Avian malaria parasites or haemosporidia are found in bird species worldwide. This special issue focuses on 3 most commonly studied genera: Haemoproteus, Plasmodium and Leucocytozoon. Seven research articles and reviews are provided to illustrate the breadth of knowledge of the diversity of avian malaria parasites in different regional habitats and across bird species, and the use of avian haemosporidian systems to examine host–parasite eco-evolutionary questions.

Mammalian haemosporidian parasites are classified in ten genera, including Plasmodium, Hepatocystis and Nycteria. A high diversity of haemosporidian parasites has been described from bats, but our understanding of their prevalence, distribution and use of hosts remain fragmented. The haemosporidian parasites of bats in Cameroon have been largely understudied, but here, bats, sampled from different habitat types of the Central Region of Cameroon, were investigated for haemosporidian infections with a combination of microscopic and molecular phylogenetic analysis. An overall prevalence of 18.1% of haemosporidian infections was detected in a total of 155 investigated bats belonging to 14 bat species. For the first time Hepatocystis and Nycteria parasites were detected in bats from Cameroon and molecularly characterized. Hepatocystis infections were exclusively identified in the epauletted fruit bat host species Epomophorus pusillus with a high prevalence of 65.5%, whereas Nycteria infections could be detected in several hosts, namely: Doryrhina cyclops (60.0%), Rhinolophus landeri (20.0%) and one Nycteris grandis. This study unveils evidence that habitat types may play a role in transmission of Hepatocystis parasites on a local scale and it adds important information on the distribution and host specificity of the neglected haemosporidian genus Nycteria.

Understanding factors that influence the spatial and temporal distributions of blood parasites is important to help predict how host species and their parasites may respond to global change. Factors that may influence parasite distributions are land cover and host dispersal patterns, which may result in exposure of a host to novel parasites, or escape from parasites of their origin. We screened golden-winged warblers from across the United States and Canada for blood parasites, and investigated whether land-use patterns or host dispersal affected the prevalence and composition of haemosporidian assemblages. Parasite prevalence varied strongly with study area, and areas with high agricultural cover had a significantly higher prevalence of Leucocytozoon and Parahaemoproteus parasites. Lineages of Parahaemoproteus and Leucocytozoon were genetically differentiated among study areas, and prevalence and composition of parasite assemblages indicated an increase in parasite prevalence and accumulation of unique parasite lineages from the southeast to the northwest. This matches the historical range expansion and natal dispersal patterns of golden-winged warblers, and suggests that golden-winged warblers may have been sensitive to novel parasites as they dispersed. The high prevalence and diversity of parasite lineages in the north-west extent of their breeding range (Manitoba) indicates that this population may face unique pressures.

Avian malaria (caused by Plasmodium spp.) and avian malaria-like infections (caused by Haemoproteus spp.) are widespread and can seriously affect the health of their bird hosts, especially of immunologically naïve individuals. Therefore, these parasites have long been in the focus of bird-parasite studies. However, the species richness and diversity of these protozoan species have only been revealed since the use of molecular techniques. Diversity and prevalence of these parasites among different bird species and even between populations of a species show a large variation. Here, we investigated prevalence of avian malaria and avian malaria-like parasites in two distant populations of a non-migratory wetland specialist passerine, the bearded reedling (Panurus biarmicus). While previous studies have shown that reed-dwelling bird species often carry various blood parasite lineages and the presence of the vectors transmitting Plasmodium and Haemoproteus species has been confirmed from our study sites, prevalence of these parasites was extremely low in our populations. This may either suggest that bearded reedlings may avoid or quickly clear these infections, or these parasites cause high mortality in this species. The remarkably low prevalence of infection in this species is consistent with earlier studies and makes bearded reedlings a possible model organism for investigating the genetic or behavioural adaptations of parasite resistance.

Haemosporidian parasites are widespread in avian species and modulate their ecology, behaviour, life-history and fitness. The prevalence of these vector-transmitted parasites varies with host intrinsic and extrinsic factors, such as host resistance and behaviour, and habitat-related characteristics. In this study, we evaluated the prevalence of avian haemosporidians in great tit Parus major populations inhabiting two areas with different degrees of exposure to aerial emissions from pulp mills, to assess if this type of pollution impacted parasite prevalence. We also compared the physiological condition of infected and uninfected individuals. Haemosporidian infection prevalence was high (58%), varied seasonally, but was not associated with air pollution exposure. Fledged birds presented higher infection rates than nestlings and infected fledged birds had higher levels of blood glutathione peroxidase activity. These results allow us to infer that infection by blood parasites may activate antioxidant defences, possibly to protect the organism from the negative oxidative stress side-effects of immune activation against parasites. Because oxidative stress is one of the mechanisms responsible for ageing and senescence and may affect fitness, the relationship between parasitism and oxidative stress markers should be further investigated through studies that include experimental manipulation of infection in model organisms.

Over a hundred years since their first description in 1913, the sparsely described malaria parasites (genus Plasmodium) of ungulates have been rediscovered using molecular typing techniques. In the span of weeks, three studies have appeared describing the genetic characterization and phylogenetic analyses of malaria parasites from African antelope (Cephalophus spp.) and goat (Capra aegagrus hircus), Asian water buffalo (Bubalus bubalis), and North American white-tailed deer (Odocoileus virginianus). Here we unify the contributions from those studies with the literature on pre-molecular characterizations of ungulate malaria parasites, which are largely based on surveys of Giemsa-reagent stained blood smears. We present a phylogenetic tree generated from all available ungulate malaria parasite sequence data, and show that parasites from African duiker antelope and goat, Asian water buffalo and New World white-tailed deer group together in a clade, which branches early in Plasmodium evolution. Anopheline mosquitoes appear to be the dominant, if not sole vectors for parasite transmission. We pose questions for future phylogenetic studies, and discuss topics that we hope will spur further molecular and cellular studies of ungulate malaria parasites.

The diversity and prevalence of malaria parasites of the genera Plasmodium and Haemoproteus were determined in the globally-threatened Aquatic Warbler Acrocephalus paludicola. Birds were sampled during migration in Portugal and at the wintering quarters in Senegal and parasites were detected using molecular methods. Only three generalist parasite lineages (Plasmodium) were found. There were no significant differences in the prevalence of parasites between sexes in Europe, but adults had higher prevalence than first-year birds, and birds in Europe had higher prevalence than those captured in Africa. When comparing with other Acrocephalus species and taking sample size into account, Aquatic Warblers had the lowest prevalence and, together with another threatened species, the Seychelles Warbler Acrocephalus sechellensis, the lowest diversity of malaria parasites. We hypothesize that the low diversity of parasites and absence of specialist lineages of Aquatic Warblers are caused by its small population size and fragmented distribution. Furthermore, Aquatic Warblers’ extreme habitat specialization may decrease their exposure to malaria parasites, but other explanations such as high mortality (which would constraint the sampling of infected birds) or, in contrast, very efficient immunological system in clearing the infections cannot be ruled out. This study contributes to explain variation in prevalence and diversity of malaria parasites among hosts.

Avian Plasmodium and Haemoproteus parasites are easily detected by DNA analyses of infected samples but only correctly assigned to each genus by sequencing and use of a phylogenetic approach. Here, we present a restriction site to differentiate between both parasite genera avoiding the use of those analyses. Alignments of 820 sequences currently listed in GenBank encoding a particular cytochrome B region of avian Plasmodium and Haemoproteus show a shared restriction site for both genera using the endonuclease Hpy CH4III. An additional restriction site is present in Plasmodium sequences that would initially allow differentiation of both genera by differential migration of digested products on gels. Overall 9 out of 326 sequences containing both potential restriction sites do not fit to the general rule. We used this differentiation of parasite genera based on Hpy CH4III restriction sites to evaluate the efficacy of 2 sets of general primers in detecting mixed infections. To do so, we used samples from hosts infected by parasites of both genera. The use of general primers was only able to detect 25% or less of the mixed infections. Therefore, parasite DNA amplification using general primers to determine the species composition of haemosporidian infections in individual hosts is not recommended. Specific primers for each species and study area should be designed until a new method can efficiently discriminate both parasites.

More than 200 species of avian Haemosporidia (genera Plasmodium, Haemoproteus, and Leucocytozoon) have been described based primarily on morphological characters seen in blood smears. Recent molecular studies, however, suggest that such methods may mask a substantial cryptic diversity of avian haemosporidians. We surveyed the haemosporidians of birds sampled at 1 site in Israel. Parasites were identified to species based on morphology, and a segment of the parasite's cytochrome b gene was sequenced. We compared 3 species concepts: morphological, genetic, and phylogenetic. Fifteen morphological species were present. Morphological species that occurred once within our dataset were associated with a unique gene sequence, displayed large genetic divergence from other morphological species, and were not contained within clades of morphological species that occurred more than once. With only 1 exception, morphological species that were identified from multiple bird hosts presented identical sequences for all infections, or differed by few synonymous substitutions, and were monophyletic for all phylogenetic analyses. Only the morphological species Haemoproteus belopolskyi did not follow this trend, falling instead into at least 2 genetically distant clades. Thus, except for H. belopolskyi, parasites identified to species by morphology were supported by both the genetic and phylogenetic species concepts.