Interpreting Waterborne Disease Outbreak Data

CDC’s Waterborne Disease and Outbreak Surveillance System (WBDOSS) is a helpful tool for monitoring illness trends, determining exposure routes and risk factors, and planning how to prevent future illness. However, not all outbreaks are recognized, investigated, or reported, and there is no way to know how many outbreaks go undetected.

Most people do not seek health care for their illnesses, and even among people who seek health care, many do not receive any laboratory testing. Laboratory testing does not always identify the germ that caused the illness, and even when a specific germ is identified, healthcare providers might not report all illnesses to public health agencies. Not all illnesses are linked to a common transmission source, so many outbreaks go undetected and unreported. There are additional challenges that need to be considered when interpreting outbreak surveillance data.

Waterborne Disease Outbreaks Are Difficult to Detect

Detecting waterborne disease outbreaks is challenging because many waterborne pathogens can also be spread in other ways (such as through food, person-to-person, or animal-to-person). Linking illness to drinking water is difficult during investigations because most people drink water every day. Recreational water (such as lakes and swimming pools) is often not considered as the source of infection, particularly when illness occurs days after the exposure or the exposure occurs while traveling.

Environmental investigations provide information on factors and deficiencies that contribute to outbreaks and strengthen evidence implicating drinking or recreational water as a common source of infection. However, these investigations might not be conducted if the evidence does not implicate water.

Outbreak Investigation Capacity and Reporting Activities Vary Across States and Localities

For state and local public health agencies to recognize, investigate, and report outbreaks, public health agencies must have the necessary financial and personnel resources. They must recognize and link cases of illness to a common contaminated water source, which requires appropriate epidemiologic, environmental, and laboratory capacity to conduct investigations. This surveillance, investigation, and reporting capacity varies across states and localities. Therefore, it is challenging to interpret reported geographic differences in the occurrence and types of waterborne disease outbreaks.

Public Health Practices Have Changed Over Time

Practices related to outbreak investigation and reporting have changed over time, and these changes make certain germs more or less likely to be detected during outbreak investigations.

For example, new diagnostic tests can increase clinicians’ awareness of certain diseases, leading to increased reporting and outbreak detection for these diseases.

Improved laboratory methods for detecting germs and microbial indicators of contamination may also mean that certain diseases are detected more often.

Generally, outbreak reporting may increase when more is known about how waterborne diseases are spread and as the ability to track and test increases. On the other hand, as local jurisdictions develop the capacity to identify illness clusters by molecular subtyping, they might investigate fewer clusters with unknown causes (for example, cases of illness without a laboratory confirmed germ diagnosis), which could lead to a decrease in waterborne disease outbreak reporting because most cases of illness are not laboratory-confirmed.

Changes in public health practice do not affect the validity of the data in surveillance reports but might limit the ability to interpret trends in the number of outbreaks and types of problems with water system across reporting periods.

Information From Laboratory Investigations Is Often Unavailable, Incomplete, or Inconsistent

Analyses of water or environmental samples for specific germs and indicators of water quality depend on the availability of certified or approved laboratories. Although many laboratories are certified to conduct standard environmental sample analyses for fecal indicators and chemicals, few laboratories have the capability to identify waterborne germs in environmental samples. In addition, the tests needed to do these analyses can be expensive. Waterborne contaminants are usually very dilute, so collecting water samples to identify germs often requires sampling large quantities of water or filtering large volumes of water through special membranes. Environmental samples also traditionally contain many other contaminants that decrease the ability to detect germs compared to common diagnostic samples, such as blood, stool, and urine.

Water samples are often collected late in an investigation or not collected at all, which limits the ability to link clinical and environmental data. Molecular epidemiologic or other laboratory testing that identifies a direct match or link between the germs in patient specimens and those in environmental water samples can establish a water-related exposure. Even with improved water sampling and pathogen testing, water samples collected during an outbreak investigation might not contain the germs that were present when people were exposed. Negative water sample tests do not rule out water as the source of the outbreak.