Parasites and their Virulence
Why do some parasites kill the host they depend upon while
others coexist with their host? Two prime factors determine parasitic
virulence: the manner in which the parasite is transmitted, and the
evolutionary history of the parasite and its host. Parasites which
have colonized a new host species tend to be more virulent than
parasites which have coevolved with their hosts. Parasites which are
transmitted horizontally tend to be more virulent than those
transmitted vertically. It has been assumed that parasite-host
interactions inevitably evolve toward lower virulence. This is
contradicted by studies in which virulence is conserved or increases
over time. A model which encompasses the variability of parasite-host
interactions by synthesizing spatial (transmission) and temporal
(evolutionary) factors is examined. Lenski and May (1994) and Antia et
al. (1993) predict the modulation of virulence in parasite-host
systems by integrating evolutionary and transmissibility factors.
INTRODUCTION
Why do certain parasites exhibit high levels of virulence within
their host populations while others exhibit low virulence? The two
prime factors most frequently cited (Esch and Fernandez 1993, Toft et
al. 1991) are evolutionary history and mode of transmission.
Incongruently evolved parasite-host associations are characterized by
high virulence, while congruent evolution may result in reduced
virulence (Toft et al. 1991). Parasites transmitted vertically (from
parent to offspring) tend to be less virulent than parasites
transmitted horizontally (between unrelated individuals of the same or
different species). Studies in whi...
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...gy determine the direction of the modulation.
The development of an inclusive, accurate model has significance
beyond theoretical biology, given the threat to human populations
posed by pathogens such as HIV (Gibbons 1994). Mass extinctions such
as the Cretaceous event may have resulted from parasite-host
interaction (Bakker 1986), and sexual reproduction (i.e. recombination
of genes during meiosis) may have evolved to increase resistance to
parasites (Holmes 1993). Parasitism constitutes an immense, if not
universal, influence on the evolution of life, with far-reaching
paleological and phylogenetic implications. A model which synthesizes
the key factors determining parasitic virulence and can predict the
entire range of evolutionary outcomes is crucial to our understanding
of the history and future of species interaction.
However, evidence such as fossils is more than enough to lend support and disprove any other theory to the development of species. Examples used by Root-Bernstein and McEachron also provide strong support to evolution. The authors detail how natural selection works in both insects and germs to create organisms better adapted to methods of control. Disease used to run rampant, until the creation of vaccines which led to many diseases becoming extinct. Root-Bernstein and McEachron note that the diseases that have survived to this day have been constantly mutating and evolving to become more resistant to any attempts at suppression. The same can be said for insects that have been consistently exposed to pesticides. The ones that survived the pesticides were able to reproduce and pass along a resistance to their offspring which in turn makes the species more resilient and better suited to their
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