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DST-F sensitivity study in naturaly infected cattle - SE3333

The development of a deployable bovine TB (bTB) vaccine for cattle forms part of Government’s comprehensive eradication strategy for bovine TB, and is a key pillar of HMG’s response to the Godfray Review (England).
To enable the CattleBCG (Bacille Calmette-Guerin) vaccine to be used alongside the current test and slaughter control policy, a compatible DIVA test (Detection of Infected amongst Vaccinated Animals) is required. During previous work, a DIVA skin test (DST-F), based on recombinant proteins not recognised by the immune response of animals vaccinated with BCG has been developed.
A Government commissioned ad hoc working group (the DIVA Skin Test Working Group) recommended a phased approach to validate the DST-F, whereby test performance would be first validated in BCG naïve animals. This project will assess how well this DST-F skin test detects cattle naturally infected with M. bovis (the bacteria that causes bovine TB).
This data is critical to understand future test perfomance to detect cattle where BCG vaccination may not have prevented natural infection (i.e. less than 100% protection). Furthermore, the data generated in the project will provide sufficient quality data to support DST-F test validation excercises and provide confidence in a BCG vaccination / DIVA test strategy to industry stakeholders and international partners to enable continued trade.
Project overview:
To date, the sensitivity of the DST-F (i.e. the proportion of truly infected animals that test positive to DST-F) is estimated at 93 %.
However, the majority of data underpinning this derives from experimentally infected cattle (infected with artificially high doses of bTB for logistical reasons), which may not accurately reflect test performance in natural settings. Indeed, limited data from naturally infected cattle, indicate a lower estimate (76 %) of test sensitivity to detect infection. This small data set, however, provided low precision with a very large range (55 % - 89 %)in the 95 % confidence intervals. This highlights a critical requirement for a definative evaluation of DST-F test performance in increased numbers of naturally infected cattle, which will provide statistically robust precision calculations of the DST-F test sensitivity in a realistic scenario.
Based on guidelines from the OIE Manual for Test Validation, we estimate a total number of 323 naturally infected reactor cattle will be required to provide adequate precision (i.e. 95 % confidence intervals within a precision of ± 5 %).
Naturally infected ‘bTB reactor’ cattle that have failed a recent tuberculin skin test (using standard OIE and UK test interpretation) will be sourced from OTF-W herds in regions of GB with endemic bTB. These animals will be housed at APHA Weybridge for a period of at least 60 days following the disclosing skin test (to conform with APHA Ops Manual procedures). Following this, they will undergo skin testing with the DST-F test. Concurrently, they will also receive the standard tuberculin skin test to generate comparative data. At the completion of the study, the cattle will undergo comprehensive post mortem inspection, with full details of bTB lesions recorded to generate pathology scores and tissue samples submitted to standard TB culture procedure to confirm infection status.
Objective 1: To generate DST-F sensitivity data in sufficient numbers of naturally infected cattle so that the 95% confidence intervals are within a precision of ± 5%.

Objective 2: Assessment of alternative test reagents, diagnostic test platforms and additional biomarkers of infection.

This objective is dependent on recruiting TB reactor animals as described in objective 1.
Time-Scale and Cost
From: 2021

To: 2023

Cost: £1,534,940
Contractor / Funded Organisations
APHA (Animal and Plant Health Agency)