Diagnostic strategies and outcomes on three New Zealand deer farms with severe outbreaks of bovine tuberculosis

SETTING: Three commercial deer herds, each containing more than 500 deer, experienced outbreaks of tuberculosis (TB) ranging from a 6% prevalence to disease levels >50%. Intensive diagnostic testing was carried out over short time intervals after the initial diagnosis of TB. OBJECTIVE: To eradicate M. bovis infection from herds of farmed red deer (Cervus elaphus) in New Zealand, which had significant outbreaks of tuberculosis (>5%), using complementary diagnostic tests and elective slaughter of all test positive animals. DESIGN: Whole herd mid cervical skin tests (MCT) were used as the primary test and a comparative cervical test (CCT) as an ancillary test. In an attempt to enhance TB eradication, ancillary blood tests comprising; lymphocyte transformation tests (LT) and enzyme-linked immunosorbent assays (ELISA) were used in parallel with MCT, or as serial tests, to complement skin testing. RESULTS: One deer herd (N), which had an acute outbreak (6%) of TB in adult stock, responded quickly to testing and the disease was eradicated within 12 months. A second more chronically infected herd (B), with low prevalence (2%) of TB initially in pregnant hinds which were retained over the breeding cycle, developed widespread uncontrollable spread of infection, that could not be contained by exhaustive testing. The final herd (S), which had an acute outbreak of TB at a prevalence >90% in young fawns, responded well to testing and became TB-free within 1 year. CONCLUSIONS: TB can be eradicated from infected farmed deer herds, using currently available TB tests, irrespective of disease prevalence. The caveat is that disease must be diagnosed early in the acute phase of infection. Undiagnosed infection over winter in breeding hinds in one herd produced a refractory infection in adult males and females and uncontrollable spread to newborn fawns. Disease could not be eradicated using comprehensive diagnostic schedules. A widespread but controllable outbreak of acute TB in another herd, appeared to have caused sufficient selective pressure on adult stock that genetically resistant adults were selected within a very short time frame (<1 year).