By Kelly A. Reynolds, MSPH, PhD
On November 10, 2017 the Centers for Disease Control and Prevention (CDC) released a review of the causes of waterborne disease outbreaks associated with drinking water in the US from 2013 to 2014, the most recent assessment period.1 The review, known as the CDC Surveillance Summary of Waterborne Disease Related to Drinking Water, is published approximately every two years and includes detailed information on reported outbreaks investigated two to three years prior. These reviews provide detailed information on outbreak causes and effects that help to identify continued failures in the delivery of safe drinking water at the tap and the expected benefit of POU water filtration.
History of the surveillance system
Since 1971, the Waterborne Disease and Outbreak Surveillance System (WBDOSS) has been used to collect data on reported waterborne disease, including drinking water and recreational water exposures. The WBDOSS consists of the CDC and the US Environmental Protection Agency (USEPA) in cooperation with the Council of State and Territorial Epidemiologists (CSTE). Data is reported to the system by public health departments and published biennially in the Morbidity and Mortality Weekly Report (MMWR). Health departments are responsible for detecting, investigating and reporting outbreaks voluntarily.
Since 2009, data has been reported via an electronic format known as the National Outbreak Reporting System (NORS). This web-based tool has enhanced reporting and improved the accuracy of the system. Under-reporting, however, is well recognized due to the extensive process involved in actually identifying a drinking-water outbreak. Recognition of waterborne outbreaks is complicated by characteristics of delayed onset of illness (generally self-limiting disease) and difficulty in distinguishing water from other potential sources, such as food.
In order for an outbreak to be documented, there must be epidemiological evidence linking two or more cases to an identifiable drinking-water source. Etiological agents may be of microbial (bacteria, viruses, protozoan parasites) or chemical origin. Toxins produced by biological agents, such as cyanobacteria, are also reportable. Studies of the affected populations, along with definitive water test results linking human illness with a source by time and location, provides strong evidence for inclusion in the surveillance database. Depending on the strength of the evidence, the CDC and US EPA weigh the validity of the outbreak data and rank the investigation into four classes with Class I investigations providing the strongest evidence of proof implicating water as the outbreak source and transmission route.
During the 2013-2014 survey period, a total of 42 outbreaks associated with drinking water were reported from 19 states, resulting in 1,006 cases of illness, 124 hospitalizations (12 percent of cases) and 13 deaths.2 The majority of outbreaks (57 percent) and all of the deaths were due to Legionella followed by 19 percent from protozoan parasites (Cryptosporidium and Giardia). Chemicals (4-Methylcyclohexanemethanol; MCHM) from an industrial spill into a public water-system supply and toxin (cyanobacterial toxin) exposures were associated with the highest number of cases, resulting in 50 percent of all illnesses, compared to 13 percent from Legionella (see Figure 1).
During the 2011-2012 survey period,3 a total of 32 outbreaks associated with drinking water were reported from 14 states, resulting in 431 cases of illness, 102 hospitalizations (24 percent of cases) and 14 deaths.2 The majority of outbreaks (66 percent) were due to Legionella followed by non-Legionella bacteria (E. coli O157:H7, Shigella sonnei and Pantoea agglomerans) and viruses (noroviruses) combined making up 16 percent of the outbreaks. Noroviruses were associated with the highest number of cases, resulting in 32 percent of all illnesses, compared to 26 percent from Legionella.
During the 2009-2010 survey period, a total of 33 outbreaks associated with drinking water were reported from 17 states, resulting in 1,040 cases of illness, 85 hospitalizations (8.2 percent of cases) and nine deaths.2 The majority (58 percent) of outbreaks were due to Legionella, followed by 12 percent from Campylobacter—both bacterial pathogens. Campylobacter was associated with the highest number of cases, resulting in 78 percent of all illnesses, compared to seven percent from Legionella. A comparison of the three most recent survey periods shows a consistent trend in Legionella as the leading cause of outbreaks and an increasing number of outbreaks in the most recently reported years (see Table 1).
The top five causes of drinking-water outbreaks are typically Giardia, Legionella, Shigella, norovirus and Campylobacter. Most are susceptible to the most basic of water treatment techniques (chlorination) and yet drinking-water outbreaks continue to increase. Legionella was added to the WBDOSS in 2001 and has increased steadily in importance and recognition as a primary etiological agent of waterborne disease. The Legionella bacterium is difficult to control as it is naturally present in the environment and persists in drinking-water distribution systems and plumbing.
Seventy-one percent of the 2013-2014 outbreaks and 75 percent of the cases occurred in water supplied by community water systems subject to federal regulatory standards. Non-community water systems were also implicated in 12 percent of the outbreaks and 12 percent of the illness cases. Private, individual water supplies only accounted for seven percent of outbreaks but 12 percent of cases were mostly associated with Cryptosporidium contamination.
Groundwater and surface-water sources were both linked to 33 percent of the outbreaks and 16 percent and 79 percent of the cases, respectively. Commercially bottled water was also responsible for one viral outbreak resulting in two illness cases.
The primary deficiency in the protection of drinking water was from Legionella in the plumbing system, causing 55 percent of all outbreaks, followed by unknown or insufficient information (17 percent). The majority of cases, however, were due to treatment not expected to remove contaminants, where the chemical contaminant was unexpectedly involved—also followed by unknown or insufficient information (14 percent). Other causes include untreated groundwater, treatment deficiencies or problems with the distribution system or premise plumbing.
Despite a multitude of precautions, drinking-water outbreaks continue to occur in the US and at an increasing rate in recent years (Figure 2). The primary cause of outbreaks, Legionella, generally contaminates premise plumbing, a point in the water supply outside the jurisdiction of the water municipal provider. Further, treatment trains are not designed to mitigate unexpected events, such as the MCHM spill. The recent survey summary of waterborne disease outbreaks in the US makes a strong case for the benefits and security of POU systems. Hundreds of illnesses and associated hospitalizations and deaths could be easily prevented with POU water treatment devices.
- Benedict KM, Reses H, Vigar M, et al. “Surveillance for Waterborne Disease Outbreaks Associated with Drinking Water–United States, 2013–2014.” MMWR Morb Mortal Wkly Rep. 2017;66(44):1216-1221. doi:10.15585/mmwr.mm6644a3.
- CDC. “Surveillance for Waterborne Disease Outbreaks Associated with Drinking Water and Other Nonrecreational Water–United States, 2009–2010.” MMWR Morb Mortal Wkly Rep. 2013:714-720. www.cdc.gov/mmwr/pdf/wk/mm6235.pdf. Accessed August 11, 2015.
- Beer KD, Gargano JW, Roberts VA, et al. “Surveillance for Waterborne Disease Outbreaks Associated with Drinking Water–United States, 2011-2012.” MMWR Morb Mortal Wkly Rep. 2015;64(31):842-848. www.ncbi.nlm.nih.gov/pubmed/26270059. Accessed December 19, 2016.
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 is WC&P’s Public Health Editor and a former member of the Technical Review Committee. She can be reached via email at [email protected]