Water-borne cryptosporidiosis: a recurring problem

Cryptosporidium , a gastrointestinal protozoan parasite, is increasingly being identified as a cause of diarrhoeal disease in humans. During the 1980s Cryptosporidium emerged as a common cause of infection in both developed and developing nations. The infection rate for individuals with diarrhoeal illness is 2 per cent in industrialized countries and 5-10 per cent in Third World countries. Cryptosporidiosis caused by C. parvum, is thought to be responsible for up to 500 million infections annually in Asia, Africa and Latin America (Current and Garcia, 1991). While cryptosporidiosis may cause acute, self-limiting diarrhoea in otherwise healthy people, infection in the immuno-compromised, such as those with AIDS, may be severe and lifethreatening. The infection rate for AIDS patients is 10 per cent in the USA and 30-50 per cent in developing countries (Petersen, 1993). Several species of the genus Cryptosporidium are known, but C. parvum is considered to be the species responsible for cryptosporidiosis in humans and domestic mammals (Current, 1987). The organism has a faecal-oral mode of transmission. It is transmitted in the form of environmentally stable, robust oocysts which have been excreted in the faeces of infected hosts (human or animal). The oocysts may be ingested in food, drink or transferred from hand to mouth. Low levels of cryptosporidial oocysts are ubiquitous in the water environment. They may be commonly detected in cool, moist environments including polluted, pristine and potable water sources (Lisle and Rose, 1995), and can survive several months in soil or water. Oocysts are resistant to most commonly used disinfectants including chlorine (Korich et al., 1990). In vitro excystation tests on Cryptosporidium oocysts showed that a free chlorine level of 16,000 mg/l was required for zero viability (Smith and Rose, 1990). Conventional filtration of water decreases the level of Cryptosporidium in water supplies (Korich et al., 1990; Madore et al., 1987), but oocysts are frequently detected at low levels in potable water (LeChevallier et al., 1991a), as they can overwhelm standard treatment barriers. There is increasing evidence that water-borne outbreaks of cryptosporidiosis are associated with transient high local concentrations of C. parvum. Oocysts are difficult to identify and enumerate using routine testing techniques and it is especially difficult to detect transient elevations using routine monitoring programmes. The paucity of information on recently identified pathogens such as C. parvum in water sources has obvious public health implications. Water supply companies cannot guarantee that their treated product is Cryptosporidium-free. The consumption of sufficient viable oocysts can result in infection of the hosts. A number of documented water-borne outbreaks have occurred in the UK, Canada and the USA. During the outbreak of cryptosporidiosis in Milwaukee, in the USA in 1993, contamination of a large municipal water supply led to approximately 400,000 people becoming ill (Fox and Lyttle, 1994; MacKenzie et al., 1994). This was the largest outbreak of water-borne disease ever reported in the USA (Berkelman, 1994). Cryptosporidium has become the most important new contaminant for control in drinking water in the USA (Lisle and Rose, 1995). The organism has a similar status in the UK and has attracted attention within the water industry as the consequence of a number of outbreaks of water-borne disease. Some researchers believe that, at present, many small outbreaks go undetected. It is alleged that, in the UK, some outbreaks are not investigated (ENDS, 1995a).