Reduced blood parasite prevalence with age in the Seychelles Warbler: selective mortality or suppression of infection?

Abstract

Avian malaria can affect survival and reproduction of their hosts. Two patterns commonly observed in birds are that females have a higher prevalence of malaria than do males and that prevalence decreases with age. The mechanisms behind these patterns remain unclear. However, most studies on blood parasite infections are based on cross-sectional analyses of prevalence, ignoring malaria related mortality and individual changes in infection. Here, we analyse both within-individual changes in malaria prevalence and long-term survival consequences of infection in the Seychelles Warbler (Acrocephalus sechellensis). Adults were less likely to be infected than juveniles but, contrary to broad patterns previously reported in birds, females were less likely to be infected than males. We show by screening individual birds in two subsequent years that the decline with age is a result both of individual suppression of infection and selective mortality. Birds that were infected early in life had a lower survival rate compared to uninfected birds, but among those that survived to be screened twice the proportion of infected birds had also decreased. Uninfected birds did not become infected later in life. Males were found to be more infected than females in this species possibly because, unlike most birds, males are the dispersing sex and the cost of dispersal may have to be traded against immunity. Infected males took longer to suppress their infection than did females. We conclude that these infections are indeed costly, and that age-related patterns in blood parasite prevalence are influenced both by suppression and selective mortality.

Age, but not sex and seasonality, influence Haemosporida prevalence in White-banded Tanagers (Neothraupis fasciata) from central Brazil

“…Our results do not support the findings of several studies showing that young individuals are more infected than adults (Weatherhead and Bennett 1991;Sol et al 2013;van Oers et al 2010).…”

“…However, other studies have also shown a decrease in parasite prevalence over host age (Hudson and Dobson 1997;Sol et al 2003;van Oers et al 2010) or no age effect (Ricklefs et al 2005;Latta and Ricklefs 2010). Because we were not able to resample individuals across time, we are unable to determine if infected juveniles managed to suppress infections later in life or if they died before becoming adults.…”

Prevalence of malaria parasites in indigenous chickens and ducks in selected districts of Bangladesh

“…Similar results were recorded by Islam et al(2013) in Bangladesh (33.3% and 3.5%), Msoffe et al(2010) in Tanzania (63% and 11%) and El-Magd et al(1988) in Egypt (60.7% and 20.0%) in adult and young birds, respectively. On the contrary, in some studies, higher prevalence was reported in young birds than in adults (Van Oers et al 2010;Hudson and Dobson, 1997). Higher prevalence in adults may be owing to long time exposure to the vectors.…”

Parasitic infection and oxidative status are associated and vary with breeding activity in the Seychelles warbler

“…The degradation products (haeme monomers) formed are capable of generating toxic oxidants [18,89]. The occurrence of such a direct oxidative effect has been confirmed for Haemoproteus [18], the genus of malaria parasite present in the Seychelles warbler [44,45]. However, it is well possible that these parasite-generated oxidant concentrations are too low to be picked up by the ROMs assay.…”