Introduction

There are three different groups of micro-organisms that can be transmitted via drinking water: protozoa, viruses and bacteria (Section 3.2) (Table 13.1). They are all transmitted by the faecal-oral route and so largely arise either directly or indirectly by contamination of water resources by sewage or, increasingly, animal wastes. It is theoretically possible, but unlikely, that other pathogenic organisms such as nematodes (roundworm and hookworm) and cestodes (tapeworm) may also be transmitted via drinking water (Gray, 2004).

Protozoa

There are two protozoa frequently found in drinking water that are known to be responsible for outbreaks of disease (Table 13.1). These are Cryptosporidium and Giardia lamblia.

13.2.1 Cryptosporidium

This parasitic protozoan is widely distributed in nature, infecting a wide range of animal hosts including pets and farm animals. However, it was only relatively recently that it was found to be a human pathogen as well. The first recorded case of human infection occurred as recently as 1976. The problem is that the protozoa form protective stages known as oocysts that allow them to survive for long periods in water while waiting to be ingested by a host. They are also able to complete their life cycle in just a single host as well as having an autoinfection capacity (Fayer and Ungar, 1986). Once infected the host is a lifetime carrier and is subject to relapses. In normal patients the protozoa give rise to a self-limiting gastroenteritis that lasts for up to two weeks, with children more at risk than adults. If the patient is immunosuppressed, infection will be life threatening. For example, it is a major cause of death among patients with AIDS (acquired immune deficiency syndrome). Two peaks in the number of infections are seen each year, one in the spring and another in the autumn.

The oocysts of Cryptosporidium are only 4–7μm in diameter and so are difficult to remove from raw waters by conventional treatment.