Intrinsic differences between horizontal and vertical transmission.
Vertical and horizontal transmission routes differ in many aspects. Theoretical models have focused on the link between the fitness of parasite and that of its host: as vertically transmitted parasites need their hosts to reproduce, they would harm themselves by increasing the host's fitness costs due to the infection (known by some people as virulence). Therefore, it is expected that, on an evolutionary scale, vertical transmission will select for low virulence. The difference between both transmission routes do not necessarily stop there.
Vertical transmission may produce infections where parasite relatedness is high, relative to that of the parasite population (Frank 1996). In a vertical infection the parasites come from a relatively limited genetic pool, that of the parent or parents of the new host. Horizontally infected parasites, on the other hand, come from a wider genetic pool, that of the host population. Differences in relatedness have further consequences. Inbreeding (and its depression) may arise as direct consequence of the increase in relatedness in vertically transmitted parasites with sexual reproduction within the host. In such parasites, the difference in relatedness between transmission routes may also lead to differences in their optimal sex allocation, with horizontal transmission increasing sperm competition and vertical transmission increasing local mate competition (viewed as two extremes of a continuum).
Hosts that acquired an infection from their mother derive, necessarily, from an infected mother. This tautology implies that the maternal effects caused by the infection per se may be stronger in vertically infected hosts than in horizontally infected hosts. Maternal effects include the effect of physiological condition (which may be poorer in infected mothers) and immunity or tolerance (which might be stronger, e.g. Little, et al. 2003), factors that can have a strong influence on the virulence of an infection.
When vertical infection occurs inside the host (e.g. many transovarial transmission in microsporidia, Dunn & Smith, 2001), the time and place of the infection differs between both transmission routes. Regarding time, vertically infecting parasites have access to the host early in the host's development. Their influence on host development and its immunity may thus be much stronger than that of horizontally infecting parasites. Regarding the place, vertical infection occurs in a relatively protected and clean environment, while horizontal infection may expose the host to concomitant infections with other pathogens present in the environment. In parasites with indirect vertical transmission, where infection from parent to offspring is external (e.g. the nematodes of fig wasps, Herre, 1993, or of Drosophila, Jaenike, 2000), the timing and environment of infection can be the same during both transmission routes.
As the results of my Ph.D. have shown, these intrinsic differences between horizontal and vertical transmission will affect the performance and, subsequently, the fitness of host and parasite. They need to be considered when approaching the study of host-parasite interactions and their coevolution, and, importantly, the study of virulence.
DUNN AM & SMITH JE (2001) Microsporidian life cycles and diversity: the relationship between virulence and transmission. Microbes and Infection 3, 381-388
FRANK SA (1992) A kin selection model for the evolution of virulence. Proceedings of the Royal Society of London Series B-Biological Sciences 250, 195-197
HERRE EA (1993) Population-structure and the evolution of virulence in nematode parasites of fig wasps. Science 259, 1442-1445
JAENIKE J (2000) Effectively vertical transmission of a Drosophila-parasitic nematode: mechanism and consequences. Ecological Entomology 25, 395-402
LITTLE TJ, O'CONNOR B, COLEGRAVE N, WATT K, & READ AF (2003). Maternal transfer of strain-specific immunity in an invertebrate. Current Biology 13, 489-492