Bovine TB and the ‘singleton protocol’: reward without risk
- W. Ray Waters, BS, DVM, PhD
- USDA, ARS, National Animal Disease Center, 1920 Dayton Avenue, Ames, Iowa 50010-0070, USA
A PIVOTAL balance for the design of disease control programmes is to achieve widescale benefits for populations based on regions/countries while minimising financial and operational burdens for individual producers. An assumption, formal or implicit, is that control programmes are periodically monitored to amend policy and procedures in accordance with changes in socioepidemiological factors. Principal control strategies for bovine tuberculosis (TB) include antemortem testing (primarily via tuberculin skin test and interferon-γ release assays), slaughter surveillance, isolation of affected herds, slaughter of infected animals or entire affected herds (depopulation or ‘stamping out’), movement control and movement testing policies.
The ‘singleton protocol’ was initiated in 1996 by the Irish Department of Agriculture, Fisheries and Food to address the shortfall in specificity of the single intradermal comparative cervical test (SICCT) for bovine TB. With the SICCT, Mycobacterium bovis purified protein derivative (PPD) and Mycobacterium avium PPD are administered intradermally at separate sites in the mid-cervical region of cattle. Seventy-two hours later, injection sites are measured for the level of reaction (induration) to provide a presumptive diagnosis concerning bovine TB infection status of the animal. As applied to cattle in Ireland, the SICCT has excellent relative specificity (ie, approximately 99.8 to 99.9 per cent [O'Reilly and MacClancy 1975]). As the test is not 100 per cent specific, a small fraction of reactors are falsely identified as M bovis-infected. The ‘singleton protocol’, as described by Murray and others (2012) in a paper summarised on p 516 of this issue, provides an opportunity for early restoration of health status for herds that meet epidemiological and test criteria as a singleton herd. Through analysis of postmortem results of cattle originating from herds with a single SICCT reactor, Murray and others …