First record of Diplotriaena obtusa (Rudolphi, 1802) Henry & Ozoux, 1909 from the Eurasian nuthatch (Sitta europaea Linnaeus, 1758)

Introduction

The nematode phylum is a highly diverse group of animals. Estimates range from 100.000 to 100 million species (Bouamer et al., 2006; Hammond, 1992; Hugot et al., 2001; Lambshead, 1993). Many species are free-living; others live as parasites in plants, arthropods and vertebrates, including humans (Anderson, 2000; Yamaguti, 1961; Yeates et al., 2009). Among parasitic nematodes, a wide range of species are found in birds as final hosts (Cram, 1927; Rebelo et al., 2024; Leung & Koprivnikar, 2016; Malik et al., 2022; Movsesyan et al., 2020; Okulewicz, 2013; Saparov et al., 2013; Santoro et al., 2016; Zhang et al., 2012). There, they can cause air sac inflammation, pneumonia, bronchopneumonia, and other diseases. Symptoms include deterioration in general condition, lethargy, and reduced flight performance (Abdu et al., 2023; Freeman et al., 2025; Leung & Koprivnikar, 2016; Sterner & Cole, 2008).

Here we report the first record of the nematode Diplotriaena obtusa in a Eurasian nuthatch (Sitta europaea) from southwestern Germany.

Material and Methods

Sampling and necropsy of the bird

The nuthatch was caught in a net on 5 September 2024 in Wangen im Allgäu, Baden-Württemberg, Germany (47°41′ N, 9°48′ E; Fig. 1) and ringed with the ring number Hiddensee C4E9091 (Fig. 1).

Fig. 1.

The bird was recognized as a juvenile male and weighed 23.2 g. It was noticed that the bird was rattling. Two days later, the nuthatch was found dead nearby and then dissected to determine the cause of death and confirm its age and sex. There were no signs of injury, for example, from being hit by a windscreen or a predator, but two nematodes were found in the area of the esophagus.

Altogether, three nematodes were fixed in ethanol (70 %) and sent to the first author as an incidental finding as part of a research program funded by members of the Saxon State Parliament to record louse flies (Freick et al., 2025). The morphological and molecular examinations and species determination were conducted at the Institute of Parasitology, University of Leipzig.

Ethical Approval and/or Informed Consent

For this study, formal consent was not required.

Results

Since no specific morphological features were recognizable via light microscopy in the adult nematodes, as might be expected in the case of Diplotriaena spp. Eggs, such as those with size, specific trident structures at the anterior end, and lateral pores at the oral opening (Freeman et al., 2025; Rentería-Solís et al., 2021, Anderson et al., 2009), were removed from a female specimen for morphological characterization. These nematodes' eggs were identified as Spirurid eggs according to Schmäschke (2025) and Beck & Pantchev (2013), with a thick, smooth wall and a larval stage inside (Fig. 2). Measurements averaged 46 μm x 30 μm.

Fig. 2.

Phylogenetic analysis of the herein obtained sequence (Isolate_Eurasian_nuthatch) for 791 bp (Fig. 3A) and 492bp (Fig. 3B) with those available from GenBank (e.g., MT129516, MT129509) employed the maximum likelihood method. The T. cati sequence (EF180059) was used as the outgroup. Only branches with support values greater than 70 % are shown (bootstrapping method; 100 bootstrap replicates). The sequence obtained was submitted to GenBank (accession number PV942266, 861 bp) and showed the highest percent identity (99.75 %) with isolates of Diplotriaena obtusa (e.g., accession number MT129516, query cover 95 %; accession number MT129509, query cover 94 %). Evolutionary analysis showed that the herein obtained sequence clustered together with other D. obtusa sequences (Fig. 3A). The specimen branched between a specimen of D. obtusa found in a blue tit from Germany (accession number: MT705333) (Fig. 3A) and the other specimens of D. obtusa (accession numbers: MT129516, MT129509, MT129524, MT129521, MT129507) which showed very high percent identities (99.75 %) and were detected in barn swallows and cliff swallows from the USA hinting on some potential genetic variations due to geographical conditions, which should be confirmed with higher-resolution markers and broader sampling. Including sequences from other Diplotriaena species in the phylogenetic analysis further supported the consensus between the isolate herein found and other isolates of D. obtusa (Fig. 3B).

Fig. 3.

Discussion

Among the nematodes affecting birds, particularly passerines, there were approximately 77 described species of the genus Diplotriaena sp. (Hong et al., 2019), to which D. obtusa found in the nuthatch was attributed. D. obtusa was recorded in different passerines of the northern hemisphere and from Madagascar, whereby, according to the frequency of publication, swallows (Hirundinidae Rafinesque, 1815) were particularly frequently observed as hosts (Tab. 1). The assignment of D. obtusa to the hosts Junco hyemalis (Linnaeus, 1758) and Quiscalus quiscula (Linnaeus, 1758) by Leidy (1885, 1886) and Walton (1927) was incorrect and based on a misidentification of the parasites (Anderson, 1959).

Until now, there has been no scientific evidence that D. obtusa has been found in the European nuthatch, of which only the species Acuaria subula (Dujardin, 1845) and D. henryi (Blanc, 1919) have been known as helminth parasites (Sitko & Okulewicz, 2010; Okulewicz, 2013).

The only available study of D. obtusa from Germany identified the blue tit as the host (Rentería-Solís et al., 2021). A report of D. obtusa parasitizing the great tit was from Poland (Rząd et al., 2023). European nuthatches live in loose associations with tits during autumn and winter, and also in similar breeding habitats (Blotzheim, 1993). It is therefore possible that the birds were infected with nematodes in habitats similar to those in which they were found. Parasitic helminths of the genus Diplotriaena sp. can cause significant pathology in their hosts (Abdu et al., 2023; Michalski et al., 2021; Stanicka et al., 2021). Many are (Leung & Koprivnikar, 2016). Sitko & Heneberg (2023) showed that the prevalence, and thus the infection intensity, of this nematode in Europe has increased in recent years.

The trivial name “air sac worm“ refers to the leading site of infection, which can also affect the body cavity (Sonin, 1975). In our case, the nematodes were found in the tracheal region, but they likely originated from the neighboring air sacs. The dissected nut-hatch showed no external injuries. However, a rattle was through-out its life, likely due to the nematode infection. This symptom may be associated with the dead bird, given the nuthatch's ringing. Although Dipoltriaena infections are usually subclinical, a high parasite load can lead to morbidity and mortality (Freeman et al., 2025; Stanicka et al., 2021). Whether the nematode infection itself was the cause of death of the bird cannot be said with certainty, as no histological or pathological evidence was available. However, it is at least possible that it contributed to the bird's death.

A limitation of this study is the small number of specimens, or, rather, the limited potential for morphometric analysis. However, this matter was mitigated by the clear identification of the parasites' eggs and subsequent molecular analysis to identify the species.

D. obtusa was found for the first time in a European nuthatch. This finding broadens the spectrum of potential hosts for this nematode species and, consequently, its distribution, as this bird species is (Blotzheim, 1993). The larvae are transmitted when they are ingested by their definitive hosts (Anderson, 2000). This occurs via invertebrates as intermediate hosts (Abdu et al., 2023; Rentería-Solís et al., 2021; Sitko & Heneberg, 2023), which are part of the European nuthatch's and many other passerines' diet (Brehm, 1867) as part of the natural chain of infection.