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    Aspects of Trichinella spiralis in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Veterinary Studies in Epidemiology at Massey University
    (Massey University, 2006) Richardson, Esther Katharine Barr
    In New Zealand, Trichinella spiralis appears absent from commercial pigs, and is not a significant cause of zoonotic disease. Surveillance testing at slaughter has not ever detected T. spiralis in a commercially raised pig in New Zealand, yet many importing countries still require individual testing of all pig carcases for export. This thesis comprises four studies designed to evaluate the risk of T. spiralis infection in commercial pigs in New Zealand. In the first study, the prevalence of T. spiralis was surveyed in selected populations of rats, cats, stoats and weasels from landfills, piggeries and Department of Conservation lands as they are considered potential reservoirs of T. spiralis for domestic pigs. No positive samples were detected in this survey. The second study investigated rodent activity and baiting efficacy on three commercial piggeries in the Manawatu region of New Zealand over 72 weeks. This study found that while baiting and on-farm sanitation can effectively control rodents, the efficacy of the control depends largely on staff commitment. The third study used a mail questionnaire sent to 123 piggeries, to survey current management practices on commercial piggeries that could pose a risk for Trichinella transmission. The survey had a 69% response rate and found that risky management practices occur infrequently in commercial piggeries. Lastly, a quantitative risk model was developed comparing individual carcass testing with alternative risk management strategies to assess the annual probability that a consumer in an importing country will eat a pork product of New Zealand origin containing at least one larva per gram of T. spiralis. Offals were found to always be safe. However, the unrestricted risk for fresh pork was over one in a million (1.87x10-5), which was above the safety threshold and therefore various risk management options were considered. The strategy of only exporting pigs reared in certified, confined commercial herds has now been accepted and is in place by the importing country. This means that individual carcass testing is no longer compulsory for every exported pig carcass.The overall conclusion from this research established a very low risk of T. spiralis infection and transmission in New Zealand commercial piggeries. These findings have directly led to successfully changing export legislation by removing compulsory carcass testing pre-export, which has advanced New Zealand’s ability to competitively export high quality pork and pork products.
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    An investigation of the aetiology and natural transmission of postweaning multisystemic wasting syndrome in pigs : a thesis presented in partial fulfilment of the requirements for the degree of Master of Veterinary Studies in Epidemiology at Massey University, Palmerston North, New Zealand
    (Massey University, 2007) Jaros, Patricia
    Postweaning multisystemic wasting syndrome (PMWS) is a wasting disease primarily affecting weaned pigs. The disease causes significant production and financial losses through increased mortality rates and reduced daily weight gain. The aetiology is controversial although reports commonly suggest that PMWS is associated with the presence of porcine circovirus type 2 (PCV2) with disease expression modified by a range of infectious and non-infectious factors. While PCV2 infection in New Zealand is ubiquitous, PMWS has behaved as a propagating epidemic since its first incursion beginning in about 1999. The initial outbreak of PMWS in New Zealand was limited to a small cluster of farms near Auckland, which were epidemiologically linked to a possible entry mechanism. A transmission study was conducted in 2005 to critically evaluate alternative hypotheses which have been proposed for the causation and epidemiology of PMWS. The study set out to investigate the natural transmission of PMWS by direct contact between PMWS-affected and susceptible pigs, while managing the influence of proposed co-factors. Six different groups, comprised of pigs from PCV2-negative and positive herds were directly exposed to possible PMWS agents at 4 and 12-weeks-of-age and compared with two groups of unexposed pigs. All experimental groups were observed daily for 8 weeks or longer and evaluated clinically and pathologically. After exposure to PMWS-affected pigs, disease characterised by wasting, dyspnoea and high case fatality rates occurred in both PCV2-positive and PCV2-negative pigs of four-weeks-of-age, but not in pigs older than 12 weeks. Histopathological lesions found in experimental groups with clinical cases were consistent with those previously reported for PMWS. A range of infectious pathogens proposed to have a modifying influence on PCV2 and to contribute to disease causation were absent as determined by molecular and serological test methods. In addition, there was not sufficient molecular evidence to explain the genomic difference between PCV2 isolates from healthy and PMWS-affected pigs. Taking this, and supporting evidence from the other experimental groups into account, the findings of this study strongly support the conclusion that a transmissible agent other than PCV2 is involved in the causality of PMWS.
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    Leptospirosis in the pig : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Massey University
    (Massey University, 1978) Ryan, Terence John
    The chapters that follow a review of the literature, describe firstly the serological and culture techniques which were used to study leptospiral infection in pigs, and secondly an attempt to define the epidemiology of leptospirosis in pigs in New Zealand. After this an investigation of cell mediated immunity in pigs which had been infected with serovar pomona is described. During the study a number of experiments were conducted with the aim of defining the optimal conditions of cell culture for these transformation experiments, and a number of these have been reported in the appendices.
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    Leptospirosis in New Zealand pig herds : an epidemiological study and a computer simulation model of endemic leptospiral infection in New Zealand with particular reference to Leptospira interrogans serovar pomona : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Veterinary Science at Massey University
    (Massey University, 1990) Bolt, Ingeborg
    A serological survey of pig sera from six regional areas throughout New Zealand indicated that 60% had titres to Leptospira interrogans serovar pomona and 13% to serovar tarassovi. Pig sera from the North Island districts had higher titres to pomona than those from the South Island districts, however the converse was true for titres to tarassovi. A serological survey of bacon weight pigs at slaughter revealed that 60% had titres to pomona, 53% to bratislava, while titres to tarassovi were undetectable. There was no significant linear association between the magnitude of corresponding pomona and bratislava titres. Pomona was isolated from 53% of pigs kidneys, however attempts to isolate bratislava were unsuccessful. The median prevalence of infection in bacon pigs from farms with endemic pomona infection, at the time of slaughter was 80%. A retrospective study of farming practices revealed that vaccination of breeding pigs had no effect on the infection status of their grower pigs at slaughter. It was also found that farms which reared their grower pigs to pork weight for slaughter were free of leptospiral infection, as were farms with less than fifty breeding sows. Cross sectional serological and cultural prevalence studies of grower pigs on farms with endemic pomona infection revealed that pigs less than ten weeks of age were not leptospiruric and had low or undetectable titres to pomona. Pigs between ten and twenty weeks of age showed an increasing prevalence of both leptospiruria and pomona titres. Further prospective studies indicated that piglets acquire passive immunity from their dams, which has a half life of around sixteen days. The majority of pigs less than fourteen weeks of age appear to be resistant to infection, thereafter the level of their passive immunity wanes and they become infected and leptospiruric. The weekly incidence of leptospiruria for pigs in an infected grower house was usually between 10% and 20%. Following infection, the intensity of leptospiruria was greatest in the first three to four weeks and it lasted for at least six weeks. Infection is believed to occur by both direct and indirect transmission of leptospires between infected and susceptible pigs. It was shown that grower pigs are at the centre of the endemic cycle of infection which is perpetuated by the transmission of infection from older infected pigs to younger susceptible pigs. This cycle of endemic infection can persist independently of the breeding herd. Experimental evidence following the artificial exposure of grower pigs to either serovar pomona or bratislava supported an hypothesis that the occurrence of bratislava titres were associated with early infection of pigs with serovar pomona. The heterologous titres were believed to be a serological cross reaction with homologous IgM antibody to which pigs had been exposed. Experimental evidence demonstrated that leptospires could survive in droplets of less than 50 um, however hamsters exposed to a leptospiral aerosol containing droplets of less than 50 um failed to become infected. Infection via the intranasal route in both hamsters and pigs showed that the infective dose of pomona was between 104 and 106 leptospires, indicating the intranasal route as a natural route for infection. Transmission of infection could therefore occur directly by infective droplets lodging in the nasal cavity. The vaccination of pigs, commencing at ten weeks of age on a farm with endemic pomona infection, revealed that multiple inoculations of a commercially available bacterin can be used to control the level of endemic infection within a grower pig herd. There was evidence to suggest that persistent passive immunity in young pigs could interfere with the efficacy of vaccination. A computer simulation model of endemic pomona infection in a pig herd [Simulated Leptospiral Infection within a Pig herd, SLIP89] was developed using the results of investigations described in this thesis by sequentially breaking down the cycle of endemic pomona infection into a series of logical events. The model repeats a number of predetermined independent and dependant events for each pig within a simulated herd. The outcome of each event is randomly determined from an appropriately selected probability distribution. Each cycle of repetition represents one week in time. The outcome generated by the simulation can be used to observe varying patterns of infection which are due to either the element of chance or the alteration of key variables within the model. The results generated by the SLIP89 must be viewed with the structure and limitations of the model in mind.
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    The epidemiology of infections by Streptococcus suis types 1 and 2 : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Veterinary Pathology and Public Health at Massey University
    (Massey University, 1988) Robertson, Ian Duncan
    An Indirect Fluorescent Antibody Test (I.F.A.T.) was developed for the Identification of carriers of Streptococcus suis. This test was more accurate than the "traditional" cultural techniques. The sensitivity for detecting S. suis type 1 from tonsillar swabs was 62%, and 76% for S. suis type 2. For nasal swabs collected from live pigs the sensitivity was 48% and 60% respectively. From the examination of 959 non specific pathogen free domesticated pigs fron 63 herds, 263 specific pathogen free (SPF) pigs from seven herds and 96 feral pigs it is concluded that all non-SPF domesticated pigs older than five weeks of age are infected with both S. suis types 1 and 2. Variation in the apparent prevalence of infection between different herds was associated with the number of pigs sampled in each herd rather than a real variation in the prevalence and was similar in both New Zealand and Australian non-SPF domesticated pigs. Three of seven SPF herds tested in Australia were free from infection with S. suis types 1 and 2. It is proposed that all SPF pigs are delivered free from infection and that infection is subsequently introduced into the piggery. Streptococcus suis was detected from numerous tissues and fluids Including the blood of normal pigs. It is suggested that the isolation of S. suis from "diseased" tissue does not infer that S. suis was the primary aetiological agent. Although both S. suis types 1 and 2 were readily detected in the environment of pigs housed in an intensive piggery, the main routes of transmission were considered to be direct contact or neonatal infection of piglets born to sows with vaginal infections. Based on serological results from using an Enzyme Linked Immunosorbent Assay (ELISA), the pattern of infection due to S. suis types 1 and 2 was similar in domestic pigs. In feral pigs the pattern of infection due to S. suis type 2 was similar to that of domestic pigs, but in the case of S. suis type 1 the prevalence and titres were considerably lower in feral pigs. It is concluded from these results, that both domestic and feral pigs are true long term carriers of S. suis type 2 whilst S. suis type 1 is maintained within a population by a series of reinfections dependent on the population density. There was no evidence of ascending uro-genital infection or signs of clinical disease in 24 bacon weight pigs inoculated with S. suis type 1 or 2 by either the intravaginal or intrapreputial routes. When two litters of previously non-infected pigs were intranasally inoculated with S. suis, no pigs developed clinical signs. The results of this experiment showed that the infective dose was less than 100 organisms by this route. Only one of eight pigs developed "classical" clinical signs after intravenous inoculation with a porcine isolate of S. suis type 2. Some pigs remained apparently normal even though high numbers of organisms were present in the blood for up to two weeks. Only two of fourteen pigs inoculated with S. suis type 2 into the cerebro-spinal fluid (C.S.F.) developed signs of clinical disease. Both these pigs were inoculated with a human strain of S. suis type 2. To explain these results it is proposed that strains of S. suis of different pathogenicity exist. This hypothesis was supported by the findings that this human strain was also pathogenic for mice, rats and rabbits whilst the porcine isolate was pathogenic only for rabbits. A cross-sectional study of various occupational groups showed a positive correlation between contact with pigs and being seropositive to S. suis type 2. No veterinary students, 10.3% of meat inspectors, 14% of dairy farmers who also kept pigs and 21.4% of pig farmers were seropositive. In exposed pig farmers the annual incidence of seroconversion could be as high as 30%. It is believed that subclinical infection of humans, as well as pigs, occurs and that isolates pathogenic for pigs are also pathogenic for other species of animals including man. It is hypothesized that S. suis is endemic in pigs at a prevalence rate approaching 100% in all countries where pigs are kept. The pathogenicity of isolates can vary and this could account for variations in the severity of disease reported throughout the world. It is concluded that S. suis is usually a commensal and is involved in a disease process only when pathogenic strains are present and after some other primary insult.
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    Epidemiology of porcine rotaviral infections : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Veterinary Virology at Massey University
    (Massey University, 1988) Fu, Zhen Fang
    The epidemiology of porcine group A rotavirus was studied in commercial herds of pigs in New Zealand. A commercial enzyme-linked immunosorbent assay (ELISA) kit (Dakopatts, Copenhagen) was found to be highly sensitive (100%) and specific (96.8%) for the detection of group A rotavirus in pig faeces when compared to electron microscopy (EM). A highly sensitive and specific ELISA test for the measurement of antibody against group A rotavirus in pigs was also developed. Of 521 faecal samples collected from Massey University Piggery in a cross-sectional survey, 47 (9%) were positive for group A rotavirus by the ELISA test. Only sucking (19.3%) and weaner (14.4%) pigs were shedding rotavirus. Rotavirus was not detected in the faeces of fattener pigs (over two months old) or adult pigs including sows and boars. Three cohort studies revealed that all pigs became infected with group A rotavirus before they were 40 days of age and shed the virus for an average of eight days. Some of the piglets shed rotavirus a second time approximately 10 days after the first period of shedding. All piglets ceased to shed the virus by two months of age. Rotaviral shedding was associated with the occurrence of milk scours in sucking piglets. Diarrhoea in weaner pigs (post-weaning diarrhoea, PWD) was more closely associated with the presence of haemolytic E.coli than with rotavirus in faeces. Infection with group A rotavirus was transmitted from piglet to piglet and from litter to litter. In one cohort of 50 piglets from five litters, shedding of rotavirus was first detected in one litter, then in the second litter two days later, and finally in other litters of piglets. Over a period of 16 days, all piglets in the five litters were infected. Group A rotavirus was also detected in dust, faeces and effluent collected from the farrowing and weaner houses, and from a weaner house which had not been used for three months. Rotavirus was not detected in any of the sows (11) during the period of investigation. Neither was rotavirus detected in fattener pigs (from two months of age to the time of slaughter), nor in the environments where old pigs (fattener and sow houses) were housed. It was not therefore possible to confirm that adult pigs, especially sows, act as carriers for rotaviral infection of young piglets as has been suggested by other workers. All the piglets acquired maternal antirotaviral antibody from their dams and the levels of antibody in piglets' sera were comparable to those in the colostrum of their dams. The maternally-derived antibody was also detected in piglets' faeces. Antibody in sera and in faeces declined rapidly after birth. Rotaviral shedding commenced in each of the cohorts when the geometric mean ELISA antibody titre fell below 1/1600 (equivalent to serum neutralizing antibody titre of 1/8 to 1/16). However, this correlation between antibody titres and protection was not observed in individual litters. In each of the cohorts studied, rotaviral shedding was usually detected initially in one or two piglets of a litter. The infection then spread to other piglets within the same litter and, finally, to piglets of other litters in the same group. Onset of rotaviral infection in particular litters was related to their location in the farrowing unit rather than to the levels of antibody. The shedding pattern of group A rotavirus was studied further in another five New Zealand piggeries and was found to be similar to that observed at the Massey University Piggery. Faecal samples from these piggeries were analyzed by polyacrylamide gel electrophoresis (PAGE) and no common electrophoretype of group A rotavirus was found in these piggeries. More than one electrophoretype of group A rotavirus was detected in three of these piggeries. Faecal samples collected from Massey University Piggery were also analyzed by PAGE and it was found that rotaviruses detected during the first two years of the present investigation had identical electrophoretypes, but one isolate detected in the third year had a distinctively different pattern. Non-group A rotaviruses were also detected for the first time in New Zealand. Nine samples had an electrophoretic pattern similar to that of group C rotavirus and one was similar to that of group B rotavirus. One of the samples containing group C rotavirus was from two litters of piglets with diarrhoea at two to four days of age. In one of these piggeries, five electrophoretypes of rotaviruses representing three groups were detected. These observations indicate that rotaviral infections are important causes of milk scours in piglets, and are probably significant in exacerbating PWD. The epidemiology of rotaviral infections is complicated by a number of factors. These include the continuous transmission of virus from pig to pig and from litter to litter, the survival of the virus in the piggery environment, the incomplete protection afforded by maternally-derived antibody, and the simultaneous circulation of different strains and different groups of rotavirus in one piggery.
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    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
    (Massey University, 1995) Wongnarkpet, Sirichai
    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 13 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 Actinobacillus 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 11 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 18 New Zealand pig herds for 1991, and from 14 Thai and 16 New Zealand herds for 1992 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. 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.