The porcine proliferative enteropathies (PPE) are a group of diseases ranging from intestinal adenomatosis (PIA), a chronic condition causing reduced growth rates in post weaning pigs, to the often fatal proliferative haemorrhagic enteropathy (PHE), resulting in intestinal haemorrhage. PHE predominantly occurs in older and heavier pigs than the chronic disease PIA. This thesis examined whether the age when susceptible pigs are infected affects the clinical response to L.intracellularis infection. The characteristic pathologic lesion of PPE is the abnormal proliferation of crypt epithelial cells in the ileum and colon. Closely associated with this proliferation is the presence of an obligately intracellular bacterium, Lawsonia intracellularis. Characterisation of L.intracellularis was performed in in-vitro co-cultures of L.intracellularis extracted from PHE-affected mucosa. The efficacy of antimicrobials to inhibit the growth of L.intracellularis in-vitro was evaluated and compared with isolates cultured in the United Kingdom. The results were analysed with respect to medication strategies currently used to control PPE in piggeries. PPE occurs in virtually all piggery management systems, including newly developed systems that are aimed at improving the herd health, such as segregated early weaning and multiple site production. PPE is currently controlled in Australia with the routine addition of antimicrobials in pig feed, in particular olaquindox. Recommendations to reduce the use of feed-based antibiotics in Australia require the development of alternate strategies to control diseases such as PPE. Sequential outbreaks of PHE reported in minimal disease herds suggested that pigs could develop immunity to disease. An experimental model of L.intracellularis infection was developed in this thesis to demonstrate that immunity to re-infection with L.intracellularis could be developed. Infection was monitored by detection of faecal shedding of L.intracellularis and serum IgG antibodies against L.intracellularis. Two in-feed antimicrobial strategies were analysed in this thesis for their ability to induce the development of immunity to L.intracellularis, while avoiding clinical signs of disease. The first strategy evaluated the use of low levels of in-feed antimicrobials to allow subclinical infection and the development of immunity. The second strategy evaluated the use of high levels of in-feed antimicrobials to terminate infection two weeks after exposure to L.intracellularis. Gaining a greater understanding of how L.intracellularis infection is spread both within and between piggeries will enable the development of management strategies to control the spread of infection. This thesis examined the possibility that other species in contact with pigs and piggeries such as rats, mice and birds may transmit infection to pigs. The transmission of infection between pigs via the faecal/oral route was also examined, as was the survival and infectivity of L.intracellularis over time. Ultimately this thesis aimed to understand the pattern of L.intracellularis infection and the survival and transmission of L.intracellularis in order to develop effective control measures for PPE, especially in minimal disease herds.
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