What is Cryptosporidium?
Cryptosporidium is a single-celled protozoan parasite belonging to the subclass Coccidia. Until recently, the only species thought to be important in human illness was referred to as Cryptosporidium parvum. But new studies have shown that several species can infect humans, including C. hominis, which is specific to humans, and C. parvum, which infects both humans and ruminants. Other species reported to infect humans include C. felis, C. canis, C. meleagridis, C. muris and C. suis.
Cryptosporidium is a parasite and requires a host in order to multiply. It was first discovered almost 100 years ago, but was not associated with human illness until 1976. It is a cause of gastrointestinal infection in humans and some other animals, especially calves and lambs, and is found worldwide. The parasite has a complex life cycle, most of which takes place within the gastrointestinal tract (mainly in the small intestine) of a single host. The transmissible stage in the cycle is a highly resistant, thick-walled spore, known as an oocyst.
What foods can be contaminated?
Cryptosporidium is mainly associated with water polluted by human or animal faeces, but oocysts have also been found in a number of unprocessed foods, notably raw milk, meat and shellfish and fresh fruit and vegetables. Any food that may come into contact with contaminated water during production, and where there is no subsequent process that will destroy oocysts, is at risk from Cryptosporidium contamination, but th parasite cannot grow in food or in water.
Oocysts are easily destroyed by heat and Cryptosporidium is not normally associated with cooked and processed foods.
Food is not considered to be a major vehicle for the transmission of the parasite. The person-to-person and animal-to-human (zoonotic) transmission routes are likely to be much more common.
How does it affect human health?
Cryptosporidium can cause an acute gastrointestinal infection in humans. It invades the cells lining the gut causing inflammation and loss of fluid. The incubation time for the infection is usually between 5-7 days, but it may vary from 2-14 days, possibly depending on the number of oocysts ingested. The main symptom is profuse watery diarrhoea, often accompanied by abdominal pain. Vomiting, fever and weight loss may also occur. Symptoms are most severe in the very young, the elderly and in immunocompromised adults, such as AIDS patients. In healthy adults, symptoms typically last for 2-4 days, but may last for up to 2-3 weeks in some cases. The infection is usually self-limiting and is resolved without medical treatment, but in vulnerable individuals, infection can be more serious and long lasting, requiring hospital treatment, and deaths have been recorded.
The infective dose is uncertain, but may be as low as 10 oocysts, or even less. A single oocyst is thought to be capable of causing disease in young lambs, and possibly also in very young children and immunocompromised adults. Infected individuals shed very large numbers of infectious oocysts in their faeces, and this may continue at a low level for several weeks after symptoms have subsided. This shedding of oocysts is the main reason why person-to-person and zoonotic transmission of the parasite are so common. Asymptomatic cases of infection have also been reported.
How common is illness?
Cases of Cryptosporidium infection are not particularly common. For example, in England and Wales between 1989 and 2010, the number of reported cases each year generally ranged from 3,000 to 6,000, with a peak of nearly 8,000 cases in 1989. Recent data for the EU refers to 2008 and shows a total of 7,032 reported cases of cryptosporidiosis from 21 countries. However, about 70% of these were from the UK, suggesting significant under-reporting in many other countries. The results also show that peaks of infection commonly occur in the autumn, or occasionally in spring. Cryptosporidiosis is a notifiable disease in the EU and in the USA.
There were 10,500 reported cases of cryptosporidiosis in the USA in 2008. The number of cases reported across the country rose dramatically between 2006 (6,479) and 2007 (11,657), in part because of a number of outbreaks associated with recreational waters. A peak in the number of cases reported typically occurred in the summer and early autumn.
There is little or no information about the proportion of reported cases that are foodborne, but it is thought likely that the majority are caused by contact with infected animals, people, or contaminated water.
Most recorded outbreaks are associated with contaminated drinking water, or recreational waters. For example, in 1993 a water-borne outbreak occurred in Milwaukee in the USA, which affected more than 400,000 people and caused an estimated 69 deaths. Foodborne outbreaks have also been recorded, usually caused by an infected food handler, or by faecal contamination, either direct or through polluted water. Outbreaks have been linked to raw produce, chicken salad, green onions and raw milk. In the USA, there have been several outbreaks linked to unpasteurised apple cider. For example, in 2003, cider made from contaminated apples caused illness in 144 people. The cider had reportedly been treated with ozone, but this had clearly not been effective.
Where does it come from?
Cryptosporidium species are all obligate parasites and thus originate from the host animal. C. hominis is thought to primarily infect humans, while C. parvum infects humans and ruminants. The primary source of Cryptosporidium is therefore the faeces of infected humans and animals, which may contain up to a billion oocysts in a single bowel movement. Infected cattle are a particularly important reservoir of C. parvum. The oocysts are extremely infectious and may be transferred to food via an infected food handler, or through polluted water used for crop irrigation or processing.
Cryptosporidium oocysts are quite difficult to remove from water, even by modern water treatment methods. Their small size (4-6 μm diameter) and resistance to chlorine enable them to pass through some water treatment plants, especially if they are present in high numbers.
How is it affected by environmental factors?
Cryptosporidium oocysts are very resistant to most environmental factors, with the exception of heat and desiccation. Oocysts can persist for months in water and in soil and have been shown to survive for hours on wet surfaces, including stainless steel. However, they are not resistant to drying and die rapidly on dry surfaces.
Cryptosporidium oocysts are not especially heat resistant and are destroyed by conventional milk pasteurisation. A temperature of greater than 73oC will cause instantaneous inactivation of oocysts.
Oocysts can survive for short periods at temperatures below 0oC, especially in water, but the commercial ice cream freezing process has been shown to cause inactivation and eventual die-off occurs at temperatures below -15oC.
Some loss of viability has been shown in acid conditions below pH 4.0. It has been reported that oocysts lost 85% of viability in 24 hours when contaminated water was used to brew beer and produce a carbonated beverage. Organic acids in fruit juice have been reported to inhibit the infectivity of oocysts, but viable C. parvum oocysts have been detected after 14 days suspended in media acidified with citric, lactic and acetic acids.
The oocysts are also remarkably resistant to many sanitisers and disinfectants, notably chlorine. One study reported survival for two hours on exposure to chlorine at 50,000 ppm.
How can it be controlled?
For food processors and caterers
Control measures for Cryptosporidium in food processing focus largely on the control of contamination in the water supply. Food processors using potable water from the public supply network should carry out a risk assessment on the consequences of mains water contamination and a ‘Boil Water Notice’ issued by the water supplier. Where there is a high risk, as in the production of raw food products, such as fresh-cut produce and salads, it may be worthwhile considering the introduction of additional on-site water treatment measures, such as charcoal or membrane filtration. Treatment with biocides such as hydrogen peroxide and chlorine dioxide may be effective, but only at concentrations well above those usually used in water treatment.
Heat processing is an effective control against Cryptosporidium oocysts in food. Normal milk pasteurisation processes are effective, as are recommended ‘Listeria cook’ processes for meat products (70 oC for at least 2 mins). Reheating cooked foods to at least 74 oC will destroy oocysts immediately.
Freezing foods for at least 7 days is an effective control, as is drying. Oocysts were reported to lose infectivity in 7 days when stored at a water activity of 0.85 at 7 oC.
Infected food handlers are also a major Cryptosporidium contamination risk for foods that do not undergo any further processing, such as sandwiches and salads. Good personal hygiene practice, especially hand washing, is an essential control and any staff suffering from gastroenteritis should be excluded from processing areas.
Are there rules and regulations?
Cryptosporidium is generally considered to be a water-borne pathogen rather than foodborne. It may therefore be covered in drinking water regulations, as is the case in the UK, but is not usually mentioned specifically in food safety and hygiene law.