Summary:

Because host populations are generally sub-divided, the epidemiology of a parasite often depends on its ability to disperse among host sub-populations and successfully invade them. To investigate the factors that determine host-mediated parasite dispersal and its subsequent transmission we carried our microcosm experiments with the bacterial parasite Holospora undulata and its host, the fresh-water ciliate Paramecium caudatum. We found that (i) the dispersal of infected hosts largely depended on both host and parasite genotypes (but not their interaction), suggesting disease spatial dynamics varies with the genetic composition of local sub-populations. Besides, (ii) shortly after the arrival of infected hosts into a healthy population parasite success was determined by the genotype of the carrier hosts and its interactions with that of the parasite and the resident hosts. This further highlights the importance of host genotype for parasite dynamics among host sub-populations. (iii) After a couple of transmission cycles, parasite genotype, in combination with resident host genotype, gained influence, matching classical ideas of parasite transmission. It thus appears that different genetic factors play on parasite dispersal between groups of hosts, its short-term spread and longer-term establishment. This study also demonstrates the role of carrier host genotype for parasite dispersal and transmission among and within host sub-populations.