Persistence of avian haemosporidians in the wild: a case study to illustrate seasonal infection patterns in relation to host life stages

Do specialist and generalist parasites differ in their prevalence and intensity of infection? A test of the niche breadth and trade-off hypotheses

Studying host specificity is crucial to understanding the ability of parasites to spread to new hosts and trigger disease emergence events. The relationship between host specificity and parasite prevalence and infection intensity, has typically been studied in the context of two opposing hypotheses. According to the trade-off hypothesis generalist parasites, which can infect a broad range of hosts, will reach a lower prevalence and infection intensity than more specialist parasites due to the higher costs to adapt to multiple host immune systems. In contrast, the niche breadth hypothesis proposes that generalists' ability to infect more host species makes them more efficient in colonising host communities and thus they are found at higher prevalences and infection intensities. This study aims to test these hypotheses using the widespread avian malaria parasites of the genera Plasmodium and the related malaria-like parasite Haemoproteus. Overall, 1188 wild house sparrows from 17 localities in southwestern Spain were screened for parasite presence and intensity of infection. For each lineage found infecting house sparrows, we estimated host specificity as i) the number of different bird taxa infected by that lineage according to the MalAvi database and ii) an index that accounts for the phylogenetic relatedness between the host species. Parasite infections were recorded in 419 house sparrows, and eight Plasmodium and three Haemoproteus lineages were identified. Prevalence was positively associated with the number of host species. Lineages found in more localities showed both higher prevalence and host range. Overall, these results support the niche breadth hypothesis in relation to blood parasites infecting house sparrows.

Factors influencing a common but neglected blood parasite prevalence in breeding populations of passerines

The occurrence of avian blood protists is affected by multiple factors that include the characteristics of the hosts, the vectors, the parasites, as well as the environmental factors. This study provides an insight into some of the factors that influence the prevalence of avian Lankesterella, neglected but common blood parasites in breeding populations of common passerines. The highest prevalences of Lankesterella infection were observed in 1 great tit (Parus major) population at 63%, 1 blue tit (Cyanistes caeruleus) population at 49% and a sedge warbler (Acrocephalus schoenobaenus) population at 33%. Prevalence was found to be significantly influenced by sampling site followed by host age, species and sex. Julian date had no significant effect on Lankesterella prevalence. Prevalence data from different sampling sites can reveal different patterns and should be combined critically. Higher prevalence in adults suggest that the infections are chronic, which helps the parasite to persist in host populations. The differences between sexes might be related to different exposure to the transmitting vectors (e. g., mites or mosquitoes) during breeding.

Delayed post-juvenile moult in malaria-infected Eurasian blackcaps

Parasites have their strongest impact on fitness when host defences deplete resources needed for other critical life-history stages, such as development, breeding or migration. Among birds, one greatly neglected stage that could be altered by parasites is post-juvenile moult (PJM), through which yearling juvenile birds replace their fast-generated, low-quality juvenile feathers with adult-like feathers after leaving the nest. The earlier the birds complete PJM, the earlier they will be prepared to withstand forthcoming challenges, such as adverse winter conditions or migration. We used data from 435 juvenile Eurasian blackcaps (Sylvia atricapilla) sampled during three years in 26 localities spanning the wide range of environmental conditions across Iberian Spain to test whether haemosporidian infections (presence and abundance in blood of parasites of the genera Haemoproteus, Plasmodium and Leucocytozoon) were related to the progress of PJM. Controlling for body condition, sex, year and date of capture, infected blackcaps (single-infected or co-infected) had lower moult scores indicative of delayed moult, especially when birds had Plasmodium infections or high intensities of Haemoproteus parasites. Our results broaden the range of fitness costs that haemosporidian parasites may have on birds, as delayed post-juvenile plumage acquisition can impact subsequent key life-history stages.

Afro-Palaearctic migrant birds rid themselves of haemoparasite infections when breeding in the temperate zone

In migratory passerines, the prevalence and parasitaemia of blood parasites often increase towards the end of the breeding season. Consequently, post-breeding migration to winter quarters can be seen as a strategy to avoid the rising pathogen pressure during this time (migratory escape). I studied haemoparasite infections in sedge warblers (Acrocephalus schoenobaenus) inhabiting natural wetlands in southern Poland, examining the same individuals just after their arrival from wintering grounds, and again during incubation or nestling feeding. The birds were found to be infected more often immediately after arrival than during subsequent sampling. Parasitaemia also decreased significantly as the season progressed. The health status of individuals appeared to improve during the breeding season, whereas the higher level of parasitaemia just after arrival suggests that infections had occurred earlier, on the wintering grounds or during migratory stopovers.

Seasonal Variation in Detection of Haemosporidia in a Bird Community: A Comparison of Nested PCR and Microscopy

In a 2-yr study on prevalence of Haemosporidia in an avian community in Ithaca, New York, USA, we tested the hypothesis that apparent seasonal variation in prevalence is influenced by the detection protocol. We confirmed a higher detection of Haemosporidia using a molecular diagnosis technique (PCR) than by microscopy; this further increased when the PCR test was triplicated. Microscopic examination and PCR techniques have different specificity and sensitivity and therefore different probabilities of detecting hemoparasites. Birds with chronic infections or sampled during winter often have very low parasitemia, and such infections may be missed by microscopy but detected by PCR. Haemosporidian prevalence was higher during the breeding season than during the nonbreeding season regardless of the method used. Detection of Leucocytozoon spp. infection from blood smears using microscopy was challenging.

Avian malaria parasite infections do not affect personality in the chestnut thrush (Turdus rubrocanus) on the Qinghai-Tibet Plateau

Personality traits, the consistent individual behavioral differences, are currently gaining much attention in studies of natural bird populations. However, associations between personality traits and parasite infections are not often investigated. Even less attention has been given to studies of birds in the high-elevation region such as the Tibetan plateau. This research aims to examine the relationship between avian malaria parasites and two personality traits in a population of the Chestnut Thrush (Turdus rubrocanus) breed in the Tibetan plateau. Our results revealed no evidence of sex bias in malaria parasite prevalence. Furthermore, we found no effect of infection status on two personality scores: activity and boldness. Additionally, no effects on the activity level or boldness were observed for different parasite lineages of Haemoproteus, Leucocytozoon, the sex of the birds, or their interactions. Similarly, we did not find any relationship between activity level and boldness with nestling numbers, sex, or their interactions. Notably, individuals with a larger number of offspring tended to display greater boldness. Our findings indicate that blood parasite infections are common in this population but do not significantly impact the personality of the birds.

Seasonal variations of intensity of avian malaria infection in the Thousand Island Lake System, China

BACKGROUND

Migratory birds play an important part in the spread of parasites, with more or less impact on resident birds. Previous studies focus on the prevalence of parasites, but changes in infection intensity over time have rarely been studied. As infection intensity can be quantified by qPCR, we measured infection intensity during different seasons, which is important for our understanding of parasite transmission mechanisms.

METHODS

Wild birds were captured at the Thousand Island Lake with mist nets and tested for avian hemosporidiosis infections using nested PCR. Parasites were identified using the MalAvi database. Then, we used qPCR to quantify the infection intensity. We analyzed the monthly trends of intensity for all species and for different migratory status, parasite genera and sexes.

RESULTS

Of 1101 individuals, 407 were infected (37.0%) of which 95 were newly identified and mainly from the genus Leucocytozoon. The total intensity trend shows peaks at the start of summer, during the breeding season of hosts and during the over-winter season. Different parasite genera show different monthly trends. Plasmodium causes high prevalence and infection intensity of winter visitors. Female hosts show significant seasonal trends of infection intensity.

CONCLUSIONS

The seasonal changes of infection intensity is consistent with the prevalence. Peaks occur early and during the breeding season and then there is a downward trend. Spring relapses and avian immunity are possible reasons that could explain this phenomenon. In our study, winter visitors have a higher prevalence and infection intensity, but they rarely share parasites with resident birds. This shows that they were infected with Plasmodium during their departure or migration and rarely transmit the disease to resident birds. The different infection patterns of different parasite species may be due to vectors or other ecological properties.

The Infection Rate of Bird-Feeding Ixodes ricinus Ticks with Borrelia garinii and B. valaisiana Varies with Host Haemosporidian Infection Status

BACKGROUND

Birds are known to maintain and spread human pathogenic borreliae, but they are common hosts of diverse parasite communities, notably haemosporidians. Only a few studies examined whether tick infestation and/or Borrelia prevalences vary with hosts' haemosporidian infection status.

METHODS

Here, we study whether Ixodes ricinus infestation rates and Borrelia infection rates in bird-feeding ticks vary according to haemosporidian infection status in a community of free-living avian tick hosts.

RESULTS

Birds of six avian species harbored the majority of ticks. Both the tick infestation prevalence and the intensity peaked during spring and summer, but while bird-feeding nymphs prevailed in spring, bird-feeding larvae dominated in summer. Almost half of the bird-feeding ticks were found to be positive for B. burgdorferi s.l. Although the majority of infections involved bird-associated B. garinii and B. valaisiana, B. garinii appears to be the dominant Borrelia strain circulating in locally breeding avian species. We detected a negative link between the hosts' haemosporidian infection status and the Borrelia infection rate of bird-feeding ticks, but the association was dependent on the host's age.

CONCLUSIONS

Our results on tick infestation intensity support the idea that more immunologically vulnerable hosts harbor more ticks but suggest that different mechanisms may be responsible for tick infestation rates among immunologically naïve and experienced avian hosts. The results on Borrelia infection rates in bird-feeding ticks are consistent with studies revealing that intracellular parasites, such as haemosporidians, can benefit from the host immune system prioritizing immune responses against extracellular parasites at the expense of immune responses against intracellular parasites. The findings of our study urge for a more robust design of parasitological studies to understand the ecology of interactions among hosts and their parasites.

Assemblages of Plasmodium and Related Parasites in Birds with Different Migration Statuses

Migratory birds spend several months in their breeding grounds in sympatry with local resident birds and relatively shorter periods of time at stopover sites. During migration, parasites may be transmitted between migratory and resident birds. However, to what extent they share these parasites remains unclear. In this study, we compared the assemblages of haemosporidian parasites in migratory, resident, and passing birds, as well as the correlations between parasite assemblages and host phylogeny. Compared with passing birds, migratory birds were more likely to share parasites with resident birds. Shared lineages showed significantly higher prevalence rates than other lineages, indicating that common parasites are more likely to spill over from the current host to other birds. For shared lineages, the prevalence was significantly higher in resident birds than in migratory birds, suggesting that migratory birds pick up parasites at their breeding ground. Among the shared lineages, almost two-thirds presented no phylogenetic signal in their prevalence, indicating that parasite transmission among host species is weakly or not correlated with host phylogeny. Moreover, similarities between parasite assemblages are not correlated with either migration status or the phylogeny of hosts. Our results show that the prevalence, rather than host phylogeny, plays a central role in parasite transmission between migratory and resident birds in breeding grounds.

Early and Late Migrating Avian Individuals Differ in Constitutive Immune Function and Blood Parasite Infections – But Patterns Depend on the Migratory Strategy

Billions of birds migrate every year. To conduct a successful migration, birds undergo a multitude of physiological adaptions. One such adaptation includes adjustments of immune function, however, little is known about intraspecies (between-individual) and interspecies (between-species) variation in immune modulations during migration. Here, we explore if early and late migrating individuals differ in their immune function, and if such patterns differ among species with short- vs. long-distance migration strategies. We quantified three parameters of baseline (constitutive) innate immune function and one parameter of baseline (constitutive) acquired immune function in 417 individuals of 10 species caught during autumn migration at Falsterbo (Sweden). Early and late migrating individuals differed in lysis and total immunoglobulins (IgY), but the patterns show different directions in long-distance migrants (LDMs) (wintering in Africa) as compared to short-distance migrants (SDMs) (wintering within Europe). Specifically, early migrating LDMs had lower lysis but higher immunoglobulin levels than late migrating individuals. In short distance migrants, there was no difference in lysis between early and late migrating individuals, but immunoglobulin levels were higher in late migrating individuals. We found no correlation between timing of migration and haptoglobin, but LDMs had lower levels of haptoglobin than SDMs. We also found that the prevalence of haemosporidian blood parasite infections decreased in LDMs, but increased in SDMs, as the autumn progressed. Taken together, our study suggests that the investment into immune function depends on the migratory strategy (short- vs. long-distance migrants), and that early and late migrating individuals of a migration strategy might invest differently in baseline immune function, potentially driven by differences in the trade-offs with timing and speed of migration. Our study highlights the potential adaptations of immune function that could help explain trade-offs with other physiological systems, and behavioural responses during migration.

Reciprocal positive effects on parasitemia between coinfecting haemosporidian parasites in house sparrows

BACKGROUND

Hosts are often simultaneously infected with several parasite species. These co-infections can lead to within-host interactions of parasites, including mutualism and competition, which may affect both virulence and transmission. Birds are frequently co-infected with different haemosporidian parasites, but very little is known about if and how these parasites interact in natural host populations and what consequences there are for the infected hosts. We therefore set out to study Plasmodium and Haemoproteus parasites in house sparrows Passer domesticus with naturally acquired infections using a protocol where the parasitemia (infection intensity) is quantified by qPCR separately for the two parasites. We analysed infection status (presence/absence of the parasite) and parasitemia of parasites in the blood of both adult and juvenile house sparrows repeatedly over the season.

RESULTS

Haemoproteus passeris and Plasmodium relictum were the two dominating parasite species, found in 99% of the analyzed Sanger sequences. All birds were infected with both Plasmodium and Haemoproteus parasites during the study period. Seasonality explained infection status for both parasites in the adults: H. passeris was completely absent in the winter while P. relictum was present all year round. Among adults infected with H. passeris there was a positive effect of P. relictum parasitemia on H. passeris parasitemia and likewise among adults infected with P. relictum there was a positive effect of H. passeris parasitemia on P. relictum parasitemia. No such associations on parasitemia were seen in juvenile house sparrows.

CONCLUSIONS

The reciprocal positive relationships in parasitemia between P. relictum and H. passeris in adult house sparrows suggests either mutualistic interactions between these frequently occurring parasites or that there is variation in immune responses among house sparrow individuals, hence some individuals suppress the parasitemia of both parasites whereas other individuals suppress neither. Our detailed screening of haemosporidian parasites over the season shows that co-infections are very frequent in both juvenile and adult house sparrows, and since co-infections often have stronger negative effects on host fitness than the single infection, it is imperative to use screening systems with the ability to detect multiple parasites in ecological studies of host-parasite interactions.

Phylogeographic Patterns of Haemoproteid Assemblages of Selected Avian Hosts: Ecological and Evolutionary Implications

BACKGROUND

While the dynamics of disease emergence is driven by host-parasite interactions, the structure and dynamics of these interactions are still poorly understood. Here we study the phylogenetic and morphological clustering of haemosporidian parasite lineages in a local avian host community. Subsequently, we examine geographical patterns of parasite assemblages in selected avian hosts breeding in Europe.

METHODS

We conduct phylogenetic and haplotype network analyses of Haemoproteus (Parahaemoproteus) lineages based on a short and an extended cytochrome b barcode region. Ordination analyses are used to examine changes in parasite assemblages with respect to climate type and geography.

RESULTS

We reveal relatively low phylogenetic clustering of haemoproteid lineages in a local avian host community and identify a potentially new Haemoproteus morphospecies. Further, we find that climate is effectively capturing geographical changes in parasite assemblages in selected widespread avian hosts. Moreover, parasite assemblages are found to vary distinctly across the host's breeding range, even within a single avian host.

CONCLUSIONS

This study suggests that a few keystone hosts can be important for the local phylogenetic and morphological clustering of haemoproteid parasites. Host spatio-temporal dynamics, both for partially and long-distance migratory birds, appear to explain geographical variation in haemoproteid parasite assemblages. This study also gives support to the idea that climate variation in terms of rainfall seasonality can be linked to the propensity for host switching in haemosporidians.

Seasonal Dynamics and Diversity of Haemosporidians in a Natural Woodland Bird Community in Slovakia

Despite the ubiquity of disease seasonality, mechanisms behind the fluctuations in seasonal diseases are still poorly understood. Avian hemosporidiosis is increasingly used as a model for ecological and evolutionary studies on disease dynamics, but the results are complex, depending on the focus (hosts, parasites, vectors) and scale (individuals, community, populations) of the study. Here, we examine the local diversity of haemosporidian parasites and the seasonal patterns of infections, parasite richness, and diversity in a natural woodland bird community in Slovakia. In 35 avian species, we detected 111, including 19 novel, haemosporidian cytochrome b lineages. The highest numbers of lineages were detected during spring and autumn, corresponding with higher avian species richness and infection prevalence in the avian community during these periods of time. Nevertheless, the haemosporidian community in the local breeders in summer was relatively stable, Haemoproteus lineages dominated in the local avian haemosporidian community, and only few parasite lineages were abundant within each genus. While prevailing Leucocytozoon infections in spring suggest that the majority of sampled birds wintered in the Mediterranean region, Plasmodium infections in spring can be due to relapses in reproductively active short-distance migrants. Multiple haemosporidian infections, both intra- and inter-generic ones, were common in the local avian community. Infection intensity peaked during summer and tended to be higher in older birds, pointing to the role of supressed immunity in reproductively active birds.

Assessment of Associations between Malaria Parasites and Avian Hosts-A Combination of Classic System and Modern Molecular Approach

Simple Summary

Throughout history, frequent outbreaks of diseases in humans have occurred following transmission from animals. While some diseases can jump between birds and mammals, others are stuck to closely related species. Understanding the mechanisms of host–parasite associations will enable us to predict the outbreaks of diseases and will therefore be important to society and ecological health. For decades, scientists have attempted to reveal how host–parasite associations are formed and persist. The key is to assess the ability of the parasite to infect and reproduce within the host without killing the host. Related studies have faced numerous challenges, but technical advances are providing solutions and are gradually broadening our understanding. In this review, I use bird malaria and related blood parasites as a model system and summarize the important advances in techniques and perspectives and how they provide new approaches for understanding the evolution of host–parasite associations to further predict disease outbreaks.

Abstract

Avian malaria and related haemosporidian parasites are responsible for fitness loss and mortality in susceptible bird species. This group of globally distributed parasites has long been used as a classical system for investigating host–parasite associations. The association between a parasite and its hosts can be assessed by the prevalence in the host population and infection intensity in a host individual, which, respectively, reflect the ability of the parasite to infect the host and reproduce within the host. However, the latter has long been poorly investigated due to numerous challenges, such as lack of general molecular markers and limited sensitivity of traditional methods, especially when analysing naturally infected birds. The recent development of genetic databases, together with novel molecular methodologies, has shed light on this long-standing problem. Real-time quantitative PCR has enabled more accurate quantification of avian haemosporidian parasites, and digital droplet PCR further improved experimental sensitivity and repeatability of quantification. In recent decades, parallel studies have been carried out all over the world, providing great opportunities for exploring the adaptation of haemosporidian parasites to different hosts and the variations across time and space, and further investigating the coevolutionary history between parasites and their hosts. I hereby review the most important milestones in diagnosis techniques of avian haemosporidian parasites and illustrate how they provide new insights for understanding host–parasite associations.

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.