Cryptosporidiosis and swimming pools

Cryptosporidiosis is a predominantly waterborne disease with infections caused by contaminated drinking water, swimming pools, water features, natural waters, or acquired by animal and human contact and a range of other routes. Cryptosporidium is a particular problem for swimming pools and drinking water because the oocysts are resistant to chlorine based disinfectants.

For the 2001 to 2010 period there were an average of 1165 cases of cryptosporidiosis during the first half of the  year (11,645 in total) compared to 2080 cases a year during the same period from  1991 to 2000 (a total of 20,796 cases for this period) There have been 9,151 fewer cases of cryptosporidiosis in the first half of the year over the last decade. This reduction in the number of cases is thought to be due to fewer Cryptosporidium parvum infections acquired from contaminated drinking water as a result of improvements in drinking water supply and regulation [1,2]. In contrast to the clear reduction in cases for the first half of the year, the number of cases in the second half of each year have shown a slight increase  from an annual average of 2,707 cases to 2,878 cases for the same time period (at total of 27,065); a total of 1714 more cases in 2001 to 2010 (total 28,779). Some of this increase resulted from swimming pool related outbreaks in Majorca in 2000 and 2003 [3]. Infections in the latter half of the year are predominantly C. hominis and are thought to be related to foreign travel and swimming pools [4].

Figure 1. Cryptosporidiosis cases per half year reported in England and Wales 1989 to 2010

Swimming pool contamination is likely to occur all year round, but outbreaks are more common in the late summer period; this may be as a result of people using  swimming pools more and also linked to holiday travel. Swimming pool outbreaks result from contamination of the water with cryptosporidium oocysts, usually from young swimmers.

Swimmers need to make sure they: 

• shower before swimming
• do not swim if they have diarrhoea 
•  try not to swallow pool water

Pool operators should:

• ensure that filters are operating well and with flocculent
• ensure there is sufficient water replacement (particularly in periods of high bather load)
• conduct filter backwashing after the pool has closed at night
• encourage pre-swim showering.

The Cryptosporidium Reference Unit in Swansea has published Cryptosporidium Swimming Pool Guidance [5].

References

1. Sopwith W, Osborn K, Chalmers R, Regan M. The changing epidemiology of cryptosporidiosis in North West England. Epidemiol Infect 2005;133(5):785-93.
2. Lake IR, Nichols G, Bentham G, Harrison FC, Hunter PR, Kovats SR. Cryptosporidiosis decline after regulation, England and Wales, 1989-2005. Emerg Infect Dis 2007;13(4):623-5.
3. Chalmers RM, Elwin K, Thomas AL, Guy EC, Mason B. Long-term Cryptosporidium typing reveals the aetiology and species-specific epidemiology of human cryptosporidiosis in England and Wales, 2000 to 2003. Euro Surveill 2009 15;14(2).
4. Chalmers RM, Smith R, Elwin K, Clifton-Hadley FA, Giles M. Epidemiology of anthroponotic and zoonotic human cryptosporidiosis in England and Wales, 2004-2006. Epidemiol Infect 2010;12:1-13.
5. Investigation of Cryptosporidium linked to swimming pools (with appendicies).  Swansea: Public Health Wales, 2011.  Available at www.publichealthwales.org/cryptopoolguidance/. 
   

Last reviewed: 15 August 2011