Parasites and their Virulence ABSTRACT 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 hostspecies tend to be more virulent than parasites which have coevolved withtheir hosts. Parasites which are transmitted horizontally tend to be morevirulent than those transmitted vertically. It has been assumed thatparasite-host interactions inevitably evolve toward lower virulence. This iscontradicted by studies in which virulence is conserved or increases overtime. A model which encompasses the variability of parasite-host interactionsby synthesizing spatial (transmission) and temporal (evolutionary) factors isexamined. Lenski and May (1994) and Antia et al. (1993) predict themodulation of virulence in parasite-host systems by integrating evolutionaryand transmissibility factors. INTRODUCTION Why do certain parasitesexhibit high levels of virulence within their host populations while othersexhibit low virulence? The two prime factors most frequently cited (Esch andFernandez 1993, Toft et al. 1991) are evolutionary history and mode oftransmission. Incongruently evolved parasite-host associations arecharacterized by high virulence, while congruent evolution may result inreduced virulence (Toft et al. 1991). Parasites transmitted vertically (fromparent to offspring) tend to be less virulent than parasites transmittedhorizontally (between unrelated individuals of the same or different species).Studies in which virulence is shown to increase during parasite-hostinteraction, as in Ebert's (1994) experiment with Daphnia magna, necessitatea synthesis of traditionally discrete factors to predict a coevolutionaryoutcome. Authors prone to habitually use the word decrease before the wordvirulence are encouraged to replace ...
