Cryptosporidiosis in farm livestock

Although diarrhoea caused by Cryptosporidium is prevalent in livestock species throughout the world relatively little is known about the species and subtypes of Cryptosporidium found in cattle on Scottish farms. In particular, little is known about the shedding profiles (age when calves become infected and duration of shedding) of the different species found in cattle and how calves become infected. There are several theories about how neonatal calves first become infected with the parasite but the role which adult cattle play in the transmission of the parasite has not been fully addressed. It was previously thought that adult cattle did not become infected with the same species of Cryptosporidium which causes disease in the young calves. Some studies have shown that this may not be true and with the advance of new techniques to discriminate species this is an area which should be revisited. In addition, it is known that it is possible for humans to become infected with Cryptosporidium and show clinical disease early in life and then again later in adulthood. In livestock however, diarrhoea caused by the parasite is generally only seen in neonatal livestock while older animals tend to be asymptomatic. It is not known if this resistance to clinical disease at an older age is due to changes in the host with an increase in age or if prior infection “immunises” the animal and provides protection against re-infection. It is also not known if infection with one isolate of C. parvum will provide protection against infection with another or if the protection formed is species/isolate specific. The main aims of this thesis were to: determine the species and subtypes of Cryptosporidium found in calves on a study farm over a one year period from birth; assess the role which adult cattle play in the transmission of the parasite to newborn calves; develop new typing tools to enable the rapid and easy differentiation of Cryptosporidium species found in cattle and to examine the host-pathogen interactions in animals given serial experimental challenges with distinct Cryptosporidium parvum isolates to determine if the resistance seen in older animals on farms is due to an increase in age or as a result of prior infection. iii A variety of different approaches were taken to achieve these aims. Longitudinal experiments carried out on a study farm revealed that in calves <9 weeks of age the most common species of Cryptosporidium is C. parvum and that all calves in the group became infected with Cryptosporidium within the first two weeks of life. Sample collection from the same animals later in life (at 6 months of age) showed that contrary to most previous studies the most common species detected at in this age group was also C. parvum although, interestingly, the subtype which the calves were shedding was not the same subtype that they were shedding previously. The longitudinal study which investigated the role of adult cattle in the transmission of Cryptosporidium also yielded some interesting results. It was found that most of the adult cattle on this farm were shedding Cryptosporidium albeit intermittently. Speciation of the positive samples revealed that, on this farm, the most predominant species of Cryptosporidium in adult cattle was also C. parvum. This is very unusual as most previous studies have not found this level of infection in older cattle and C. parvum is not usually found in this age group. A number of different subtypes were found in adult cattle and some animals shed more than one subtype over the course of the study. This contradicts prior findings which demonstrated that only one subtype is found on a single farm. The experimental infection trial involving infection of young (<1 week old) and older (6 week old) lambs with distinct C. parvum isolates demonstrated that an increase in age at primary infection reduces the effect of clinical disease. Animals which were infected at <1 week of age were re-challenged at 6 weeks of age with either a homologous or heterologous infection. Results revealed that previous exposure does not protect against re-infection with the same or a different isolate of C. parvum. This study also demonstrated that an increase in infective dose leads to a shorter pre-patent period and that there are variations in the clinical manifestations of different isolates of the same Cryptosporidium species.