Cryptosporidium is a genus of parasitic protozoa that can cause cryptosporidiosis, a gastro-intestinal illness in humans, cattle and some other animals. In people, it causes abdominal pain, diarrhoea, nausea, headaches and fever, but the infection is usually self-limiting and resolves within a few weeks. In immunocompromised patients, the infection can be more serious; it can become prolonged and rarely, fatal as a result of dehydration caused by chronic diarrhoea. These protozoa complete their life cycles in one host and their oocysts (spores) are highly infectious.
It is primarily a waterborne disease spread through the faecal-oral route; the oocysts can also be transmitted by handling infected animals or person-to-person contact. The parasite can be found in soil, water or food and may be transmitted through surfaces that have been contaminated with the faeces of infected people or animals; Cryptosporidium requires a host in which to multiply and cannot grow in foods or water.
The majority of water treatment plants cannot completely guarantee removal of all Cryptosporidium oocysts from the water as the oocysts are very small and resistant to chlorine, the disinfectant commonly used in these plants, so rendering much of the treatment process irrelevant. Since The Water Supply (Water Quality) (Amendment) Regulations 1999 as amended by The Water Supply (Water Quality) (Amendment) Regulations (SI 2000 No./ 31854 came into force there has been a reduction in reported cases, as demonstrated by health surveillance data. The extent to which recreational waterways such as lakes, and private wells continue to pose an occasional risk to health remains unclear, but other sources of contamination remain a cause for concern.
Cryptosporidium is inactivated by UV, heat, freezing and desiccation, so heat-treated, frozen and dried foods should be safe unless contaminated after processing.
Cryptosporidium was described by Tyzzer in 1907, but its importance was only realised in the 1950’s when the parasite was associated with diarrhoea in turkeys and in the 1970s by veterinary workers investigating the cause of scours (severe and serious diarrhoea) in young farm animals. Medically unimportant until the 1970’s, Cryptosporidium was then recognised as a potential human pathogen when the first cases in humans were diagnosed in immunecompromised patients (a young child and an AIDS patient), reported in 1976 by Nime et al. and Miesel et al. Over 2000 people were affected with Cryptosporidiosis in 1984 during an outbreak in Braun Station, Texas; the first time Cryptosporidium was recognised as a waterborne pathogen.
In the early 1980s Cryptosporidium was first detected in Europe and was later identified as causing serious infections in severely immunocompromised patients including patients with AIDS. Human cryptosporidiosis is now widely recognised as an endemic enteric pathogen with illness in more than 90 countries and 6 continents and is one of the leading causes of human diarrheal disease. Infection rates are highest in developing countries and amongst children aged 1-5 years in developed countries. More than 1 million people have since been affected in documented outbreaks; the largest occurred in Milwaukee, Wisconsin in 1993.
Cryptosporidium is an obligate parasite (it must have a host to complete its lifecycle); a very small protozoa belonging to the Coccidia genus which has emerged over the past decades as a major waterborne pathogen. While humans and livestock are the main sources of infection, it is found in a variety of vertebrate hosts including mammals, fish and birds. In humans it causes cryptosporidiosis, an intestinal infection; it also infects many animal species, causing symptomatic illnesses and morbidity leading to important economic losses .
At least eight Cryptosporidium species and two Cryptosporidium genotypes (cervine and monkey) have been found to infect humans, with over 90% of human infections being due to Cryptosporidium hominis and Cryptosporidium parvum. The main species encountered in Europe (and particularly the UK) is C. hominis, whereas in the Americas, Australia and Africa C. parvum is the most identified genotype, although studies have shown a growing number of species can cause human disease. C. hominis is probably specific to humans, with transmission via direct person to person contact through the faecal oral route due to poor hygiene. Two-thirds of cases are in young children aged 1-5 years and, along with the other immunocompromised groups such as the elderly, are more at risk from severe disease requiring hospitalisation.
Many animal species can be infected with C. parvum (recent studies suggests that whether the younger or older animals are infected may depend on the animal species), and transmission is via animals to humans, as well as from humans to humans. Cryptosporidiosis can also be passed on as a secondary infection as it requires a low infective dose of less than 100 organisms.
Infection or illness depends on several factors based on the host, pathogen and environment, including the immune status of the host and the genotype of Cryptosporidium. The main site of cryptosporidiosis infection is the small intestine. Invasion of the host cells is restricted to the luminal border and leads to issue damage, which triggers the immunological and inflammatory responses of the host. Diarrhoea then occurs due to impaired intestinal absorption of fluids and nutrients and the disruption in the normal ion flux.
Cryptosporidium parasites may also spread throughout the gastrointestinal and respiratory tracts, particularly in immunocompromised groups. Extra-gastrointestinal cryptosporidiosis has been reported in both immune-competent individuals and immunocompromised people including the pancreas, liver and bile ducts. Respiratory involvement and sinusitis have been described in severe cases.
Oocysts are excreted in huge numbers (106 to 107 per gram stool) and are capable of surviving for long periods in the environment and while they can be found in soil, water or on food, they are most typically transferred through faecally contaminated water. Cryptosporidium may be more common in surface water than ground water because surface waters are more vulnerable to direct contamination from fertiliser, sewage and industrial discharges and runoff. Farmyard manure may contain high numbers of cryptosporidial oocysts and, consequently, water may be contaminated by manure or slurry washed from fields into rivers and vegetable crops may be contaminated by direct manuring of the fields in which they are grown. Well-managed and stored manure and slurry is effective in reducing infectivity through raised temperature and ammonia levels.
Cryptosporidium has been isolated from fresh vegetables, irrigation water, contaminated drinking water, raw meat, fruit juices, unpasteurised milk and swimming pools.
The lifecycle of Cryptosporidium is complex and, with the exception of one stage, is completed in a single vertebrate host in one to eight days. Cryptosporidium is excreted in the faeces of an infected host in the form of oocysts (see diagram below, stage 1).
Mature thick walled oocysts are comprised of a tri-laminate wall structure with a multilayered inner zone (that thin walled oocysts lack) which maintains the viability of the sporozoites. The sporozoites are the source of infection in their hosts. It is possible that there are two methods of infection; autoinfection may be possible in warm aqueous solutions without exposure to pancreatic enzymes and re-infects the host (possibly including sites outside of the intestine including the testicle, conjunction of the eye and respiratory tract) and the thick walled oocysts which have a high environmental resistance and are excreted in the faeces, where they can infect another host.
In stage 2, excystation (thinning of the oocyst wall) occurs following ingestion by the host animal – the wall on either side of the suture in the oocysts retracts and coils inwards following exposure to the stomach acid, bile salts and body temperature of the host. C. parvum and C. hominis oocysts release four motile sporozoites (motile, infective stage) into the small intestine.
The released slender, crescent shaped sporozoites contain secretary apical organelles required for adhesion, penetration and inclusion in host epithelial cells lining the luminal surfaces of the digestive and respiratory tracts. At the next stage (stage 3), as the sporozoites actively invade the cell wall, vacuoles form near the anterior end of the sporozoites and cluster together, surrounding the parasite and forming a parasitophorous vacuole which fuses with the hosts’ cell membrane to form a host-parasitic interface. The sporozoites within are intracellular but are not directly in contact with the host cell cytoplasm (extracytoplastic). As this process progresses, the sporozoites becomes spherical (called a trophozoite).
The asexual multiplication (metrogony, stage 4) occurs when the trophozoite nucleus divides into meronts. Type I meronts develop nuclei which are incorporated into merozoites. In theory, each mature merozoite leaves the meronts to infect another host cell, developing into either a type I or II meronts. Type II meronts produce four merozoites.
This is followed by a sexual reproduction stage (gamogony) where the type II meronts initiate sexual reproduction. The meronts differentiate into either male microgamonts (sperm equivalent) or female macrogamonts (ovum equivalent) (stage 5). Microgamonts become multinucleate, with each nucleus being incorporated into a microgamete; macrogamonts remain uninucleate and become macrogametes. The microgametes then detect the macrogametes by an unknown method and penetrate the membranes of both the host cell and macrogamete membrane.
The nucleus of the fertilised macrogamete develops into an oocyst (stage 6) in situ by undergoing meiosis with four sporozoites developing. The oocysts are then either released into the intestinal tract and are excreted in the faeces, or if they are in the respiratory tract are excreted in the respiratory or nasal secretions. They do not need any further maturation, unlike many other Coccidian protozoa.
The life cycle is repeated when sporulated oocysts, 4-6µ in diameter , are excreted by an infected host and are subsequently ingested by a new host and the sporozoites excyst within the small intestine .
Thick walled oocysts are appreciably resistant to natural decay in the environment as well as to most disinfection processes. They can remain viable for about 18 months in a cool, damp or wet environment, including sea water. They are quite common in rivers and lakes, especially where there has been sewage or animal contamination and can survive for several months in standing water. Oocysts can survive for months in soil and for up to one year in low-turbidity water. Oocysts appear to develop an enhanced impermeability to small molecules when in contact with feces which might increase the robustness of the oocysts when exposed to environmental pressures .
Drying at ambient temperatures effectively reduces their infectivity; they are destroyed by freezing and they are also heat sensitive. A temperature of 60°C inactivates oocysts in 5-10 minutes. Oocysts are remarkably resistant to many common disinfectants, including chlorine-based compounds. Very high concentrations of most disinfectants may be effective, but such levels are not practical for water treatment due to cost and taste. This parasite is however susceptible to ozone and UV light.
Simplified diagram of Cryptosporidium lifecycle
Human cryptosporidiosis occurs in developed and developing countries, urban and rural areas, and in temperate as well as tropical climates. Infections have appeared from contaminated drinking supplies, contaminated food, swimming pools and other recreational waters, daycares and as a result of foreign travel.
The average incubation is 7-10 days (range 1-28) with symptoms appearing 5-10 days after becoming infected by ingestion of the oocysts. Very low doses are able to initiate an infection, probably fewer than 100 oocysts.
The illness is characterised by profuse, watery diarrhoea with abdominal pain. It can also cause vomiting, weight loss, loss of appetite, headache, fatigue, respiratory problems and a low-grade fever. Typically, the illness is self-limiting resolves in 2-3 weeks but it can last for up to 50 days. Only supportive treatment is available, and this will only be required in serious cases. However, in severely immunocompromised patients, e.g. AIDS sufferers, the infection may become chronic and serious, sometimes fatal. In these cases, other organs and tissues may become infected, including the biliary tract and respiratory system. No antibiotic treatment has yet been shown to be effective in clinical use, although some encouraging results following the use of paromomycin have been reported.
Mature oocysts are excreted in faeces in very high levels during diarrhoea and are immediately infectious. Excretion may continue for some weeks after the cessation of diarrhoea.
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 this scale of potential harm that puts Cryptosporidium in the first league of emerging pathogens. As the oocysts can survive in cold, dark bodies of water and pipes in the water supply, it can strike a large number of people and presents a threat to life for the elderly, very young and immunocompromised.
The first identified outbreak in the UK was in Ayrshire in 1988 and affected 27 people. This was followed in 1989 by a major outbreak affecting over 500 people in the Swindon area. Major outbreaks are rare; however, many cases are thought to be unreported. Cryptosporidiosis may show a seasonal distribution with swimming pool outbreaks continuing to occur, with incidence peaking in late summer and autumn when swimming pool use is highest. Cryptosporidium is the most common enteric pathogen in children under 5 years old. In one 2-year UK study, Cryptosporidium was found twice as often as Salmonella spp. in children aged 1 to 5 years. It is also recognised as a frequent cause of "traveller's diarrhoea" especially after travel to parts of Southern Europe. Since 2002, there have been 4 Cryptosporidium outbreaks in the United Kingdom.
Wales, August 2009
Illness in children and adults who swam at the Merthyr Tydfil Leisure Centre in August 2009; Cryptosporidium oocysts were introduced into the Leisure Pool of the swimming pool complex late in the morning of Saturday 22nd August. The most likely cause of this was a faecal accident involving the smearing of faeces on the toddler slide. A total of 106 cases of Cryptosporidium associated with this outbreak were identified, 45 of which were confirmed through laboratory testing .
Northamptonshire, England, June – July 2008
23 cases of cryptosporidiosis were confirmed as being infected with C. cuniculus. On 30th June 2008, the Cryptosporidium oocysts found in the reservoir water of Pitsford Reservoir were confirmed as being of the rabbit genotype Cryptosporidium cuniculus. Subsequently, a dead rabbit was found in a treated water tank at the water treatment works. The genotype of Cryptosporidium oocysts in the rabbit’s large bowel was indistinguishable from that of the oocysts found in the water.
Staffordshire, England, October 2007
39 confirmed cases were identified – the most plausible explanation was the infection was ingested in multiple swimming pools. One large swimming pool was most frequently visited by swimmers and was therefore considered a significant contributor to transmission because of substandard filtration and maintenance systems.
The 39 confirmed and 18 probable cases of cryptosporidiosis had a significant impact on the affected population and households in terms of hospitalisation and absence from schooling or work.
North West Wales 2005
231 confirmed cases of Cryptosporidium hominis. Environmental investigations, confirmed by microbiological testing, identified several routes by which Cryptosporidium hominis from sewage treatment systems in the Cwellyn catchment could have entered the reservoir and subsequently the mains water supply from Llyn Cwellyn .
Grampian, Scotland, January 2002
The overall conclusion is that the outbreak of Cryptosporidium was caused by the contamination of drinking water supplied from the Invercannie Treatment Works due to a reduction in the effectiveness in the slow sand filtration system. There were 143 cases .
Although Cryptosporidium is resistant to chlorine, the multi-barrier approach to water treatment adopted by the industry includes effective barriers to the organism.
It is unclear how the oocysts are 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 the oocyst harm. Whatever its defence, it is effective, as 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 being to those people who use private water supplies (these supplies are tested). Research in the United Kingdom suggests effective screening requires very frequent analysis of large volumes of water samples. Private wells are typically less advanced than those of water treatment plants meaning the protozoa is able to pass easily, often escaping detection by hiding in the biofilm of the water system.
In the UK, water companies adopt a formal risk-based approach to assessing and managing Cryptosporidium and any other substance that could constitute a potential danger to human health. Companies carry out routine monitoring of treated water and on the rare occasions where Cryptosporidium is detected in the treated water, companies immediately investigate the cause and make corrections while consulting with local health protection units to determine measures needed to protect public health
The complete removal of Cryptosporidium oocysts from water supplies is difficult to achieve in conventional water treatment plants. The oocysts are resistant to the normal chlorine disinfection treatment and, as they are very small (4-6 μm diameter), some pass through the flocculation and sand filtration systems. Control measures may be different in supplies from bore holes as compared to those derived from surface waters, although even those from bore holes may be susceptible to oocyst contamination. Sources derived from surface water will inevitably be susceptible to run-off from land contaminated by animals as well as, on occasion, human sewage. Modern drinking water treatment combines a multi-barrier approach of good maintenance and design of the filtration systems with UVC, ozone or ultrafiltration as a tertiary treatment to inactivate including protozoan oocysts. Careful control of the treatment facility and processes, especially during back-flushing to clear the filtration beds , are essential. In practice, many conventional water treatment plants cannot guarantee the complete removal of these protozoa from water supplies and the fairly high operating costs and inability to deal with seasonal fluctuations in pathogen load reinforce the need to investigate new water treatment technologies and control strategies .
In view of this, both English and American public health authorities have advised severely immunocompromised people to boil drinking water in order to reduce the chance of acquiring waterborne cryptosporidiosis. It is sufficient simply to bring water to the boil to eliminate Cryptosporidium. It should be stressed that, to be effective, the guidance must be followed consistently for all water used for drinking or for washing foods intended for consumption without cooking. It should also be noted that bottled water is not guaranteed free of Cryptosporidium.
While rare, there have been outbreaks of water-borne cryptosporidiosis in both the UK and the US and these have generally been shown to be caused by inadequate water treatment or by breaches of the integrity of distribution systems. Increased understanding of Cryptosporidium contamination and the considerable amounts of monitoring data that are now available have helped water suppliers to establish effective means of controlling Cryptosporidium in drinking water. However, there is a need for constant vigilance. The latest report from the independent Drinking Water Inspectorate (DWI) has found that the quality of drinking water remained exceptionally high in 2011. Around two million samples were taken in England and Wales, with 99.96% meeting or exceeding the standards set by the DWI.
Cryptosporidium cannot grow in food. Although no commercial foodborne outbreaks have been recorded, oocysts will survive in wet/moist foods if they become contaminated. Raw milk, raw sausages and offal together with fruit, vegetables, salad products and filter feeding shellfish are at risk if in contact with manure, sewage or contaminated water and therefore may become contaminated. Cooked foods are not thought to be at risk; the normal recommended time and temperature for controlling bacterial food poisoning (cooking to an internal temperature of 70°C for 2 minutes) will probably inactivate Cryptosporidium. Heat processed foods have never been shown to be a source of infection and studies show standards thermal treatments are effective. Oocysts will not survive freezing.
There is a potential danger that infected food handlers could contaminate food; people with symptoms must not handle foods and advice should be taken on when they can restart such work (usually at least 48 hours after symptoms resolve). Personal hygiene is very important with this illness because the infective dose is so low.
Raw milk, fresh pressed apple juice, uncooked meat products, uncooked (and possibly unwashed) green onions and fresh produce have all been implicated in cryptosporidiosis outbreaks.
Unlike most bacterial enteric pathogens, Cryptosporidium is difficult to detect; it is not possible to grow Cryptosporidium routinely in foods or beverages to levels where they may be readily detected; hence, examination of foods for this protozoan has in the past been considered impractical.
Techniques for sampling and analysis are complicated and time consuming, requiring the filtration of large volumes of water (100 - 1000 litres), followed by several stages of elution, isolation and concentration of the oocysts, and then identification and enumeration. Organisms in water samples can be concentrated by filtration and by immunomagnetic separation. Deposits are then usually examined by microscopy with differential staining techniques being available to distinguish viable from non-viable oocysts. Immunoassays and PCR-based procedures are also used.
Initial testing does not provide information on whether the oocysts are viable and therefore capable of causing disease, this requires further testing. Identification of the species sub-type can be helpful in tracing the source of contamination - this requires much additional work.
Farm management practices to reduce the occurrence of Cryptosporidium:
- Feeding practices that promote good hygiene including the use soap/detergent when washing bottles and feeding equipment.
- Housing practices that promote good hygiene to prevent infection of animals.
- Reducing infection by steam cleaning of equipment where possible/practice (oocysts inactivated above 65°C).
- Isolating ill animals.
- Sanitary disposal of animal manure. Flies are strongly attracted to animal manure and may be carriers of oocysts in their exoskeleton and digestive tracts[18).
- Optimising management and storage of manure and slurry to reduce the infectivity of oocysts through raised temperature and ammonia levels.
- Reducing run off from animal farms into drinking water supplies, for example by planting strips of grass or vegetation. This may help to trap sediment and therefore reduce the organic matter contamination.
- Processing including water suppliers, sectors that use fresh product and operations in which contaminated process or waste water could be used.
- All water that is to be used in direct contact with food/food contact surfaces must be of potable quality and should be free of pathogenic microorganisms.
- A consistent supply of food-safe water for use in direct contact with foods and food contact surfaces is required.
- Treatments for bottled natural mineral waters and bottled spring waters that are effective in reducing Cryptosporidium include physical removal (filtration or decanting), the addition or removal of carbon dioxide and treatment with ozone. A multi-barrier approach is the most effective at reducing or eliminating Cryptosporidium oocysts.
- When water to be used for drinking, bottling or vending can be treated prior to use, microfiltration membranes or similar can be incorporated in conjunction with other measures (such as UV treatment, reverse osmosis or pasteurisation/distillation) to reduce further the likelihood of Cryptosporidium contamination.
- Water to be used as an ingredient (juices, beer and so on) or in the making of baby formula should be filtered and /or other treatment systems used.
- Adequate available hand washing facilities and training on correct hand washing for employees.
- A risk management approach (e.g. HACCP) should be utilised and Cryptosporidium included in the assessment when assessing the risk and requirement/decision on the technologies to be used, taking into account:
- Water source
- pH (naturally acidity, carbonation)
- Temperature of product
- Presence of preservatives
- Final consumer
- Whether chemical technologies are sufficient or physical removal methods are required
- Manual of Diagnostic Tests and Vaccines for Terrestrial Animals 2010, OIE, http://www.cabi.org/ahpc/default.aspx?site=160&page=3323
- Environmental Protection Office of Water March 2001, Cryptosporidium: Drinking Water Health Advisory
- Cryptosporidium species a "new" human pathogen. D P Casemore, R L Sands, A Curry, J Clin Pathol 1985;38:1321-1336 doi:10.1136/jcp.38.12.1321
- Enteric Protozoa: Giardia and Cryptosporidium. Prepared by the Federal-Provincial-Territorial Committee on Drinking Water. http://www.hc-sc.gc.ca/ewh-semt/consult/_2010/giardia-cryptosporidium/draft-ebauche-eng.php#ii
- Rev Argent Microbiol. 2009 Jun-Sep;41(3):185-96. Cryptosporidiosis: an emerging zoonosis. Del Coco VF, Córdoba MA, Basualdo JA
- Microbes Infect. 2004 Jul;6(8):773-85.A review of the biology and epidemiology of cryptosporidiosis in humans and animals. Ramirez NE, Ward LA, Sreevatsan S.
- Cryptosporidiosis: A report on the surveillance and epidemiology of Cryptosporidium infection in England and Wales, Gordon Nichols et al.
- Cryptosporidium and Cryptosporidiosis, Second Edition, edited by Ronald Fayer, Lihua Xiao, ISBN-10: 1420052268
- Appl. Environ. Microbiol. November 1992 vol. 58 no. 11 3494-3500 Survival of Cryptosporidium parvum oocysts under various environmental pressures. L J Robertson, A T Campbell and H V Smith
- Eurosurveillance, Volume 15, Issue 33, 19 August 2010.
- Photocatalytic inactivation of Cryptosporidium parvum on nanostructured titanium dioxide films. O. Sunnotel, R. Verdoold, P. S. M. Dunlop, W. J. Snelling, C. J. Lowery, J. S. G. Dooley, J. E. Moore and J. A. Byrne
- Chin James. Control of Communicable Diseases Manual. 17th Ed, 2000; American Public Health Association
- Drinking Water 2011. A report by the Chief Inspector of drinking water. http://dwi.defra.gov.uk/about/annual-report/2011/index.htm.
- Summary: Lake IR, Nichols G, Bentham G, Harrison FCD, Hunter PR, Kovats RS. Cryptosporidiosis decline after regulation, England and Wales, 1989–2005. Emerg Infect Dis [serial on the Internet]. 2007 Apr [date cited]. http://wwwnc.cdc.gov/eid/article/13/4/06-0890.htm
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