The epidemiology of Johne's disease in New Zealand farmed deer, including validation of abattoir-based surveillance : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Veterinary Clinical Science at Massey University, Palmerston North, New Zealand
Johne’s disease (JD), a fatal granulomatous enteritis predominately affecting ruminants, is
caused by Mycobacterium avium subspecies paratuberculosis (MAP) and has become a
potentially serious economic risk to New Zealand deer farmers. This research investigates the
incidence of clinical JD on affected deer farms, along with potential risk factors that may be
adopted by farmers for its control. Additionally, research was undertaken to establish a national
abattoir surveillance mechanism for MAP, and to validate criteria used for that purpose.
Personal interview of 174 farmers in a 2005/2006 national case-control study, followed by a
2007 longitudinal postal questionnaire, allowed characterization of clinical JD at the herd- and
age-class levels and identification of risk factors significantly associated with herd-level MAP
infection and farmer-diagnosed clinical JD in young deer. The mean within-herd clinical JD
annual incidence rate was 1%, with age-class incidence rates highest (2.0%) in yearling hinds
and stags. The proportion of herds affected with clinical JD increased from 2005 to 2007,
showed a seasonal trend, and was higher in the South versus North Island. Species other than
deer, specifically beef cattle and sheep, were variably associated with herd-level MAP infection
and clinical JD in young deer. As the first investigation of herd-level MAP and clinical JD in
farmed deer, this research considerably increased our knowledge specific to this species. This
information may inform control measures and direct further research aimed at reducing the
prevalence and transmission of MAP and development of clinical JD in farmed deer.
Another research aim was to validate abattoir MAP surveillance in deer through meat inspector
identification of ‘abnormal’ lymph nodes (LN). A pilot cross-sectional study found 94.6% of
‘abnormal’ LN were MAP-positive through culture or histopathological examination. A 55 mm
circumference cut-point defining an ‘abnormal’ LN was then established and the sensitivity,
specificity and level of agreement of inspector detection were estimated at 13.3%, 99.9% and
‘fair’ (? = 0.32), respectively. As an adjunct to this validation, the prevalence of and risk factors
associated with seven histopathological features of grossly ‘normal’ LN from herds classified at
‘low’ and ‘high’ MAP risk were described. This research allows confident and informed use of
conclusions drawn from the national abattoir-based surveillance scheme. However, animal-level
prevalence is currently underestimated with use of ‘abnormal’ LN as the sole criterion and
further inspector training is required.
The research presented in this thesis provides the foundation for future examination of MAP and
clinical JD in farmed deer, its control and surveillance.
Content removed due to copyright restrictions:
Hunnam, J. C., Wilson, P. R., Heuer, C., Mackintosh, C. G., West, D. M., & Clark, R. G. (2011).
Histopathology of grossly normal mesenteric lymph nodes of New Zealand farmed red deer
(Cervus elaphus) including identification of lipopigment. Veterinary Pathology, 48(2), 525-529.