Waterborne outbreaks of cryptosporidiosis Rachel

INTRODUCTION Human infection with the protozoan parasite Cryptosporidium causes the gastrointestinal disease cryptosporidiosis. Of the 25 or so species currently recognised, 15 have so far been reported in humans, of which some are established as human pathogens: C. parvum, C. hominis (which are the most commonly detected species in human cryptosporidiosis worldwide) and C. meleagridis are supported by human infectivity and clinical outcome data from feeding trials in adult volunteers [13], C. cuniculus (formerly the rabbit genotype) caused a drinking waterborne outbreak in the United Kingdom (UK) [4], and C. felis and C. canis were associated with diarrhoea in children in a shanty town in Peru [5]. Dose response studies have shown similar ranges for some C. parvum isolates compared with C. hominis, and small numbers (< 10) of parasites ingested can cause disease [1, 2]. Other Cryptosporidium species are rarely reported human infections, or have never been found in humans, and many are considered adapted to farmed animal or wildlife hosts [6] (Table 1). Transmission is by the faecal-oral route, from either humans or animals, depending on the Cryptosporidium species; for example, C. hominis has a human infection cycle while C. parvum also has susceptible animal hosts causing mainly gastrointestinal disease in young ruminants. The natural host for C. cuniculus is the European rabbit (Oryctolagus cuniculus) [13] and for C. felis, cats and C. canis, dogs. Although C. meleagridis was originally identified in farmed turkeys [14] current distribution and risk factors for human acquisition are not known; many cases report no contact with birds, the parasite species has a wide host range and other bird-restricted species are not considered a threat to human health. Although some Cryptosporidium spp. are highly infectious person-toperson, it is the parasite’s ability to survive in the environment and its resistance to chlorine disinfection that support transmission via drinking and recreational waters, and other vehicles such as food. Table 2 shows the human risk factors for acquisition of Cryptosporidium spp. and how these relate to settings where outbreaks have occurred. Symptoms of cryptosporidiosis, which usually occur between 2 to 12 (usually 5 to 7) days after ingestion of oocysts (the transmissive stage of the life cycle), include watery diarrhoea, abdominal pain, nausea and/or vomiting, low grade fever and malaise, and may last for up to three weeks during which time apparent recovery may be followed by