Epidemiological studies to inform control strategies for paratuberculosis in farmed deer : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Massey University, Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Palmerston North, New Zealand
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Date
2010
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Massey University
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Abstract
Paratuberculosis, caused by Mycobacterium avium subsp. paratuberculosis (MAP), occurs
in a range of ruminant species, and has been diagnosed in wild and domesticated
deer worldwide. The disease process in other ruminants is chronic and fatal, with the
highest clinical disease incidence generally seen in older animals. However, in farmed
deer, disease incidence is highest in young animals, occurring as an acute syndrome in
deer as young as eight months of age. The deer industry in New Zealand is concerned
about the on-farm impact of paratuberculosis, and the consequences for the venison market
should MAP be classified as a zoonosis. Research is thus directed at investigating
tools for paratuberculosis control, to reduce the threat to the industry.
The aim of the research presented in this thesis was to provide epidemiological evidence
that can be used to inform strategy, at industry and farm-level, for control of paratuberculosis
in deer.
A survey of the deer slaughter population established a baseline prevalence of MAP infection,
against which the effects of control initiatives can be measured. Infection was
widespread in individuals (45%) and herds (59%), suggesting control rather than eradication
as the goal of any industry programme.
On-farm disease control was investigated in a randomised controlled trial of vaccine efficacy
in young naturally-infected deer. Vaccination reduced the incidence of clinical
disease and subclinical pathology; no significant effect on mean production parameters
was seen. There was no effect of vaccination on faecal MAP excretion, indicating vaccination
may not reduce infection prevalence. Vaccinated deer had an increased risk of
testing positively to diagnostic screening tests for bovine tuberculosis. Non-specificity
was resolved by ancillary testing, but such tests come at an increased financial and test
sensitivity cost.
Paratuberculosis control at the industry level may involve schemes to classify herd infection
status. For this purpose, the sensitivity and specificity of individual faecal culture and
an IgG1 ELISA (Paralisa) to detect young deer infected with MAP was estimated using
Bayesian latent class analysis. Paralisa and faecal culture had sensitivity of 19%and 77%,
and specificity of 94% and 99%, respectively. Improved diagnostics are therefore needed
if herd infection status is to be classified in a sensitive, specific, cost-effective and timely
system.
The studies contribute to knowledge on different aspects of paratuberculosis control in the
New Zealand farmed deer population, providing an evidence base for informed decisionmaking
at farm and industry level.
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Keywords
Deer diseases, Paratuberculosis, Farmed deer, Mycobacterium avium subsp. paratuberculosis, Vaccination, Epidemiology, New Zealand