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.