeks to months and may even grow in warmer seasons.

At the time of the outbreak, Alamosa’s water utility was in the process of adding chlorine disinfection technology to their treatment works. Chlorine is known to be an effective treatment against Salmonella, and other enteric bacteria, but in the absence of routine disinfection contamination of the source water or water in the distribution system is possible, leaving no additional treatment barriers prior to consumption at the tap. To clear the distribution network of any residual contaminants, Alamosa has been purging the pipes with high concentrations (25 ppm) of chlorine. This treatment, however, may cause additional problems where heavy metals leach from the piping materials and create additional health hazards. The recovery process has been slow for the citizens of Alamosa. The lab turn-around time for Salmonella and metals analysis is about 5 days and repeat testing is needed to make certain the water is safe again for consumption.

Increased Environmental Prevalence
In May, 2006, Salmonella made headlines following a study by Consumer Reports magazine that found a spike in Salmonella-contaminated chicken. In a 2003 survey of whole chickens sold in grocery stores 49% were positive for either Salmonella or Campylobacter compared to 83% in 2006. Both bacteria are harmful enteric pathogens causing diarrhea and related gastrointestinal symptoms. Salmonella has long been recognized as a hazard with improperly cooked poultry products, including eggs, but what does all of this have to do with water?

Water and foodborne disease are sometimes hard to differentiate in outbreak investigations. Unless the clinical isolate is also detected in a particular food or water source, determining exposure is problematic after days have often passed before patients seek treatment or definitive testing. Contaminated water plays a role in the persistence and spread of pathogens between and among domestic animals that may transmit the organisms via direct contact, food consumption, or by contributing to the level of environmental contamination.

In the Consumer Reports study, 525 chickens were tested from at least 4 major chicken manufacturers. Although the consumer reports study has been criticized by the US Department of Agriculture (USDA) for not collecting enough samples to draw conclusions relative to an increased incidence, the fact remains that Salmonella was not hard to find in grocery-sold chicken products. What’s more, among all brands, 84% of the Salmonella showed resistance to one or more antibiotics.

In August, 2006 the federal Centers for Disease Control and Prevention (CDC) published USDA figures of the presence of Salmonella enterica serotype Enteritidis in broiler chicken carcass rinses increased four-fold during a recent 6-year sampling period (2000 through 2005). Likewise, the proportion of establishments with Salmonella Enteritidis–positive rinses increased nearly three fold during the same period.

Many questions remain, are we at increased risk of Salmonella exposure in food or water? Has Salmonella found additional environmental niches? Are the changes in these, and other, bacterial strains increasing the risk of infection following exposure?

Looking Ahead
Any microorganism that has a shared host (i.e., animals and humans) and multiple environmental reservoirs is difficult to control. Another challenge with Salmonella is that a carrier state can occur in about 4% of those infected. A person who is a “carrier” is permanently infected and can spread the disease to others but has no symptoms of the disease themselves. Typically, the bacteria is sustained in the carrier’s gallbladder and secreted in the stool. Approaching the various exposure routes, with targeted interventions, will have the greatest impact on improving public health. Following the USDA/Food Safety Inspection Service’s survey of Salmonella in broiler chickens, the CDC recommended interventions to prevent continued emergence of the pathogen. Water is likely to be a targeted route- whether it be treatment of the water that the chickens drink or the rinse water used, and re-used, during processing.

The use of chlorine to disinfect water is considered one of the greatest public health advances of the 20th century.2 A 300% decrease in typhoid (caused by S. typhi) was noted after the initial use of chlorine in U.S. cities. Eventually, chlorine disinfectants virtually eliminated typhoid, cholera and bacterial dysentery in the United States. Although conventional water treatment processes, when properly administered, should eliminate Salmonella they are more resistant to treatment than the bacterial indicators (i.e., E. coli) used to routinely monitor drinking water. In addition, municipal treatment is the final barrier prior to tap water consumption and provides no protection from contamination of drinking water in the distribution system, where Salmonella regrowth is a concern.

For the most complete protection from waterborne pathogens, a point-of-use (POU) water treatment device device should be considered. Systems utilizing reverse osmosis, distillation, disinfection, filtration, and other methods, are effective for removal of harmful bacteria. Salmonella is a relatively large enteric bacteria (0.6 micron) and can be effectively removed by conventional filtration. Many commercial POU devices are available with a 0.2 micron absolute pore size rating for microfiltration capabilities. Disinfection treatments (ozone, chlorine, UV, etc.) provide an added measure of protection.


  1. USEPA (2006) Prepublication of ground water rule federal register notice. national primary drinking water regulations: ground water rule. 40 CFR Parts, 9, 141, and 142. EPA-HQ-OW-2002-0061; FRL-RIN-2040- AA97.
  2. CDC. Ten great public health achievements—United States, 1900-1999. Morbidity and Mortality Weekly Report, 48(12)241-243.

About the author
Dr. Kelly A. Reynolds is an associate professor at the University of Arizona College of Public Health. She holds a Master of Science degree in public health (MSPH) from the University of South Florida and a doctorate in microbiology from the University of Arizona. Reynolds has been a member of the WC&P Technical Review Committee since 1997. She can be reached via email at [email protected].


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