Aonchotheca annulosa is a valid species

The earliest record of capillariid nematodes from wild rodents is by Dujardin (Reference Dujardin1845), who described a species from the intestine of rats in Northern France, the males of which possessed a relatively long spicule (0.95 mm). Dujardin named these nematodes Calodium annulosum. Travassos (Reference Travassos1915) moved the species to the genus Capillaria, as Capillaria annulosa, and then López-Neyra (Reference López-Neyra1947) to Aonchotheca. Hence this species is now usually known as A. annulosa. The relatively long spicule of A. annulosa has been reaffirmed on multiple occasions (e.g. Mészáros, Reference Mészáros1977 [1.00–1.02 mm]; Mészáros, Reference Mészáros1978 [1.00 mm]; Bain and Wertheim, Reference Bain and Wertheim1981 [1.14 mm]; Moravec and Baruš, Reference Moravec and Baruš1991 [1.11–1.27 mm]; Umur et al., Reference Umur, Moravec, Gurler, Bolukbas and Acici2012 [0.86–1.08 mm]) in consonant material. Thus, the modern concept of A. annulosa is clearly linked to Dujardin's original record by the presence of a distinctive long spicule and by the typical site of infection in the small intestine of Eurasian rodents. Aonchotheca annulosa may therefore be considered an uncomplicated and valid taxon.

Aonchotheca halli has precedence on a short-spicule morphotype mostly occurring in Eurasian cricetids

In the same monograph of 1845, Dujardin also described female worms from the intestine (Diesing, Reference Diesing1851) of Mus sylvaticus (= A. sylvaticus) which he referred to as ‘Trichosome du mulot (Mus sylvaticus)’. Six years later Diesing (Reference Diesing1851) created the name Trichosomum muris sylvatici Dujardin, based on some part of the specimens collected by Dujardin from wood mice. Diesing specified Dujardin as the authority for this taxon, but as Dujardin did not formally name his specimens, Diesing has been taken as the authority by subsequent authors. Diesing (Reference Diesing1851) provided only very sparse and ambiguous information to add to the minimal description provided by Dujardin, and together these works lack any diagnostic morphological information useful in distinguishing between the Aonchotheca morphotypes found in present-day Eurasian rodents. In particular, no information was provided by either author on male worms, which were lacking in Dujardin's original collection. In all likelihood, Diesing's taxon was the same species, A. annulosa, as the specimens Dujardin had found in the intestine of rats, given the preference of A. annulosa for the small intestine of murine hosts (see also below). Thus, T. muris sylvatici of Diesing might be regarded as a junior synonym of A. annulosa. Alternatively, it could be argued that the information surrounding T. muris sylvatici is so sparse and inconclusive that it should instead be considered a nomen nudum (i.e. effectively lacking an establishing description [International Commission on Zoological Nomenclature, 1999]).

Despite its poor support in evidence, T. muris sylvatici Diesing (Reference Diesing1851) was moved to the genus Capillaria Zeder, 1800 by Travassos (1915; see also Moravec, Reference Moravec1982). Thereafter it became stabilized within Capillaria or Aonchotheca (which is the generic classification we accept here) by later authors who arbitrarily linked it to small-spicule morphotypes that were not described for the first time until the early 20th century. At times 3 different versions of the specific name have been employed: muris sylvatici, muris-sylvatici or murissylvatici. In fact, the earliest record of a rodent-infecting Aonchotheca-like capillariid clearly different to A. annulosa was by Kalantarian (Reference Kalantarian1924) who described material with a short spicule (0.1928 mm; approximately one-quarter of the length found in A. annulosa), that she recovered from the migratory or grey dwarf hamster (Nothocricetulus migratorius (= Cricetulus migratorius) [Pallas, 1773]) in Armenia. This was named Capillaria halli by Kalantarian (Reference Kalantarian1924) but was later synonymized with C. murissylvatici, by Teixeira De Freitas and Lent (Reference Teixeira De Freitas and Lent1936). These latter authors gave the spicule length for C. murissylvatici as exactly the same as given earlier by Kalantarian (Reference Kalantarian1924) for C. halli, although they did not attribute this measurement to her work. Teixeira De Freitas and Lent (Reference Teixeira De Freitas and Lent1936) included bank voles and wood mice, alongside the migratory hamster, as hosts of C. murissylvatici, likely on the basis of records by Baylis (Reference Baylis1928) and Elton et al. (Reference Elton, Ford, Baker and Gardiner1931) mentioned further below.

Following the work of Teixeira De Freitas and Lent (Reference Teixeira De Freitas and Lent1936), reports of C. halli-like short-spicule morphotypes tended to be attributed to C. murissylvatici. Roman (Reference Roman1939) reported worms occurring in large numbers within tumour-like developments of the gastric mucosa of French voles (primarily bank voles). He provided comprehensive measurements of gastric specimens from bank voles that confirmed the relatively short spicule of the male worms (0.187–0.247 mm and mean of 0.219 mm). In this publication and in his review of 1951 (Roman, Reference Roman1951) Roman further established one concept of C. murissylvatici as a parasite of the gastric mucosa of microtine voles. This concept was followed by several later authors, who placed the species either in Capillaria or Aonchotheca. This included Justine and de Roguin (Reference Justine and de Roguin1990) who again confirmed the short spicule length (0.200–0.215 mm) in stomach-dwelling forms.

Aonchotheca annulosa in the small intestine has likely often been misidentified

Aonchotheca nematodes are long, thin, filamentous worms that penetrate the intestinal mucosa. Because of their fragility, their length and their location in the mucosa, they are easily fragmented upon recovery from the host. Moreover, they are not easy to identify precisely by those who are not closely familiar with the taxon. It is perhaps not surprising, therefore, that additional confusion has arisen in the literature about whether the worms observed in rodent intestines were A. murissylvatici or A. annulosa. This confusion has been further exacerbated by reports in journals that are not easily accessed even today (especially those from East European university and society journals), and hence relevant studies have not always been available to those describing and/or reporting on these species.

The stage for confusion between the stomach and small intestine-specialist Aonchotheca species may have been partly set by Elton et al. (Reference Elton, Ford, Baker and Gardiner1931). Following Baylis (Reference Baylis1928) these authors reported what they called ‘Capillaria? muris-sylvatici’ from the small intestines of both wood mice and bank voles. Given the site of infection, this is in fact likely to have been A. annulosa. Furthermore, Elton et al. (Reference Elton, Ford, Baker and Gardiner1931) reported ‘Capillaria or Hepaticola sp.indet.’ from the stomachs of wood mice, but, given the site of infection, this is likely to have been the A. halli morphotype associated with A. murissylvatici by many authors (see above), or a Eucoleus species. Several authors working on the epidemiology of helminths in wild rodents from the British Isles (Lewis, Reference Lewis1968a; Canning et al., Reference Canning, Cox, Croll and Lyons1973; Langley and Fairley, Reference Langley and Fairley1982; O'Sullivan et al., Reference O'sullivan, Smal and Fairley1984; Montgomery and Montgomery, Reference Montgomery and Montgomery1988, Reference Montgomery and Montgomery1990; Abu-Madi et al., Reference Abu-Madi, Behnke, Lewis and Gilbert2000; Loxton et al., Reference Loxton, Lawton, Stafford and Holland2016; Stuart et al., Reference Stuart, Paredis, Henttonen, Lawton, Ochoa Torres and Holland2020), as well as those working in continental Europe (Sołtys, Reference Sołtys1949; Erhardová and Ryšavý, Reference Erhardová and Ryšavý1955; Tenora and Baruš, Reference Tenora and Baruš1955; Tenora and Zavadil, Reference Tenora and Zavadil1967; Bjelić-Čabrilo et al., Reference Bjelić-Čabrilo, Popović, Šimić and Kostić2009, Reference Bjelić-Čabrilo, Kostić, Popović, Cirković, Aleksić and Lujić2011; Movsesyan et al., Reference Movsesyan, Nikoghosian, Petrosian, Vlasov and Kuznetsov2018) followed this lead, reporting A. murissylvatici from the small intestines of rodents (see Table 1). Thus, although A. annulosa has been recorded only rarely in the British Isles (James, Reference James1954; Wakelin, Reference Wakelin1968; Jackson et al., Reference Jackson, Friberg, Bolch, Lowe, Ralli, Harris, Behnke and Bradley2009; Behnke unpublished observations) this likely reflects underreporting due to misidentification with A. murissylvatici. On the European mainland, despite some likely misidentification (discussed by Moravec [Reference Moravec2000]), A. annulosa is nonetheless recognized as a frequent parasite of the small intestine from studies of bank voles (Tenora and Zejda, Reference Tenora and Zejda1974; Mészáros, Reference Mészáros1978; Milazzo et al., Reference Milazzo, Casanova, Aloise, Ribas and Cagnin2003a; Grzybek et al., Reference Grzybek, Bajer, Bednarska, Alsarraf, Behnke-Borowczyk, Harris, Price, Brown, Osborne, Siński and Behnke2015), and of other rodent species (Moravec, Reference Moravec2000).

Table 1. Chronological list of papers reporting Aonchotheca murissylvatici

For each publication, we give the name of the parasite as stated in that paper. Note, that this is not intended to be a comprehensive list of all papers, but the majority of studies undertaken in the British Isles are covered, and representative/key studies from abroad have also been included. We have not included all recent studies where no information was given on the criteria for species assignment nor on the gastrointestinal site from which the worms were recovered. Initially, the species was recorded as Trichosomum muris sylvatici by Diesing (Reference Diesing1851). It was then moved to the genus Capillaria Zeder, 1800 by Travassos (Reference Travassos1915), and subsequently to a new genus, Aonchotheca, by López-Neyra in 1947, although as is evident above, for over 60 further years many authors continued to refer to Capillaria murissylvatici. The hosts reported above are: Apodemus sylvaticus, Apodemus uralensis, Rattus norvegicus, Nothocricetulus migratorius (syn. Cricetulus accedula), Microtus arvalis, Microtus pennsylvanicus and Myodes glareolus. Clethrionomys is taken to be a junior synonym of Myodes and records previously named Cricetulus migratorius are assigned to Nothocricetulus migratorius.

^a The original version of the name as in publication.

^b Not stated whether the high frequency and intensity were for worms in the stomach or small intestine. Sympatric wood mice were not infected.

^c In these studies the intestinal site is not given in the paper but has been confirmed by correspondence with the authors.

In contrast to authors who understood A. murissylvatici to be a small intestinal parasite, others, following Texeira De Freitas and Lent (Reference Teixeira De Freitas and Lent1936) and Roman (Reference Roman1951), were quite definite that A. murissylvatici is a parasite of the stomach. Among the earliest was Thomas (Reference Thomas1953) who found heavy infections in the stomachs of bank voles in Scotland, and then the work of Kisielewska (Reference Kisielewska1970a, Reference Kisielewska1970b, Reference Kisielewska1983) in Poland emphasized that A. murissylvatici is a bank vole stomach specialist. Others include Murúa (Reference Murúa1978), Justine and de Roguin (Reference Justine and de Roguin1990) and more recently, Miljević et al. (Reference Miljević, Čabrilo, Budinski, Rajičić, Bajić, Bjelić-Čabrio and Blagojević2022).

Other relevant Aonchotheca species in Eurasia

Aonchotheca annulosa and A. halli are the only Aonchotheca species to be frequently reported in Eurasian rodents (with nominal records of A. murissylvatici most likely to be one or the other), if A. wioletti (Rukhlyadeva, 1950) is accepted as a junior synonym of A. halli (following Justine and de Roguin, Reference Justine and de Roguin1990). However, an assemblage of Aonchotheca-like species is known in glirids (dormice) (Justine et al., Reference Justine, Ferté and Bain1987; Veciana et al., Reference Veciana, Chaisiri, Morand and Ribas2016) whose members could potentially be confused with A. halli and should be kept in mind given the low specificity of many capillariids. Of these forms in glirids, both A. myoxinitelae (Diesing, Reference Diesing1851) and A. legerae (Justine et al., Reference Justine, Ferté and Bain1987) appear to have a longer spicule than A. halli (>0.26 mm). Furthermore, both have more prominent structures around the vulval opening and shorter ejaculatory ducts (Pisanu and Bain, Reference Pisanu and Bain1999), amongst other potential differentiating features (Justine et al., Reference Justine, Ferté and Bain1987; Pisanu and Bain, Reference Pisanu and Bain1999). Tenoranema alcoveri Mas-Coma and Esteban, Reference Mas-Coma and Esteban1985, also from glirids, which is close to Aonchotheca and whose generic characters require further clarification, has a spicule that overlaps that of A. halli, but is distinguished by the presence of complex digitiform rays supporting the bursa (Mas-Coma and Esteban, Reference Mas-Coma and Esteban1985; Justine et al., Reference Justine, Ferté and Bain1987). Apart from these species in glirids, 2 additional rodent-infecting species occur in Eurasia. This includes the relatively poorly known A. armeniaca (Kirschenblat, 1939), which has a long spicule (1.10 mm) and infects the small intestine of Citellus spp. in Armenia (Veciana et al., Reference Veciana, Chaisiri, Morand and Ribas2016), and the more recently described Aonchotheca yannickchavali, Veciana et al., Reference Veciana, Chaisiri, Morand and Ribas2016 which is distinguished by a very large, tube-like projection associated with the vulva and infects the intestine of Bandicota species in Thailand (Veciana et al., Reference Veciana, Chaisiri, Morand and Ribas2016). Moreover, it should be borne in mind that several poorly known species with some similarity to A. halli have been recorded in a variety of non-rodent mammalian hosts in Eurasia. This includes A. musimon Pisanu and Bain, Reference Pisanu and Bain1999 and A. bilobata (Bhalerao, 1933) in ungulates and A. speciosa (Beneden, 1873) in bats. The existence of this additional sympatric diversity emphasizes the importance of future molecular studies to further elucidate the relationships of A. halli to similar forms in non-rodent hosts.

Recommendations

In order to survive in the hostile environment of the mammalian intestine, helminths evolve to become specialists in parasitizing specific regions of the intestine where they can best resist acidity, host enzymes and other defences against invasion by microorganisms (Schad, Reference Schad1963; Crompton, Reference Crompton1973; Sukhdeo and Sukhdeo, Reference Sukhdeo and Sukhdeo1994; Sukhdeo and Bansemir, Reference Sukhdeo and Bansemir1996). In our view it is unlikely, therefore, that a species that has become a specialist for survival in the duodenum, will be able to cope equally well in the stomach.

As justified in the sections above, the most parsimonious explanation of the confusion in the literature is that the commonly-occurring gastrointestinal Aonchotheca fauna of European rodents is made up of 2 site-specialist species. One is a gastric mucosal specialist, living intertwined in the glandular, pyloric region of its host's stomach and while perhaps occasionally worms in heavy infections may spill over into the duodenum, this nematode is not usually a resident of the small intestine. The other is an intestinal specialist species, A. annulosa, also usually with a mucosal burrowing habit. We propose that most of the previous reports of ‘A. murissylvatici’ from the small intestine were in fact A. annulosa. We further propose that the stomach-dwelling species should be referred to by the name Aonchotheca halli (Kalantarian, Reference Kalantarian1924). As explained above, this has precedence on the stomach-dwelling short-spicule, cricetid-specialist morphotype, replacing ‘A. murissylvatici’ which has very dubious support in evidence. Although the type specimens of A. halli and its original description are not available (despite our efforts to locate these), Texeira De Freitas and Lent's redrawings of Kalantarian's specimens are sufficient to link these to the small-spicule gastrointestinal Aonchotheca morphotype occurring in Eurasian cricetids. This shift in nomenclature is in line with rules on precedence in the International Zoological Code of Nomenclature (International Commission on Zoological Nomenclature, 1999). Moreover, rather than disturbing stability and precedent, this in fact draws a line under more than 100 years of incorrect, inconsistent and biologically confusing use of the name A. murissylvatici and its variants, within which a large proportion of nominal records are likely to have been misidentifications of A. annulosa. One additional advantage of suppressing A. murissylvatici will be that the instability associated with the exact form of the name (muris sylvatici/muris-sylvatici/murissylvatici) will also be curtailed.

The host-specificity, geographical distribution, zoonotic potential and identification of A. annulosa and A. halli

Given clarification of the species concepts for these Aonchotheca morphotypes common in wood mice and bank voles, a clearer picture can be formed of their broader host and geographical distributions. To this end we carried out a literature search of host and geographical records. We accepted all records of A. annulosa from the intestine of hosts. For A. halli, we only accepted records of ‘A. murissylvatici’ from the stomach of hosts, or in the case of intestinal records, where a positive identification had been made based on morphological criteria. From summaries of the resulting data (Figs 1 and 2) it can be seen that A. annulosa (Fig. 1) has the wider host range of the 2 species. Whilst it is most often recorded in Apodemus and Rattus it can also infect a range of other murines but also cricetids, sciurids, mustelids and insectivores. Moreover, it has been recorded more than once in primates (baboons and monkeys) in zoological park settings (Moravec, Reference Moravec2000; Umur et al., Reference Umur, Moravec, Gurler, Bolukbas and Acici2012). As previously recognized, this wide host range could be indicative of transboundary and zoonotic potential. The distribution of this species extends across a very wide area of Eurasia, with records from Atlantic islands likely indicating a propensity to be spread anthropogenically via commensal murine hosts (e.g. rats). In contrast to A. annulosa, A. halli has narrower specificity infecting primarily bank voles, but also some other cricetids and, in fewer cases, murines (Fig. 2) . Although reliable records of this species are less numerous, it also seems to have a very wide distribution within Eurasia (Fig. 2).

Figure 1. Geographical and host distribution of Aonchotheca annulosa (Dujardin, Reference Dujardin1845). Top panel shows locations of nominal records of A. annulosa in Eurasia. Bottom panels show pie charts representing the proportional frequency of records of A. annulosa at different host taxonomic levels. Based on 91 records drawn from publications cited in the main text and also Balfour (Reference Balfour1922), Lewis (Reference Lewis1927), Bernard (Reference Bernard1961), Mészáros and Murai (Reference Mészáros and Murai1979), Mas-Coma and Feliu (Reference Mas-Coma and Feliu1981), Mészáros et al. (Reference Mészáros, Habijan and Mikes1983), Mészáros and Štollmann (Reference Mészáros and Štollmann1984), Jirouš (Reference Jirouš1985), Justine (Reference Justine1989), Mascato et al. (Reference Mascato, Rey, Bos, Peris, Paniagua and Blanco1993), Afonso-Roque (Reference Afonso-Roque1995), Asakawa and Tenora (Reference Asakawa and Tenora1996), Casanova et al. (Reference Casanova, Miquel, Fons, Molina, Feliu, Mathias, Torres, Libois, Santos-Reis, Collares-Pereira and Marchand1996), Milazzo et al. (Reference Milazzo, de Bellocq, Cagnin, Casanova, di Bella, Feliu, Fons, Morand and Santalla2003b), Fuentes et al. (Reference Fuentes, Sáez, Trelis, Galán-Puchades and Esteban2004, Reference Fuentes, Sainz-Elipe, Sáez-Durán and Galán-Puchades2010), Pisanu et al. (Reference Pisanu, Jerusalem, Huchery, Marmet and Chapuis2007, Reference Pisanu, Lebailleux and Chapuis2009), Ondríková et al. (Reference Ondríková, Miklisová, Ribas and Stanko2010), Salvador et al. (Reference Salvador, Guivier, Xuéreb, Chaval, Cadet, Poulle, Sironen, Voutilainen, Henttonen, Cosson and Charbonnel2011), Kirillova (Reference Kirillova2011, Reference Kirillova2012), Romeo et al. (Reference Romeo, Ferrari, Saino, Wauters and Lanfranchi2012, Reference Romeo, Wauters, Ferrari, Lanfranchi, Martinoli, Pisanu, Preatoni and Sainu2014), Debenedetti et al. (Reference Debenedetti, Sáez-Durán, Sainz-Elipe, Galán-Puchades and Fuentes2014, Reference Debenedetti, Sainz-Elipe, Sáez-Durán, Galicia, Imaz, Galán-Puchades and Fuentes2015), López González (Reference López González2014), Meshkekar et al. (Reference Meshkekar, Sadraei, Mahmoodzadeh and Mobedi2014), Čabrilo et al. (Reference Čabrilo, Jovanović, Bjelić-Čabrilo, Budinski, Blagojević and Vujošević2016, Reference Čabrilo, Jovanović, Bjelić-Čabrilo, Budinski, Blagojević and Vujošević2018), Martínez-Rondán et al. (Reference Martínez-Rondán, Ruiz de Ybáñez, Tizzani, López-Beceiro, Fidalgo and Martínez-Carrasco2017), Galán-Puchades et al. (Reference Galán-Puchades, Sanxis-Furió, Pascual, Bueno-Marí, Franco, Peracho, Montalvo and Fuentes2018), Mazhari et al. (Reference Mazhari, Moosavi, Mostafavi, Esfandiari, Mobedi, Rahimi Esboei and Mowlavi2019), Islam et al. (Reference Islam, Farag, Hassan, Enan, Mohammadi, Aldiqs, Alhussain, Al Musalmani, Al-Zeyara, Al-Romaihi, Yassine, Sultan, Bansal and Mkhize-Kwitshana2024). Points either represent records from specific localities or central points for a general area, depending on the precision given in the respective publications.

Figure 2. Geographical and host distribution of Aonchotheca halli (Kalantarian, Reference Kalantarian1924). Top panel shows locations of records of ‘A. murissylvatici’ where these were from the stomach, or where there was a definite morphological identification. Bottom panels show pie charts representing the proportional frequency of records of A. halli at different host taxonomic levels. Based on 30 records drawn from publications cited in the main text or Fig. 1 and also Asakawa et al. (Reference Asakawa, Yokoyama and Fukumoto1983, Reference Asakawa, Hasegawa, Ohnuma, Tatsushima and Ohbayashi1992, Reference Asakawa, Koyasu, Harada, Krishna, Mekada and Oda1997).

For the practical purposes of future identification, these 2 species differ in several morphological criteria (and accurate descriptions can be found in Moravec [Reference Moravec2000] and Justine and de Roguin [Reference Justine and de Roguin1990]) but, if male worms are available, can be most easily distinguished by the difference in the length of spicules: A. annulosa has spicules that are about 4 times longer than those of A. halli (c. 0.20–0.25 vs c. 1.00 mm) (Table 2, Fig. 3). In females, A. halli has a small vulval appendage (Justine and de Roguin, Reference Justine and de Roguin1990; see also Read, Reference Read1949) and A. annulosa lacks such an appendage but usually has an elevated anterior vulval lip (Moravec and Baruš, Reference Moravec and Baruš1991) (Fig. 3). A key for the identification of the Aonchotheca species found in European rodents is provided in Table 2. For unambiguous differentiation of A. annulosa and A. halli that does not depend on skilled microscopic observation, the use of 18S rRNA gene DNA sequencing might be recommended. Our preliminary molecular results (manuscript in prep.) suggest this will easily distinguish between these 2 species. In a forthcoming study, we will be depositing reference sets of sequences for both species that will facilitate molecular identification.

Figure 3. Diagnostic features in Aonchotheca halli (Kalantarian, Reference Kalantarian1924) (A–D) and Aonchotheca annulosa (Dujardin, Reference Dujardin1845) (E–G). Scale bars indicate 0.1 mm. (A) Posterior region of male worm. (B) Posterior extremity of male worm with spicule in lateral view. (C) Terminal region of female reproductive tract. (D) Anterior region of female worm. (E) Posterior region of male worm, with partly evaginated spicule sheath, in sublateral view. (F) Terminal region of female reproductive tract. (G) Anterior region of female worm. A–D are based on specimens from the stomach of bank voles in Cornwall, UK; E–G are based on specimens from the intestine of wood mice (Apodemus sylvaticus) in Nottinghamshire, UK.

Table 2. A dichotomous key for the identification of Aonchotheca species in European rodents

Note: not included in the key, in wider Eurasia other records include the poorly known A. armeniaca, which has a spicule c. 1.1 mm in length and infects the small intestine of Citellus spp. and A. yannickchavali which infects the stomach of Bandicota spp. in Thailand and is distinguished by a very large tube-like projection associated with the vulva (see Veciana et al., Reference Veciana, Chaisiri, Morand and Ribas2016). Measurements are given in mm unless otherwise indicated.