Competitive co-existence of vertically and horizontally transmitted parasites

Questions: Can vertically transmitted parasites (VTPs) serve as indirect mutualists by excluding more virulent, horizontally transmitted parasites (HTPs)? How does the efficiency of vertical transmission affect: (1) the equilibrium number of hosts infected by each type of parasite; (2) the total number of hosts in the population; and (3) virulence of the horizontally transmitted parasite? How does reproductive output by the horizontally transmitted parasite affect the frequency of infection by both parasite types? Methods and assumptions: We used a deterministic computer simulation to determine hostpopulation dynamics following introduction of a vertically transmitted parasite. We assumed that the vertically transmitted parasite precludes infection by the horizontally transmitted parasite. We allowed for the possibility that virulence of the horizontally transmitted parasite depends on host density. Conclusions: Vertically transmitted parasites can serve as indirect mutualists by excluding more virulent horizontally transmitted parasites. As such, selection would be expected to favour VTP strains that most efficiently exclude horizontally transmitted parasites. In addition, the total number of hosts at equilibrium increases (and HTP frequency decreases) with the efficiency of vertical transmission. Increasing the reproductive output of the horizontally transmitted parasite has little effect on the frequency of that parasite at equilibrium, but greatly increases the frequency of hosts infected with the vertically transmitted parasite. When virulence of the horizontally transmitted parasite is density dependent, the vertically transmitted parasite indirectly increases virulence by increasing host density at equilibrium.