Waterborne transmission of infectious agents.

Waterborne transmission is a highly effective means for spreading infectious agents to a large portion of the population. Several water-related modes of transmission of infectious agents are discussed. Infection and development of clinical symptoms depend on a number of specific and nonspecific host factors, such as age, immune status, gastric acidity, nutritional status, vitamin A deficiency, and possibly genetic predisposition. The majority of poliovirus and hepatitis A virus infections in young children are asymptomatic. Most of the information on the risk factors and etiologic agents of waterborne disease comes from investigations of waterborne-disease outbreaks by state and local health departments and the surveillance program maintained by the CDC and the Environmental Protection Agency (EPA). The commonly recognized waterborne pathogens consist of several groups of enteric and aquatic bacteria, enteric viruses, and three enteric protozoa. The most recent list of candidate contaminants was released in February 2005 and includes nine microorganisms: adenoviruses, caliciviruses, coxsackieviruses, echoviruses, Aeromonas hydrophila, Mycobacterium avium and Mycobacterium intracellulare (together referred to as the M. avium complex [MAC]), Helicobacter pylori, cyanobacteria (blue-green algae) and other toxin-producing freshwater algae, and microsporidia (Enterocytozoon and Septata). The section on the classification of water-related diseases, deals with water-related transmission of infectious agents associated not only with microbiological water quality but also with water availability, sanitation, and hygiene. Finally, for infectious agents with multiple transmission routes, it may be difficult to determine the attributable risk associated with waterborne transmission compared to other routes of transmission, especially in areas where waterborne diseases are endemic.