Résumé
1) Co-infections by multiple parasites are common in natural populations.
Some of these are likely to be the result of sequential rather than
simultaneous infections. The timing of the co-infections may affect their
competitive interactions, thereby influencing the success of the parasites
and their impact on the host. This may have important consequence for
epidemiological and eco-evolutionary dynamics. 2) We examined in two
ecological conditions the effect of sequential co-infection on the outcome
of infection by two microsporidians, Vavraia culicis and Edhazardia aedis,
that infect the mosquito Aedes aegypti. The two parasites have different
transmission strategies: V. culicis is transmitted horizontally either
among larvae or from adults to larvae, while E. aedis can be transmitted
horizontally among larvae or vertically from females to their eggs. 3) We
investigated how the timing and order of the co-infection and how the
host’s food availability affected the parasite’s transmission potential
(the percentage of individuals that harboured transmissible spores) and
the host’s juvenile survival, its age at emergence and its longevity. 4)
The outcome of co-infection was strongly affected by the order at which
the parasites arrived. In co-infections, V. culicis had greater horizontal
transmission if it arrived early, whereas the transmission potential of E.
aedis, either vertical or horizontal, was not affected by the competitor
V. culicis. The availability of food determined the duration of infection
leading to variation in mortality and in the transmission potential. For
both parasites low food decreased juvenile survival, delayed emergence to
adulthood and increased horizontal transmission potential. High food
increased juvenile survival and the probability of emergence with higher
vertical transmission for E. aedis. Overall, our results suggest that
early infection favours transmission and that (i) V. culicis plastically
responded to co-infection, (ii) E. aedis was not affected by co-infection
but it was more susceptible to factors extending or decreasing the time it
spent in the host (time of infection and food). 5) Our results emphasise
the complexity of the impact of co-infection on host-parasite
interactions. In particular, the timing and order of sequential
co-infections can result in different within-host dynamic and modify
infection outcomes.