Browsing by Author "Norton, Solis"

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    Bovine tuberculosis in the Brushtail possum (Trichosurus vulpecula) : behaviour and the development of an aerosol vaccinator : a thesis presented in partial fulfilment of the requirements for the degree of Master of Applied Science at Massey University
    (Massey University, 2001) Norton, Solis
    The Australian brushtail possum (Trichosurus vulpecula) is a wildlife reservoir of tuberculosis (Mycobacterium bovis) in New Zealand. The disease is endemic over one third of the country. Possum control operations have reduced the prevalence of disease in livestock but have not fully controlled infection in wildlife or geographic spread of the disease. The disease is transmitted to livestock when they investigate the unusual behaviour of terminally ill possums. Reduction of disease incidence in possums through vaccination with bacille Calmette-Guerin (BCG) has shown promise both in pen trials and field studies. Integration of vaccination into existing control programmes may reduce transmission of tuberculosis among possums, and from possums to livestock. There are two parts to this thesis. Part one is a longitudinal, behavioural study of tuberculous and nontuberculous wild possums. Part two is a description of an aerosol delivery device (aerosol vaccinator) designed to administer aerosolised BCG vaccine to possums in the wild, and a record of its progressive development. The aim of part one was to identify aspects of behaviour of tuberculous possums that may influence disease transmission to livestock. Twenty two tuberculous and eight healthy possums were observed. Possums were radio tracked weekly and live trapped at bimonthly intervals on a 56 hectare site in the Wairarapa, New Zealand. Generally possums remained within their activity range apart from infrequent long distance forays. Possums were weak, lethargic and uncoordinated during the terminal stages of disease which lasted for one to three weeks. Only three possums made long forays when terminally ill with tuberculosis. The carcasses of 17 tuberculous possums were recovered of which 15 were in dense scrub or on long grass under scrub and two were on pasture. Of these 17 carcasses, 14 were within or near (<200m) to their activity range. Most tuberculous possums died in their activity range and in scrub. These possums represent little risk of infection to livestock, but a risk to other wildlife. However, the small number of tuberculous possums that died on pasture present an important risk to livestock. Interactions between diseased and healthy possums during long distance forays may cause considerable geographical spread of tuberculosis. Part two, the development of an aerosol vaccinator, consisted of pen and field trials. The aim of pen trials was to evaluate the willingness of possums to investigate novel objects and the influence of social hierarchy on this investigative behaviour. It also allowed refinements to aerosol vaccinator design. Four captive colonies were used. In each colony most possums (80%) showed minimal neophobia and would actively investigate novel objects. A small proportion (20%) would not approach a novel object. A loose social hierarchy existed with one dominant animal and a changeable middle order. In two of the four colonies, there was one possum clearly at the bottom of the social order. Social hierarchy did not affect the proportion of a colony which could investigate a novel object or vaccinator. However it did effect the order in which individual possums would investigate. The aim of the field trials was to evaluate the efficacy of an aerosol vaccinator with possums in the wild. During five field trials, the proportion of the possum population marked with dye from the device steadily increased and ranged from 0%–34%. Trials were conducted over eight months and during this period a total of 56% of the study population was marked with dye. Some possums would repeatedly use a vaccinator. These results justify further research into aerosol vaccination of wild possums with BCG. Three key avenues for future research include determining the proportion of a possum population which will use the device, developing an aerosol container suitable for dispensing BCG vaccine and determining whether the combination of vaccinator and aerosol vaccine elicits a protective immune response. The aerosol vaccinator may be use [sic] to deliver aerosolised materials other than BCG to possums. It may also be altered to suit use by other species.
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    The epidemiology of Johne's disease in New Zealand dairy herds : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Massey University
    (Massey University, 2007) Norton, Solis
    Johne's disease (JD), caused by Mycobacterium avium subspecies paratuberculosis (MAP) is a chronic, debilitating enteritis of cattle, other domestic livestock and some wildlife species. JD was first identified in the late 1800s and today it is a worldwide problem in dairy cattle. Heavily infected cows have reduced milk production, a higher risk of removal from the herd and low slaughter value. Several countries have implemented national level control strategies. In New Zealand, JD was first reported in 1912 and today the prevalence of infected dairy herds is thought to be high. To improve our understanding of the epidemiology of JD and to evaluate the feasibility of a national control strategy, four studies were conducted. The first study was a questionnaire based case-control study to identify associations between management practices and the occurrence of clinical JD on farms from four regions of New Zealand. The second study was on the effect of sub-clinical JD on milk production and the risk of removal from the herd in four dairy herds over four milking seasons. The effect of misclassification of disease status on productivity estimates was also studied. In the third study diagnostic test result data from the productivity study was combined with a novel Bayesian regression model to estimate performance of the ELISA and faecal culture tests as a function of covariates and utilising repeated tests on individual cows. Finally, results from these three studies were used to adapt an existing JD simulation model, 'JohneSSim', to represent the epidemiological behaviour of JD in New Zealand dairy herds. Control strategies for the disease were simulated and evaluated based on their cost effectiveness. Of the 427 farmers responding to the questionnaire, 47% had suspected clinical cases of JD in their herd in the preceding 5 years. Only 13% of suspected infected herds had an average incidence of greater than 0.5 cases per 100 cow years at risk. The disease was not considered a serious problem by 20% of herd managers who reported the presence of disease in the preceding 5 years. The presence of Jersey cows in the herd and the purchase of bulls had strong positive associations with the presence of clinical JD. Grazing calves in the hospital paddock, larger herds, the purchase of heifers, and the use of induction were also positively associated with JD. In the productivity study the herd-level prevalence of JD by ELISA and/or faecal culture ranged from 4.5% (95% CI 2.6-6.9) to 14.2% (95% CI 9.2-20.6). Daily milksolids production by JD positive cows was 0.8% (95% CI -6.1%-4.5%) less than that of JD negative cows. However in herd D, JD positive cows produced 15.5%, (95% CI 6.75%-24.2%) milksolids less than JD negative herd mates daily. This equates to a loss of 53kg of milksolids/305 day lactation, or NZD 265/lactation, given a price of NZD 5/kg of milksolids. In herd D only, the annual hazard ratio of removal for JD positive cows was significantly increased. It was 4.7 times and 1.4 times higher in cows older than 5 years and younger than 5 years. The results were insensitive to misclassification. Analysis of the diagnostic test data demonstrated the strengths of our Bayesian regression model. While overall estimates of sensitivity and specificity by this method were comparable to estimates by existing methods, it showed a broad trend of increasing sensitivity in higher parity groups and higher sensitivity in early, relative to late, lactation. It also showed that estimates of prevalence may in fact decline with repeated, relative to single, testing. Our novel approach demonstrated trends that could not be shown by existing methods, but could be improved by application to a larger data set. Simulation showed that control strategies for JD based on either test-and-cull, vaccination, breeding for genetic resistance, or removal of offspring from clinically affected cows, were not cost effective for the average infected herd. Improvement of the hygiene associated with calf management provided the greatest reduction in the within-herd prevalence of JD. While JD is present in a high proportion of New Zealand dairy herds, the incidence of clinical cases is usually low, and most farmers consider it to be of little importance. However, JD causes significant losses in productivity in some herds. The disease would probably be best controlled on a herd-by-herd basis, given the limited success of national-scale control programs for JD in other countries. The education of dairy farmers regarding risky management practices, and the offer of a risk assessment to farmers wishing to control the disease, would provide a combination of wide reaching and targeted approaches, of low cost, for JD control. It seems likely that JD will persist in some capacity in the years ahead, but will remain of minor concern next to major animal health issues, such as infertility and mastitis. Clarification of the effect of genetic strain on the virulence of MAP may help explain differences in the effect of the disease between herds. This knowledge could then be used to further improve the efficiency of JD control.