Epidemiology and Infection

Zoonotic infections

Farm-scale risk factors for bovine tuberculosis incidence in cattle herds during the Randomized Badger Culling Trial

A. C. MILLa1a2 c1, S. P. RUSHTONa1, M. D. F. SHIRLEYa1, A. W. A. MURRAYa2, G. C. SMITHa2, R. J. DELAHAYa2 and R. A. McDONALDa2

a1 School of Biology, Newcastle University, Newcastle Upon Tyne, UK

a2 Food and Environment Research Agency, Sand Hutton, York, UK

SUMMARY

We analysed the incidence of cattle herd breakdowns due to bovine tuberculosis (Mycobacterium bovis) in relation to experimental badger culling, badger populations and farm characteristics during the Randomized Badger Culling Trial (RBCT). Mixed modelling and event history analysis were used to examine the individual risk factors. The interdependencies of covariates were examined using structural equation modelling. There were consistent findings among the different analyses demonstrating that during a badger culling programme farms experiencing: reactive culling, larger herd sizes, larger holdings and holdings with multiple parcels of land were all at greater risk of a herd breakdown. Proactive culling reduced risks within the culling area, but we did not assess any potential effects in the periphery of the treatment area. Badger-related variables measured prior to the start of culling (number of social groups and length of badger territorial boundaries) did not consistently point to an increase in risk, when set against a background of ongoing badger culling. This could be because (1) the collected variables were not important to risk in cattle, or (2) there were insufficient data to demonstrate their importance. Our findings highlight the difficulty in identifying simple predictors of spatial variation in transmission risks from badger populations and the consequent challenge of tailoring management actions to any such field data.

(Accepted February 25 2011)

(Online publication March 25 2011)

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