Bovine tuberculosis (bTB) poses significant health risks to livestock and humans alike, particularly nutrition-dependent communities. Various control measures have been employed, but challenges persist, particularly concerning effective diagnostic tests for detecting the disease among Bacillus Calmette-Guérin (BCG)-vaccinated cattle. A recent study published by researchers highlights the strengths and weaknesses of using tuberculin and defined-antigen cocktails to diagnose bTB.
The study, conducted at the Animal Health Institute near Addis Ababa, Ethiopia, compared traditional tuberculin tests with novel defined-antigen cocktails, namely ESAT-6, CFP-10, and Rv3615c. This research targeted the pressing issue of differentiable immune responses after BCG vaccination, which, until now, has complicated the detection of bTB due to false-positive test results.
With 134 cattle (67 vaccinated and 67 unvaccinated) studied over the course of one year, researchers employed skin tests every four months and interferon-gamma release assays (IGRAs) every two months. Strikingly, prior to exposure to the disease, both DIVA (Differentiation of infected from Vaccinated Animals) tests and tuberculin showed 100% specificity for unvaccinated control animals. Post-exposure assessments revealed comparative sensitivity of 46% for the DIVA skin test, 45% for the DIVA IGRA, and 47% for the comparative cervical tuberculin (CCT) test, highlighting the inadequacy of these tests to detect infections reliably.
The results have significant ramifications, with researchers stating, "test-and-slaughter control strategies using tests with low sensitivity are not expected to be effective in controlling bTB in high-prevalence herds, and highlight an urgent need to improve the sensitivity of diagnostic tests for bTB in these settings."
The need for reliable diagnostic methods is underscored by the increasing incidence of bTB, particularly within intensive animal farming systems. The prevalence rates of bTB are especially high, with 21.6% identified among Holstein-Friesian breeds. This situation compromises both public health and food security, particularly within resource-limited settings.
Control efforts currently rely heavily on tuberculin testing and the culling of animals deemed infected. This established strategy has seen varying successes worldwide, with Australia managing to eradicate bTB by implementing stringent testing and culling protocols. Yet, this approach remains impractical for countries like Ethiopia, where resources are limited, and cattle farming is integral to the local economy. The study's authors advocate for BCG vaccination as it both reduces disease severity and offers economic viability by preventing slaughter.
While BCG has shown efficacy, its deployment requires diagnostic tests capable of differentiative identification of infected animals among vaccinated cohorts—a challenge met by introducing DIVA tests. These tests aim to distinguish true vaccinations from genuine infections and, if validated, could complement BCG vaccination strategies, enhancing control efforts within resource-limited contexts.
Although the study finds DIVA tests and tuberculin tests to elicit similar responses, both still fall short of the desired sensitivity levels, particularly against the backdrop of rising bTB infections. Notably, both DIVA and tuberculin demonstrated sensitivity rates of barely 50%. This analysis highlights, "the sensitivity of the current bTB diagnostic tests is insufficient to confidently direct control schemes, especially under high prevalence conditions. Further refinement of these diagnostic methods is required to improve outcomes significantly."
With approximately 30% bTB prevalence noted across extensive herds within Ethiopia, the drive to improve diagnostic capabilities has never been more pressing. Integrative strategies pairing vaccination with sensitive diagnostic tools may emerge as the only feasible solution, steering farmers away from full herd culling practices.
Researchers hope to work on refining DIVA tests for broader and more reliable applicability across different geographical settings, which could potentially bolster efforts to mitigate the impact of bovine tuberculosis worldwide.