Cryptosporidium is a protozoan parasite of the class Coccidia. Of two main types reflecting different seasonal peaks of infection, it is found world-wide and excreted in large numbers by a diverse variety of hosts including birds, fish and mammals and is able to live for long periods in animal droppings in fields, reservoirs and pipes. With a low infective dose - less than 10 organisms and possibly only one - it causes a self-limiting, but nonetheless very unpleasant gastrointestinal illness in humans, among whom two-thirds who contract the illness (Cryptosporidiosis) are children. The elderly and otherwise immune-compromised are also at increased risk.
First identified in 1907, Cryptosporidium (`hidden spore`) is so named due to its very small (4-6 microns) size and the ease with which it can accordingly escape detection in samples. Detection methods have improved a lot since it was first found, but it is still a difficult and expensive process.
Acting through the faecal-oral route (`bum to tum`), without a cure it has earned a worldwide reputation as a serious threat to public health. Although just as common as a food poisoning organism and one that can be acquired during farm visits, at petting zoos and from swimming pools, it is Cryptosporidium in drinking water supplies that has attracted the most attention as it is readily transmitted to large numbers of people in water and it is immune to conventional chlorination. Indeed, chlorination may actually increase the risk, as chlorine encourages the hard-coated oocyst to release the disease-causing sporozoites. Ultra violet light, ozone and membrane filtration are, however, effective against the organism.
Often striking communities which had previously enjoyed safe water, in 1987, an outbreak of Cryptosporidium hit Carrollton, a university town in rural Georgia, USA, affecting 13,000 people out of a total population of 65,000 with diarrhoea, stomach cramps and vomiting. In 1993, the City of Milwaukee, Wisconsin, suffered on an even bigger scale when an estimated 403,000 residents fell ill through contamination of the water system. Four thousand were hospitalised at an estimated cost of $54 million. By 1992 reported cases of Cryptosporidium in the United States had reached the same level as those of Giardia, until then the most common protozoan contaminant in water.
Cases in Europe also increased and since the first major outbreak, in Swindon in 1989, there have been over a dozen more reported in the United Kingdom, including from borehole supplies that had never failed any microbiological standards previously. The last major outbreak was traced to an infected rabbit that fell into a water storage tank.
Following the first series of major outbreaks, governments and their water suppliers had to find answers to many questions. In the UK, a series of official reports (firstly by Badenoch and then by Bouchier) provided a lot of useful information and tests have shown that source protection allied to attention to detail during treatment and not reusing backwash water are the best methods to reduce Cryptosporidium numbers in supply water. Where outbreaks do occur, boiling the water will inactivate the organism.
Though there is no known safe level for Cryptosporidium in treated water, it is nonetheless now rare in well maintained public water networks with multiple barrier treatment and robust detection systems in place and the greatest danger is to people who use private water supplies. These supplies are, of course, tested microbiologically for faecal organisms but, for reasons of cost and practicality, not routinely for Cryptosporidium. With little source protection and no, or inadequate, treatment systems in place and, often, a risk from the proximity of water sources to young stock animals, farm waste and septic tanks, the organism can be a serious problem for users of private supplies who might well be advised to connect to a public supply where available.