Epidemiological studies of cryptosporidiosis : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Veterinary Pathology, Massey University, Palmerston North, New Zealand
An interpretive overview of the literature on intestinal cryptosporidiosis in humans and domestic mammals (Chapter 1) is followed by two studies of the population genetic structure of the protozoan parasites Cryptosporidium parvum and Cryptosporidium hominis (Chapter 2), five epidemiological studies of cryptosporidiosis in foals, calves and humans in New Zealand (Chapter 3), and an investigation of a serendipitous outbreak of cryptosporidiosis among a class of veterinary students, which occurred at the end of 2006 (Chapter 4). The analysis of the population genetic structure of C. parvum and C. hominis indicates the existence of a significant genetic segregation of geographically separated parasite populations, consistent with allopatry. The results do not conform to a simplistic model that considers all C. parvum as multi-host anthropozoonotic agents, and provide statistical support to the idea of the occurrence of anthroponotic cycles that do not involve cattle. Rather than conforming to a rigid paradigm of either a clonal or a panmictic species, data are consistent with the co-occurrence of clonal and recombinatorial diversification in C. hominis, and perhaps C. parvum. The results of the epidemiological studies in New Zealand suggest cryptosporidiosis caused by C. parvum is relatively common in young foals and calves. In the time and space frames underlying these studies, humans, calves, and foals were infected with a genetically homogeneous C. parvum population. This feature is in accordance with previous reports that have indicated C. parvum as the dominant species in humans during the peaks of incidence of cryptosporidiosis in winter and spring, and support the view that the peaks are in large part attributable to direct and/or indirect zoonotic transmission of C. parvum. Finally, the outbreak of cryptosporidiosis among a class of veterinary students highlighted the potential hazard for explosive large-scale outbreaks in New Zealand. The results of the investigation were consistent with point-source exposure and zoonotic transmission of a rare C. parvum subtype through direct contact with calves during a practicum.