Evolutionary ecology of transmission strategies in protozoan parasites
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Date
24/11/2011Author
Pollitt, Laura C.
Metadata
Abstract
In recent years there has been growing interest in applying frameworks from
evolutionary ecology to understand infectious disease. It is becoming increasingly
apparent that the interactions between parasites within the host environment can
shape parasite phenotypes underlying infection dynamics and transmission.
However, the spread of the disease will crucially depend on both within-host and
between-host dynamics. Bridging these scales is challenging and for vector borne
parasites, such as malaria and trypanosomes, will involve gaining a much better
understanding of infection dynamics both within the host and vector. I apply
evolutionary ecology frameworks including social evolution, life history theory,
and phenotypic plasticity to investigate how parasite phenotypes are shaped by
within-host and within-vector environments and examine the implications for inhost
survival and between-host transmission. Specifically, I demonstrate that;
1. Within the host;
i. In accordance with theory malaria parasites detect and respond to the
presence of competitors by altering reproductive strategies to
maximise in-host survival. Furthermore, these strategies are fine
tuned in response to variation in the within-host environment,
including the availability of resources.
ii. The reproductive investment strategies of malaria parasites can be
applied to explain the transmission strategies of African
trypanosomes. This shows how general evolutionary frameworks can
be applied to a novel parasite species and demonstrates the
explanatory power of an evolutionary approach.
iii. The complexity of the within-host environment poses specific
statistical challenges for examining the temporal dynamics of parasite
life history traits that are often not adequately dealt with, potentially
leading to type 1 errors. Methods to evaluate levels of autocorrelation
and how to deal with it are applied to datasets of within-infection
dynamics.
2. Within the vector;
i. Malaria parasites undergo programmed, apoptotic cell death. The
occurrence of, and putative explanation for, apoptosis in protozoan
parasites is controversial. I demonstrate the importance of
quantitative methods and parasite ecology in testing the evolutionary
explanations for parasite apoptosis.
ii. The links between within-host dynamics and within-vector dynamics
are complex and can lead to counter-intuitive implications for the
success of between-host transmission. Density-dependent processes
result in diverse fitness costs to parasites of crowding. More broadly,
these processes could explain why parasites undergo apoptosis.
In general my results demonstrate, across vertebrate hosts and insect vectors, how
the interactions between parasites and with their environment shapes traits
important for the transmission of infectious disease.
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