The validity of the genus Cyathostomum and its type species C. tetracanthum have had a contentious history (Lichtenfels, 1975; Hartwich, 1986; Gibbons and Lichtenfels, 1999). The validity of Cyathostomum and its type species were finally established recently by the International Commission on Zoological Nomenclature (ICZN) (Opinion 1972 on Case 3075, June 2001). A few highlights of the history of the controversy and its solution are summarized here. This history is primarily about the generic level classification of the Cyathostominea. The history of species level differences will be discussed following the keys to species of the various genera.
Molin (1861) established Cyathostomum, with Strongylus tetracanthus as the type species, by monotypy. Looss (1900) recognized that Molin’s C. tetracanthum included several species and restricted the name to the most common species found by him in Egypt. Railliet (1923) proposed that Cyathostomum was a homonym of Cyathostoma Blanchard, 1849 and substituted Trichonema Cobbold, 1874. For many years both Trichonema and Cyathostomum were used for overlapping groups of species by various experts. Mclntosh established (International Commission ruling; Hemming, ed., 1943) that Cyathostomum was not a homonym of Cyathostoma and listed a synonymy of the type species (McIntosh, 1951). Lichtenfels (1975) reviewed the history of the controversary and followed McIntosh’s recognition of Cyathostomum, with C. tetracanthum as the type species. Hartwich (1986) discovered Molin’s (1861) type series of C. tetracanthum and determined that the species designated by Looss (1900) as C. tetracanthum is not present among Molin’s specimens. Hartwich selected and renamed C. catinatum Looss, 1900 as the true C. tetracanthum and renamed Looss' C. tetracanthum as C. aegyptiacum. In the interest of stability, the Sun City Workshop (Lichtenfels et al., , 1998) voted (with the concurrence of Dr. Gerhard Hartwich) to ask the International Commission on Zoological Nomenclature to validate the names of these species in use prior to Hartwich's proposal (Gibbons and Lichtenfels, 1999). Only Dvojnos and Kharchenko (1994) had followed Hartwich and referred to C. catinatum as C. tetracanthum. The ICZN ruled (Opinion 1972, 2001):
(1) Under the plenary power all previous fixations of' type specimens for the nominal species Strongy1us tetracanthus Mehlis, 1831 are hereby set aside and the specimen no. 087757.00 in the U.S. National Parasite Collection, Beltsville, Maryland, collected by A. Looss in 1899, is designated as the neotype.
(2) The name Cyathostomum Molin, 1861 (gender: neuter), type species by monotypy Strongy1us tetracanthus Mehlis, 1831, is hereby placed on the Official List of Generic Names in Zoology.
(3) The following names are hereby placed on the Official List of Specific Names in Zoology:
(a) tetracanthus Mehlis, 1831, as published in the binomen Strongylus tetracanthus and as defined y the neotype designated in (1) above (specific name of the type species of Cyathostomum Molin, 1861),
(b) catinatum Looss, 1900, as published in the binomen Cyathostomum catinatum.
(4) The following names are hereby placed on the Official Index of Rejected and Invalid Generic Names in Zoology:
(a) Cylichnostomum Looss, 1901 (a junior objective synonym of Cyathostomum Molin, 1861),
(b) Cylicostomias Railliet, 1901 (a junior objective synonym of Cyathostomum Molin, 1861).
(5) The following names are hereby placed on the Official Index of Rejected and Invalid Specific Names in Zoology:
(a) hexacanthum Wedl, 1856, as published in the binomen Sclerostoma hexacanthum (a junior objective synonym of Strongy1us tetracanthus Mehlis, 1831
(b) aegyptiacum Railliet, 1923, as published in the binomen Trichonema aegyptiacum and as defined by the lectotype designated by Gibbons & Lichtenfels (1999) (a junior objective synonym of Strongylus tetracanthus Mehlis, 1831).”
As can be seen in the lists of synonyms in the Outline Classification preceding this section (Table 1), many additional genera have been coined for the species collectively known as Cyathostomum, sensu lato. Major contributions and schemes of classification for this group were made by Ihle (1922), Ershov (1943), McIntosh (1951), K'ung (1964), Lichtenfels (1975), Hartwich (1986), Dvojnos and Kharchenko (1994), Lichtenfels et al. (1998) and Zhang and Kung (2002). In the following paragraphs the systems of these workers are described briefly.
The system of Ihle (1922) organized 20 species in seven groups — five of which he designated as subgenera — all in the genus Cylicostomum, which is a synonym of Cyathostomum, Ihle's five subgenera were Cylicostomum, Cylicocercus, Cylicocyclus, Cylicostephanus, and Cylicodontophorus. His other groups were the Brevicapsulatum-group and the Montgomeryi-group. Cram (1924) raised all the subgenera of Ihle (1922) to generic rank, placed the Brevicapsulatum-group in the genus Cylicobrachytus Cram, 1924, and the Montgomeryi-group in Cylicotoichus Cram, 1924.
The system of Mclntosh (1951) was essentially identical to that of Ihle (1922) as modified by Cram (1924) except that Cylicotoichus was omitted (probably because C. montgomeryi is a parasite of the zebra not known from domestic equines) and Cylicotetrapedon Ihle, 1925, was added.
The system of Ershov (1943) divided Cyathostomum, sensu lato into five genera including Trichonema, Cylicocyclus, Cylicodontophorus, Petrovinema Ershov, 1943, and Schulzitrichonema Ershov, 1943.
In 1964 K'ung reorganized Cyathostomum, sensu lato. He substituted Trichonema for Cylicostephanus and accepted Cyathostomum, Cylicocyclus, Cylicodontophorus, Cylicotetrapedon, Petrovinema, and Skrjabinodentatus Tshoijo, 1957.
Lichtenfels (1975) modified the earlier schemes evolved by Ihle, Cram, and Mclntosh as follows:
1. The genus Cylicocercus, which was distinguished primarily by the bent female tail, was eliminated by placing three species (C. alveatum, C. catinatum, and C. pateratum) in Cyathostomum and one species (C. goldi) in Cylicostephanus.
2. The species of Cylicotetrapedon, which were distinguished by the presence of teeth in the esophageal funnel, were included in Cylicostephanus, as suggested by Foster (1936).
3. The two species of Cylicobrachytus (C. prionodes and C. brevicapsulatum) were placed in Cylicocyclus following Ershov (1939) and K'ung (1964).
4. Cylicodontophorus ultrajectinus was moved to Cylicocyclus following Ershov (1939).
Lichtenfels (1975) modified the scheme of Ershov (1943) as follows:
1. The species of Trichonema were assigned to either Cyathostomum or Cylicostephanus.
2. Two species, C. pateratum and C. sagittatum, were moved from Cylicodontophorus to Cyathostomum.
3. The genus Schulzitrichonema Ershov, 1943, distinguished by teeth in the esophageal funnel and identical to Cylicotetrapedon Ihle, 1925, was eliminated and the species assigned to Cylicocyclus (C. leptostomus) or Cylicostephanus (C. asymetricus and C. goldi).
4. Petrovinema was eliminated by transferring the two species to Cylicostephanus.
5. Cylicodontophorus ornatum was moved to Cylicostephanus.
The system of Lichtenfels (1975) differed from the system of K'ung (1964) as follows:
1. Trichonema, was not accepted as a substitute for Cylicostephanus.
2. The species of the genus Petrovinema were included in Cylicostephanus.
3. The species of the genus Cylicotetrapedon were included in Cylicostephanus, except for C. leptostomum, which was placed in Cylicocyclus.
Hartwich (1986) restudied the Cyathostominea, providing useful new cephalic characters (see section on Morphology, below), a detailed history and a revision of the generic classification of the tribe. Except for his ill-fated proposal (see above) to change the names of Cyathostomum catinatum (to C. tetracanthum) and C.tetracanthum (to C. aegyptiacum), his classification of the tribe differed little from that of Lichtenfels (1975). Changes in the classification proposed by Hartwich (1986) included the establishment of 2 new genera: Coronocyclus to including 4 species - C. coronatus, C. labiatus, C. labratus and C. sagittatus – that Lichtenfels (1975) had included in Cyathostomum; and, Parapoteriostomum to include P. mettami, P. euproctus and P. schuermanni. Hartwich (1986) also differed with Lichtenfels (1975) by recognizing Petrovinema Ershov, 1943 as separate from Cylicostephanus. In addition, Hartwich (1986) did not include Gyalocephalus in Cyathostominea.
The complete classification of the strongylids of horses provided by Dvojnos and Kharchenko (1994), unique in its descriptions of parasitic fourth-stage larvae of many species, included a generic classification of the Cyathostominae of horses that differed somewhat from that of Hartwich (1986). Dvojnos and Kharchenko (1994) recognized 2 genera proposed by Tshoijo, in Popova, 1958 - Tridentoinfundibulum for the single species T. gobi, and Skrjabinodentus for S. caragandicus and S. tshoijoi, species not studied by Hartwich, - and differed with him by not recognizing Parapoteriostomum and by recognizing Cylicotetrapedon Ihle, 1925 for 2 species – C. asymetricus and C. bidentatus. Unfortunately, Dvojnos and Kharchenko (1994) followed the ill-fated proposal of Hartwich (1986) (see above) to rename C. catinatum as C. tetracanthum and to rename C. tetracanthum as C. aegyptiacum. Thus, users of the 1994 paper by Dvojnos and Kharchenko must be careful to avoid the confusion of names for these species used therein. To our knowledge, Dvojnos and Kharchenko (1994) were the only authors to follow the ill-fated proposal of Hartwich (1986) to rename these species.
In part, to avoid the confusion expected by the proposed renaming of C. tetracanthum and C. catinatum, a series of international workshops were convened at meetings of the World Association for the Advancement of Veterinary Parasitology in 1997, 1999 and 2001 (Lichtenfels, et al., 1998; 2002). The first of these workshops, at Sun City, Republic of South Africa, resulted in 2 major advances: 1) A checklist of 93 genus and species level names for 51 recognized species of the Cyathostominea (Lichtenfels et al., 1998); and, 2) An agreement, in the interest of stability, to ask the International Commission on Zoological Nomenclature (ICZN) to validate the names Cyathostomum tetracanthum and Cyathostomum catinatum of Looss, 1900. This nomenclatural adjudication was required to resolve an ambiguity over which species should bear the names C. tetracanthum and C. catinatum following the discovery by Hartwich (1986) of long-lost types of the former species. The ICZN granted the request to preserve the common usage prior to Hartwich’s (1986) discovery and to validate the names of Looss (1900) (Opinion 1972 on Case 3075 was published in Bulletin of Zoological Nomenclature, June 2001; details above). The Checklist of Recommended generic and specific names for the Cyathostominea was published (Lichtenfels et al., 1998) and differed only slightly from the names used in this treatise. The 1998 Checklist included the same 14 genera as in the present treatise (Table 1), and differs from the generic system of Dvojnos and Kharchenko (1994) in not recognizing Cylicotetrapedon, and by recognizing Parapoteriostomum. The number of species recognized in the present treatise (50) differs from the 1998 checklist by the addition of Cylicocyclus asini, a new species described by Matthee, Krecek and Gibbons (2002), and Cylicocyclus adersi and Cylicocyclus gyalocephaloides, 2 recently redescribed species listed in the 1998 checklist as species inquirendae; and by the recognition herein of only 3 of the 8 species of Cylindropharynx listed in the 1998 checklist.
In illustrated keys to nematode parasites of horses, Zhang and Kung (2002) followed the generic classification of the Cyathostominea recommended by Lichtenfels et al. (1998).