Introduction
Cryptosporodiosis is a zoonotic disease that occurs worldwide caused by a protozoa Cryptosporidium spp. There are currently 19 recognised species and 40 genotypes of Cryptosporidium spp(1). They infect the epithelial cells in the microvili of the gastrointestinal tract. C. hominis and C. parvum primary site of infection is the small intestine, while C. muris, C. andersoni and C. serpentis the gastric mucosa and C. baileyi the respiratory tree and cloaca in chicken. Immunocompromised or young humans and animals are more susceptible to clinical disease.
The sporulated oocyst, the infective stage, is excreted in faeces of an infected animal. It has a tough trilaminar wall that protects four sprorozoites from harsh weather conditions for extended period of times. Oocysts survive longest in wet conditions in temperatures between 5-15 °C, in which they can be viable for up to 6 months(2). The oocysts lose infectivity when heated at 62.4°C for 5 minutes, 72.4°C for 1 minute, frozen at -70°C or exposed to ultraviolet light(2). Oocysts can also lose infectivity by exposure to chemicals including hydrogen peroxide, chlorine dioxide, ammonia, ozone and bromine, chlorine and and iodine related compounds. However, the required concentrations to achieve this are very high, making chemicals an expensive and toxic option. It is also time consuming, and can only achieve desired results at high temperatures.
Infection occurs via fecal-oral route transmission, when a human or animal ingests a sporulated oocyst.
Cryptosporidiosis in humans
Cryptosporidiosis in humans is caused mainly by C. parvum and C. hominis. A study conducted on 13,000 people revealed that C. hominis attributed to 50.29% while C. parvum attributed to 45.6% of infections. 0.5% of people had mixed infections of both C. parvum and C. hominis(3). A separate study of 2414 people in England also revealed that C. hominis and C. parvum make up the majority of cases in humans, coming up to a wooping 98.7% collectively.
The mean incubation period for C. parvum infection is 7-8 days (10) and 5.3 days for C. hominis(11). Clinical disease is dose dependent. Infections may be subclinical. In clinical infections, symptoms include sudden onset of voluminous watery diarrhoea with or without mucous, abdominal pain, nausea, vomiting, mild fever, anorexia, malaise, fatigue and weight loss(13-14). While some infections are self-limiting after 6-21 days, symptoms may also persist for over a month. Oocysts continue to shed 1-15 days after clinic signs have ceased (15).
Infection is prevalent in immunocompromised individuals with HIV, hematological malignancies, bone marrow transplant, chemotherapy, solid-organ transplants, hemodialysis or primary immunodeficiency diseases(17). Subclinical, transient and self-limiting infections are seen, but most cases are chronic and severe. Diarrhoea is more voluminous and explosive, reaching up to 17L of excreta per day(18) and may even lead to death(19).
While C. hominis is mostly related to human-human transmission, C. parvum is found in humans, wild animals and livestock(4-9). This is further supported by a study done in the UK where C. hominis was found to be most prevalent in urban areas with higher proportion of young children and related to diaper-changing. In contrast, C. parvum infections were found mostly in people living near agricultural land(16).
Due to the high density of sheep farming near water catchment areas in the UK, drinking-water borne C. parvum infections have been implicated.
Cryptosporidiosis in lambs
C. parvum targets young lambs and calves less than 2 months old and causes production losses. The disease is manifested as acute watery diarrhea that lasts between 4-16 days(20). Infected animals are often found to be dehydrated, depressed and weak with high oocyst shedding even after the clinical signs have dissolved. The oocysts are then washed into these water bodies as surface water after rain. Obviously, in spring when more lambs are born and there is higher rainfall, a higher burden of Cryptosporidium in the water reservoirs is to be expected.
Drinking-water borne transmission of Cryptosporidiosis in humans.
A study was conducted in North Cumbria, England between 1996-2000 after an outbreak of Cryptosporidiosis in residents of Allerdale and Copeland. It was concluded that drinking unboiled tap water in these areas was a significant risk for a human infection(24). Infection was seen more in children than adults. However, we need to consider that this particular outbreak, which had happened in spring, was merely a higher spike of cases of every outbreak that has been happening every spring in the past few years. This may be due to higher rainfall and surface run-off, which led to contamination of oocysts in nearby water sources, or more likely, it was due to the higher frequency of contact between children and animals. These areas are part of a national park and are popular destinations for school children during this period to experience mingling and touching livestock animals. We cannot discount the possibility that the number of outbreaks in this study was actually due to direct contact instead of drinking contaminated water as suggested by this study. This theory is partially supported by a study done in Melbourne and Australia, which failed to detect a robust link between sporadic Cryptosporidiosis associated with drinking tap water. However, it may be useful to note that this could be attributed to the Australia’s effective water treatment processes and Melbourne’s strict regulations that protect water catchment areas from contamination (25), which is a good point of consideration for Cryptosporidiosis management in the UK.
Another study conducted in the UK via client-filled surveys is consistent with this theory, and concluded that Cryptosporidiosis infections were more likely to occur due to travel outside of UK, contact with an infected person or touching a cattle, than drinking tap water (26). Authors also noted that the restrictions enforced during the FMD outbreak, which led to reduced contact between cattle and humans, had dramatized the results even further, and emphasized that indeed the infections were due to direct contact.
After a series of outbreaks, the U.K. Drinking Water Inspectorate incorporated regulations in 2000 for stricter control of Cryptosporidium. A risk assessment was conducted to identify supplies at risk of contamination, and for each of this facility, water supplier is to include a water treatment process that can monitor and ensure oocyst count of 1/10L or lower. This translates to physical filters that can effectively remove oocysts, either infective or otherwise. Under these regulations, outbreaks were not reported, until 2005, when an outbreak occurred due to contamination in a water supply that was not previously identified as at risk.
It was concluded from a study found in Loch Lomond, Scotland that risk of contracting Cryptosporidiosis is directly linked to drinking unfiltered water from the tap (29). Data in this study strongly suggested implementation of physical filtration systems will substantially lower number of cases.
Recommendation
To reduce C. parvum burden in water supply, we propose instead 2 separate plans as opposed to removal of sheep from water catchment areas.
Farms with high Cryptosporidium spp. burden should be advised to improve their biosecurity, waste management and hygiene standards. Newborn animals should receive sufficient colostrum so that they will be able to combat enteropathogens, as multiple infections are largely responsible for increasing the severity of Cryptosporidiosis. Where possible, sick animals should be isolated and handling is minimized. Faeces should be washed into a separate and controlled drainage that does not join with the water supply. Livestock living areas and feeding troughs should be kept clean and dry to reduce contamination of feed and the possibility of transmitting the disease to other animals. It was also suggested that livestock living areas and surfaces are washed with hot water and detergent, followed by disinfection with chemicals such as hydrogen peroxide (21).
However, as it is impossible to completely eliminate contamination of water sources due to the connectivity of cattle and sheep farm with water bodies, effective water treatment will also be required. Water treatment processes should include new processes that physically filter water effectively. Water suppliers can also look into disinfection using UV light.
Conclusion
It should be noted that water-borne C. parvum infections in humans are small in percentage as compared to Cryptosporidium spp. infections transmitted via other methods, such as human to human or cattle to human. As such, to effectively eliminate Cryptosporidium infection of any species, we suggest a wholesome approach that will tackle all the different sources of infection.
As we can see, many studies have shown high risk of being infected by drinking unboiled tap water or untreated water. However, it is to note that the contamination is a result of many factors aside from just sheep farming, including cattle farming. There is not enough evidence to create a robust link between sheep farming near water catchment areas and Cryptosporidiosis in humans. Furthermore, removal of livestock farming will bring about many more problems in the long run, such as vast areas of highlands not grazed upon and becoming unmanageable. It is wiser to consider alternative options such as better water treatment processes nationwide and effective waste management and husbandry practices in farms as these methods are both practical and sustainable.