Download the role of environmental transmission

2017_54: Tuberculosis in cattle and badgers – the
role of environmental transmission
Supervisors: Professor Christl Donnelly ([email protected]) and
Professor Rosie Woodroffe (Zoological Society of London)
Department: School of Public Health
Models of infectious diseases – especially those involving wildlife – typically assume
that pathogens are transmitted through direct contact between hosts. However,
transmission through the environment can lead to profoundly different dynamics, with
implications for disease management. This studentship will use statistical and
dynamical modelling to explore how different assumptions about transmission
mechanisms affect the dynamics and control of bovine tuberculosis (TB) in Britain.
Currently, TB management practices tacitly assume that its causative agent
(Mycobacterium bovis) is directly transmitted. For example, cattle which test positive
for the disease are physically separated from test-negative cattle, but pasture they
have grazed is not managed as potentially contaminated. However, new evidence
suggests that M. bovis can be transmitted through the environment. Cattle can
evidently acquire from wild badgers (Donnelly et al. 2006), but new data show that the
two species very seldom meet (Woodroffe et al. 2016). Transmission through the
shared environment, which has been demonstrated experimentally between deer and
cattle (Palmer, Waters & Whipple 2004), is the most likely explanation for these
findings.
While the frequency of direct transmission largely reflects biological characteristics of
the host species, environmental transmission can be strongly affected by aspects of
the abiotic environment which influence pathogen survival. For example, the dynamics
of anthrax (Bacillus anthracis) are driven mainly by soil pH and rainfall patterns
(Hampson et al. 2011). Similar factors may influence TB dynamics, if environmental
transmission plays an important role. For example, incidence-based statistics on cattle
TB
show
substantial
unexplained
periodicity
(Figure
1
in
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/55262
3/bovinetb-statsnotice-quarterly-14sep16.pdf), which may reflect interannual variation
in weather conditions.
For more information on how to apply visit us at www.imperial.ac.uk/changingplanet
Science and Solutions for a Changing Planet
Drawing on extensive published and unpublished datasets, including an ongoing field
project monitoring M. bovis in cattle, badgers, and the environment, the project will
address four questions:
(1) Is variation in climatic conditions associated with the geographic spread of cattle
TB, its increasing incidence, or periodicity?
(2) Is variation in climatic conditions associated with spatio-temporal variation in the
prevalence of M. bovis infection in badgers?
(3) How does assuming that either cattle or badgers can acquire infection from
bacteria that have persisted in the environment (regardless of whether those bacteria
originated from cattle or badger sources) influence the projected dynamics of M. bovis
infection in the two species?
(4) How does an assumption of environmental transmission, either within or between
species, influence the projected outcome of management such as cattle test-andslaughter, badger culling, and farm husbandry?
References
Donnelly, C.A., Woodroffe, R., et al. (2006) Positive and negative effects of
widespread badger culling on cattle tuberculosis. Nature, 439, 843-846.
Hampson, K., Lembo, T., et al. (2011) Predictability of anthrax infection in the
Serengeti, Tanzania. Journal of Applied Ecology, 48, 1333-1344.
Palmer, M.V., Waters, W.R. & Whipple, D.L. (2004) Investigation of the transmission
of Mycobacterium bovis from deer to cattle through indirect contact. American Journal
of Veterinary Research, 65, 1483-1489.
Woodroffe, R., Donnelly, C.A., et al. (2016) Badgers prefer cattle pasture but avoid
cattle: implications for bovine tuberculosis control. Ecology Letters, 19, 1201-1208.
For more information on how to apply visit us at www.imperial.ac.uk/changingplanet