Avian Malaria in Wild Birds: Vector-Borne Transmission and Diagnostic Tools

Avian malaria is a vector-borne disease caused by protozoan parasites of the genus Plasmodium (phylum Apicomplexa, order Haemosporida). These parasites infect erythrocytes and other tissues of avian hosts, leading to pathologies ranging from subclinical carriage to acute mortality. The disease is transmitted exclusively by mosquitoes (family Culicidae). In wild bird populations, Plasmodium infections can alter host fitness, influence population dynamics, and pose a significant threat to conservation efforts, particularly for naive species on islands [1, 2]. This article provides a detailed examination of vector-borne transmission mechanisms and the panel of diagnostic tools available for detecting avian malaria in wild birds.

Vector-Borne Transmission Cycles

The transmission of avian malaria requires competent mosquito vectors that ingest Plasmodium gametocytes during a blood meal from an infected bird. Within the mosquito midgut, fertilization occurs, and the resulting ookinete penetrates the gut wall to form an oocyst. Sporozoites develop within oocysts and migrate to the salivary glands, from which they are injected into a new avian host during subsequent feeding. The extrinsic incubation period depends on temperature, humidity, and parasite species.

The primary vectors belong to genera Culex, Aedes, Anopheles, and Culiseta. In New Zealand, for example, native Culex pervigilans and introduced Aedes notoscriptus have been shown to carry Plasmodium DNA in their thoraxes, indicating potential vector competence [3]. In the Hawaiian Islands, Culex quinquefasciatus is the principal vector of Plasmodium relictum (lineage GRW4). Experimental transfection of Wolbachia wAlbB into this mosquito did not alter its vector competence, demonstrating the robustness of the transmission system [4].

Seasonal and climatic factors modulate transmission intensity. In the Thousand Island Lake system in China, infection intensity peaks in early summer and during the overwintering season, coinciding with host breeding and migration [5]. Wintering migratory birds often show higher prevalence, likely reflecting infections acquired at breeding grounds in colder climates where some Plasmodium lineages, such as Plasmodium circumflexum, are adapted [6].

Avian malaria parasites are not uniformly distributed across hosts. A global meta-analysis estimated a pooled worldwide prevalence of 16% in wild birds, with higher rates in adults and migratory species [7]. In the Republic of Korea, a study of 1043 rescued wild birds found a prevalence of 7.19%, with 30 distinct Plasmodium lineages identified [6]. In a tropical dry forest of Guatemala, prevalence reached 38%, with Plasmodium more common than Haemoproteus [8]. These figures underscore the importance of accurate diagnostics for both surveillance and conservation management.

Diagnostic Tools for Avian Malaria

Diagnosis of avian malaria relies on three principal approaches: microscopic examination of blood smears, molecular detection of parasite DNA, and serological assays. Each method has distinct strengths and limitations.

Microscopic Examination of Blood Smears

Light microscopy of Giemsa-stained thin and thick blood smears remains the classical method for diagnosing avian malaria. Thin smears allow species identification based on the morphology of intraerythrocytic stages (trophozoites, schizonts, gametocytes) and the presence of hemozoin pigment. Thick smears increase sensitivity for low-level parasitemia.

In Colombian wild birds, microscopy detected Plasmodium spp. in 32.6% of 46 individuals, with coinfections by Haemoproteus and Leucocytozoon [9]. However, microscopy underestimates true prevalence, especially in species or individuals with low parasitemia. A study in Hawai`i found that most qPCR-positive birds had cycle threshold (Ct) scores >= 35, which corresponded to parasitemia levels rarely visible by microscopy [10]. For the introduced Japanese white-eye (Zosterops japonicus), microscopy missed up to 80% of infections, leading to severe underestimation of prevalence [10]. Therefore, microscopy alone is insufficient for epidemiological surveys.

Molecular Detection: Conventional and Quantitative PCR

Polymerase chain reaction (PCR) targeting the mitochondrial cytochrome b (cytb) gene is the gold standard for sensitive and specific detection of avian Plasmodium. Nested PCR protocols amplify a 479-base pair fragment that can be sequenced for lineage assignment. This method has been applied globally: in Portugal, a wildlife rehabilitation center used nested PCR to detect haemosporidians in 31% of dead passerine birds [11]; in Japan, 21.1% of injured wild birds tested PCR-positive [12]; and in Brazil, novel lineages were discovered in Atlantic Rainforest birds using molecular techniques [13].

Quantitative PCR (qPCR) allows estimation of parasitemia by comparing Ct values to standard curves. This is particularly useful for monitoring infection intensity over time and for assessing disease severity. Linking qPCR data to microscopy estimates, Seidl et al. [10] generated predictive models that corrected historical prevalence data. For example, native Hawaiian honeycreepers (Himatione sanguinea) had higher average parasitemia and were thus more likely to be detected by microscopy than introduced species with low parasitemia. Such modeling is essential for interpreting long-term datasets.

The MalAvi database provides a curated repository of cytb lineages, enabling the identification of known and novel haemosporidian sequences [13, 14]. For instance, Plasmodium relictum lineage GRW11 was identified in a fatal case of avian malaria in a feral pigeon in Switzerland [15], while lineage pCOLL7 (Plasmodium relictum) was experimentally transmitted to Eurasian siskins [14]. These molecular tools have also revealed associations between Plasmodium and Matryoshka RNA viruses (MaRNAVs), opening new avenues for studying parasite-virus interactions [14].

Serological and Immunohistochemical Methods

Serology can detect past or current exposure to Plasmodium. Enzyme-linked immunosorbent assays (ELISA) for detecting anti-Plasmodium antibodies are less commonly used in wild birds compared to PCR due to the lack of species-specific reagents. However, a promising approach involves detecting antibodies against the carbohydrate alpha-Gal, which are generated in response to gut microbiota and correlate with resistance to avian malaria [16]. This method is not yet standard but offers a non-molecular tool for field studies.

Immunohistochemistry using anti-P. falciparum HSP70 antibody cross-reacts with avian Plasmodium antigens in tissue sections. Muchaamba et al. [15] used this antibody to confirm Plasmodium infection in lung, liver, and spleen of a pigeon with fatal avian malaria. Chromogenic in situ hybridization (CISH) with genus-specific probes effectively visualizes exoerythrocytic stages (phanerozoites) in organs [17] and can distinguish between Plasmodium, Haemoproteus, and Leucocytozoon [11].

Comparative Summary of Diagnostic Methods

Conservation Implications

Avian malaria is a major threat to endangered bird species, especially on islands where naive populations have evolved in the absence of the parasite. In Hawaii, the introduction of *P. relictum* and its mosquito vector drove multiple honeycreeper species to extinction or severe decline. However, some populations of Hawaii `Amakihi (Hemignathus virens) have evolved tolerance, maintaining high parasitemia without mortality [2]. Management strategies include vector control through sterile insect techniques and genetic modification of mosquitoes, though the latter approach has not yet achieved reduction in vector competence [1, 4].

Illegal wildlife trade also facilitates the spread of avian malaria. In Peruvian Amazonas, 18.5% of trafficked white-winged parakeets harbored P. relictum GRW04, a lineage known to cause population declines when introduced to new regions [18]. Such findings stress the need for health surveillance at wildlife trade checkpoints.

Zoo populations are similarly at risk. In Thailand, malaria outbreaks in Humboldt penguins (Spheniscus humboldti) have been documented, and haemosporidian infections are common in captive avian collections [19]. Monitoring using sensitive molecular tools is essential for early detection and treatment.

Diagnostic Workflow for Wild Bird Surveillance

The following Mermaid diagram illustrates a recommended diagnostic workflow for avian malaria surveillance in wild bird populations.

flowchart TD
    A[Blood sample from wild bird], > B{Immediate field conditions?}
    B, >|Yes - blood smear| C[Prepare thin and thick smears<br>Fix with methanol, stain with Giemsa]
    B, >|No - store sample| D[Collect blood in EDTA or ethanol<br>Transport to laboratory]
    C, > E[Microscopic examination<br>Identify Plasmodium stages]
    D, > F[DNA extraction<br>Nested PCR cytb amplification]
    E, > G{Parasitemia > 0.1%?}
    G, >|Yes| H[Species ID by morphology]
    G, >|No| F
    F, > I[Gel electrophoresis]
    I, > J[Positive band?]
    J, >|Yes| K[Sequence PCR product<br>Assign lineage via MalAvi]
    J, >|No| L[Report negative<br>Consider qPCR if suspicion high]
    K, > M[Quantify by qPCR if needed]
    M, > N[Parasitemia estimate]
    N, > O[Integrate with host data<br>Prevalence, intensity, lineage]
    L, > O
    H, > O

This workflow emphasizes that microscopy and molecular methods are complementary. For large-scale surveillance, especially in migratory or endangered species, PCR-based methods are preferred due to higher sensitivity. Sequencing of positive amplicons is essential for lineage identification and for detecting novel or invasive strains.

Conclusion

Avian malaria in wild birds is maintained by complex vector-host-parasite interactions that are modulated by climate, migration, and human activity. Accurate diagnosis is the cornerstone of understanding these dynamics and informing conservation interventions. While traditional microscopy remains valuable for morphological identification and parasitemia estimation, molecular tools, particularly nested PCR and qPCR, provide superior sensitivity and the ability to characterize parasite diversity. Emerging techniques such as qPCR-based parasitemia modeling and immunohistochemistry enhance our ability to detect subclinical infections and tissue pathology. The combination of these diagnostic methods, applied within a structured workflow, is essential for monitoring avian malaria in wild bird populations and for mitigating its impact on vulnerable species.

References

[1] Miranda Paez, A., Chalkowski, K., Zohdy, S., et al. "Management of avian malaria in populations of high conservation concern." Parasites & Vectors, 2022. URL: https://www.semanticscholar.org/paper/b1bc7c003572691e2a461008d86950fd182c656e

[2] Atkinson, C., Saili, K.S., Utzurrum, R.C.B., et al. "Experimental Evidence for Evolved Tolerance to Avian Malaria in a Wild Population of Low Elevation Hawai‘i ‘Amakihi (Hemignathus virens)." EcoHealth, 2013. URL: https://www.semanticscholar.org/paper/9baba589cceaac57620e5abe1571944354772814

[3] Schoener, E., Tompkins, D., Howe, L., et al. "New insight into avian malaria vectors in New Zealand." Parasites & Vectors, 2024. URL: https://www.semanticscholar.org/paper/abc390da542dc2fbeb229ee40dc50ea96adb480e

[4] Kilpatrick, A.M., Seidl, C.M., Ipsaro, I.J., et al. "Transinfection of Wolbachia wAlbB into Culex quinquefasciatus mosquitoes does not alter vector competence for Hawaiian avian malaria (Plasmodium relictum GRW4)." bioRxiv, 2024. URL: https://www.semanticscholar.org/paper/cd7cd1e06bd3fd9b670d4d79b09090fcefb304af

[5] Han, Y., Hellgren, O., Wu, Q., et al. "Seasonal variations of intensity of avian malaria infection in the Thousand Island Lake System, China." Parasites & Vectors, 2023. URL: https://www.semanticscholar.org/paper/bd375fe9e475707706c01d9ec076c4d6c82fe104

[6] Kim, M.C., Wut Hmohn, Z.Z., Han, J.I. "The Prevalence and Genetic Diversity of Avian Malaria in Wild Birds in the Republic of Korea." Animals, 2025. URL: https://www.semanticscholar.org/paper/cd3a2d1b5fd64aac538f44b76768893e6de3184d

[7] Yan, W.L., Sun, H.T., Zhao, Y.C., et al. "Global prevalence of Plasmodium infection in wild birds: A systematic review and meta-analysis." Research in Veterinary Science, 2024. URL: https://www.semanticscholar.org/paper/0e75d8145445bc35df04b5067d1b17deb686e588

[8] Bustamante-Castillo, M., De León-Robles, E., Barrios-Izás, M., et al. "Avian malaria in wild birds within a tropical dry forest area in Guatemala." Neotropical Biology and Conservation, 2025. URL: https://www.semanticscholar.org/paper/a99bca46dd208be2c3e6540cf66f7d996fff4d02

[9] Arroyave Pérez, A.O., Cifuentes-Rincón, A., Roldán Carvajal, I.J., et al. "A morphological survey of avian Haemosporida in Colombian wild birds." Revista Colombiana de Ciencias Pecuarias, 2025. URL: https://www.semanticscholar.org/paper/765317a6f889f8d94e890c4339fc3a2a1498d2a5

[10] Seidl, C.M., Ferreira, F.C., Parise, K.L., et al. "Linking avian malaria parasitemia estimates from quantitative PCR and microscopy reveals new infection patterns in Hawai'i." International Journal of Parasitology, 2023. URL: https://www.semanticscholar.org/paper/7fa19caae7df4f8dd92b8a8910397c905e26e4cb

[11] Himmel, T., Harl, J., Matt, J., et al. "A citizen science-based survey of avian mortality focusing on haemosporidian infections in wild passerine birds." Malaria Journal, 2021. URL: https://www.semanticscholar.org/paper/c09d3f2e3c5855134afa95e6b720e98b61a0ca9d

[12] Inumaru, M., Murata, K., Sato, Y. "Prevalence of avian haemosporidia among injured wild birds in Tokyo and environs, Japan." International Journal for Parasitology: Parasites and Wildlife, 2017. URL: https://www.semanticscholar.org/paper/831572651817c31910b0f08179b0d17576bfbba6

[13] Jardim, T.H., Rossi, M., Santos, H., et al. "Avian malaria in birds from Atlantic Rainforest: a record of Plasmodium cathemerium and other novel lineages." Anais da Academia Brasileira de Ciências, 2025. URL: https://www.semanticscholar.org/paper/ed2d43405191ab8c2a4daf9eccbeaf3ccbc22f3c

[14] Valkiūnas, G., Iezhova, T., Chagas, C., et al. "New experimental insights into the biology of avian malaria parasites." Malaria Journal, 2026. URL: https://www.semanticscholar.org/paper/e05652f1fa737fe7aed1dab5609b5005ba429e45

[15] Muchaamba, G., Venugopal, K., Gaechter, B., et al. "Avian malaria in a feral-pet pigeon: a case report." Malaria Journal, 2024. URL: https://www.semanticscholar.org/paper/b29ce3682c8193b0cc90b342e5632e0a7348c9e4

[16] Palinauskas, V., Mateos-Hernández, L., Wu-Chuang, A., et al. "Exploring the Ecological Implications of Microbiota Diversity in Birds: Natural Barriers Against Avian Malaria." Frontiers in Immunology, 2022. URL: https://www.semanticscholar.org/paper/f4460ec3592883b9aa41f2d8cfa2881fdb051626

[17] Ilgūnas, M., Bukauskaitė, D., Palinauskas, V., et al. "Mortality and pathology in birds due to Plasmodium (Giovannolaia) homocircumflexum infection, with emphasis on the exoerythrocytic development of avian malaria parasites." Malaria Journal, 2016. URL: https://www.semanticscholar.org/paper/4ed495d0347c0e21a40eb20ff74439c461547353

[18] Marzal, A., Magallanes, S., Salas-Rengifo, T., et al. "Prevalence and diversity of avian malaria parasites in illegally traded white‐winged parakeets in Peruvian Amazonas." Animal Conservation, 2023. URL: https://www.semanticscholar.org/paper/483421c6d3987f03dfe054fe80a144f05bf0499c

[19] Rahmah, S.A., Jairak, W., Thanee, S., et al. "Malaria outbreaks in Humboldt penguins and haemosporidian infections in avian hosts in zoos across Thailand." Current Research in Parasitology & Vector-Borne Diseases, 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41626102/

[20] Gomes, J., Leitão, M., Louro, M., et al. "Avian Malaria in wild birds from a wildlife rehabilitation center in Central Portugal." Veterinary Parasitology: Regional Studies and Reports, 2023. URL: https://www.semanticscholar.org/paper/46842636bf0ee997a2711bec161899f98c977f8e

[21] Yuda, P. "Detection of avian malaria in wild birds at Trisik Beach of Yogyakarta, Java (Indonesia)." Annals of Parasitology, 2019. URL: https://www.semanticscholar.org/paper/1362653d62486876b89b0bd4af97568f51611f49

[22] Fix, A., Waterhouse, C., Greiner, E., et al. "PLASMODIUM RELICTUM AS A CAUSE OF AVIAN MALARIA IN WILD-CAUGHT MAGELLANIC PENGUINS (SPHENISCUS MAGELLANICUS)." Journal of Wildlife Diseases, 1988. URL: https://www.semanticscholar.org/paper/d836b7468e20aa252de3f3ebc8a1dfd1bf02aea6

[23] Dominoni, D.M., Albalawi, B., Mladenova, M., et al. "Phenology underlies apparent urbanisation effects on avian malaria in juvenile songbirds." bioRxiv, 2025. URL: https://www.semanticscholar.org/paper/9f83bbad56fde279a42f9e6fd0c6daf340fea7c0

[24] Yuda, P. "High Prevalence Level of Avian Malaria in the Wild Population of the Java Sparrow." Biota, 2019. URL: https://www.semanticscholar.org/paper/44a185da046401f25082b6b43961a06b0d156f75

[25] Rivero de Aguilar, J., Barroso, O., Bonaccorso, E., et al. "Associations among MHC genes, latitude, and avian malaria infections in the rufous‐collared sparrow (Zonotrichia capensis)." Ecology and Evolution, 2024. URL: https://www.semanticscholar.org/paper/2e7a47edcf4305b5859e37aff4cb979ae2415c3f

[26] Dos Anjos, C.C., Damato, B.A., Ferioli, R.B., et al. "A new Plasmodium Marchiafava & Celli, 1885 (Apicomplexa: Haemosporida) species in Cory's Shearwater (Calonectris borealis) stranded in a coastal area in Brazil." Systematic Parasitology, 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42142167/

[27] Podmokła, E., Pluciński, B., Dubiec, A. "Avian Malaria Infections Are Not Associated With Host Genetic Diversity: A Meta-Analysis." Molecular Ecology, 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42141800/

[28] Pigeault, R., Bichet, C., Bellot, P., et al. "Chronic exposure to a common azole fungicide is associated with reactivation of chronic Plasmodium infections in farmland birds." International Journal for Parasitology: Parasites and Wildlife, 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42006681/

[29] Garrigós, M., Jiménez-Peñuela, J., Saavedra, I., et al. "Interactions between urbanization, malaria infection and avian cloacal microbiome." Environmental Research, 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41740703/

[30] Oliva-Vidal, P., Muriel, J., Marzal, A., et al. "Unraveling Occurrence Patterns and Diversity of Avian Malaria Parasites in Iberian Obligate and Facultative Scavenger Birds." Integrative Zoology, 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41684191/