Major Trends In US Waterborne Disease Outbreaks

In the United States, the CDC, EPA and the Council of State and Territorial Epidemiologists maintain a national database known as the Waterborne Disease and Outbreak Surveillance System (WBDOSS) to track waterborne disease outbreaks (WBDOs). In a recent publication issued by researchers at the CDC, EPA and other public health groups, the major trends in WBDOs were presented for data collected over the period of 1971 to 2006 by the WBDOSS (Craun, et al. 2010).

In reporting WBDOs during this period, all drinking water-related outbreaks were tracked. This includes exposures to waterborne diseases through ingestion, inhalation and skin contact. Legionella outbreaks, which comprised a large portion of inhalation-related outbreaks, have only been tracked since 2001.

From 1971 to 2006, the most commonly implicated microbiological agents of WBDOs were parasites and non-Legionella bacteria. These groups represented 18.4% and 13.6% of the cases, respectively. Within the parasite category, the protozoan, Giardia intestinalis, represented the overwhelming majority (86%) of cases. Most of the bacterial disease outbreaks were caused by Shigella and Salmonella, which are bacteria typically associated with sewage.

The single largest outbreak of WBD occurred in 1993 in Milwaukee where 403,000 people were sickened by the protozoan parasite, Cryptosporidium. This outbreak was the largest occurrence of waterborne disease in the nation’s history and prompted the EPA to strengthen the Surface Water Treatment Rule.

Of the outbreaks involving drinking water, the majority of incidents involved contaminated groundwater where the water was either untreated or inadequately treated. These figures have remained relatively steady, suggesting that efforts to ensure the safety of groundwater systems during the period tracked have had little impact in reducing WBDOs linked to groundwater. This may change in the future with the 2009 implementation of the EPA’s Groundwater Rule. By contrast, outbreaks involving surface water declined significantly over this period, presumably due to increased efforts to ensure safe drinking water from surface water sources.

One of the emerging trends observed by the researchers concerned increasing outbreaks associated with premise plumbing deficiencies. Premise plumbing is plumbing that is associated with a building or dwelling (e.g. house, apartment complex, hospital, etc.) receiving water from a utility and is not considered part of the distribution system monitored by the utility. Greater attention needs to be focused on these sources of WBDOs in the future.

Another trend observed concerns WBDOs associated with Legionella bacteria. Legionella are Gram-negative bacteria that cause a type of pneumonia. They are generally distributed by airborne droplets. Legionella bacteria were associated with all respiratory-type WBDOs from 2001 to 2006. Since Legionella have only been tracked since 2001, the number of Legionella-related WBDOs is probably undestimated. In all of these cases, Legionella occurred in premise plumbing and other water distribution infrastructure not associated with public utilities.

By tracking and reviewing major trends in WBDOs, public health officials and regulators can determine the efficacy of efforts to improve water treatment and infrastructure. Monitoring of emerging water quality issues such as WBDOs in premise plumbing and rising Legionella outbreaks can focus efforts to reduce risks associated with these infections either through improved maintenance and monitoring protocols and/or regulatory action.

LEGEND’s network of laboratories supports these efforts through offering a wide range of analytical services for waterborne pathogens, including sewage-related microorganisms and Legionella. For more information on these services, please contact LEGEND’s Phoenix, Arizona laboratory.

References:

Craun, Gunther et al. “Causes of Outbreaks Associated with Drinking Water in the United States from 1971 to 2006”. American Society for Microbiology, 2010.