Evolution of Plastic Transmission Strategies in Avian Malaria

Abstract

Malaria parasites have been shown to adjust their life history traits to changing environmental conditions. Parasite relapses and recrudescences—marked increases in blood parasite numbers following a period when the parasite was either absent or present at very low levels in the blood, respectively—are expected to be part of such adaptive plastic strategies. Here, we first present a theoretical model that analyses the evolution of transmission strategies in fluctuating seasonal environments and we show that relapses may be adaptive if they are concomitant with the presence of mosquitoes in the vicinity of the host. We then experimentally test the hypothesis that Plasmodium parasites can respond to the presence of vectors. For this purpose, we repeatedly exposed birds infected by the avian malaria parasite Plasmodium relictum to the bites of uninfected females of its natural vector, the mosquito Culex pipiens, at three different stages of the infection: acute (∼34 days post infection), early chronic (∼122 dpi) and late chronic (∼291 dpi). We show that: (i) mosquito-exposed birds have significantly higher blood parasitaemia than control unexposed birds during the chronic stages of the infection and that (ii) this translates into significantly higher infection prevalence in the mosquito. Our results demonstrate the ability of Plasmodium relictum to maximize their transmission by adopting plastic life history strategies in response to the availability of insect vectors.

Malaria parasites prepare for flight

“…These findings support the predictions of mathematical models also presented by Cornet et al [3]: seasonality can select for the evolution of a 'plastic strategy' (Box 1), in which parasites invest in transmission by upregulating within-host growth only when vectors (and susceptible hosts) are available (Figure I in Box 1). This avoids wasting resources, or causing too much harm to the host, during periods when investing in transmission would not be rewarded.…”

“…Previous studies have not supported this hypothesis [2], but the puzzle is now one step closer to being solved. Cornet et al [3] reveal that parasites of the avian malaria Plasmodium relictum detect when mosquitoes blood feed on their host and respond by enhancing transmission.…”

Overlap in the Seasonal Infection Patterns of Avian Malaria Parasites and West Nile Virus in Vectors and Hosts

“…This may have been associated with prior persistent avian Plasmodium infections in host tissues, the recrudescence of infections into the bloodstream associated with stress from reproduction, 15,26 and increased vector activity. 62 Soon thereafter, the increased vector blood meals from American robins in late June 7 may have driven the increase in the infection rate of CHI07PL among Culex vectors. Increasing mosquito abundance may have facilitated the transmission of these parasites to naive juvenile robins that were numerous following peak breeding, leading to an observed rapid increase in prevalence.…”

Maternal effects, malaria infections and the badge size of the house sparrow

“…Furthermore, an avian malaria infection is not just characterised by the presence of the parasite in the blood: males with higher levels of parasitemia may face bigger tradeoffs, a fact we could not control for by using absence/ presence data for infections. However, since parasitemia is not constant during the course of an infection but is characterised by an initial acute phase before dropping to low levels with occasional relapses, it remains unclear how informative single measures in time of parasitemia taken from adult birds would be Cornet et al (2014). Coinfections by other parasites may also play an important role: if badge size represents an integrative signal of the overall long-term parasite burden of an individual, as has been shown for bill colour in blackbirds (Biard et al 2010), malaria status on its own may be insufficient in influencing badge size significantly.…”