COS 25-6
When Judas meets Typhoid Mary: modeling how a “need to be social” affects transmission dynamics of virulent pathogens
A challenge in disease ecology is to predict disease dynamics on the basis of behavioral interactions among hosts. Home range characteristics and contact patterns within and between groups can be measured for populations not currently undergoing an epizootic and used to construct models of potential disease transmission. Such models’ predictions are likely to fail, however, if disease affects behavioral interactions. Parasite manipulation of host behavior is receiving increasing attention, but another mechanism is through disease impacts on group membership. If a virulent pathogen spreads rapidly within a group, animals remaining after other group members die may be inclined to join another group (as Judas goats do), potentially bringing disease with it. An individual-based simulation model was developed to examine the effect of a strong inclination to be a member of a group on disease transmission dynamics, as a function of typical group size (4, 12, or 36), group turnover rate (0.2 to 0.5 per time step), and the time scale of disease (mean duration of infectivity 0.1 to 1 step). Groups had circular bivariate normal utilization distributions situated randomly in space, and contact rates were 10 or 100 fold greater within than between groups (with perfect home range overlap).
Results/Conclusions
With no inclination to join other groups, transmission rate showed a nonlinear (decelerating) relationship with host density due to interaction neighborhood constrained by home ranges. In the most favorable scenario (100:1 within:between contact ratio, group turnover 0.2/step, and infectivity 0.1 time steps), forcing any remnant animals to join another group resulted in nearly complete frequency dependence and frequent quasiextinction (5% of original density) of the host. With higher group turnover rates, and longer infectious periods (lower virulence and transmissibility), the circumstance of only one infected animal remaining in a group occurred rarely, so transmission reverted toward the base scenario. Lesser compartmentalization of contacts also substantially reduced the effect of social need. The effect of group size was not straightforward: smaller groups led to more frequent between-group contact and more frequent infected remnants. In larger groups, however, stochastic extinction of the pathogen within the group occurred less frequently, driving groups to low numbers. Our findings suggest that under favorable conditions, a “need to be social” can produce frequency dependent transmission, but the restrictive combination of conditions necessary to be truly favorable are likely to occur in few wild populations.