Further Description and First Genetic Characterization of Oswaldofilaria bacillaris (Nematoda: Onchocercidae) Infecting the Pantanal Caiman (Caiman yacare), with Insights into the Phylogeny of Oswaldofilaria

Acta Parasitologica (2026) 71:75 https://doi.org/10.1007/s11686-026-01264-7 RESEARCH Further Description and First Genetic Characterization of Oswaldofilaria bacillaris (Nematoda: Onchocercidae) Infecting the Pantanal Caiman (Caiman yacare), with Insights into the Phylogeny of Oswaldofilaria Glória M. C. Lacerda1 · Gustavo M. do Carmo1 · Lorena G. Ailán-Choke2 · Fernando Paiva3 Luiz E. R. Tavares3 · João A. de Araújo-Filho4 · Samuel C. Ribeiro5 · Felipe B. Pereira1 · Received: 15 January 2026 / Accepted: 4 March 2026 / Published online: 24 March 2026 © The Author(s) 2026 Abstract Purpose Oswaldofilaria bacillaris is the type species of the genus, but its morphology remains little known and partially contradictory. Moreover, there are no genetic data and scanning electron microscopical observations on the species, like in most Oswaldofilaria spp. The phylogenetic relationships among Oswaldofilaria spp. are practically unknown. This study aimed to evaluate the detailed morphology of O. bacillaris, provide its first genetic characterization, and discuss the phylogenetic relationships within Oswaldofilaria. Methods Parasites infecting the body cavity of Caiman yacare in Pantanal wetlands, Mato Grosso do Sul, Brazil, were collected and processed for morphological studies using light and scanning electron microscopy. Genetic characterization was based on partial sequences of 18S and 28S rDNA. A phylogeny was reconstructed based on 28S sequences including all Oswaldofilaria available. Results Nematodes were assigned to O. bacillaris mainly based on the oral opening markedly displaced ventrally. Males had seven pairs of caudal papillae encircling the cloaca, and their two most posterior pairs were hardly visible, which is also characteristic in other congeners. Moreover, a delicate area rugosa previously unreported in the species was observed. Oswaldofilaria bacillaris formed a monophyletic assemblage with the congeners, being sister to O. chabaudi, and O. petersi was basal, these last two parasites of lizards. Conclusion The present findings strengthened the specific diagnosis of O. bacillaris and elucidated the morphological contradictions. The phylogeny reinforced the hypotheses that Oswaldofilaria emerged in lizards and colonized crocodilians by host switch, and reduction in number of caudal papillae in males is a derived feature. Keywords Filarid · Oswaldofilariinae · Integrative taxonomy · Reptile · Crocodilia · Brazil Felipe B. Pereira 1 Programa de Pós-Graduação em Parasitologia, Universidade Federal de Minas Gerais, Av. Antônio Carlos 6627, Pampulha, Belo Horizonte, MG 31270-901, Brazil 2 Centro de Ecología Aplicada del Litoral, Consejo Nacional de Intestigaciones Científicas y Técnicas, Ruta Provincial 5, Km 2,5, Corrientes CP. 3400, Argentina 3 Instituto de Biociências, Universidade Federal de Mato Grosso do Sul, Av. Costa e Silva s/n, Campo Grande, MS 79070-900, Brazil 4 Departamento de Ciências Biológicas, Centro de Ciências Biológicas e da Saúde, Universidade Regional do Cariri, Rua Vicente Alexandrino de Alencar, 348, Centro, Campos Sales, Crato, CE 63150-000, Brazil 5 Laboratório de Biologia e Ecologia de Animais Silvestres— LABEAS, Instituto de Formação de Educadores, Universidade Federal do Cariri—UFCA, Rua Olegário Emidio de Araujo, s.n, Brejo Santo, Brazil 13 75 Page 2 of 9 Acta Parasitologica (2026) 71:75 Introduction Materials and Methods Onchocercidae Leiper, 1911 (Spiruromorpha: Filairoidea) is a diverse family of nematodes with complex life cycles, in which the transmission is mediated by hematophagous arthropods [1, 2]. The microfilaria of onchocercids, a larval stage prior to L1, is the key element for their transmission and has been reported in all organ systems as well as in most tissues of their definitive hosts, although the preferred site varies according to the species [1]. Onchocercids infect a wide spectrum of hosts, including amphibians, reptiles, birds and mammals [2, 3]. Within Onchocercidae, the subfamily Oswaldofilariinae Chabaud & Choquet, 1953 incudes 7 genera of parasites mostly in lacertilian reptiles, except for the genus Oswaldofilaria Travassos, 1933 that has been reported also in crocodilians [4–6]. Oswaldofilaria is among the most diverse genera of onchocercids and currently comprises 15 valid species, which occur in Australia, Africa and South America, representing a “gondwanian-type” distribution [4, 6]. The following congeners have been reported in crocodilians: O. bacillaris (Molin, 1858), O. kanbaya Manzanell, 1986, O. medemi Marinkelle, 1981 and O. vesterae Bain, Kouyaté & Baker, 1982 [4, 7–10]. Oswaldofilaria bacillaris and O. medemi are the only species known to infect crocodilians in South America until now [4]. Oswaldofilaria bacillaris (= Filaria bacillaris) was originally described parasitizing Melanosuchus niger (Spix, 1825) and Caiman crocodilus (Linnaeus, 1758) from Brazil [7]. The morphology of this species has been investigated since then, but some morphological aspects remain poorly known and seemingly variable [11, 12]. This variability could be better understood with the help of genetic data, but there is no such data available for O. bacillaris. The lack of genetic data is also observed in most species of Oswaldofilaria, in which only two of the 15 species have been genetically characterized, in addition to a few sequences not identified to species level. This panorama makes it difficult to understand the phylogenetic relationships among Oswaldofilaria spp., since some of their morphological features are reduced and seemingly show evolutionary convergence [3, 13, 14]. During parasitological analysis of a Caiman yacare (Daudin, 1801) (Crocodylia: Alligatoridae) found freshly dead at one lagoon of the Pantanal wetlands, State of Mato Grosso do Sul, Brazil, some specimens of O. bacillaris were found infecting its body cavity. The morphology of these parasites was evaluated in detail using light and scanning electron microscopy, and genetic characterisation based on two nuclear ribosomal markers was performed. The results are presented herein. Collection, Processing and Morphological Evaluation of Parasites 13 On 18 August 2018 one specimen of C. yacare (about 1.5 m of total length) was found freshly dead by researchers of the Universidade Federal de Mato Grosso do Sul, at a marginal lagoon of the federal highway BR 262, which crosses Pantanal wetlands, municipality of Corumbá, State of Mato Grosso do Sul, Brazil. Host nomenclature and classification are according to [15]. The reptile was immediately necropsied in the field and the filariae were found still alive in the body cavity. The nematodes were washed in saline, fixed in hot 4% formalin and preserved in 70% ethanol. The middle body part of one male specimen was excised and fixed in molecular-grade 96–99% ethanol for genetic studies. For morphological examination using light microscopy, specimens were cleared in glycerine and obs (...truncated)