Enteric viruses are obligate human pathogens. That is, they replicate only when within the human host. Their structures may allow prolonged survival outside the human body, in the environment. There are over 120 enteric viruses that may be found in sewage. Table B.1 lists some of the better described viruses, including the enteroviruses (polio-, echo-, and coxasackieviruses), hepatitis A virus, rotavirus, and Norwalk virus, and the annual incidence of disease and case mortality rates for all sources of exposure (Bennett et al. 1987). Virus numbers reported in sewage vary greatly and reflect the variation in infection in the population excreting the agent, the season of the year (outbreaks of viral disease are often seasonal), and methods used for their recovery and detection. Table B. shows virus numbers that have been reported in sewage. Treatment reduces but does not eliminate viral contamination (Melnick and Gerba 1980, Rose and Gerba 1990, Asano et al. 1992).
Over 100 outbreaks of hepatitis and viral gastroenteritis have been associated with the consumption of sewage contaminated shellfish in the United States (Richards 1985). The reported outbreaks have increased from less than 10 in the years 1966-1970 to more than 50 in the years 1981-1985. Although this apparent increase could be due to reporting artifacts, the number reported most certainly represents a great underestimate because of the long incubation period for hepatitis A and the difficulty in tracing the source. From 1983-1989, the incidence of hepatitis A increased 58 percent with 14.5 cases in 100,000 in the United States. An estimated 10 percent of these cases may be due to foodborne transmission, including shellfish (CDC 1990).
Viral outbreaks due to recreational exposure to contaminated waters have been documented in the United States. Between the years 1986 and 1988, 41 percent of these were an undefined gastroenteritis and likely of a viral etiology (CDC 1990). Shigella and Giardia were also predominant causes of recreational outbreaks of disease. Viruses (entero- and rotaviruses) have been isolated from recreational waters in the absence of any discharge from a wastewater treatment plant (Rose et al. 1987).
The parasites of primary public health concern for wastewater exposure are the protozoa and helminths. The helminths include roundworms (Ascaris), hookworms, tapeworms, and whipworms. These organisms are endemic in areas where there is inadequate hygiene and their transmission is generally associated with untreated sewage, untreated sludges, and night soil, with very little documentation of waterborne transmission.