Revista Argentina de Parasitología

First record of Contracaecum sp. (Nematoda: Anisakidae) larvae in Eunectes notaeus with a hypothesis on the role of yellow anaconda in the parasite's transmission

Mariané B. Mañez¹^,²*, Lucas E. Garbin¹^,³, Juan F. Draque⁴, Mariano Barros⁴, Patricio Micucci⁴, Tomas Wallers⁵, Julia I. Diaz¹^,²

INTRODUCTION

Anisakidae nematodes, particularly Contracaecum Railliet & Henry, 1912, are widely distributed parasites of aquatic ecosystems, infecting a broad range of vertebrate and invertebrate hosts worldwide (Anderson, 2000; Garbin et al., 2023). Although fishes constitute the main paratenic host, third-stage larvae of Contracaecum have also been reported encysted in amphibians and reptiles, indicating that these vertebrates may contribute to parasite transmission in aquatic food webs (Silva Trejo et al., 2025).

The yellow anaconda, Eunectes notaeus Cope, 1862, is a large semiaquatic snake widely associated with wetlands in South America (McCartney-Melstad et al., 2012). In Argentina, it is found in the provinces of Formosa, Chaco, Corrientes, northern Santa Fe, northern Entre Ríos, and southern Misiones (Micucci et al., 2006). Its broad generalist diet (Barros et al., 2011; Miranda et al., 2016; Waller et al., 2007) and dual role as both predator and prey integrate E. notaeus into multiple trophic pathways within wetland food webs, making it a potentially important paratenic host for parasites with indirect life cycles. Despite these ecological characteristics, parasitological and health records for E. notaeus remain remarkably scarce (Fernandes & Kohn, 2014; Mañez et al., 2026).

The present study documents for the first time the occurrence of Contracaecum third-stage larvae in adults of the yellow anaconda, E. notaeus, from Bañado La Estrella, Formosa Province, Argentina. Based on these findings we hypothesise about the role of semi-aquatic snakes as paratenic hosts in the transmission dynamics of anisakid nematodes within Neotropical wetlands.

MATERIALS AND METHODS

A total of 150 adult anacondas provided by the "Yellow Anaconda Management Program" (Programa Curiyú) were examined for parasites; therefore, no unnecessary deaths occurred during this research. The sampling was conducted in conjunction with the biological research campaigns of the Fundación Biodiversidad during June 2019, May 2021, June 2022, and May 2023. Each snake was categorised as an adult, defined as individuals with a snout-vent length > 160cm. Specimens were placed in a supine position and dissected by the hunters themselves along the midline, beginning at the ventral region of the jaw and continuing to the cloaca, following the skinning protocol established by the Management Program for each harvest season (Micucci et al., 2006). The coelomic cavity, serous membranes, mesenteries, and musculature were inspected for nematode larvae, which were preserved individually in 4% formalin or 70% alcohol.

The nematodes were counted, cleared in lactophenol, mounted on temporary slides, and observed with an Olympus BX51® optical microscope (Tokyo, Japan), photographed with a Q Imaging Go3 camera using the Q Capture Pro 6.0 software, and measured using a drawing tube. To obtain greater detail on certain external structures, three specimens were dehydrated in a gradual series of alcohols, dried using the critical-point technique, metallised, and mounted for proper observation of their external characteristics under a Jeol/SET 100® scanning electron microscope (SEM) (Davidovich, 2022; Santana et al., 2023). Measurements are expressed in millimetres (unless otherwise indicated), with the mean followed by the range in parentheses. Larvae identification followed the morphological criteria of Anderson (2000).

The parasitological descriptors prevalence (P), mean abundance (MA) and mean intensity (MI) were interpreted and calculated overall and discriminated for each year according to Bush et al. (1997). To further characterise the distribution of parasite intensity among infected hosts, the median and mode were also calculated. The 95% confidence interval (CI) for prevalence was estimated using the exact binomial method, while 95% CIs for mean abundance and mean intensity were obtained using bias-corrected and accelerated (BCa) bootstrap procedures with 2000 replicates. The median and mode were calculated only among parasitized hosts, as descriptors of infection intensity. The 95% CI for the median was estimated using a non-parametric method based on order statistics, with RStudio (version 2026.08) and the ParasiteR package (Furlan et al., 2026; Reiczigel et al., 2019).

To evaluate whether parasite prevalence varied significantly across sampling years (2019, 2021, 2022, and 2023), we performed a Chi-squared test of independence using a 4×2 contingency table, with year as the grouping variable and parasitized/non-parasitized individuals as the response categories. Statistical analyses were performed using RStudio, and statistical significance was set at p < 0.05.

Statement of Animal Ethics

The collection and use of biological samples were authorized by Resolution No. 40/15 of the Ministry of Production and Environment of Formosa, along with permits issued through provisions 0151/19, 050/21, 052/22, and 049/23. These permits established the requirements for accessing biological resources and their derivatives, under both ex situ and in situ conditions, within the jurisdiction of Formosa Province. All animals were treated following the Reference ethical framework for biomedical research: ethical principles for research on laboratory, farm, and wild animals (Consejo Nacional de Investigaciones Científicas y Técnicas [CONICET], 2005).

RESULTS

A total of 196 third-stage larvae of nematodes were found and identified as belonging to the genus Contracaecum.

Nematoda, Anisakidae Skrjabin & Karokhin, 1945
Contracaecum Railliet & Henry, 1912
Contracaecum sp. L3 (Figs.1-3)

Description. Based on 12 specimens. Larvae are creamy white, robust, and widest at the midbody. Total body length 18.56 (10.20-28.40); body width 0.6 (0.4-0.7); three rudimentary lips with a well-developed boring tooth; nerve ring from anterior end 0.27 (0.2-0.35), and body width at the level of the nerve ring 0.27 (0.24-0.45); excretory pore located at the base of the lips; oesophagus length 2.01 (1.30-2.60); intestinal caecum length 1.68 (0.90-2.00); ventricular appendix length 0.46 (0.38-0.56), tail length 0.13 (0.08-0.20) (excluding the mucron) (1% of body length). Oesophagus / body length ratio 10.83%, intestinal caecum / body length ratio 9.05%, ventricular appendix / body length ratio 2.48%, tail length/body length ratio 0.70%.

Host. Eunectes notaeus Cope, 1862.

Locality. Bañado La Estrella, Formosa, Argentina.

Site of infection. Larvae encysted in the serosa of the stomach.

Population parameters. Overall values and those for each year are presented in Tables 1 and 2. Parasite prevalence did not differ significantly among sampling years (χ² = 7.25, df = 3, p = 0.064).

Remarks. The larvae recovered from Eunectes notaeus agree closely with previous Contracaecum L3 descriptions in their general morphology, including the presence of a robust body, a well-developed intestinal caecum and ventricular appendix, a short conical tail bearing a terminal mucron, and the relative positions of the nerve ring and excretory pore. In the Neotropical region, third-stage larvae of Contracaecum have been reported encysted in several snake species, including the northern Mexican garter snake Thamnophis eques (Reuss) and the blackbelly garter snake Thamnophis melanogaster (Peters) from Mexico (Jiménez-Ruiz et al., 2002; Silva-Trejo et al., 2025); and the arrow-headed coral snake Micrurus alleni Schmidt and the double-banded false coral snake Erythrolamprus bizona Jan from Costa Rica (Goldberg & Bursey, 2004). Compared with the most comprehensive morphometric data currently available for snake-derived Contracaecum larvae, the specimens recovered from E. notaeus show broad overlap in body length, body width, intestinal caecum length, and tail length, but generally possess a longer oesophagus than larvae reported from T. melanogaster (1.30-2.60 vs. 0.39-0.57) (Silva-Trejo et al., 2025). The largest specimens recovered from E. notaeus also exceeded the maximum body length reported for third-stage larvae recovered from T. melanogaster (10.20-28.40 vs. 7.95-25.45).

DISCUSSION

Snakes are known to act as hosts for Contracaecum L3; however, reports of these larvae in snakes remain scarce, and most studies only document their occurrence without providing a detailed morphological description (Goldberg & Bursey, 2004; Jiménez-Ruiz et al., 2002; Silva-Trejo et al., 2025). To our knowledge, there are no previous records of Contracaecum larvae infecting species of Eunectes. Thus, the present finding represents the first record for this host genus and expands the known diversity of reptilian hosts for anisakid nematode larvae in the Neotropical region.

Various aquatic invertebrates, particularly crustaceans, together with fishes, act as intermediate hosts for Contracaecum species (Anderson, 2000). Parasitological studies on freshwater fish from Formosa Province remain scarce. Larvae of Contracaecum sp. were recorded in the spotted sorubim Pseudoplatystoma corruscans (Spix & Agassiz), the red-bellied piranha Pygocentrus nattereri Kner, and the white pim catfish Pimelodus albicans Valenciennes in the Pilcomayo River, Salta Province (Ramallo et al., 2020). Similarly, Contracaecum sp. larvae have been reported in the trahira Hoplias malabaricus Bloch, the dorado Salminus brasiliensis Cuvier, and P. albicans in other provinces of Argentina (Mancini et al., 2014; Ramallo & Ailán-Choke, 2022). Although there are no published records of these parasites in fish from Formosa, the host species mentioned are part of the ichthyofauna of Bañado La Estrella (Dománico et al., 2014), suggesting that suitable intermediate or paratenic fish hosts are likely present in the wetland and may contribute to the local transmission cycle of Contracaecum.

Although fish represented a relatively small proportion of the diet of E. notaeus in samples collected during the high-water season at Bañado La Estrella (Waller et al., 2007), their consumption likely represents one of the routes by which this snake acquires Contracaecum larvae. Additionally, field reports indicate that yellow anacondas, like other snakes, occasionally consume dead fish (DeVault & Krochmal, 2002; Sazima & Strüssmann, 1990; Strüssmann, 1997). In this sense, between October and December, the Bañado La Estrella undergoes progressive drying, which may result in localised fish mortality events (Miranda et al., 2016). Because Contracaecum larvae may remain viable in the tissues of recently deceased fish, scavenging during these events may represent an additional route of larval acquisition. In this context, the similar prevalence of larvae across annual samplings suggests that fish consumption is a sustained behaviour over the years.

The localisation of larvae encysted within the gastric serosa of snakes is compatible with the behaviour of anisakid larvae in paratenic hosts, in which larvae migrate through host tissues and remain encapsulated until transmission to a suitable definitive host (Anderson, 2000). Although there are no published records of predation on E. notaeus by birds in Argentina, several groups of birds including herons, egrets, owls, and other raptors have been documented preying upon snakes and other reptiles (Kahl, 1971; Martínez et al., 2019; Santana et al., 2025). Among these the Jabiru Jabiru mycteria (Lichtenstein), which occurs in sympatry with E. notaeus in Bañado La Estrella, is known to include snakes in its diet, and has been reported as a host of Contracaecum spp. (Pinto et al., 2004). These ecological associations provide a plausible link between E. notaeus and avian definitive hosts. In this extensive floodplain, seasonal flooding promotes trophic interactions among aquatic invertebrates, fishes, reptiles, and piscivorous birds, thereby facilitating parasite transmission (Dománico et al., 2014). However, the large body size of adult E. notaeus may act as an ecological filter for trophic transmission, as larger snakes are likely to be less frequently consumed by potential predators than smaller (juvenile) individuals, potentially representing a dead-end host for the parasite.

Beyond documenting a new host record, we hypothesise that juvenile semi-aquatic yellow anacondas may play a role as paratenic hosts in the transmission dynamics of Contracaecum in Neotropical wetlands. Further studies integrating morphological and molecular data on Contracaecum larvae, along with dietary analyses and parasitological surveys of sympatric fishes, reptiles, and birds, will clarify the role of semi-aquatic snakes in maintaining anisakid nematode transmission networks in Neotropical wetlands. Because prey availability and trophic interactions vary markedly throughout the annual flood pulse, year-round studies combining dietary and parasite data will also be important to clarify how seasonal hydrological dynamics influence parasite transmission in these systems.

ACKNOWLEDGEMENTS

The authors would like to thank Nicolas Oswald, and Victoria Lichtschein from the Yellow Anaconda Management Program; Guillermo Panisse, Noralí Pagnutti, Agustín Abba, Lautaro Fuentes, and Virginia Monges from CEPAVE, for their assistance. We thank Patricia Sarmiento of Scanning Electron Microscope Service at the Museo de La Plata (FCNYM, UNLP). This study was authorised by Resolutions No. 0151/19, 050/21, 052/22, and 049/23 of the Dirección de Recursos Naturales y Manejo del Ministerio de la Producción y Ambiente of Formosa, Argentina (Ministry of Production and Environment's Department of Natural Resources and Management).

FUNDING

This research was partially funded by Universidad Nacional de La Plata (N996 to JID), and Fundación Biodiversidad (to the Yellow Anaconda Management Program).

COMPETING INTERESTS

The authors declare no conflicts of interest.

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Recibido: 26 de julio de 2026. Aceptado: 16 de septiembre de 2026.