The fieldwork formed part of a larger bio-surveillance initiative called CamBatRat, which sampled 2,461 bats at 233 sites across Cambodia between December 2016 and February 2020. From this archive, the team selected 1,180 individuals representing 34 species for retrospective screening of blood samples. DNA was extracted and initially tested using a polymerase chain reaction targeting a fragment of the parasites’ mitochondrial cytochrome b gene. Positive samples were then characterized with additional markers spanning the parasites’ three genomes: mitochondrial genes cytb and cox1, an apicoplast marker called clpC, and two nuclear loci, ef2 and pat. This multilocus approach matters because some bat haemosporidians carry highly rearranged mitochondrial genomes that can defeat standard amplification protocols, and because different genes evolve at different rates, offering complementary windows into parasite evolutionary history.

Sequence-confirmed infections turned up in 152 bats belonging to eleven species from four families: Emballonuridae, Hipposideridae, Pteropodidae and Vespertilionidae. Two parasite genera dominated the findings. Polychromophilus, a genus restricted to bats, was confined to a handful of host species, infecting the vespertilionids Scotophilus kuhlii, Scotophilus heathii and Myotis rosseti as well as the hipposiderid Hipposideros diadema. Prevalence differed dramatically between the two Scotophilus species: 54 percent of S. kuhlii were infected, compared with just 4 percent of S. heathii. Hepatocystis, a genus closely related to mammalian Plasmodium, showed a broader footprint, occurring across three bat families and infecting, among others, 42 percent of Cynopterus brachyotis and 23 percent of Hipposideros cf. grandis individuals examined.

The most taxonomically significant discovery came from the emballonurid bat Taphozous melanopogon, in which 18 percent of sampled individuals carried Hepatocystis. This is the first record of the genus in the family Emballonuridae, substantially expanding its known host range. Because emballonurid bats are widely distributed across the Old World yet rarely documented as haemosporidian hosts, the finding suggests that parasite diversity in this family has gone largely unnoticed. Infections were also newly recorded in the fruit bats Megaerops niphanae and Eonycteris spelaea. By contrast, no infections were detected in bats from the families Molossidae, Megadermatidae and Rhinolophidae, although the authors caution that the molossid sample was spatially concentrated and therefore less conclusive.

Phylogenetic analyses revealed two contrasting evolutionary patterns. The Polychromophilus lineage associated with Scotophilus bats proved remarkably genetically conserved: haplotype networks built from cytb, cox1, clpC and ef2 sequences showed identical or near-identical parasites in hosts from Cambodia, Thailand and Madagascar, thousands of kilometres apart. This lineage forms a well-supported monophyletic group that does not cluster with any currently recognized Polychromophilus species, hinting that it may represent a distinct evolutionary lineage with taxonomic significance. Such extreme genetic uniformity across such a vast range could reflect either recent dispersal or long-term evolutionary conservatism, and in either case points to strong host specificity at the level of the host genus.

Hepatocystis told a very different story. Parasites from Cambodian bats fell into several deeply divergent clades, and lineages infecting hipposiderid bats did not even form a single group. Most strikingly, cytb sequences from the Taphozous-associated parasites formed a well-supported clade with Hepatocystis lineages infecting Southeast Asian squirrels of the genus Callosciurus, despite substantial mitochondrial divergence between the host-associated groups. Analyses at additional loci placed the Taphozous lineage basal to other Hepatocystis taxa, though the authors note that interpretation is limited by the scarcity of comparable rodent-derived reference sequences. Taken together, the patterns suggest that the evolutionary history of Hepatocystis may involve multiple historical host transitions among mammalian groups, with Asian bats and rodents playing a more prominent role than previously appreciated.

Within the genus Cynopterus, the parasite picture was equally complex. Hepatocystis from C. brachyotis and C. sphinx did not form a single group but instead split into two deeply divergent clades differing by 30 to 41 nucleotide substitutions, with no clear structure by host species or geography. The clades also included parasites from Megaerops and Eonycteris hosts. This suggests that host specificity in Asian bat Hepatocystis may be looser than classical morphology-based taxonomy implies, or that recognized morphospecies conceal cryptic diversity, a possibility previously raised for African bat parasites. Because no blood smears were collected during the original sampling, the Cambodian parasites could not be confidently assigned to described morphospecies, underscoring the need to integrate morphological and molecular data in future work.

To understand what drives infection, the team fitted a Bayesian generalized linear mixed model to 589 individuals of ten host species from 84 sites, treating host species and sampling site as crossed group-level effects. The result was clear: host species accounted for 26.3 percent of the variance in infection probability and sampling site for 25.3 percent, while the measured covariates added essentially nothing. Knowing a bat’s species and capture location predicted infection as well as any combination of host traits, landscape attributes and climate variables. The strongest single effect was seasonal, with infection probability substantially lower in the wet season than the dry season, a direction that runs counter to a simple vector-abundance expectation, since the presumed Culicoides midge vectors should thrive when wet-season larval habitat is most available.

One landscape variable did stand out: infection probability declined with increasing human population density within a two-kilometre radius of the capture site. The authors interpret this as consistent with reduced availability of vector habitat in built-up areas, since Culicoides midges depend on aquatic and semi-aquatic larval sites and have limited dispersal ranges, making transmission highly sensitive to conditions close to the roost. Habitat fragmentation showed a weaker, statistically ambiguous negative trend, and land cover class at the site showed no association. Among individual host attributes, sex and body condition were unrelated to infection, while reproductive individuals showed a suggestive but inconclusive elevation in infection probability. Notably, residual variation among sites was not distance-structured, meaning that infection hotspots were not clustered spatially in any simple way.

The broader implications reach into both evolutionary biology and surveillance strategy. Because bats host parasites from at least seven genera, collectively representing roughly a third of all described mammalian haemosporidian species, they are a key system for understanding how malaria parasites have diversified across vertebrate hosts. The Cambodian data reinforce the view that diversification within Hepatocystis may have occurred substantially among Asian mammalian hosts, a hypothesis that will require broader sampling of Asian rodents and additional bat families to test. On the ecological side, the finding that infection is structured principally by host identity and fine-scale roost conditions rather than by landscape composition indicates that roost ecology and vector biology are the most promising targets for understanding transmission. Distinguishing among competing explanations for the seasonal pattern, whether vector phenology, temperature or shifts in host demography, will require paired vector sampling and bloodmeal analysis alongside host screening, work the current study design did not include but which the authors identify as the logical next step.

Subject of Research: Diversity, host associations and ecology of haemosporidian malaria parasites infecting bats in Cambodia

Article Title: High diversity and novel host associations of bat haemosporidian parasites in Cambodia: insights from a nationwide bat survey and multilocus analyses

Article References: Schaer, J., Werb, O., Ch´ng, L., Low, D. H. W., Borthwick, S. A., Furey, N., Frias, L., Pimsai, A., Yim, R., Chor, K., Cheang, D., Smith, G. J. D., Hitch, A., & Mendenhall, I. H. (2026). High diversity and novel host associations of bat haemosporidian parasites in Cambodia: insights from a nationwide bat survey and multilocus analyses. Parasites & Vectors, 19(1), Article 436. https://doi.org/10.1186/s13071-026-07732-3

Image Credits: AI Generated

DOI: 10.1186/s13071-026-07732-3

Keywords: bats, haemosporidian parasites, Hepatocystis, Polychromophilus, malaria, Cambodia, Southeast Asia, phylogenetics, host associations, Culicoides, parasite ecology, molecular survey

The scale of the survey is one of its strengths. Between 2019 and 2024, the team collected fish at two sampling points along the Tocantins River, one near the riverbank close to domestic effluent discharges and another closer to the main channel where the current runs stronger. Of the 262 hosts examined, 69 were Triportheus albus and 193 were Triportheus trifurcatus. The fish were caught with casting nets, transported to the Laboratory of Ecology and Limnology at the State University of the Tocantina Region of Maranhão, and necropsied so that their gills could be carefully removed. Parasites were dislodged using heated distilled water at approximately 65 degrees Celsius, fixed in 70 percent ethanol, and later mounted in Hoyer’s medium for microscopic examination of their sclerotized attachment structures, the hardened hooks and bars that taxonomists use to distinguish species.

What emerged from the gills was a strikingly diverse parasitic fauna. Nine species of Monopisthocotyla were recorded on Triportheus albus and thirteen on Triportheus trifurcatus, all belonging to the genera Anacanthorus, Ancistrohaptor, and Jainus, a group of dactylogyrid flatworms that cling to gill filaments with remarkable host specificity. Overall, 71.01 percent of T. albus and 76.17 percent of T. trifurcatus were parasitized, with mean intensities of 8.04 and 7.05 parasites per infected fish respectively. One genus stood out above all others: Jainus spp. showed the highest prevalence and mean abundance in both hosts, infecting 57.14 percent of T. albus and 62.89 percent of T. trifurcatus. This dominance of a single taxon would prove central to understanding how the two parasite communities differ in their internal organization.

That difference became clear when the researchers applied a battery of community ecology indices. Triportheus trifurcatus hosted the richer and more balanced community, with a Shannon-Wiener diversity index of 1.642, Pielou’s evenness of 0.640, and Simpson’s diversity of 0.743. Triportheus albus, by contrast, showed lower values on all three measures, and correspondingly higher dominance: its Simpson’s dominance index reached 0.443 and its Berger-Parker index, which measures the proportional abundance of the single most common species, hit 0.622, compared with just 0.427 in T. trifurcatus. In plain terms, the parasite community on T. albus is a lopsided affair in which Jainus spp. calls the shots, while T. trifurcatus supports a more democratic distribution of parasite species. The lesson, the authors argue, is that communities with similar total parasite loads can be organized in fundamentally different ways depending on how individuals are apportioned among taxa.

Species sharing between the two hosts added another layer of intrigue. Four parasite species were recorded on both fish, two occurred exclusively on T. albus, and six were found only on T. trifurcatus. Among the shared species, Anacanthorus chaunophallus and Ancistrohaptor spp. showed significantly higher prevalence in T. trifurcatus, with chi-square tests yielding p-values of 0.008 and below 0.001 respectively. This pattern of partial host specificity, in which closely related fish carry overlapping but distinct parasite faunas, is a hallmark of dactylogyrid flatworms in the Neotropics and suggests that even subtle differences in host biology, gill architecture, or ecology can shape which parasites establish persistent populations.

Perhaps the most consistent finding of the entire study was the aggregated distribution of nearly every parasite population. Seven of the nine species recorded on T. albus and nine of the thirteen on T. trifurcatus showed statistically significant aggregation, meaning that most parasites were concentrated in a minority of host individuals rather than spread evenly through the fish population. Jainus spp. displayed the strongest clustering of all, with a dispersion index of 8.91 in T. albus and 4.65 in T. trifurcatus. Aggregation is one of the most universal patterns in parasite ecology, generally attributed to variation in host susceptibility, differences in exposure, or localized environmental conditions that favor transmission. For schooling fish like Triportheus, dense aggregations of hosts moving together likely amplify these effects, allowing the free-swimming larval stage, called an oncomiracidium, to encounter gills with high efficiency.

What the study did not find is equally telling. Neither host sex nor hydrological season nor sampling site had a significant effect on parasite abundance or richness. Prevalence in T. albus rose from 71.21 percent in the dry season to 100 percent in the rainy season, and mean abundance climbed from 5.88 to 8.00, yet the difference was not statistically significant. The same held for T. trifurcatus, whose mean abundance rose from 9.56 to 11.30 between seasons without reaching significance. Host sex showed no effect at all, with p-values ranging from 0.695 to 0.795 across tests. The only biometric signal was a weak but significant positive correlation between standard length and parasite richness in T. trifurcatus, a Spearman coefficient of 0.148 with p equal to 0.039, hinting that larger fish, with more gill surface area and longer cumulative exposure, accumulate slightly more parasite species over time.

The authors interpret this seasonal stability with a provocative hypothesis: the cascade of hydroelectric dams that now regulates the Tocantins River may be flattening the natural pulse of the river, and with it the seasonal dynamics of its parasites. In a free-flowing river, seasonal floods would dilute infective larvae and disperse host schools, creating predictable peaks and troughs in transmission. By damping those peaks, dam regulation could stabilize parasite-host interactions year-round, allowing continuous reproduction of Monopisthocotyla and constant levels of infestation. The researchers caution that absence of statistical significance does not prove absence of ecological effect, and they acknowledge a key limitation: only three T. albus individuals were collected during the rainy season, which sharply reduced the statistical power of the seasonal comparison for that species. Still, the idea that artificial flow regimes can restructure parasite communities is consistent with growing evidence from other impacted rivers and adds a new dimension to debates over the ecological costs of large dams in Amazonia.

Beyond their theoretical interest, these parasites may serve as sentinels of ecosystem health. Monopisthocotyla are ectoparasites with direct life cycles, no intermediate host, and intimate dependence on water quality, which makes their abundance and community structure sensitive to pollution, eutrophication, and hydrological change. The consistently high prevalence observed near Imperatriz, in a stretch of river receiving domestic effluents and subject to upstream dam regulation, may itself reflect chronic environmental stress. Because the Tocantins-Araguaia basin has been far less studied than the Amazon or Paraná systems, this work establishes a crucial ecological baseline against which future changes, whether from new dams, expanding agriculture, or climate shifts, can be measured. The study also underscores the value of combining population-level descriptors such as prevalence, intensity, and aggregation with community-level indices such as diversity, evenness, and dominance, since each scale reveals different aspects of how parasite assemblages are organized.

For a basin under intensifying anthropogenic pressure, where hydroelectric construction and altered flow regimes have already reshaped fish migrations and habitats, the humble gill worms of Triportheus offer something rare: a quantifiable, repeatable, and rapidly responding biological indicator. As the authors note, future studies with broader spatial coverage, deeper taxonomic resolution, and molecular tools such as DNA barcoding could refine these basins and resolve the species-level identities of taxa currently known only by genus. In the meantime, the message from the Tocantins is clear: to understand the health of a great river, sometimes the best instruments are the parasites riding on the gills of its smallest fish.

Subject of Research: Population and community ecology of Monopisthocotyla gill parasites infecting Triportheus fish in the Tocantins-Araguaia River basin, Brazil

Article Title: Population and Component Community Descriptors of Monopisthocotyla Parasitizing Triportheus spp. (Actinopterygii: Triportheidae) in the Tocantins-Araguaia River Basin, Brazil

Article References: Saraiva, D. D. J., da Costa, A. P., Viana, D. C., Nogueira, R. D. M. S., & Cohen, S. C. (2026). Population and Component Community Descriptors of Monopisthocotyla Parasitizing Triportheus spp. (Actinopterygii: Triportheidae) in the Tocantins-Araguaia River Basin, Brazil. Acta Parasitologica, 71(5), Article 208. https://doi.org/10.1007/s11686-026-01372-4

Image Credits: AI Generated

DOI: 10.1007/s11686-026-01372-4

Keywords: Monopisthocotyla, Triportheus, Tocantins-Araguaia basin, fish parasites, Dactylogyridae, parasite ecology, community structure, bioindicators, Neotropical fish, dams, Brazil, gill parasites

The setting is the Medina-Salas River, a torrential mountain stream that feeds into the Salí-Dulce endorheic basin, the largest closed basin in Argentina, stretching from the northwest of the country toward its center. The basin harbors approximately 93 fish species of both native and introduced origin, predominantly of Paranoplatense derivation, and within Tucumán Province alone around 66 native species have been recorded. Many of these fishes carry economic, recreational, and sanitary importance. Yet the upper basin is under intense pressure from agro-industrial activity, urbanization, and dams that sever the migratory routes of fish. Against this backdrop, the parasitic fauna of the Medina-Salas River had never been systematically explored, even though earlier work by Ramallo had described new nematode species from fishes in the wider region.

Between May 2019 and March 2021, the team sampled three sections of the lower Medina-Salas River, all situated at roughly 800 meters above sea level and within about three kilometers of one another, ensuring broadly comparable environmental conditions. Using hand nets, drag nets, and electrofishing equipment, they collected 951 individual fish representing seven host species: Bryconamericus iheringii, Jenynsia tucumana, Jenynsia lineata, Trichomycterus corduvensis, Trichomycterus alterus, Trichomycterus spegazzinii, and Heptapterus qenqo. Each fish was weighed, measured, and then dissected, with the body cavity and all organs examined under a stereomicroscope. Helminths recovered from the intestine or abdominal cavity were identified taxonomically using light microscopy, with nematodes cleared in lactophenol for detailed study. Voucher specimens were deposited in the helminth collection of the Fundación Miguel Lillo in San Miguel de Tucumán.

The results paint a picture of a parasitic community dominated overwhelmingly by nematodes. Of the 951 fish examined, 223 individuals, or 23.5 percent, were infected with at least one parasite taxon, yielding a total of 1,269 endoparasites. Six taxa were identified in all: five nematodes and a single cestode, an Eucestoda genus and species that could not be resolved further. Only one taxon, the nematode Contracaecum sp., was found in a larval stage; the remaining 99.4 percent of collected specimens were adults. Two of the adult nematodes, Rhabdochona acuminata and Spirocamallanus huacraensis, occurred in more than one host species, infecting five and two host species respectively, while the larval Contracaecum appeared in two hosts as well.

Abundance varied dramatically across host-parasite pairings. The single most abundant parasite was Rhabdochona fabianae, a nematode apparently specific to the characid fish Bryconamericus iheringii, with 670 specimens recovered. Close behind was Cucullanus pinnai in the heptapterid catfish Heptapterus qenqo, with 522 specimens. At the other extreme, larval Contracaecum in Jenynsia lineata was represented by a single individual, and Spirocamallanus huacraensis contributed only two specimens in each of its two Trichomycterus hosts. The nematode Cucullanus pinnai in H. qenqo stood out as the most successful parasite in the system, achieving a prevalence of 86.8 percent, a mean intensity of 7.9 worms per infected fish, and a mean abundance of 6.9 worms per fish examined, the highest values recorded for any taxon in the study.

Most parasite taxa displayed the classic aggregated distribution that characterizes macroparasite infections: a small number of host individuals carry the bulk of the parasites, while most hosts harbor few or none. This pattern was quantified using the discrepancy index, which ranges from zero, when all hosts carry similar parasite burdens, to one, when all parasites are concentrated in a single host. Interestingly, the two most abundant associations, Cucullanus pinnai in H. qenqo and Rhabdochona fabianae in B. iheringii, deviated from strong aggregation, suggesting a more even spread of infection across those host populations.

Ecological analyses were conducted at two levels: the component community, meaning all helminths within a host population, and the infracommunity, meaning all helminths within a single host individual. Across the board, richness and diversity were low. Component communities of B. iheringii and Trichomycterus alterus consisted of a single parasite species each, producing the lowest diversity values. The most diverse component community belonged to Jenynsia tucumana, followed by Trichomycterus corduvensis, although even these communities were weakly dominated by a single species, as reflected by Berger-Parker index values of around 0.6 compared with 0.9 in the more strongly dominated communities. At the infracommunity level, richness ranged from zero to two parasite taxa per fish, with most infected fish carrying only a single species. Species accumulation curves reached an asymptote for six of the seven host species, indicating sufficient sampling effort, but the curve for Jenynsia lineata did not plateau, suggesting its true parasite richness may be underestimated.

The composition of these communities offers a window into the trophic ecology of the hosts. Because most endohelminths have complex life cycles requiring two or more hosts, whether a fish harbors adult or larval parasites reveals its position in the food web. The larvae of Contracaecum, a genus whose adults mature in piscivorous birds, appeared only in the two Jenynsia species, and at low prevalences of 4.2 percent and 1.3 percent. These omnivorous livebearers feed on microcrustaceans such as copepods and amphipods as well as aquatic insect larvae, and the authors suggest that microcrustaceans serve as first intermediate hosts while the fish act as paratenic hosts that could pass the parasite onward to fish-eating birds. By contrast, the remaining host species carried only adult worms, consistent with their roles as definitive hosts feeding on infected invertebrate prey.

The transmission routes of the dominant nematodes fit this framework. Rhabdochona species are known to use mayflies as principal intermediate hosts, occasionally caddisflies or stoneflies, which explains infections in insectivorous fishes such as B. iheringii and the benthic catfishes of the genus Trichomycterus. Spirocamallanus species generally employ copepods as intermediate hosts, again implicating the invertebrate-rich diet of the catfishes. The striking prevalence of Cucullanus pinnai in Heptapterus qenqo, a species whose diet remains undocumented, may involve ingestion of odonate, mayfly, or megalopteran nymphs carrying larval stages, although previous work has hypothesized a heteroxenous cycle involving other fish as intermediate hosts. The authors call for integrative morphological and molecular studies to untangle the full life cycle. Notably, no digeneans were found at all, despite freshwater gastropods, potential first intermediate hosts, being recorded in the area, a puzzle the researchers attribute to the many biotic and abiotic factors governing trematode transmission.

The low richness and diversity observed across all communities mirror patterns reported in other Neotropical freshwater systems, though some comparable ecosystems host far richer parasite faunas, underscoring how strongly host ecology, diet, habitat use, and local availability of intermediate hosts shape transmission dynamics. Phylogenetically related and sympatric host species, such as the two Jenynsia and several Trichomycterus species, shared similar parasite communities, including the generalist R. acuminata, a pattern consistent with host switching over evolutionary time. Six new host-parasite records emerged from the survey. Because the Medina-Salas River faces ongoing disturbance from agriculture and aggregate extraction, the authors argue that this baseline dataset provides a critical reference point for future studies using parasite communities as indicators of environmental change in one of Argentina’s most ecologically significant, and most threatened, river basins.

Subject of Research: Ecological diversity of endohelminth parasites in freshwater fish from the Medina-Salas River, a tributary of the Salí-Dulce basin in northwestern Argentina

Article Title: Diversity of endohelminth parasites of fish from Sali-Dulce River tributary (Tucumán province, Northwestern Argentina)

Article References: Ailán-Choke, L. G., Cancino, F., & Ramallo, G. (2026). Diversity of endohelminth parasites of fish from Sali-Dulce River tributary (Tucumán province, Northwestern Argentina). Acta Parasitologica, 71(5), Article 213. https://doi.org/10.1007/s11686-026-01395-x

Image Credits: AI Generated

DOI: 10.1007/s11686-026-01395-x

Keywords: endohelminth parasites, freshwater fish, Nematoda, Salí-Dulce basin, Argentina, Neotropical region, parasite ecology, Tucumán, component communities, host-parasite interactions, Acta Parasitologica, biodiversity