What should a good epidemiological question look like?
M. Shaw
School of Biological Sciences - Plant Science Laboratory - L4 TOB2 -
Earley Gate - Whiteknights - Readings - Berks RG6 6AS – UNITED KINGDOM
The question defining epidemiology is perhaps
'Why has my crop become diseased? To
answer this question requires good etiology and well-defined questions about
pathogen life-cycle, host, environment and management, susceptible to answer by
experiment. Assumptions about etiology
and life-cycle based on circumstantial evidence are often accepted and appear
often to be wrong: examples from my own
work include Mycosphaerella graminicola - assumed to be trash-borne, but
with air-borne ascospores critical - and Botrytis cinerea - assumed to
be airborne and localised, but in some crops seed-borne and systemic. With
correct etiology and life cycle it is frequently assumed that all we then need
to know for management is some simple function of the environment, usually as
measured by commercial instruments. In practice this is extremely hard to
investigate, because there are logical and/or statistical gaps between the
information available and the inferences we seek; it will only be true if
environment (and not host, other organisms, or intrinsic biology) is what
limits pathogen populations. Statistically, there are major problems due to the
correlation of environment in different locations, and the corresponding lack
of degrees of freedom in what otherwise seem large experiments. Logically there
are problems to do with what we measure and how we reduce an infinity of
dimensions in environmental records to manageable dimensions. My examples will
include the same pathogens as before: in which long-term changes in M.
graminicola may be mediated by acid rain; while conditions leading to
disease in plants infected by B. cinerea are as much to do with the host
physiology as the environment, but the cumulated environment determines the
host physiology. My conclusion is that scientifically we should recognise that
in some areas our knowledge is distressingly limited; and in giving advice and
in politics we should plan for the unexpected, and constantly look for the
unexpected assumption we did not know we had - but which has now proved
unfounded.