Introduction

The horse, Equus caballus, the ass, Equus asinus, zebras and their hybrids are hosts to a great variety of nematode parasites, most of which have traveled the world with their hosts. The nematodes normally parasitic in equids fall into seven suborders, 12 families, 29 genera, and 83 species. The great majority (64 of 83 species) are members of a single family, the Strongylidae. The 64 species, in19 genera, of the family Strongylidae, are the most common and economically important nematode parasites of horses. The 19 non-strongylid nematode species, scattered in 10 different families, are for the most part well known and easily identifiable (Lichtenfels, 1975) and are not included here.
The Strongylidae (common name strongylids) of horses—nematodes with a well-developed buccal capsule, a mouth collar with two leaf-crowns, and a strongyloid (common name of Superfamily Strongyloidea) copulatory bursa — can be separated (Lichtenfels, 1980) into two subfamilies: Strongylinae (common name strongylins), usually large or medium-sized with a globular or funnel-shaped buccal capsule; and Cyathostominae (common name cyathostomins), usually small to medium-sized with a cylindrical buccal capsule. In this treatise 14 species of Strongylinae of domestic equids are organized in 5 genera: Strongylus, Oesophagodontus, Triodontophorus, Bidentostomum and Craterostomum. This 5-genera system is accepted by most taxonomists except that Skrjabin and his students subdivided the genus Strongylus into three genera following Ershov (1943). The reasons we do not follow Ershov (1943) are given in the Discussion of the genus Strongylus. The 14 species of strongylins are relatively easy to identify, in comparison to the 50 closely related species of cyathostomins.
The 50 species of the Tribe Cyathostominea (all species of the subfamily Cyathostominae parasitic in equids) (Lichtenfels et al., 1998) are organized in 14 genera: Cyathostomum Molin, 1861 sensu stricto; Coronocyclus Hartwich, 1986, Cylicocyclus Ihle, 1922; Cylicodontophorus Ihle, 1922; Tridentoinfundibulum Tshoijo in Popova, 1958; Cylicostephanus Ihle, 1922; Skrjabinodentus Tshoijo in Popova, 1958; Petrovinema Ershov, 1943; Parapoteriostomum Hartwich, 1986; Poteriostomum Quiel, 1919; Gyalocephalus Looss, 1900; Hsiungia Kung and Yang, 1964; Caballonema Abuladze, 1937; and Cylindropharynx Leiper, 1911. More detailed discussion of the bases for plac¬ing various species in particular genera can be found in the Discussion following each key to species.
Because the cyathostomins have been historically difficult to identify, and because they have emerged as the most significant nematode pathogens of horses (Herd, 1990), we provide below a brief nomenclatural and taxonomic history and an introduction to the morphology of this group.
The Cyathostominea are the most common nematode parasites of horses and can cause considerable morbidity and mortality (Herd, 1990). Research activity on these nematodes is high because: 1) Larval cyathostominosis (previously cyathostomosis or cyathostomiasis), a syndrome in which large numbers of larvae emerge from the walls of the large intestine and caecum and cause severe colitis that may result in death, is recognised increasingly (Mair, 1994; van Loon et al., 1995); 2) Resistance to anthelmintics within the Cyathostominea has been reported widely (Fisher et al., 1992); and, 3) Biological control prospects, using nematode-trapping fungi, appear to be promising (Larsen et al., 1996).
Infections with these nematodes typically consist of very large populations and numerous species. A total of 50 species of cyathostomins are recognized as valid species parasitic in horses, donkeys, and zebras worldwide (Lichtenfels et al., 1998; Mathee et al., 2002), but 10 of these species have been reported only from zebras or donkeys, and a few others have been reported only rarely (Lichtenfels et al., 1998). Surveys worldwide have reported about 16 to 24 species of the Tribe Cyathostominea in most regions and from 4 to 14 species with a prevalence of 50 % or higher (Reinemeyer et al., 1984; Carvalho, et al., 1998: Lyons et al., 1999; Lichtenfels et al., 2001). However, the prevalence of the less common species is greatly underestimated. Chapman et al. (2002) reported that 9 to 15 species were found in a single animal when 200 worms were identified, but the number increased to 20 to 29 when all nematodes in a 5% aliquot were identified. Thus, most individual horses carry a burden of 5 to 10 common species (and several to many less common species), including many thousands (sometimes more than 100,000) of lumen-dwelling adult nematodes. In addition, populations of developing larval stages in the walls of the large intestine may be as large or larger than populations of adults in the lumen (Reinemeyer et al., 1984; Bucknell, et al., 1995).
The major challenges to understanding and controlling these parasites are the species complexity of the nematode populations, our inability to identify eggs in feces and the difficulty in identifying larvae on pasture. Adult nematodes can be identified to species by only a few authorities using comparative anatomy. Larval stages are exceptionally difficult to identify and eggs are impossible to identify to even the subfamily level. Research worldwide on the development of diagnostic DNA markers, on the testing of biological and biochemical control agents is hampered by the need to collect specimens from sacrificed horses. However, recent studies in Scotland (Kaye et al., 1998; McDonnell et al., 2000; Hodgkinson et al., 2001,) and in Australia (Hung et al., 2000), have examined molecular relationships of these species with a view to: 1) Preparing a predictive classification; and, 2) Developing molecular markers for use in identification of both pre-parasitic and parasitic stages.
This increased attention to strongylid nematode parasites of horses has resulted in the need for updated diagnostic keys to these parasites using readily recognizable characters and the most recent literature on their systematics.
This treatise is intended to serve as a basic working tool—providing easy identifications to genus and species of adult strongylid nematodes of equids. All strongylid nematodes normally parasitic in horses, the ass (and their hybrids), and zebras are included
This treatise consists of illustrated keys to genera and to species. The keys are illustrated with line drawings and halftone photomicrographs of each species. Illustrations are original unless noted otherwise.
A short discussion of the systematics of the genus and species is provided for each genus following the species descriptions. Species diagnoses and a synonymy of each species is provided. Geographic distribution, prevalence, and location in host are also given for each species.