Cryptosporidium and Giardia: new challenges to the water industry

The protozoan parasites Cryptosporidium parvum and Giardia intestinalis have emerged as significant waterborne pathogens over the past decades. Many outbreaks of waterborne cryptosporidiosis and giardiasis have been recorded,primarily in the United States and the United Kingdom.Chapter 1 gives an overview on the currently available knowledge on the parasites, the disease, the transmission through drinking water and the measures to prevent waterborne transmission. The disease caused by Cryptosporidium and Giardia consists of a self-limiting diarrhoea that lasts for several days in the majority of cases, but the burden of disease and the mortality are high in the immunocompromised part of the infected population. Several characteristics of the parasites facilitate their waterborne transmission: they are very resistant to environmental stress and to chemical disinfection, they can be transmitted from livestock and wildlife to man and their infectivity is high, so even a dose of 1 (oo)cyst gives a discrete probability of infection. The abundance and size of drinking waterborne outbreaks in developed countries show that transmission of Giardia and Cryptosporidium by drinking water is a significant risk. In the case of Cryptosporidium, the absence of an adequate cure for immunocompromised patients increases the problem. Although the outbreaks receive most attention, low-level transmission of these protozoa through drinking water is very likely to occur. Cysts and oocysts are regularly found in drinking water, although only a small proportion may be viable and infectious to man. A major drawback for the determination of the health significance of (oo)cysts in (drinking) water is that methods for a sensitive and specific detection of infectious (oo)cysts, with a consistently high recovery are not available. The cause of drinking water contamination with these parasites that led to the reported outbreaks was not limited to obvious treatment inadequacies or post treatment contamination, but also occurred in apparently well-treated water. Moreover, in several outbreaks, the coliforms, the parameter that was used to demonstrate the microbiological safety of drinking water did not warn against parasite breakthrough through the treatment, particularly because the coliforms were more efficiently eliminated by disinfection than both parasites. Surveys of surface water show that these parasites are ubiquitously present in the aquatic environment, even in pristine environments. Hence, all surface water treatment systems have to deal with these protozoa. These developments raised concern over the safety of Dutch drinking water with regard to Cryptosporidium and Giardia. Considering this situation, the Dutch drinking water companies and government initiated a research programme to determine the (im)probability of transmission of Cryptosporidium and Giardia through drinking water (Chapter 2). The protozoa have changed the philosophy in the developed countries towards safe-guarding of drinking water from monitoring of the ‘end-product’ drinking water to monitoring raw water and the efficiency of the treatment. Furthermore, the extreme resistance of these organisms implies that a “zero-risk” is no longer achievable. Treatments should be designed to reduce the (oo)cyst concentrations in the raw water as far as possible and preferably include filtration step(s). This implies that information on the parasite concentrations in the raw water is necessary, as well as information on the removal efficiency of the treatment. Quantitative risk assessment provides a tool for the combination of information on raw water quality (concentrations detected, recovery of the detection method, viability) and treatment efficiency (removal by different steps in the treatment).