Hidden Nematodes Reveal the Parasite Secrets of an Argentine Mountain River

Parasites are among the most successful and widespread forms of life on Earth, yet they remain chronically overlooked in conservation planning and ecosystem management. A new study published in Acta Parasitologica has delivered the first comprehensive ecological survey of endohelminth parasites infecting freshwater fishes in northwestern Argentina, focusing on a mountain tributary of one of the country’s most important river basins. The research, led by Lorena G. Ailán-Choke of the Centro de Ecología Aplicada del Litoral (CONICET) and the Universidade Estadual Paulista in Brazil, together with Fabiana Cancino and Geraldine Ramallo of the Fundación Miguel Lillo in Tucumán, examined nearly a thousand fish specimens and revealed a surprisingly simple but ecologically telling parasitic world.

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