Higher plasma corticosterone is associated with reduced costs of infection in red-winged blackbirds

Changes in hypothalamic-pituitary-adrenal axis function, immunity, and glucose during acute Plasmodium relictum infection in house sparrows (Passer domesticus)

Hosts of the same species vary in physiological responses to the same parasite, and some groups of individuals can disproportionately affect disease dynamics; however, the underlying pathophysiology of host-parasite interactions is poorly understood in wildlife. We tested the hypothesis that the hypothalamic-pituitary-adrenal (HPA) axis mediates host resistance and tolerance to avian malaria during the acute phase of infection by evaluating whether individual variation in circulating glucocorticoids predicted resistance to avian malaria in a songbird. We experimentally inoculated wild-caught house sparrows (Passer domesticus) with naturally sourced Plasmodium relictum and quantified baseline and restraint-induced circulating corticosterone, negative feedback ability, cellular and humoral immune function, and baseline and restraint-induced glycemia, prior to and during acute malaria infection. During peak parasitemia, we also evaluated the expression of several liver cytokines that are established pathological hallmarks of malaria in mammals: two pro-inflammatory (IFN-γ and TNF-α) and two anti-inflammatory (IL-10 and TGF-β). Although most of the host metrics we evaluated were not correlated with host resistance or tolerance to avian malaria, this experiment revealed novel relationships between malarial parasites and the avian immune system that further our understanding of the pathology of malaria infection in birds. Specifically, we found that: (1) TNF-α liver expression was positively correlated with parasitemia; (2) sparrows exhibited an anti-inflammatory profile during malaria infection; and (3) IFN-γ and circulating glucose were associated with several immune parameters, but only in infected sparrows. We also found that, during the acute phase of infection, sparrows increased the strength of corticosterone negative feedback at the level of the pituitary. In the context of our results, we discuss future methodological considerations and aspects of host physiology that may confer resistance to avian malaria, which can help inform conservation and rehabilitation strategies for avifauna at risk.

Experimental reduction of haemosporidian infection affects maternal reproductive investment, parental behaviour and offspring condition

When hosts have a long coevolutionary history with their parasites, fitness costs of chronic infection have often been assumed to be negligible. Yet, experimental manipulation of infections sometimes reveals effects of parasites on their hosts, particularly during reproduction. Whether these effects translate into fitness costs remains unclear. Here, we present the results of an experimental study conducted in a free-ranging population of red-winged blackbirds (Agelaius phoeniceus) naturally experiencing a high prevalence of haemosporidian infections, with more than 95% of breeding adults infected with parasites from one or more haemosporidian genus. To assess effects of infection during reproduction, we manipulated adult red-winged blackbird females' parasite burden by administering an anti-haemosporidian medication before onset of egg-laying. Experimental reduction of infection resulted in significant benefits to mothers and their offspring. Medicated females laid heavier clutches, invested more in incubation and provisioning behaviour, and produced more fledglings than control females. Nestlings of medicated females had higher haematocrit, higher blood glucose, and lower reactive oxygen metabolites than nestlings of control females. Overall, our results provide evidence that, even in a species with high prevalence of infection, parasites can lead to decreased maternal investment and offspring quality, substantially reducing fitness.

Trade-Offs (and Constraints) in Organismal Biology

AbstractTrade-offs and constraints are inherent to life, and studies of these phenomena play a central role in both organismal and evolutionary biology. Trade-offs can be defined, categorized, and studied in at least six, not mutually exclusive, ways. (1) Allocation constraints are caused by a limited resource (e.g., energy, time, space, essential nutrients), such that increasing allocation to one component necessarily requires a decrease in another (if only two components are involved, this is referred to as the Y-model, e.g., energy devoted to size versus number of offspring). (2) Functional conflicts occur when features that enhance performance of one task decrease performance of another (e.g., relative lengths of in-levers and out-levers, force-velocity trade-offs related to muscle fiber type composition). (3) Shared biochemical pathways, often involving integrator molecules (e.g., hormones, neurotransmitters, transcription factors), can simultaneously affect multiple traits, with some effects being beneficial for one or more components of Darwinian fitness (e.g., survival, age at first reproduction, fecundity) and others detrimental. (4) Antagonistic pleiotropy describes genetic variants that increase one component of fitness (or a lower-level trait) while simultaneously decreasing another. (5) Ecological circumstances (or selective regime) may impose trade-offs, such as when foraging behavior increases energy availability yet also decreases survival. (6) Sexual selection may lead to the elaboration of (usually male) secondary sexual characters that improve mating success but handicap survival and/or impose energetic costs that reduce other fitness components. Empirical studies of trade-offs often search for negative correlations between two traits that are the expected outcomes of the trade-offs, but this will generally be inadequate if more than two traits are involved and especially for complex physiological networks of interacting traits. Moreover, trade-offs often occur only in populations that are experiencing harsh environmental conditions or energetic challenges at the extremes of phenotypic distributions, such as among individuals or species that have exceptional athletic abilities. Trade-offs may be (partially) circumvented through various compensatory mechanisms, depending on the timescale involved, ranging from acute to evolutionary. Going forward, a pluralistic view of trade-offs and constraints, combined with integrative analyses that cross levels of biological organization and traditional boundaries among disciplines, will enhance the study of evolutionary organismal biology.

Physiological and morphological correlates of blood parasite infection in urban and non-urban house sparrow populations

In the last decade, house sparrow populations have shown a general decline, especially in cities. Avian malaria has been recently suggested as one of the potential causes of this decline, and its detrimental effects could be exacerbated in urban habitats. It was initially thought that avian malaria parasites would not have large negative effects on wild birds because of their long co-evolution with their hosts. However, it is now well-documented that they can have detrimental effects at both the primo- and chronical infection stages. In this study, we examined avian malaria infection and its physiological and morphological consequences in four populations of wild house sparrows (2 urban and 2 rural). We did not find any relationship between the proportions of infected individuals and the urbanisation score calculated for our populations. However, we observed that the proportion of infected individuals increased during the course of the season, and that juveniles were less infected than adults. We did not detect a strong effect of malaria infection on physiological, morphological and condition indexes. Complex parasite dynamics and the presence of confounding factors could have masked the potential effects of infection. Thus, longitudinal and experimental studies are needed to understand the evolutionary ecology of this very common, but still poorly understood, wild bird parasite.

Exogenous glucocorticoids amplify the costs of infection by reducing resistance and tolerance, but effects are mitigated by co-infection

Individual variation in parasite defences, such as resistance and tolerance, can underlie heterogeneity in fitness and could influence disease transmission dynamics. Glucocorticoid hormone concentrations often change in response to fluctuating environmental conditions and mediate changes in immune function, resource allocation and tissue repair. Thus, changes in glucocorticoid hormone concentrations might mediate individual variation in investment in resistance versus tolerance. In this study, we experimentally increased glucocorticoid concentrations in red-winged blackbirds ( Agelaius phoeniceus) that were naturally infected with haemosporidian parasites, and assessed changes in resistance and tolerance of infection. Glucocorticoid treatment increased burdens of Plasmodium, the parasite causing avian malaria, but only in the absence of co-infection with another Haemosporidian, Haemoproteus. Thus, glucocorticoids might reduce resistance to infection, but co-infection can mitigate the negative consequences of increased hormone concentrations. Glucocorticoid treatment also decreased tolerance of infection. We found no evidence that the inflammatory immune response or rate of red blood cell production underlie the effects of glucocorticoids on resistance and tolerance. Our findings suggest that exogenous glucocorticoids can increase the costs of haemosporidian infections by both increasing parasite numbers and reducing an individual's ability to cope with infection. These effects could scale up to impact populations of both host and parasite.

Critical Roles of Endogenous Glucocorticoids for Disease Tolerance in Malaria

During malaria, the hypothalamic-pituitary-adrenal (HPA) axis is activated and glucocorticoid (GC) levels are increased, but their essential roles have been largely overlooked. GCs are decisive for systemic regulation of vital processes such as immune responses, vascular function, and metabolism, which are crucial in malaria. Here, we introduce GCs in general, followed by their versatile roles for disease tolerance in malaria. A complementary comparison is provided with their role in sepsis. Finally, potential translational implications are considered. The failed clinical trials of dexamethasone against cerebral malaria in the past have diminished the interest in GCs in malaria. However, the issue of relative corticosteroid insufficiency has barely been explored in malaria patients, but may hold promise for a better understanding and treatment of specific malaria complications.