Detection of a local Mycobacterium bovis reservoir using cattle surveillance data

Abstract

The incidence of bovine tuberculosis (TB, caused by Mycobacterium bovis) in cattle has been associated with TB in badgers (Meles meles) in parts of England. The aim was to identify badger-associated M. bovis reservoirs in the Edge Area, between the High- and Low-Risk Areas for cattle TB. Data from badger TB surveys were sparse. Therefore, a definition for a local M. bovis reservoir potentially shared by cattle and badgers was developed using cattle TB surveillance data. The performance of the definition was estimated through Latent Class Analysis using badger TB survey data. Spatial units (25 km² ) in the Edge Area were classified as having a reservoir if they had (i) at least one TB incident in at least three of the previous 7 years, (ii) at least one TB incident in a cattle herd confirmed by post-mortem tests as due to M. bovis infection and not attributable to cattle movements in the previous 2 years and (iii) more confirmed TB incidents than un-confirmed in the previous 2 years. Approximately 20% of the Edge Area was classified as having a local M. bovis reservoir using the cattle-based definition. Assuming 15% TB prevalence in Edge Area badgers, sensitivity for the local M. bovis reservoir definition varied from 25.7% [95% credible interval (CrI): 10.7%-85.1%] to 64.8% (95% CrI: 48.1%-88.0%). Specificity was 91.9% (CrI: 83.6%-97.4%). Over 90% of the local reservoir was in stable endemic TB areas identified through previous work and its spatial distribution was largely consistent with local veterinary knowledge. Uncertainty in the reservoir spatial distribution was explored through its recalculation in spatial units shifted in different directions. We recommend that the definition is re-evaluated as further data on badger infection with M. bovis become available.

Keywords: Mustelidae; Mycobacterium bovis; cattle; disease reservoirs; latent class analysis.

FIGURE 1

Badger density in the Edge Area and locations of samples from badgers with TB confirmed by post‐mortem tests or no confirmed TB detected in different surveys. Badger samples from the border area in England are also shown. The border area extends up to 14 km beyond the Edge Area boundary. Its precise width is dependent upon where hexagonal spatial units crossing the Edge Area boundary are bisected.

FIGURE 2

Herd density in 25‐km² hexagonal spatial units in the Edge Area and TB (OTF‐W and OTF‐S) incidents in cattle in 2018 and 2019

FIGURE 3

The estimated sensitivity for the detection of a local M. bovis reservoir in 25‐km² hexagonal spatial units by number of samples for three levels of badger infection prevalence, assuming random mixing and random sampling

FIGURE 4

Venn diagram of 25‐km² hexagonal spatial units within the Edge Area that fulfilled at least one criterion for a local M. bovis reservoir. TB = OTF‐W and/or OTF‐S incidents. Counts include hexagonal spatial units entirely within the Edge Area or touching the border of the Edge Area and in England. The spatial distribution of the 230 hexagons that fulfilled the three criteria for a reservoir within the Edge Area are shown in Figure 5.

FIGURE 5

Original map of local reservoir of Mycobacterium bovis with buffer across 25‐km² hexagonal spatial units in the Edge Area

FIGURE 6

Final map showing a local reservoir of Mycobacterium bovis infection in the Edge Area using the cattle‐based definition with buffer. The depth of colour indicates the number of times (up to a maximum of five) a grid point was within 25‐km² circular spatial units defined as having a local reservoir using the cattle‐based definition. Spatial units have centroids 2 km apart