Cory's Shearwater (Calonectris borealis), a migratory seabird from the Procellariidae family, is widely distributed across the Atlantic Ocean. It breeds on islands of Azores, Madeira, Canary, and Berlengas, and winters in coastal areas of South America and Africa. This species' wide habitat range contributes to pathogen exchange across global regions. Plasmodium, a genus of apicomplexan parasites causing avian malaria, affects many wild bird species worldwide. Although rare in seabirds, Plasmodium infections can significantly impact these hosts. We identified and characterized a new Plasmodium species in an adult male C. borealis found live-stranded on the northern coast of São Paulo, Brazil, during the daily activities of the Santos Basin Beach Monitoring Project (PMP-BS). The bird was rescued and kept in rehabilitation for 61 days but was euthanized due to poor prognosis. Blood smears revealed parasitemia varying from 0.04% until 2.09%. Blood and tissue samples were tested using PCR targeting a cytochrome b (cytb) gene fragment. The obtained sequence was 100% similar to Plasmodium lineage pLK06. Phylogenetic analysis placed the parasite within the Novyella clade, with morphological features closely related to P. vaughani. Necropsy showed hepato- and splenomegaly, while histopathology confirmed hepatitis and pulmonary hemorrhage. These findings, combined with clinical relapse despite anti-malaria treatment, indicate the parasite's pathogenicity in C. borealis. This represents the first haemosporidian infection in this seabird and the first identification of the pLK06 lineage in Procellariiformes, resulting in the description of Plasmodium borealis sp. nov. These findings expand knowledge of parasite diversity in seabirds and highlight the need for continued surveillance in understudied avian groups.

The Fatu Hiva monarch (Pomarea whitneyi) is a Critically Endangered (IUCN) passerine species restricted to a single island in French Polynesia, with fewer than 20 individuals remaining. In the Pacific region, avian malaria caused by Plasmodium relictum lineage GRW4 is a major conservation concern because it has been linked to severe population declines and extinctions in Hawai'i and poses a documented threat to endemic birds in New Zealand. On 17 June 2022, a recently fledged juvenile Fatu Hiva Monarch (Pomarea whitneyi) was found dead in its natural habitat on Fatu Hiva. A necropsy was performed on site in humid tropical conditions, with real-time online guidance from an expert based in Switzerland. Gross examination revealed severe emaciation, an empty gastrointestinal tract, diffuse pallor with scant intravascular blood and hepatosplenomegaly with dark discolouration. Despite advanced autolysis having occurred, cytologic tissue imprint and blood-smear assessment revealed high parasitaemia with intraerythrocytic developmental stages and abundant birefringent malarial pigment. Histopathology confirmed a hemozoin-producing haemosporidian infection due to the presence of widespread polarisation-positive and Prussian blue-negative intracytoplasmic pigment in erythrocytes and macrophages. Molecular testing was performed on dried blood pressed onto household filter paper, that had been stored and shipped at ambient temperature for several weeks. Nested cytochrome b polymerase chain reaction and sequencing confirmed a single infection with the GRW4 lineage of Plasmodium relictum. This first confirmed fatal case of avian malaria in this critically depleted island endemic species highlights the urgent need to integrate disease surveillance and vector management into conservation planning. The results suggest that acute severe P. relictum (GRW4) infection was the proximate cause of death, likely occurring against a background of prolonged negative energy balance and chronic stress exposure. Although the direction of causality cannot be fully determined post-mortem, aggressive malaria prevention measures remain justified to avoid further demographic decline in the critically low Fatu Hiva monarch population. To effectively reduce risk, vector control measures must be combined with measures to enhance host resilience and continuous surveillance to monitor transmission and guide adaptive management. Furthermore, this case study shows that a scientifically defensible, lineage-level diagnosis can be achieved using simple, low-cost samples and techniques adapted for remote tropical fieldwork, even when carcass quality is compromised by warm, humid conditions.

Understanding vector-borne pathogen transmission is a priority due to its impact on human and animal health. While mosquitoes are key hosts of avian malaria, most studies focus on birds. This study examines how environmental factors influence Plasmodium communities in Culex pipiens and Culex perexiguus and compares lineage composition and spatial patterns. Mosquitoes were collected from 16 sites in southwest Spain over two years. Plasmodium detection was performed using nested PCR, and lineages were identified by SANGER sequencing. Linear mixed models tested environmental effects on Plasmodium communities, and correspondence analysis and Mantel tests explored interspecific and spatial relationships. In Cx. pipiens, Plasmodium prevalence increased with NDVI, while lineage richness declined with land surface temperature. In Cx. perexiguus, prevalence and lineage richness increased with minimum temperature, and diversity decreased with land surface temperature. Plasmodium communities differed between mosquito species, with community similarity correlating with geographical distance only in Cx. pipiens. These findings suggest that environmental factors shape Plasmodium communities in mosquitoes, and that vector identity influences lineage structure and distribution. Integrating vector ecology and environmental context is essential to better understand avian malaria transmission dynamics in changing environments.

Avian malaria parasites and related haemosporidians are diverse and widespread vector-borne parasites that circulate within complex host-vector networks. Despite their ecological importance, their diversity and circulation within mosquito communities remain poorly understood in temperate North America. The Midwestern United States, located along major North American migratory flyways, supports diverse avian habitats and mosquito assemblages, providing an ideal setting to investigate these interactions. Here, we characterized the occurrence, phylogenetic diversity, and mosquito-lineage associations of avian haemosporidians in mosquitoes. A total of 233 pools comprising 6170 unfed female mosquitoes from seven species were collected from 38 protected natural areas in northern, central, and southern Illinois. Samples were screened for avian haemosporidian DNA by nested PCR targeting the mitochondrial cytochrome b gene. Positive samples were analyzed by multiplex PCR and sequencing to identify parasite lineages and assess potential mixed infections. Phylogenetic relationships were reconstructed, and mosquito-lineage associations were visualized using a bipartite network. Infection rates were estimated using maximum likelihood estimation (MLE) and minimum infection rate (MIR), and factors associated with infection prevalence in pooled samples were evaluated using pooled-binomial regression. Of 233 pools, 44 (18.9%) were positive for avian haemosporidians. Eleven lineages were identified, including nine Plasmodium and two Haemoproteus lineages; one Plasmodium lineage was novel. pTUMIG03 (Plasmodium unalis) was the most frequently detected lineage, whereas pSYAT05 (P. vaughani) showed the broadest mosquito distribution. Culex restuans harbored the highest lineage diversity and infection rate among the mosquito species examined. Mosquito species was a significant predictor of infection prevalence estimated from pooled samples, and Cx. restuans was most strongly associated with avian Plasmodium detection. Additionally, a deer-associated non-avian Plasmodium lineage was incidentally detected in two pools. Mosquito populations in Illinois harbor a diverse assemblage of avian haemosporidians, with heterogeneous lineage distributions across mosquito taxa. Culex restuans emerged as the species most strongly associated with avian Plasmodium prevalence, suggesting an important role in local enzootic circulation. These findings expand current knowledge of avian haemosporidian diversity in North American mosquito communities and provide a basis for future studies integrating mosquito surveillance, avian host sampling, and vector competence experiments.

Malaria parasites in the genus Plasmodium are ubiquitous infectious agents. Most avian species effectively manage infection and experience limited population effects, with a notable exception in the Hawaiian Islands. Since Hawaiian honeycreepers evolved over ~7 million years without blood parasites, the introduction of Plasmodium relictum had devastating consequences-including widespread extinctions and extirpations. However, honeycreeper species have been differentially affected by malaria. Using whole genome sequences for 35 O'ahu 'amakihi (Chlorodrepanis flava), we conducted a genome-wide association study to identify single nucleotide polymorphisms (SNPs) correlated with avian malaria infection. Infection status was determined using real-time PCR on blood samples. We identified 582 SNPs associated with malaria infection. Annotations included long ncRNAs, genes associated with transcription regulation, apoptosis, T-cell and complement activation, autophagosome assembly and transport, and intracellular calcium flux. By identifying genes regulating intracellular calcium ions, a key signaling molecule in Plasmodium replication, our work shows that host limitations on parasite replication may not solely depend on the adaptive immune response. Host responses can also involve restricting intracellular components essential for completion of parasite life cycles. This adds to a growing wildlife disease literature indicating a broad range of mechanisms of disease resistance rather than common pathways.

Species distribution modeling for honeycreepers on Kauaʻi, Hawaiʻi informs conservation action

Genetic diversity is essential for the adaptive potential of species, particularly in the context of parasitic infections, which exert strong selective pressures on host populations. While reduced genetic diversity is often linked to increased susceptibility to infection, the extent to which this applies across different genetic markers and ecological contexts remains unclear. In this meta-analysis, we synthesized data from 35 studies encompassing 27 bird species and 85 effect sizes to examine the relationship between individual genetic diversity and avian malaria infections caused by Plasmodium, Haemoproteus, and Leucocytozoon. We focused on the influence of genetic marker type (functional vs. neutral), parasite genus, infection assessment method, host migratory behaviour, and latitude. Contrary to expectations, we found no significant association between individual genetic diversity and either infection prevalence or intensity, regardless of marker type. Meta-regression analyses further revealed that none of the tested moderators-including parasite genus, infection assessment method, migratory behaviour, or latitude-significantly influenced this relationship. Similarly, analyses restricted to MHC data showed no effect of additional moderators such as MHC class or genotyping method. These findings suggest that broad measures of genetic diversity may not reliably predict infection outcomes in wild bird populations. It is likely that such measures obscure finer genetic effects from specific alleles (e.g., MHC variants). Moreover, reliance solely on parasite genetic material may underrepresent infection status. We recommend that future studies integrate general diversity metrics with allele-specific analyses and incorporate antibody-based diagnostics to more accurately capture infection dynamics and host-parasite co-evolution in natural systems.

Aedeomyia squamipennis is the only Neotropical representative of a genus recognized for its ornithophilic behavior and potential involvement in the transmission of avian malaria and arboviruses. Despite its broad distribution in South America, ecological information on its habitat preferences and seasonal patterns remains scarce. This study investigated the spatial distribution and climatic factors associated with Ad. squamipennis in three habitat types within the Coastal Plain of southern Brazil, an ecologically relevant region for migratory and overwintering birds. Mosquitoes were sampled monthly from August 2014 to May 2015 in urban, rural, and forested environments across three municipalities. Abundance patterns were compared using nonparametric tests, and correlations with mean temperature, precipitation, and relative humidity were assessed. A total of 1,156 specimens were collected, with higher abundances in rural (79.1%) and forested areas (20.7%), whereas urban environments contributed only 0.2% of captures. Ad. squamipennis abundance was significantly higher in rural habitats compared with other areas (p < 0.05). Population peaks showed a strong positive correlation with temperature (r = 0.84; p = 0.0026) and a negative correlation with relative humidity (r = -0.86; p = 0.0020), indicating that warmer and less humid conditions favored adult activity. Precipitation showed a negative correlation with mosquito abundance in Rio Grande (r = -0.441; p = 0.202) and in São Lourenço do Sul (r = -0.590; p = 0.056), with no statistically significant association during the study period. Ad. squamipennis was primarily associated with rural flooded areas and adjacent forested habitats. Temperature and humidity were the main drivers of its population dynamics, highlighting seasonal fluctuations in abundance and implications for parasite transmission. These findings support integrated monitoring of ornithophilic mosquitoes and advance the understanding of ecological interfaces relevant to avian parasite circulation within a One Health framework.

BackgroundDespite much research on avian malaria, the details of development of Plasmodium species in birds remain insufficiently investigated. This study aimed to gain new experimental information on avian malaria parasites regarding their host specificity, molecular characterization, persistence, and association with Matryoshka RNA viruses (MaRNAV), which are linked to haemosporidian infections.MethodsThree species of widespread avian Plasmodium—P. circumflexum pTURDUS1, P. vaughani pSYAT05, and P. relictum pCOLL7—were found in naturally infected wild birds of the Muscicapidae, Turdidae and Phylloscopidae, respectively. The parasites were transferred to non-infected Eurasian siskins Spinus spinus (Fringillidae) by inoculation of infected blood, and infections were monitored up to 185 days. Microscopy, histology, chromogenic in situ hybridization (CISH), PCR, RNA sequencing and bioinformatic methods were applied in this study.ResultsAll recipient birds were susceptible to these pathogens. Extensive histology and CISH testing did not detect any tissue stages in all birds, suggesting that long-lasting persistence is mainly due to erythrocytic merogony. However, in the case of pCOLL7, parasitaemia consisted almost exclusively of gametocytes, presenting an unusual mode of parasitaemia in avian malaria. Based on morphological and phylogenetic data, pCOLL7 was linked to P. relictum. Morphological characterization showed that taxonomic characters of blood stages of all three parasite species are relatively stable across host species, supporting the value of the morphological characters of blood stages in Plasmodium species identification in birds belonging to different families. Furthermore, this study detected and associated a Matryoshka RNA virus (MaRNAV-9) with avian Plasmodium parasites for the first time. This virus was found in association with the bird malaria parasite P. (Giovannolaia) circumflexum (pTURDUS1) widespread in Eurasia, which can be used as a model host-parasite system to better understand the biology of MaRNAVs and their relationship with haemosporidians.ConclusionsThis study provides new information to better understand diagnostics and persistence of avian malaria parasites in avian hosts. It also contributes to new knowledge about the diversity of MaRNAVs associated with Plasmodium pathogens, opening opportunities for detecting these viruses in association with malaria parasites.Supplementary InformationThe online version contains supplementary material available at 10.1186/s12936-026-05879-z.

Avian malaria and invasion success: Contrasting infection patterns in two introduced Eurasian sparrows in North America (Passer domesticus and Passer montanus).

Haemosporidians, including Leucocytozoon spp., Plasmodium spp., and Haemoproteus spp., are vector-borne parasites that cause avian malaria and malaria-like diseases in birds. These infections can result in anaemia, reduced body condition, impaired reproductive success, and mortality, thereby acting as selective agents within host populations. Climate change is expected to influence dynamics of some haemosporidians, particularly in northern regions, by altering vector distributions and extending transmission seasons. In this study, tawny owls (Strix aluco; n=27) from central Norway, were screened for Leucocytozoon spp., Haemoproteus spp.,and Plasmodium spp., using polymerase chain reaction (PCR). Samples were also screened for herpesvirus and Chlamydia spp., via PCR, to assess potential co-infections. Ten individuals (37%) tested positive for Leucocytozoon spp., with sequencing confirming the lineages STAL3 and STAL1, both previously detected in raptors. While seven individuals (26%) were positive in the Haemoproteus/Plasmodium PCR, sequencing was unsuccessful and infection could not be confirmed. Neither herpesvirus nor Chlamydia spp. were detected. This study provides the first molecular evidence of haemosporidian infections in raptors in Norway, underscoring the importance of continued surveillance of avian haemosporidians in resident northern species, to detect potential climate-driven changes in infection prevalence and associated health impacts.

ABSTRACTBackground and Aim:Avian malaria caused by Plasmodium spp. is an important vector-borne disease affecting poultry production in tropical and subtropical regions. Thai native chickens (Gallus gallus domesticus) and fighting cocks represent valuable genetic and economic resources in Thailand, yet information on the molecular epidemiology of avian malaria in northeastern Thailand remains limited. Kalasin Province contains diverse ecological environments with freshwater reservoirs, irrigated agricultural systems, and backyard poultry farming, which may facilitate transmission of haemosporidian parasites. Molecular tools targeting mitochondrial genes provide higher sensitivity than microscopic examination and allow accurate phylogenetic characterization. This study aimed to determine the molecular prevalence of Plasmodium spp., compare infection rates between free-range Thai native chickens and intensively managed fighting cocks, and analyze the phylogenetic relationships of circulating parasites using the mitochondrial cytochrome c oxidase subunit III (COXIII) gene.Materials and Methods:A cross-sectional survey was conducted from January to April 2025 using 181 blood samples collected from Thai native chickens (n = 112) and fighting cocks (n = 69) across 18 districts of Kalasin Province, Thailand. Genomic DNA was extracted using a commercial kit, and Plasmodium infection was detected by polymerase chain reaction targeting the mitochondrial COXIII gene. Positive amplicons were sequenced, and phylogenetic relationships were reconstructed using the Maximum Likelihood method. Differences in prevalence between host types and districts were evaluated using the Chi-square test, with p < 0.05 considered significant.Results:The overall molecular prevalence of Plasmodium spp. infection was 62.98% (114/181). Infection was higher in Thai native chickens (67.85%) than in fighting cocks (55.07%), but the difference was not statistically significant (p > 0.05). In contrast, prevalence varied significantly among districts (p < 0.05), ranging from 20% to 100%. Sequencing analysis revealed two haplotypes of Plasmodium. Phylogenetic analysis showed that all isolates clustered within Clade A and shared 99%–100% nucleotide identity with Plasmodium juxtanucleare. Haplotype I was dominant and detected in multiple chicken breeds, whereas Haplotype II formed a minor lineage closely related to P. relictum.Conclusion:This study confirms the hyper-endemic circulation of P. juxtanucleare in Kalasin Province and demonstrates genetic diversity within the local parasite population based on COXIII gene analysis. Similar infection rates in free-range and intensive systems indicate that environmental vector exposure plays a major role in transmission. These findings highlight the importance of molecular surveillance and vector control strategies and provide baseline data for future One Health studies on avian haemosporidian infections in Thailand.

Avian haemosporidian diversity and transmission across birds and mosquitoes in Botswana.

Abstract After expanding their range from the Netherlands to Germany, introduced Egyptian Geese ( Alopochen aegyptiaca , Tadornini) started to breed in Germany in 1981. Egyptian Geese are well-adapted to an aquatic lifestyle, which brings them into close contact with some of the vectors of avian malaria, such as biting insects in the order Diptera. As research on avian malaria in Anseriformes remains scarce in Europe, the prevalence and diversity of haemosporidians in Egyptian Geese were analyzed in 62 blood samples taken from 14 adult and 48 juvenile birds in Stuttgart, southwestern Germany. Blood was collected during the annual capture of family groups. The samples were tested for Plasmodium , Haemoproteus and Leucocytozoon using the nested PCR protocol developed by Hellgren et al. (2004). Surprisingly, none of the sampled geese tested positive for haemosporidian parasites, despite infections being observed in Egyptian Geese in their native range.

Abstract Goose farming is gaining importance due to the economic value of meat and by-products; however, despite frequent reports of Plasmodium spp. infections in waterfowls, data on systemic biochemical responses in domestic geese remain limited. This study investigated biochemical alterations associated with natural Plasmodium spp. infection in Anser anser domesticus , focusing on acute phase proteins (APPs), thiol–disulfide homeostasis (TDH), and serum protein profiles. Blood samples were collected from geese with Plasmodium infection defined by combined microscopic examination and PCR (n = 35) and from uninfected geese (n = 20) Serum levels of APPs [Serum amyloid A (SAA), haptoglobin (Hp), total sialic acid (TSA)], TDH parameters [Native thiol (NT), total thiol (TT), disulfide (DS)], and serum proteins (total protein, albumin, globulin) were analyzed. Plasmodium -infected geese showed significantly higher levels of APPs than uninfected geese, indicating a strong acute phase response. Meanwhile, thiol components (TT, NT, DS) were significantly lower in Plasmodium -infected geese, reflecting infection-associated oxidative stress. However, redox ratios remained stable, suggesting preserved redox balance through compensatory antioxidant mechanisms. No significant differences were observed in serum protein concentrations. Correlation analysis revealed that Hp positively correlated with DS and negatively with NT/TT%, highlighting an interaction between inflammation and oxidative imbalance. Principal component analysis (PCA) showed that multivariate differentiation between infected and uninfected geese was mainly associated with protein-related and thiol-based redox parameters. In conclusion, Plasmodium infection triggers a distinct inflammatory and oxidative response in domestic geese, without affecting total serum protein levels. The combined use of APPs and TDH markers provides a promising non-invasive approach for monitoring infection-related physiological responses in avian malaria, supporting their potential utility in veterinary practice.

Over the past decades, the widespread use of pesticides has raised concerns about their effects on non-target organisms. Among pesticides, fungicides remain comparatively understudied, despite representing nearly 40% of total pesticide sales in Europe. Wild birds living in or close to farmlands are frequently exposed to tebuconazole, a triazole fungicide widely used in agroecosystems. While its effects on avian physiology and life-history traits have been the subject of several studies, the downstream effects that this agrochemical may exert on host-parasite interactions remain poorly understood. Here, we investigated whether chronic exposure to tebuconazole alters the dynamics of avian malaria chronic infection in house sparrows (Passer domesticus). Wild-caught adults were maintained under semi-natural conditions and malaria infection was monitored before and after an eleven-week exposure period to tebuconazole ingestion, starting in late autumn and ending in mid-winter. At the beginning of the experiment, 70% of the birds were chronically infected with malaria. Then, while prevalence declined in control individuals during winter, as typically expected in passerines living in temperate climates, it increased by 20% in tebuconazole-exposed individuals. No significant difference in parasite load was observed between control and exposed birds, suggesting that tebuconazole did enhance parasite replication rate but may instead have promoted the reappearance of parasites from exo-erythrocytic stages. Our findings highlight that exposure to tebuconazole can influence chronic malaria dynamics. Future work should assess how combined exposure to multiple pesticides and environmental stressors might exacerbate these effects, with broader implications for host-parasite interactions in agricultural landscapes.

Abstract Avian malaria parasites and related haemosporidians include a highly diverse group of parasites infecting wild birds worldwide, with variable knowledge depending on bird taxa. Although comparatively less studied than terrestrial species, avian malaria infections in seabirds are generally rare, a pattern attributed to several non-mutually exclusive hypotheses, including limited exposure to vectors and differences in host life history traits. Here, we screened blood parasites by combining molecular detection and observation of blood smears in the highly endangered Bermuda Petrel (Cahow) Pterodroma cahow, sampled during the 2022 breeding season (January–April). In addition, we quantified the blood cell types to provide baseline information on the immunological status of this species. Furthermore, few blood samples of the White-tailed Tropicbird Phaethon lepturus catsbyii breeding in sympatry on the same archipelago were also analyzed for parasite presence. Our results support the absence of blood parasites of the genera Plasmodium , Haemoproteus , and Leucocytozoon as well as filaroid nematodes. The absence of blood parasite infections in both species may reflect low exposure to vectors in offshore marine and terrestrial environments, although other explanations, such as the absence of parasites able to infect these host species, are also possible. We found no significant differences in the relative abundance of monocytes among islets, although differences were observed, and a significantly higher total abundance of leucocytes in female Bermuda Petrels than males. Differential parasite exposure among islets and differential life history traits between sexes could account for these results. Negative records of parasite infections in wild birds, including endangered species, are essential for accurately characterizing global infection patterns and understanding host–parasite associations in avian communities.

Abstract During an infection, the genetic composition of the pathogen population in the host might range from clonal to genetically diverse, depending on the genetic variation of the inoculum, mutation events, genetic drift, and host selection during the course of the infection. Variation in the genetic diversity of avian malaria parasites, both within and between individual hosts, is not well documented. To shed light on this, we used avian malaria to investigate how changes in the predominant haplotype varied between infection events and also within single infections over time. This was done by experimentally inoculating nine birds with the blood of one bird infected with Plasmodium relictum (lineage SGS1), and sequencing 1.03 Mb (13%) of the parasite exome before and after peak infection. In each infected bird, different haplotypes became established and changed in relative frequency over time. Several birds did not effectively suppress parasitemia, and we identified exclusive non-synonymous SNPs in gene families with functions related to immune evasion and host cell invasion in those infections, suggesting that some haplotypes might be harder for some individual hosts to suppress than others. We speculate that these haplotypes existed at low relative frequencies in the donor bird, but only increased in frequency (i.e., became runaway haplotypes) in a subset of susceptible host individuals. Alternatively, those SNPs might represent novel mutations that appeared independently in only a subset of host individuals. Deeper sequencing of the initial inoculum and the infections in all host individuals would be required to distinguish between these two hypotheses.

Avian malaria is a globally distributed vector-borne infectious disease caused by haemosporidian parasites capable of driving population declines and even species extinctions, posing major challenges for conservation biology. However, its occurrence in scavenger birds, particularly vulture species, remains poorly understood. We conducted active monitoring in northeastern and central Spain, collecting 383 blood samples from free-living birds of all age classes, including obligate (all European vultures) and facultative (red and black kites) scavenger species, to assess the occurrence of avian malaria parasites and identify their lineages using a nested-PCR protocol. Overall haemosporidian prevalence was 3.4% (13/383), with Leucocytozoon and Plasmodium detected at values of 2.3% and 1.6%, respectively. Among positive birds, Leucocytozoon spp. (69.2%) were more common than Plasmodium spp. (46.1%), including two co-infected individuals. Red kites, cinereous, bearded, and Egyptian vultures harbored both Plasmodium and Leucocytozoon, while black kites and griffon vultures only showed Plasmodium and Leucocytozoon, respectively. Black kites exhibited the highest haemosporidian prevalence (8.3%), followed by cinereous vultures (5.3%), red kites (4.8%), Egyptian (4.6%), bearded (1.9%), and griffon vultures (1.3%). Interestingly, we report 10 new host-parasite interactions and describe a novel Leucocytozoon lineage (lGYPBAR01) infecting all European vultures except the griffon vulture. We also detected Plasmodium relictum pSGS1 in an adult red kite, the first confirmed occurrence of this lineage in adults of the species. Our findings highlight a previously overlooked threat to avian scavengers and suggest that global warming may further facilitate the spread of haemosporidian-infected vectors, underscoring the urgent need for targeted conservation measures and further research into parasite dynamics in vulnerable wild populations.

Two aspects of host infectiousness shape pathogen transmission and distribution but are underappreciated: the relationship between pathogen load and infectiousness, and variability in pathogen load within species. We quantified the relationship between host pathogen load (parasitemia) for avian malaria (Plasmodium relictum) and infectiousness for biting Culex quinquefasciatus mosquitoes with experimental infections in canaries (Serinus canaria). Using this relationship, we estimated the infectiousness of 17 bird species in 11 communities in Hawai'i and quantified the relative contributions of infection stage (acute versus chronic) to transmission. We show that infectiousness to mosquitoes increased with parasitemia, temperature, and time since feeding. The relationship's gradual (low) parasitemia slope resulted in a wide range of parasitemias being partly infectious, and high within-host species variability in parasitemia led to extensive overlap in infectiousness among hosts. Disproportionate mosquito host utilization (inferred from relative infection prevalence) elevated the importance of a few host species, yet broad overlap in species infectiousness resulted in similar total infectiousness across most bird communities. This similarity likely contributed to avian malaria's widespread distribution throughout Hawai'i despite diverse host community assemblages. Our findings highlight the importance of both the shape of the pathogen load-infectiousness relationship and within-species variability in determining a pathogen's host range, transmission intensity, and spatial spread.