So what is the public concern about? Cryptosporidium means 'hidden spore'; transmitted readily by water systems, the organism is named after the ease with which it can escape detection in samples. Without a cure, it has earned a worldwide reputation as a serious threat to public health.
First detected in Europe only as recently as the 1980s, Cryptosporidium has often struck communities, which had until then enjoyed safe water with apparently high standards of filtration. In 1987, Cryptosporidium hit Carrollton, a university town in rural Georgia, USA. Thirteen thousand people out of a total population of 65,000, many of them students, fell ill with diarrhoea, stomach cramps and vomiting. The authorities discovered that every household on the water supply system received contaminated water, yet the town's filtration system complied fully with national filtration standards. In 1993 the City of Milwaukee, Wisconsin, suffered on an even bigger scale. 400,000 residents fell ill through contamination of the water system with four thousand residents hospitalised at an estimated cost of $54 million. There have also been over a dozen outbreaks reported in the United Kingdom.
What worries health authorities is Cryptosporidium's resistance to many traditional treatments including chlorination, rendering much of the treatment process irrelevant. A single-celled organism, it has many attributes of the 'nightmare bug' -it is difficult to detect, able to live for long periods in cold dark reservoirs and pipes, strikes large numbers of people and presents a threat to life for the elderly, very young and immuno-compromised.
It is unclear how it is able to survive chlorination although it is believed that its protective 'oocyst' membrane plays a role. One theory is that the organism might use a pump mechanism to expel toxins from its inside before they cause it harm. Whatever its defence, it is effective: a study by the US Environmental Protection Agency suggested that Cryptosporidium could even live on Chlorine products.
Although difficult to detect it is still relatively rare in most well maintained public water systems. The greatest danger is to people who use private water supplies. These supplies are, of course, tested. Research* in the United Kingdom suggests effective screening requires very frequent analysis of water samples. With less advanced filtration systems in private water supplies, the organism is able to pass easily, often escaping detection by hiding in the biofilm of the water system.
By 1992 cases of Cryptosporidium in the United States had reached the same level as those of Giardia, until then the most common microbial contaminant in water supplies. Cases in Europe also increased. With Cryptosporidium's apparent resistance to disinfection and its ability to slip through any but the best kept filtration systems, the onus is on health agencies and water suppliers to invest in effective detection systems. As a contaminant of the water supply, the organism has the potential to infect large numbers of people particularly where water supplies serve densely populated urban areas. It is the scale of potential harm that puts Cryptosporidium in the first league of emerging pathogens.
Cryptosporidium in water supplies
Cryptosporidium is a very small protozoan parasite of the Coccidia genus. It has a worldwide distribution and is found in a large number of hosts including birds, fish and mammals. The organism can cause an unpleasant, self-limiting, gastrointestinal illness in humans. Two thirds of people who contract the illness (Cryptosporidiosis) are children.
The 1989, in the United Kingdom, an outbreak in Swindon was the first time the country's public health authorities became aware of the potential scale of the problem. Cryptosporidium is excreted in huge numbers and is capable of surviving for long periods in the environment. It can also be passed on as a secondary infection as it requires a low infective dose of less than 100 organisms. The volume of water sample required to screen the water supply has to be large, often as much as 20 litres. With such a small number of organisms diffused in a large volume of water, it is easy to see how the organisms escape detection.
Health action
Following the first series of major outbreaks, governments and their water suppliers had to find answers to many questions. Most research since however, has not provided simple answers. Tests have shown that source protection allied to attention to detail during treatment is the best method to reduce Cryptosporidium numbers in supply water. There is no known safe level for treated water and no health related standard for water quality looks forthcoming.
Private water supplies
This is where the greatest risks lie. Many private water supplies have little source protection and no treatment. What treatment is available is generally ineffective. There is also greater risk from the proximity of farm waste and septic tanks. It is far better to prevent contamination entering a water supply than relying on treatment at the end of the pipe. This emphasises the need for frequent testing and the benefits of converting to well-maintained public water supplies. Further information on private water suppplies can be in found in Cryptosporidium and giardia lamblia in private water supplies by David Clapham & Natasha Franklin. Environmental Health. Issue no. 6/6. June 1998.