Update on rabbit Cryptosporidium

Researchers in the UK have recently published the final results from a research program to characterise the ‘rabbit genotype’ of Cryptosporidium and determine the contribution of this genotype to human infections in the UK.*

The study, conducted through the UK Cryptosporidium Reference Unit, was one of several investigations prompted by an outbreak attributed to contamination of a drinking water supply by a rabbit that gained entry to a tank within the Pitsford Water Treatment Works. The event was initially detected due to routine continuous monitoring of oocyst levels in finished water, rather than recognition of cryptosporidiosis cases in the community.

Although the levels of oocysts detected in the finished water were low (less than 0.1 oocysts per litre on average), the incident triggered a 10-day boil water notice in June/July 2008 which affected over 250,000 people in the East Midlands region of England. A total of 23 cases of cryptosporidiosis attributable to the rabbit genotype were eventually identified by genetic typing of oocysts in faecal specimens from people suffering gastroenteritis.

These included two cases with delayed symptom onset dates which may have arisen by secondary transmission within households rather than directly from consumption of contaminated water.

The outbreak raised questions over the significance of rabbits as a reservoir of potentially human-infectious Cryptosporidium as no instances of human infection by the so-called 'rabbit genotype' had been reported in the published literature prior to this event.

Overall, the research undertaken in this program has shown that C. cuniculus from rabbits currently makes up a minor fraction of human cases of illness, but has the potential to cause waterborne outbreaks given the right circumstances. The potential for rabbits to contribute to water contamination must therefore be considered in water quality management strategies.

Information is needed on the density of oocysts produced per gram of rabbit faeces and the weight of faeces produced per infected rabbit per day in order to assess the relative importance of rabbits compared to other catchment animals which may also be host to human-infectious Cryptosporidium strains.

The discovery of this ‘new’ source of risk does not invalidate past Cryptosporidium water testing results or risk assessments based on such results as C. cuniculus oocysts would have been detected by currently used techniques, but probably misclassified as C. hominis.

You can read more about this research in issue 63 of WQRA's HealthStream.The reference for the paper reported on is included below.

*Sporadic human Cryptosporidiosis caused by Cryptosporidium cuniculus, United Kingdom, 2007-2008  United Kingdom. Chalmers RM et al. (2011) Emerging Infectious Diseases 17(3): 536-538.