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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 which virulence is shown to increase during parasite-host interaction as in Eberts 1994 experiment with Daphnia magna necessitate a synthesis of traditionally discrete factors to predict a coevolutionary outcome Authors prone to habitually use the word decrease before the word virulence are encouraged to replace the former with modulate which emphasizes the need for an inclusive predictive paradigm
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