Health Stream Literature Summary - Issue 56 - December 2009

Cryptosporidium surveillance and risk factors in the United States.
Yoder, J.S. and Beach, M.J. Experimental Parasitology. doi:10.1016/j.exppara.2009.09.020

Cryptosporidiosis was made a nationally notifiable disease in the United States in 1994, however there is no national system or repository for collecting stool samples from infected patients for molecular testing. From 1995 through 2007, the total number of reported cryptosporidiosis cases increased from 2972 for 1995 to 11,657 for 2007. From 1995 through 2004, the reported incidence of cryptosporidiosis remained relatively stable however since 2005 the reported incidence has increased significantly. The increase in the number of cases may be due to a combination of factors including: an actual increase in incidence, improved surveillance for this organism, improved awareness about cryptosporidiosis and likely exposures and changing healthcare testing practices resulting from the licensing of nitazoxanide, the first ever treatment for cryptosporidiosis. The highest incidence of cryptosporidiosis occurs in children aged 1-9 years and among adults aged 30-39 years, and it is believed that transmission from children to their caregivers may occur. The importance and severity of cryptosporidiosis among people with AIDS has declined significantly since the introduction of highly active anti-retroviral therapy. The reported incidence of cryptosporidiosis cases is ten times higher in summer than in winter, especially in children under 10 years. This increase coincides with increased outdoor activities such as swimming and might reflect use of community swimming venues by younger children. Cryptosporidium has now become the leading cause of recreational water-associated outbreaks of gastroenteritis in the United States. Transmission through recreational water is aided by the substantial number of Cryptosporidium oocysts that a single person can shed; the extended periods of time that oocysts can be shed; the low infectious dose; the resistance of Cryptosporidium oocysts to chlorine and the prevalence of inadequate pool operation and maintenance, particularly of children's wading pools.

The first reported US drinking water-associated cryptosporidiosis outbreak was in 1984 and the first US recreational water-associated outbreak in 1988. There have been 143 waterborne cryptosporidiosis outbreaks reported to CDC from 1984 through 2007 as part of the national Waterborne Disease and Outbreak Surveillance System. Of these, 15 were associated with drinking water and 128 were associated with recreational water. Of the recreational water outbreaks, 114 (89.1%) were in treated venues such as swimming pools and 14 (10.9%) were in untreated venues such as lakes. Large numbers of case were involved in some of these outbreaks, including an outbreak of over 3000 cases in New York State during 2005 linked to a recreational water fountain. The fountain was treated with filtration and chlorination however this was not sufficient to inactivate Cryptosporidium. Outbreaks of cryptosporidiosis associated with recreational water facilities can also expand into community-wide outbreaks with multiple modes of transmission. In a 2003 outbreak in Kansas, Cryptosporidium was spread into community pools by ill swimmers and then subsequently spread through childcare facilities. There were 96 confirmed cases identified and greater than 600 probable cases of cryptosporidiosis. In 2005 a cryptosporidiosis outbreak among over 800 persons that affect adjoining communities in Kentucky and Ohio showed multiple modes of transmission. The majority of cases (over 70%) were associated with treated recreational water use, however community-wide, person-person transmission also occurred. These outbreaks show how Cryptosporidium, can adopt multiple modes of transmission that cause the expansion of these recreational water outbreaks into larger community-wide outbreaks.

Food-borne outbreaks of cryptosporidiosis are relatively uncommon, accounting for only 0.2% (15/7650) of reported foodborne outbreaks from 1998-2007. Person-to-person transmission of cryptosporidiosis has been reported in childcare facilities and children have subsequently transmitted the infection to their parents and other persons in the community. Zoonotic cryptosporidiosis outbreaks have also been reported and have occurred in children and adults following contact with animals.

There have been several studies that have characterised risk factors associated with Cryptosporidium transmission. Those at increased risk of infection include: persons who have ingested contaminated recreational (e.g., lake, river, pool or hot tub) or drinking water; persons who have contact with infected animals; close contacts of infected persons (e.g., those in the same family or household or in day care settings); and travellers to disease-endemic areas. Case-control studies have been conducted in recent years in the US, UK and Australia to identify risk factors associated with sporadic cryptosporidiosis. All three studies show that contact with persons with diarrhoea, particularly incontinent individuals such as young children, and contact with cattle, especially calves are risk factors. The US and UK studies also found travel abroad as a risk factor while the Australian study excluded persons had had recently travelled abroad to better characterise endemic diseases. The US study also identified swimming in fresh water as a risk and the Australian study found swimming in a chlorinated swimming pool a risk factor. In the UK and Australian studies a dose dependent risk in drinking unboiled drinking water was identified and this was also reported in a regional UK study. A UK case-control study using GIS and genotyping of over 3000 stool samples demonstrated that Cryptosporidium hominis transmission was associated with urban areas of higher socioeconomic status and many young children and Cryptosporidium parvum was associated with areas with poorer water treatment and those areas likely to have large amounts of Cryptosporidium applied to land from manure. There have been several additional studies that have found contact with cattle to be a significant risk factors for sporadic cryptosporidiosis. During England's 2001 Foot and Mouth Disease outbreak, millions of animals were slaughtered and heavy restrictions were placed on access to the countryside. During this time, the countries with the heaviest restrictions experienced an 82% reduction in the number of cryptosporidiosis cases, particularly within the transmission peak occurring during the spring calving season, this suggests a strong association between cattle contact and human disease in the UK. As Cryptosporidium cannot multiply in food, the risk of foodborne cryptosporidiosis results mainly from direct contamination of food by an ill person during preparation or faecal contamination of food, unhygienic food production, storage, processing or preparation processes. The actual cryptosporidiosis burden in the United States is likely to be underestimated as not all infected person are symptomatic, those who are symptomatic do not always seek medical care, health-care providers seldom include laboratory diagnostics in their evaluation of non-bloody diarrhoeal disease, a majority of laboratories do not test for Cryptosporidium unless it is specifically requested, case reports are not always completed for positive laboratory results and reports are not always forwarded to public health officials. The true burden of cryptosporidiosis in the US is not known however CDC has estimated that 836,212 cases occur annually.

There is no formalised national prevention plan in the US for cryptosporidiosis however multiple efforts should be focused on reducing the transmission of the parasite including those focusing on the route of transmission. The risk of zoonotic transmission could be reduced by improving hygiene on farms, sequestration of animals waste and reducing contact of individuals with ill animals. Where children are in contact with livestock, adequate hand-washing facilities should be available and used. Person-to-person and foodborne transmission of Cryptosporidium in outbreak settings might be reduced by following cryptosporidiosis control guidelines. For drinking water transmission, the US Environmental Protection Agency (EPA) has implemented multiple regulations designed to improve the safety of surface water supplies, including multiple regulatory changes following the 1993 Milwaukee outbreak. Since 1993, no Cryptosporidium outbreaks associated with community water systems that use surface water have occurred. There have been two reported groundwater outbreaks however and the signing of the EPA's Groundwater Rule in 2006 has been an increased effort to protect against Cryptosporidium outbreaks in public water systems using groundwater supplies. Cryptosporidium remains the leading cause of diarrhoeal illness outbreaks in recreational water, especially in chlorinated venues. To reduce transmission of the pathogen in chlorinated swimming pools will require new technology known to inactivate Cryptosporidium such as ultraviolet light or ozone inactivation or improvement in parasite removal using improved filtration technology or filtration enhancers/flocculants that improve removal. Improved operation and maintenance is required as well as improved training for operators and staff. The public will also need to be actively educated to reduce the amount of people who still swim despite being ill with diarrhoea. In the US there is no uniform national pool code for governing design, construction, operation and maintenance of swimming pools and other treated recreational water venues. In 2007 the CDC initiated sponsorship of the development of a national Model Aquatic Health Code (MAHC). The objective of the MAHC is to change the various swimming pool regulations used by health departments into uniform national guidelines to ensure the health and safety of those swimming.

There is still much knowledge to be gained about the epidemiology of cryptosporidiosis in the United States. National surveillance systems are generally passive in nature and require improvement. More information on the clinical aspects and exposure histories is required. The lack of a national laboratory-based system to collect stool samples for molecular typing and comparisons has prevented a more detailed understanding of the role of genotypes/species and subtypes in transmission of sporadic disease or in causing outbreaks. The US molecular data had mainly been collected for outbreaks and not for sporadic cryptosporidiosis. The CDC is developing CryptoNet, a sequence-based national surveillance system to collect and track sporadic cryptosporidiosis transmission in the US. A more detailed molecular understanding is required of the links between sporadic cryptosporidiosis, outbreaks and the simultaneous circulation of zoonotic Cryptosporidium which may enable the development of a more specific plan for reducing Cryptosporidium transmission in the future. In order to decrease the future transmission of cryptosporidiosis, efforts need to be integrated between epidemiology, laboratory, environmental heath and behavioural science research along with improved surveillance and health communication.

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