Contact heterogeneity in deer mice: implications for Sin Nombre virus transmission

Abstract

Heterogeneities within disease hosts suggest that not all individuals have the same probability of transmitting disease or becoming infected. This heterogeneity is thought to be due to dissimilarity in susceptibility and exposure among hosts. As such, it has been proposed that many host-pathogen systems follow the general pattern whereby a small fraction of the population accounts for a large fraction of the pathogen transmission. This disparity in transmission dynamics is often referred to as '20/80 Rule', i.e. approximately 20 per cent of the hosts are responsible for 80 per cent of pathogen transmission. We investigated the role of heterogeneity in contact rates among potential hosts of a directly transmitted pathogen by examining Sin Nombre virus (SNV) in deer mice (Peromyscus maniculatus). Using foraging arenas and powder marking, we documented contacts between wild deer mice in Great Basin Desert, central Utah. Our findings demonstrated heterogeneity among deer mice, both in frequency and in duration of contacts with other deer mice. Contact dynamics appear to follow the general pattern that a minority of the population accounts for a majority of the contacts. We found that 20 per cent of individuals in the population were responsible for roughly 80 per cent of the contacts observed. Larger-bodied individuals appear to be the functional group with the greatest SNV transmission potential. Contrary to our predictions, transmission potential was not influenced by breeding condition or sex.

Figure 1

Host frequency of the number of unique contacts per individual deer mouse, as estimated by powder marking. (During the mark–recapture study, five randomly selected male deer mice were powder marked at all 12 sites for two nights of each sampling period, with a total of 99 male mice being observed in spring and autumn, 2005. Of those marked, 25 mice had at least one contact with another deer mouse during the two nights observed, with a maximum of four unique contacts observed for an individual deer mouse. The frequency distribution of unique contacts per individual deer mouse, as estimated by powder marking, was non-normal and highly aggregated, k=0.54.)

Figure 2

Host frequency of the number of unique contacts per individual, as estimated by PIT tag and foraging arenas. (Deer mice (n=300) were marked with PIT tags at five sites during spring and autumn, 2005 (three nights per period). Contacts between deer mice were approximated using foraging arenas equipped with PIT antennae and data loggers. The frequency distribution of unique contacts per individual deer mouse, as estimated by PIT tag, was non-normal and highly aggregated, k=0.41.)

Figure 3

Host frequency of the average duration of contacts per individual (by 5 s intervals). (Deer mice (n=300) were marked with PIT tags at five sites during spring and autumn, 2005 sampling periods (three nights per period). Duration of each contact between deer mice was recorded using foraging arenas equipped with PIT antennae and data loggers. For each individual, the average duration of contact was estimated as the mean of all contacts observed in which that mouse was involved. The frequency distribution of average duration of contacts per individual deer mouse was non-normal and highly aggregated, k=0.29.)

Figure 4

Host frequency of contacts-by-duration per deer mouse by SNV infection (by 15 unit intervals). (Contacts-by-duration was estimated by multiplying the number of unique contacts per individual by the average duration of contacts per individual; this index served as an aggregate measure of the number interactions deer mice had with other individuals as well as the length of time spent in contact with others. The frequency distribution of contacts-by-duration per individual deer mouse was non-normal and highly aggregated, k=0.25. As such, the top 20 per cent of individuals have 80 per cent of all contacts, all of which were longer than 10 s in duration. Black, SNV seropositive; White, SNV seronegative.)