Cryptosporidium is a genus of protozoan parasites. Some species infect mammals including cattle, sheep, rodents, cats and dogs, but also birds, fish and reptiles. It can cause diarrhoea in humans, and protracted diarrhoea in people with an immune deficiency. Faecal-oral transmission can occur directly through person-to-person and animal-to-person routes or indirectly through environmental vehicles including water and food. Outbreaks have been reported in healthcare facilities and daycare centres, within households, among bathers and water sports participants in lakes and swimming pools, and in municipalities with contaminated public water supplies or people served by private water supplies [1]. The disease in humans is predominantly caused by the species Cryptosporidium hominis and C. parvum, although a number of other species are also pathogenic for humans.

Cryptosporidium oocysts can resist harsh environmental conditions (heat, cold or chemical insult) for extended periods of time and can survive for months in moist soil or water. Furthermore, oocysts can survive most common water disinfection procedures, including chlorination [2]. Water distribution systems and swimming pools are particularly vulnerable to contamination with Cryptosporidium and thus pose a considerable threat to public health. Oocysts can, however, be effectively removed by well operated filtration, or killed by UV treatment.

Surveillance of cryptosporidiosis in Europe

Data on cryptosporidiosis cases are collected and recorded by health agencies in several European countries, and the confirmed cases from 16 countries reported to the European Basic Surveillance Network (BSN) in 2005 are presented in Table 1. The reporting is based on the case definition described in EC decision 2002/253/EC, i.e. a clinical description characterised by diarrhoea, abdominal cramps, loss of appetite, nausea and vomiting, or laboratory confirmation of oocysts in stool, intestinal fluid or small-bowel biopsy specimens, or antigen in stool. A total of 7,960 cases were reported to the BSN in 2005, with 70% reported from the United Kingdom. However, the highest incidence was observed in Ireland with 13.7 cases per 100,000. Only five of the 16 countries reported age specific incidence, which revealed an elevated risk among individuals younger than five years of age (5.7 cases per 100,000) and five to 14-year-olds (2.5 cases per 100,000) compared to older age groups (incidence =<1 cases per 100,000) (Figure 1).

It is difficult to compare counts and incidence between countries due to differences in detection, investigation, case definitions, recording and the procedural/legal basis of reporting. The extent to which routine diagnostic laboratories around Europe screen for Cryptosporidium is unclear, but it is likely that there are substantial differences in ascertainment between countries. Furthermore, the reported cases are likely to underestimate the actual burden of cryptosporidiosis due to the insensitivity of passive surveillance. Thus, the currently available data represent only 60% of European countries, and are likely to be biased by the conditions of reporting.

Seasonality

Figure 2 illustrates the percentage of cryptosporidiosis cases per month for individual countries. While this shows differences between months, the data are for a single year only and do not necessarily reflect regular seasonal trends. A peak is observed in the autumn for most countries. However, Ireland saw an increase in spring, and the number of cases in Spain peaked in summer. For certain countries, the available data are sparse (reflecting limitations in laboratory testing and surveillance in these countries). Surveillance data for multiple years would be necessary to confirm the seasonality of the results, but such data are not available on a European level. An attenuated increase in spring cases is observed in the United Kingdom and Sweden. Evidence from England and Wales suggests that cases of cryptosporidiosis in the spring have mainly been caused by C. parvum, while cases in the autumn are frequently caused by C. hominis [3,4]. The seasonality of cryptosporidiosis has changed over the years within England and Wales and the spring peak has substantially decreased since 2001 [3,4]. The autumn cases may be caused by holiday travel and swimming pool use, but the evidence is poor.

Routine surveillance in North West England over 17 years showed that the majority of cases occurred in spring and autumn [4,5]. The introduction of Cryptosporidium drinking water regulations in 1999 that came into effect in 2000/01 together with substantial additional investment in drinking water treatment has led to a reduction in the cases in the spring, but had only a negligible effect on the cases in late summer. Data from eight health authorities in North West England that had previously had regular spring increases have shown a dramatic reduction in these spring cases since 2001, compared to seven control health authorities, where there had never been a regular spring increase (Figure 3). This suggests that improved water treatment such as filtration of previously unfiltered water has resulted in a substantial reduction in the disease [4,5].

A relatively small proportion (2%) of the sporadic and epidemic cases of gastrointestinal infections suffered in Europe is estimated to be waterborne [1], and the case count differs by country (Table 1). The number of reported waterborne infections varies greatly and is probably affected by the quality of the public water supply and sewage disposal systems, and the nature of the surveillance systems for these diseases. Several Cryptosporidium outbreaks associated with public water supplies in Europe have been reported in the literature and selected examples are presented in Table 2.

In 208 of 710 waterborne disease outbreaks officially reported in Europe between 1986 and 1996, the causative agent was identified through epidemiological investigations; of these, Cryptosporidium was implicated in one outbreak in Croatia, 13 in England, one in Spain, and one in Sweden [1]. Cryptosporidium has been linked to drinking water supplies in a number of European Union member states. This issue was examined as part of the European project MedVetNet called Cryptnet (http://www.cryptosporidium.it/index.php?id=04). A recent report on cryptosporidiosis in England and Wales identified 149 cryptosporidiosis outbreaks between 1983 and 2005, 55 of which were linked to municipal drinking water supply, six to private water supplies, 43 to swimming pools, and 16 to contact with animals [3].

Preventing cryptosporidiosis infections

In most European countries chlorine is used to disinfect drinking water and to prevent bacterial growth in the water distribution system. Alternative methods such as ozone (O₃) or UV are also very effective processes of inactivation. In addition, chlorine dioxide is currently used in drinking water in Belgium, France, Germany and Italy to inactivate Cryptosporidium. Although standard chemical disinfection has limitations, flocculation (a process by which fine particulates are caused to clump together into floc) and filtration can remove Cryptosporidium oocysts if carried out properly. Particles suspended in water tend to be negatively charged and repel each other. Coagulation with aluminium sulfate, iron (II) sulphate or iron (III) chloride eliminates this natural charge so that oocysts attract each other and coagulate, building larger particles that will eventually precipitate. Sedimentation and filtration can then provide an effective barrier for Cryptosporidium. Membrane filtration can further improve the quality of the drinking water. Heavy rainfall can cause water drainage systems to overflow and strain water treatment capacity, leading to Cryptosporidium contamination of the water supply, treatment plant, or distribution network [2]. Water catchment management and temporary abandonment of water sources have both been useful in reducing the contamination of source waters, and the World Health Organization (WHO) Water Safety Plans are being used to improve drinking water quality.

In summary, cryptosporidiosis can be a life-threatening disease in immuno-compromised individuals and is of concern in young children. The seasonal BSN data and the longitudinal surveillance from England indicate recurrent exposure of the general public to Cryptosporidium. However, evidence from North West England shows that improvements in drinking water treatment can substantially reduce the number of cryptosporidiosis cases. These data illustrate opportunities for communicable disease control of this rarely reported, but potentially severe disease. Improvements in investigation, detection, case definition, recording and reporting of cryptosporidiosis are important in assessing the disease burden and in identifying outbreaks. Targeted interventions such as upgrading drinking water treatment plants require timely and complete surveillance data in order to assess risks using Water Safety Plans and to monitor the effectiveness of interventions [18,19].

Acknowledgements

The authors would like to thank Dr. Edward van Straten from the Surveillance Unit at the European Centre for Disease Prevention and Control for technical assistance with the data. We would also like to thank the anonymous reviewers for their detailed comments and critical feedback.