The epidemiological investigation of pig diseases : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Massey University

A series of epidemiological studies was carried out to identify methods of improving productivity of New Zealand and Thai pig herds. Disease surveillance at slaughter of 2,807 finisher pigs from 3 piggeries in the North Island of New Zealand was conducted over a 1 3 month period. This was used to establish and apply procedures suited to New Zealand conditions, for monitoring the subclinical status of 8 production-limiting diseases and 1 zoonotic disease. Enzootic pneumonia, sarcoptic mange and oesophagogastric ulcer were the most commonly observed lesions. Seasonal patterns were found for enzootic pneumonia, pleurisy and ascariasis lesions. Seasonal effects were found to be associated in part with temperature variation. The effectiveness of simultaneous administration of commercial Mycoplasma hyopneumoniae and A ctinobacillus pleuropneumoniae vaccines was assessed in 380 pigs from an indoor commercial piggery. These vaccines produced significant improvement of growth rate during the high risk period for clinical pneumonia, and increased slaughter weight. Enzootic pneumonia lesions were significantly reduced by more than 50%, but pleurisy and pleuropneumonia lesions were not significantly reduced. No evidence of synergy between the vaccines in influencing lesion severity for pleuropneumonia was detected. A longitudinal study of thirty cohort pigs was conducted within the vaccination trial, to describe the epidemiological pattern of subclinical A. pleuropneumoniae infection of healthy pigs. A. pleuropneumoniae was first isolated at 4 weeks of age from one vaccinated pig. The incidence of A. pleuropneumoniae infection reached a maximum of 54% and 40% at 1 1 weeks of age in vaccinated and control pigs. No evidence was found to support the hypothesis that infection with M. hyopneumoniae increases susceptibility to A. pleuropneumoniae infection. Pig production data from 16 Thai and 1 8 New Zealand pig herds for 1 99 1 , and from 1 4 Thai and 16 New Zealand herds for 1 992 were analysed to define opportunities for improving productivity in temperate and tropical environments. In Thailand, potential areas for improving productivity are particularly through increasing total litter size and improved management of breeding procedures. In New Zealand , potential areas for improvement are particularly reduction of stillbirths and pre­ weaning mortality, and reduction of sow mortality. A new method of graphical presentation of important productivity parameters was used to clearly demonstrate differences in performance between the two countries. ii PigFIX, a fertility investigation expert system linked to a computer-based herd recording system, was developed to provide diagnostic guidance on likely causes of fertility problems and offer guidance on possible corrective action. A novel approach was used in developing the method by which PigFIX assessed reproductive performance. Graphical and text-based reports were developed to show the user what conclusions had been drawn in the analysis. PigFIX was shown in verification studies on six herds to produce conclusions which agreed with a human expert on identification of major reproductive problems. iii Acknowledgements I left Thailand in November 1991 to undertake the Summer English Language Course at Victoria University of Wellington, and subsequently undertoo k my PhD in Veterinary Epidemiology at Massey University, New Zealand. The time has passed quickly and I am most grateful for the friendship, hospitality and support that have been offered to me and my wife by so many people during our stay in New Zealand. I would like to thank my chief supervisor, Professor Roger Morris, for convincing Kasetsart University, Thailand to allow me to study in the Epidemiology group at Massey. Without him I would still be working in my home country, and would not have had an opportunity for advanced study overseas. He has encouraged and guided me with enthusiasm throughout my period of research and thesis writing. I also appreciated his willingness to arrange a number of international trips to meet other epidemiological workers and scientists in various areas of interest to me. I wish to thank Associate Professor Roger Marshall, my former co-supervisor until his retirement, for his encouragement, support and his expertise in Bacteriology. Special thanks to Dr Dirk Pfeiffer, my co-supervisor, who offered his expertise in statistics and computer science, and gave me his constructive criticism and advice whenever required. I am grateful to Mr Stan Fenwick for his support, guidance and constructive criticism in Bacteriology. Thanks to Mr Neil Christensen for his help and suggestions with regard to slaughter checks. Thanks are due also to Associate Professor M R Alley for his suggestions in Pathology. I also want to thank Mark Stem, computer scientist and programm er who took ideas for a program and turned them into computer reality. I am grateful to Faris Sharpe for his long term assistance in the post-mortem room. Special thanks to Lynn Cullin ane, Magda Gwozdz, Jan Schrama and Peter Wildbore for their help in microbiological techniques, media preparation and ordering all experimental materials. Special thanks to Fiona Dickinson for her expertise in Word Perfect and her help with preparation of the manuscript. I thank Robyn O'Connor for her help with administrative matters, and Vanessa Tilson for her suggestions in refIning style of poster presentations, and technical assistance. These studies could not have been possible without the cooperation of Thai and New Zealand pig farmers who allowed me to use their data or animals, and Kiwi Bacon Company of Longbum and