Bovine tuberculosis breakdown duration in cattle herds: an investigation of herd, host, pathogen and wildlife risk factors

Abstract

Background Despite rigorous controls placed on herds which disclose ante-mortem test positive cattle to bovine tuberculosis, caused by the infection of Mycobacterium bovis, many herds in Northern Ireland (NI) experience prolonged breakdowns. These herds represent a considerable administrative and financial burden to the State and farming community. Methods A retrospective observational study was conducted to better understand the factors associated with breakdown duration, which was modelled using both negative binomial and ordinal regression approaches. Results Six explanatory variables were important predictors of breakdown length in both models; herd size, the number of reactors testing positive in the initial SICCT test, the presence of a lesioned animal at routine slaughter (LRS), the count of M. bovis genotypes during the breakdown (MLVA richness), the local herd-level bTB prevalence, and the presence of herds linked via management factors (associated herds). We report that between 2008 and 2014, mean breakdown duration in NI was 226 days (approx. seven months; median: 188 days). In the same period, however, more than 6% of herds in the region remained under movement restriction for more than 420 days (13 months); almost twice as long as the mean. The MLVA richness variable was a particularly important predictor of breakdown duration. We contend that this variable primarily represents a proxy for beef fattening herds, which can operate by purchasing cattle and selling animals straight to slaughter, despite prolonged trading restrictions. For other herd types, the model supports the hypothesis that prolonged breakdowns are a function of both residual infection within the herd, and infection from the environment (e.g. infected wildlife, contiguous herds and/or a contaminated environment). The impact of badger density on breakdown duration was assessed by including data on main sett (burrow) density. Whilst a positive association was observed in the univariate analysis, confounding with other variables means that the contribution of badgers to prolonged breakdowns was not clear from our study. We do not fully reject the hypothesis that badgers are implicated in prolonging bTB breakdowns via spillback infection, but given our results, we posit that increased disease risk from badgers is unlikely to simply be a function of increasing badger density measured using sett metrics.

Evaluation of Risk Factors Associated With Herds With an Increased Duration of Bovine Tuberculosis Breakdowns in Castilla y Leon, Spain (2010–2017)

“…Both survival and case-control studies showed that a countylevel herd prevalence above 0.2% in the year prior to the start of the bTB breakdown was significantly associated with an increased risk of experiencing chronic bTB breakdowns. This result is in agreement with previous studies that identified proximity to infected neighbors as a risk factor for persistent bTB infection in Spain (18) and Ireland (14) or prolonged outbreak duration in Northern Ireland (10,36). Local sources of infection that could contribute to this effect include local movements (51), contact with infected cattle and wildlife reservoirs (52), or environmental sources of M. bovis (53), thus contributing to disease recurrence or local persistence (6,12,54).…”

“…Herds with ≤2 bTBpositive herd tests were not considered to select the threshold in order to focus on problematic herds. The median bTB breakdown for all bTB-positive herds in Castilla y Leon duration was 133 days, which is in agreement with values reported in Northern Ireland (10,36).…”

“…Among the factors that may substantially extend the time to recover OTF status and thus hamper eradication programs are the chronic nature of the disease, the presence of wildlife reservoirs, and the lack of performance of diagnostic tools. Several studies have conducted risk factor analyses for chronic bTB herd breakdowns in Europe (10,(35)(36)(37)(38), but factors associated with disease persistence in infected herds had not been characterized at this level yet in Spain. Survival and case-control analyses performed in this study revealed the impact of both farm and breakdown characteristics on the probability of disease persistence in infected farms.…”

Tuberculosis in badgers where the bovine tuberculosis epidemic is expanding in cattle in England

“…not only a source of infection to cattle—the ‘target’ host—but able to maintain infection without reintroduction from cattle) 2 , 9 , and even less is known about the role of badgers in cattle infection in the Edge Area. Thus the expansion of the cattle epidemic in the Edge Area could be the result of cattle-to-cattle, badger-to-badger, cross species transmission, or, indeed, other even less well-understood transmission routes such as through soil and farm waste 10 – 13 ; and the relative contributions of each potential transmission route, or if they differ between regions in the Edge Area, are unknown.…”

Future Risk of Bovine Tuberculosis (Mycobacterium bovis) Breakdown in Cattle Herds 2013–2018: A Dominance Analysis Approach

“…Recrudescence of infection is a known feature of bTB epidemiology in the UK and Ireland [ 14 , 15 , 16 , 17 , 18 , 19 ], and elsewhere [ 20 , 21 , 22 , 23 ]. Herds that experience a breakdown (that is, when one or more animals present with evidence of bTB via antemortem tests or postmortem surveillance) tend to be at a greater risk of future breakdown.…”