Scientists have devised a method for predicting how rising global
temperatures are likely to affect the severity of diseases mediated by
parasites. Their method can be applied widely to different host-pathogen
combinations and warming scenarios, and should help to identify which
infectious diseases will have worsened or diminished effects with rising
temperatures.
The proof-of-concept method, which was road-tested using the water
flea (Daphnia magna) and its pathogen (Ordospora colligata) as a model
system, uses a long-standing biological concept known as the metabolic
theory of ecology to predict how a wide range of processes - all of
which influence host-parasite dynamics - are affected by temperature.
The scientists, led by William C. Campbell Lecturer in Parasite
Biology at Trinity College Dublin, Professor Pepijn Luijckx, and
graduate student Devin Kirk from the University of Toronto, have just
published their results in leading international journal PLOS Biology (see: http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.2004608" rel="nofollow - http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.2004608 ).
Professor Luijckx said: "Rising temperatures due to global warming
can alter the proliferation and severity of infectious diseases, and
this has broad implications for conservation and food security. It is
therefore really important that we understand and identify the diseases
that will become more harmful with rising temperatures, with a view to
mitigating their impacts."
Unfortunately this has always been very difficult -- because
temperature affects many processes in the host and the pathogen in
different ways, it is hard to predict the cumulative effect that a rise
(or drop) in temperature will have. For example, while host immune
function and pathogen infectivity may be higher as temperatures rise,
pathogen longevity may be lower. Additionally, to predict the severity
of disease, we need data that doesn't always exist on the temperature
sensitivity of all the processes involved, especially for newly emergent
diseases.
The solution -- the metabolic theory of ecology
The metabolic theory of ecology can be used to predict how various
biological processes respond to temperature. It is based on the idea
that each process is controlled by enzymes, and that the activity and
temperature dependence of these enzymes can be described using simple
equations. Even with limited data, the theory thus allows for the
prediction of the temperature dependence of host and pathogen processes.
Professor Luijckx said: "By using the metabolic theory of ecology we
can estimate the thermal dependence of each individual process, step by
step, and calculate a final prediction of disease severity at
different, changing temperatures. Until now, no study has shown if this
works for simple - unicellular - pathogens growing within their host,
but we have been able to show that the method works very well in the
model system we used."
In their study, the scientists used the water flea and its pathogen
and measured how processes such as host mortality, aging, parasite
growth and damage done to the host changed over a wide temperature
range. They used these measurements to determine the thermal
dependencies of each of these processes using metabolic theory.
The results showed that the different processes had unique
relationships with temperature. For example, while damage inflicted to
the host per pathogen appeared to be independent of temperature, both
host mortality and pathogen growth rate were strongly dependent -- but
in opposite ways.
Professor Luijckx added: "What is exciting is that these results
demonstrate that linking and integrating metabolic theory within a
mathematical model of host-pathogen interactions is effective in
describing how and why disease interactions change with global warming."
"Due to its simplicity and generality, the method we have developed
could be widely applied to understand the likely impact of global
warming on a variety of diseases, including diseases affecting
aquaculture, such as salmonid diseases like Pancreas disease, pathogens
of bee pollinators, such as Nosema, and growth of vector-borne and
tick-borne diseases in their invertebrate hosts, such as malaria and
Lyme disease."
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