final report - ARCHIVE: Defra

Environment and epidemiology
Transmission of bTB from badgers to cattle
4.4.4 The Krebs Report concluded that ‘evidence strongly supports the view that badgers are a cause of herd
breakdowns’ but that, owing to the indirect nature of the relevant evidence, this point could not be
regarded as proven (Krebs et al., 1997, p. 121). Since then, the case for badger-to-cattle transmission
has been considerably strengthened, primarily by evidence provided by the ‘Four Areas Trial’ in Ireland
and by the RBCT in England that culling of badgers affects the incidence of bTB in cattle (Bourne,
2007; Jenkins et al., 2008a). However, the transmission mechanism remains poorly understood. It
could be argued that the implementation of blanket methods of control such as culling or vaccination
of badgers does not require that we first understand the transmission mechanism. However, knowing
how badgers make contact with cattle would assist in the development of more precisely targeted
methods of control.
4.4.5 Thinking to date has focussed on two issues, namely whether: (1) contact between badgers and cattle is
direct (i.e. involves direct transmission of an aerosol produced by a badger) or indirect (i.e. transmission
results from contamination of the environment with Mb by badgers); and (2) contact occurs while
cattle are at pasture or as a consequence of visits by badgers to farm buildings. These possibilities, in
combination, yield four potential, non-exclusive badger-to-cattle transmission scenarios. The relative
importance of these four scenarios is unknown.
4.4.6 There are circumstantial reasons for supposing that direct transmission via aerosol contact may be
more important than indirect transmission via contaminated feed or soil. In particular, infection in
both cattle and badgers is most commonly associated with the respiratory tract; substantially smaller
numbers of bacilli are required to infect cattle by this route; and Mb survives sufficiently well whilst
airborne for aerosol transmission to be effective (for references see Wilsmore & Taylor, 2008, p. 29).
4.4.7 Observational studies suggest that direct contact between badgers and cattle, while the latter are grazing
on pasture, is rare (for references see Wilsmore & Taylor, 2005). However, one case has been reported
of badgers and cattle simultaneously feeding from a trough containing concentrated feed (Garnett et
al., 2002). Somewhat more convincing evidence of opportunities for direct transmission has been
obtained from observations of badgers in or around farm buildings. Some farms have been shown
to be highly attractive to badgers, which visit in order to consume stored feed, primarily in the form
of grain or concentrated cattle feed (Garnett et al., 2002, 2003; Tolhurst, 2006). In doing so, badgers
sometimes come into sufficiently close proximity to housed cattle for respiratory contact to be possible.
However, the evidence of such contact remains essentially anecdotal since only a small number of
direct contacts between badgers and cattle have so far been observed.
4.4.8 Rather more attention has been given to the possibility of indirect transmission and, in particular, to the
idea that cattle, while at pasture, become infected while grazing on grass that has been contaminated
by tuberculous badgers. While early work suggested that cattle tend to avoid grass contaminated with
badger excreta (Benham & Broom, 1991), a subsequent study shows that they do graze active badger
latrines when the sward length in the rest of the field has been reduced (Hutchings & Harris, 1997).
Cattle may also graze grass that has been contaminated by single deposits of urine or faeces, as happens
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