Water quality and waterborne protozoa

Abstract

The nature of waterborne outbreaks has changed during the past 100 years (3, 4, 8, 9, 27, 28). This changing scenario of epidemiology and etiology has included: i) a decrease in individual cases and average number of cases per outbreak; ii) an increase in the number of total outbreaks and of those in smaller community water systems; iii) a change of etiology (Figure 2); and iv) a change in the effectiveness of predicting outbreaks by conventional testing methods. Current problems regarding waterborne outbreaks include the inadequacy of current EPA guidelines, the failure of coliform levels to adequately predict waterborne outbreaks, the need for coagulation, filtration, and disinfection to remove Giardia and Cryptosporidium cysts, and the improper design and poor quality of operation of some water treatment facilities (9). The EPA proposals to deal with these problems include providing new criteria for the filtration and disinfection of all surface waters, revising coliform level regulations, and stressing the necessity of the use of multiple barrier systems (10). Importantly, no properly operated water treatment plant using conventional (multiple barrier) treatment with some form of filtration (direct filtration, slow sand filtration or diatomaceous earth filtration) with disinfection has been implicated in a waterborne outbreak of giardiasis or cryptosporidiosis (3). To better understand the role of Giardia and Cryptosporidium in waterborne outbreaks, we must study several technological and epidemiological questions including: i) what is the prevalence of these parasites in surface waters; ii) when recovered, are the cysts viable; iii) what species is recovered and is it transmissible to humans; iv) what role do wild and domestic animals play in contaminating surface waters; and v) what alternatives exist for removal or inactivation of protozoan cysts. Procedures for recovering protozoan cysts from the environment are insensitive, expensive, and tedious (7, 22). Improvements in such techniques are essential for better delieation of the aforementioned questions. While protozoans may play a significant role in waterborne outbreaks, these cases may account for a relatively small proportion of the total Giardia and Cryptosporidium cases in the United States. These waterborne outbreaks, however, undoubtedly fuel the endemic and occasional epidemic nature of these infections in the general population, especially in day-care centers. A better understanding of the environmental dynamics of these protozoan parasites will enable us to limit the spread and disease caused by these agents.