first described in 1907 , cryptosporidium remained obscure for many decades . during the 1960s and 1970s it was increasingly recognised as an important pathogen of neonatal ruminants   and humans , but it was not until the emergence of aids in the early 1980s that cryptosporidium was identified as a life - threatening parasite in immunocompromised patients . today , it is known to be one of the most serious causes of waterborne diarrhoea ; it is a most difficult organism to control and it is one of the most heavily researched protozoan parasites . in contrast , we are only beginning to understand the importance of cryptosporidium on the island of ireland . this article summarises what we know about the epidemiology of the parasite on the island and how the unique combination of land use , infrastructure and geophysics on the island may affect it . the disease is transmitted via long - lived , thick - walled oocysts of 3 m to 8 m diameter , excreted in the faeces of infected humans and animals . the infectious dose is dependent on the species and immune status of the host and on the cryptosporidium species / strain , but generally considered to be very low . infection experiments in 29 healthy volunteers indicated an infectious dose of 132 oocysts ( median value ) , while mathematical modelling suggested it may be as low as one oocyst . upon ingestion , invasive stages or sporocysts emerge from the oocysts and invade cells in the microvillous border of the intestinal epithelium and , to a lesser extent , extra - intestinal epithelia ( figure 2 ) . situated intracellularly , just beneath the host cell membrane , but extracytoplasmic and enclosed within parasitophorous vacuoles , the parasites multiply through repeated cycles of asexual reproduction and re - invasion , thereby destroying the functional integrity of the brush border . ultimately , sexual reproduction occurs resulting in the formation of thick - walled oocysts that are shed with the faeces . thin - walled oocysts may occur also ; these are believed to be capable of excysting within the same host and causing auto - infection in immunodeficient hosts . stained cryptosporidium oocysts ; ( top ) modified ziehl - neelsen stain , viewed under bright light , and ( bottom ) phenol - auramine stain , viewed under fluorescent fitc filter . according to the geological survey of ireland , in many parts of the island there is only a shallow layer of soil and subsoil over karst limestone . in other areas , the general soil map of ireland   indicates heavy soils that can be prone to either rapid surface runoff after rainfall or channeling of water along large pores , known as preferential flow . in all these situations , instead of being filtered by slow passage through a substantial volume of soil , rainwater that may hold a significant contaminant load rapidly reaches surface water reservoirs or groundwater the water level in the lake was shown to rise rapidly after rainfall events , indicating that areas within the catchment have extremely poor permeability , leading to excessive overland flow and facilitating direct ingress of contaminants into the lake ( report , ) . incidentally , three years later the lake water , which is used as a source for drinking water , was linked to one of the largest recorded outbreaks of cryptosporidiosis in the republic of ireland . long survival periods in the environment , combined with quick passage to surface and ground waters , mean that many of the oocysts are still viable when they reach reservoirs of drinking water . several studies investigating waterbodies and associated shellfish , on the island of ireland found that contamination with oocysts was chiefly restricted to more densely populated areas ( belfast , dublin , sligo ) and the river shannon , where low levels of oocyst contamination were widespread . this widespread occurrence of oocysts in irish waterbodies needs to be considered in view of the fact that the chief disinfection treatment used for drinking water in ireland is chlorination , which does not kill cryptosporidium . filtration , the only reliable method to remove the parasite from drinking water , is not included in regular water treatment . this fact , combined with the high stocking rates that are carried on dairy farms in ireland and the agricultural practices , soils and hydrology found on the island , could mean that ireland is highly vulnerable to large - scale outbreaks should , for whatever reason , environmental contamination with oocysts increase . the most common clinical signs in humans are diarrhoea , abdominal cramps , fever , nausea and vomiting , which may result in weight loss and dehydration . sometimes , headache , weakness , fatigue , myalgia and inappetence are also reported . in immunocompetent patients , these patients usually recover within one to two weeks ; however , shedding of oocysts may continue for several weeks after symptoms cease and clinical relapses may occur up to 14 days after resolution of the initial symptoms . in contrast , immunocompromised persons suffer chronic enteritis , lasting as long as the immune impairment . in these patients , the disease may go through cycles of resolution and recurrence , or it may be persistent and become life - threatening . in immunocompromised hosts , the parasite can also infect other organs , resulting in hepatitis , pancreatitis , cholecystitis , cholangitis and conjunctivitis . respiratory infections are associated with coughing , wheezing , croup , hoarseness and shortness of breath over the last number of years , there have been several outbreaks of cryptosporidiosis on the island of ireland . between april 2000 to april 2001 , there were three outbreaks associated with drinking - water in the greater belfast area and neighboring communities , involving at least 476 cases in total . officials blamed northern ireland 's antiquated system of water pipes , which allowed the ingress of human sewage from a septic tank and of wastewater from a blocked drain . in the spring of the following year , an outbreak of cryptosporidiosis in county westmeath was traced to the drinking water distribution network serviced by lough owel . the water from this spring - fed lake is generally considered to be of pristine quality . following basic gross solid removal and disinfection , it enters the mains unfiltered . primary land use in the catchment is grassland farming and it is thought that heavy rains in april 2002 may have increased surface water runoff , facilitating the ingress of animal excrement into the lake . during the outbreak , 26 cases were confirmed , a further seven were reported but not confirmed , and four people were admitted to hospital . in 2005 , there were two outbreaks of cryptosporidium on the island of ireland . between march and may , 30 cases of cryptosporidiosis were confirmed in carlow and residents were advised to consume only boiled or bottled water for a period of six weeks . in june , in addition to these larger outbreaks , there have been numerous smaller outbreaks , frequently indistinguishable from stochastic cases . according to the communicable disease surveillance centre , northern ireland , rates of cryptosporidiosis in northern ireland have risen since 1997 and in 2000 , it was the highest in the uk with 417 reported cases ( figure 3 ) . since then , it has decreased somewhat , with between 125 and 140 reported cases per annum in the last three years ( communicable disease surveillance centre , n. ireland website : http://www.cdscni.org.uk/surveillance/gastro/cryptosporidium_sp.htm ) . recent trends of reported cases in ( top ) northern ireland from the communicable disease surveillance centre , northern ireland and ( bottom ) in the republic of ireland from infoscan ( report of waterborne cryptosporidiosis subcommittee of the scientific advisory committee , 2004 ) . in the republic of ireland , cryptosporidiosis in humans became a notifiable disease on january 1 , 2004 , under new infectious disease legislation . two hospital studies carried out before that date identified cryptosporidiosis as the second most common cause of childhood gastroenteritis after e. coli , with cryptosporidium being detected in 4.0% to 4.3% of stools of children under 12 years old suffering from gastroenteritis   ( figure 3 ) . according to estimates made by the health protection surveillance centre for the period before 2004 , cryptosporidium accounted for approximately 8% of laboratory - confirmed cases of gastroenteritis in children under two years old . the number of cases reported in the republic of ireland since january 2004 can be viewed on the hpsc website http://www.ndsc.ie/notifiablediseases . in both jurisdictions , the majority of cases involve children under five years of age . in common with the uk , there is a significant urban - rural divide with a higher incidence amongst the rural population , indicating farmers and animal health workers as occupational risk groups . in addition , private water schemes , more common in remote areas , are known to be at higher risk from contamination than publicly managed water supplies in urban areas . there is also a marked seasonal pattern with a peak in spring / early summer and often a further smaller rise in the autumn . reports of natural infections with c. hominis have been restricted to humans so far , with the exception of two cases identified last year in scottish cattle . in contrast , c. parvum infects most , if not all , mammals including humans and is a major pathogen of calves . recent studies suggest that a number of other zoonotic species , particularly the avian pathogen , c. meleagridis , also cause diarrhoea in humans . morphologically indistinguishable , the various species are most easily discriminated genetically , mostly by polymerase chain reaction ( pcr ) combined with restriction fragment length polymorphism ( rflp ) analysis   ( figure 4 ) . most work to date on the biology and pathogenicity of cryptosporidium in relation to human infection has been done using c. parvum . therefore , compared to c. parvum , little is known of the biology of invasion of the human - restricted c. hominis . recent studies carried out in the children 's research centre , our lady 's hospital for sick children , crumlin , dublin , indicate that there is diversity in the mechanisms used by c. parvum and c. hominis to infect cells of different origin . more specifically , it was shown that host cell invasion by the two species was mediated by different receptor - ligand interactions . for instance , while a galactose - n - acetylgalactosamine ( gal / galnac)-specific sporozoite epitope is crucial for host cell invasion by c. parvum , , hashim et al .   found that in vitro , c. hominis interacted with potential host cells via a gal / galnac - independent mechanism . these data have important implications for understanding the pathogenesis of cryptosporidiosis and for improved chemotherapy . identification of c. hominis ( lanes 2 , 3 and 5 , 6 ) and c. parvum ( lanes 4 and 7 ) by pcr - rflp . digestion with the endonuclease sspl results in products of approx 500 and 250 bp in both spp . digestion with the endonuclease vspl renders products of approx 590 ( c. hominis ) and 610 bp ( c. parvum ) . , c. parvum and c. maleagridis in the human population is dependent on the geographical region . for example , in north and south america c. hominis appears to be more prevalent , while in europe zoonotic infections with c. parvum are more common . in latin america , the prevalence of c. meleagridis may exceed that of c. parvum . on the other hand , in the uk this species accounted for only 0.3% of clinical cases . generally , it is thought that the prevalence of zoonotic species reflects the importance and type of farming and agriculture in a region . farming practices , such as pasture grazing , the storage and spreading of slurry / manure , or events such as lambing and calving , all contribute to environmental contamination with oocysts which may enter surface waters either directly or through surface runoff . in fact , it has been suggested that c. parvum is more common in areas where a greater proportion of surface water is used in the public water supply . the spring peak of cryptosporidiosis cases is usually attributed to c. parvum , as it coincides with the calving and lambing seasons . in contrast , c. hominis , a strictly human parasite , is significantly more common in patients infected during the late - summer / autumn peak and in those with a history of foreign travel . infections with this parasite are due to contamination with human faeces originating from leaking waste water pipes or septic tanks . other common routes of transmission are recreational facilities , particularly swimming pools . considering the important role of the agricultural sector on this island and the pronounced spring peak , one would assume that c. parvum is the most prevalent species in the human population here . a study of 39 positive samples collected in the greater belfast area during 1998 reported that c. parvum accounted for the greatest number of sporadic cases : 87.2% . however , two of the three outbreaks in the same geographical area between april 2000 and 2001 were ascribed to c. hominis and only one to c. parvum . to date , there has been no report of c. meleagridis infection in ireland . clearly , in order to improve our understanding of the epidemiology of the parasite , there is an urgent need for a reference facility to provide active surveillance , including routine typing .
cryptosporidiosis , a protozoal disease which causes significant morbidity in humans , is one of the chief causes of diarrhoea in neonatal ruminants . although the parasite poses a significant threat to public health and animal health in ireland , its epidemiology on the island is only poorly understood . 
 environmental studies have shown the waterborne parasite to be widespread in some untreated waterbodies around ireland . 
 the island 's hydrogeological situation , combined with high stocking rates of livestock and the absence of filtration from regular water treatment , render it vulnerable to large - scale outbreaks . 
 this review discusses the parasite in the irish context and underlines the need for a reference facility to provide active surveillance on the island .