Medium stomach worms in ungulates, usually referred to the Ostertagiinae (Trichostrongyloidea), remain economically important parasites among domestic and free-ranging livestock throughout the world. Diversity for this abomasal parasite fauna is relatively well defined for those nematodes occurring globally in cattle, sheep, goats, and some farmed cervids (Hoberg et al., 2001; Haigh et al., 2002), although accurate taxonomy continues to be hindered by confusion over the application of polymorphism among males representing single species (Dróżdż, 1995; Hoberg et al., 1999). Concepts for species-limits have also been changing in this group as a consequence of cryptic diversity revealed through molecular systematics combined with detailed comparative morphology (Criscione et al., 2005). Such is best exemplified within Teladorsagia Andreeva and Satubaldin, 1954 in which T. circumcincta (Stadelman, 1894) has been shown to include an array of cryptic species in domestic and free-ranging artiodactyls (Hoberg et al., 1999; Leignel et al., 2002; Grillo et al., 2007); the occurrence of species complexes is predicted to be a generality across other related genera among the ostertagiines but remains to be evaluated. An accurate definition of diversity, including reliable taxonomy encompassing morphological and molecular criteria extending to population genetics, in conjunction with comprehensive data for host and geographic distributions, are the cornerstones for understanding epizootiolgy and patterns of emerging disease across a mosaic of rapidly changing environments and for development of efficient and robust measures for control of parasitism in ungulates (Hoberg, 1997; Brooks and Hoberg, 2000, 2006, 2007; Hoberg, Polley et al., 2008).

Although the global fauna and its intricate structure of endemic and introduced species in most biogeographic regions is becoming understood (Hoberg et al., 2004; Hoberg, Polley et al., 2008), previously unrecognized genera and species continue to be discovered, particularly in Africa (Hoberg and Abrams, 2008; Hoberg, Abrams, and Ezenwa, 2008). One third of known genera of ostertagiines are endemic to Africa; yet, comprehensive surveys for many species of ungulates have not been conducted, and parasitological examinations have often been based on relatively few hosts or have focused on relatively restricted geographic zones. Over the last century, however, considerable details of the African ostertagiine and trichostrongyloid fauna in both domestic and free-ranging ungulates have been revealed (e.g., Le Roux, 1931; Mönnig, 1932; Round, 1962, 1968; Sachs and Sachs, 1968; Gibbons, 1974, 1977); notable here are the substantial body of surveys and articles addressing parasite faunas among domestic and free-ranging ungulates from South Africa by J. Boomker, I. G. Horak, and their colleagues and from eastern Africa by L. M. Gibbons and L. F. Khalil (summarized in Hoberg, Abrams, and Ezenwa, 2008).

The global ostertagiine fauna currently encompasses 14 genera (Hoberg and Abrams, 2008); aspects of this proposed generic taxonomy adopt components of a diversity of opinions presented for these nematodes over the past 70 yr (e.g., Travassos, 1937; Skrjabin et al., 1954; Andreeva, 1956; Dróżdż, 1965; Durette-Desset and Chabaud, 1981; Durette-Desset, 1982, 1983, 1989; Gibbons and Khalil, 1982; Jansen, 1989; Durette-Desset et al., 1999). Among the ostertagiines, 8 genera are characterized by males in which the lateral rays of the copulatory bursa describe a 2-2-1 pattern (defined by Durette-Desset and Chabaud, 1981; Durette-Desset, 1983). Specifically, these include Africanastrongylus Hoberg, Abrams and Ezenwa, 2008, Cervicaprastrongylus Gibbons and Khalil, 1982, Hamulonema Hoberg and Abrams, 2008, Hyostrongylus Hall, 1921, Mazamastrongylus Cameron, 1935, Sarwaria Dróżdż, 1965, Spiculopteragia (Orloff, 1933), and Teladorsagia (e.g., Durette-Desset, 1983; Gibbons and Khalil, 1982; Hoberg, Abrams, and Ezenwa, 2008; Hoberg and Abrams, 2008). Among 6 remaining genera, a 2-1-2 bursal pattern is characteristic, being represented in species of Camelostrongylus Orloff, 1933, Longistrongylus Le Roux, 1931, Marshallagia (Orloff, 1933), Orloffia Dróżdż, 1965, Ostertagia Ransom, 1907, and Pseudomarshallagia (Roetti, 1941).

In the current study, we propose establishment of a new genus among the ostertagiines to accommodate unique nematodes in ungulates from eastern Africa originally collected by Dr. John Bindernagle during the 1960s. These specimens were initially examined by W. W. Becklund and M. L. Walker at the U.S. National Parasite Collection in the late 1960s and early 1970s, but definitive taxonomic decisions were not developed. Studies of materials in the Bindernagle Collections were reinitiated during broader investigations of the ostertagiines, which led to proposals for Africanastrongylus and Hamulonema (Hoberg and Abrams, 2008; Hoberg, Abrams, and Ezenwa, 2008). Here, we diagnose the remaining new generic-level taxon evident in these collections, and explore aspects of the history and distribution for ostertagiines in the African and global faunas.

MATERIALS AND METHODS

Specimens of a previously undescribed ostertagiine nematode included 10 males and 9 females in a kob, Kobus kob (Erxleben), and 2 females in a kongoni (hartebeest), Alcelaphus buselaphus jacksoni (Pallas), from Uganda collected by J. Bindernagle in 1964. All materials representing this unrecognized species and new genus are held in the U.S. National Parasite Collection, U.S. Department of Agriculture, Beltsville, Maryland (USNPC); specimens are held in vials in a solution of 70% ethanol with 4% formalin by volume.

As a basis for comparisons within the Ostertagiinae, specimens of Longistrongylus banagiense (Gibbons, 1972), L. curvispiculum (Gibbons, 1973), L. meyeri Le Roux, 1931, L. sabie (Mönnig, 1932), L. schrenki (Ortlepp, 1939), and L. thalae (Troncy and Graber, 1973) were examined. Observations, notes, and line drawings from morphological studies of the type specimens for L. albifrontis (Mönnig, 1931), L. namaquensis (Ortlepp, 1963), L. sabie, and L. schrenki by W. W. Becklund and M. L. Walker (U.S. National Parasite Collection, unpublished documents) were also used in our current evaluations. A male specimen of Pseudomarshallagia elongata (Roetti, 1941), permanently mounted in glycerine jelly; transverse sections; and dissected spicules were examined. Redescriptions of P. elongata Roetti, 1941 by Graber and Delavenay (1978), Gibbons (1981a), and Gibbons and Khalil (1982) were also used for comparisons because of the unavailability of the type specimens or larger series of vouchers for evaluation. Specimens and sources for species of ostertagiine nematodes used in comparative morphological studies are listed (Table I). Specimens of Haemonchinae from the USNPC used in the current study are listed in Hoberg et al. (2004). Additionally, multiple lots of Graphidium strigosum (Dujardin, 1845) from leporid hosts included USNPC 16895, 18065, and 66335. These latter specimens were necessary to establish the context for certain morphological attributes among the Ostertagiinae.

Nematodes were prepared as temporary whole mounts cleared in phenol-alcohol (80 parts melted phenol crystals and 20 parts absolute ethanol) and examined with interference contrast microscopy. The synlophe was studied in whole mounts, with particular attention to the pattern of ridge systems in the cervical zone and their extent posteriad in males and females consistent with prior studies among the ostertagiines (Lichtenfels et al., 1988; Hoberg et al., 1999). Transverse sections were hand cut with a cataract knife and mounted in glycerin jelly. Sections were used to examine structure and to count the number of ridges in a single male and female at the esophageal-intestinal junction (EIJ), one fourth, midbody, and three fourths of total body length as determined from the anterior. Sectioning was restricted to 2 specimens because of the limited number of worms that had been collected. Additional counts of ridges were based on reconstructions from whole-mounted specimens.

The male specimens were evaluated on the basis of the copulatory bursa, spicules, and genital cone. Bursal ray patterns were determined and described using the system of Durette-Desset and Chabaud (1981) and Durette-Desset (1983). Papillae of the genital cone and rays of the bursa followed the numbering system of Chabaud et al. (1970). The structure of the ovejectors was evaluated based on recent definitions and descriptions among related nematodes (Lichtenfels et al., 2003). Owing to the difficulty in consistently discerning the division of the sphincter region S2 from the vestibule, the anterior and posterior portions of the vestibule, relative to the vulva, and adjacent portions of the sphincters (S1 and S2), were measured as a single unit. All measurements are given in micrometers, unless specified otherwise. In the description and tables the sample size (n =) is followed by the range and mean ± 1 SD in parentheses.

Taxonomy for hosts follows Wilson and Reeder (1993) in all text and tables. Host listings have been modified from those reported in the original literature to reflect current use and understanding of ungulate taxonomy.

RESULTS

Field collections for survey of helminth diversity in ungulates from eastern Africa revealed the occurrence of abomasal nematodes. Kob, and kongoni from the West Acholi District of Uganda, were found to be naturally infected with nematodes representing a previously undiagnosed genus and undescribed species of ostertagiine with a 2-1-2 bursal form.

MORPHOLOGICAL DIAGNOSIS AND DESCRIPTION

DISCUSSION

Robustostrongylus aferensis gen. nov. et sp. nov. represents a new ostertagiine, among those with a 2-1-2 bursal pattern, in which males are monomorphic. Assessment of distribution, diversity, and records of occurrence for ostertagiines across Africa (summarized in Hoberg and Abrams, 2008; Hoberg, Abrams, and Ezenwa, 2008) suggests the absence of polymorphism among males of R. aferensis as defined for this subfamily (e.g., Dróżdż, 1995). Across the global ostertagiine fauna, monomorphic males are known from 4 of 7 genera where a 2-1-2 bursal pattern is characteristic (Camelostrongylus, Longistrongylus, Pseudomarshallagia, and Robustostrongylus) and from 6 of 8 genera defined by a 2-2-1 bursal formula (Africanastrongylus, Cervicaprastrongylus, Hamulonema, Hyostrongylus, Mazamastrongylus, and Sarwaria) (Hoberg and Abrams, 2008). Interestingly, polymorphism seems to be most often manifested among ostertagiines with distributions in the Palearctic and across the Holarctic, i.e., Marshallagia, Orloffia, Ostertagia, Spiculopteragia, and Teladorsagia. Perhaps this reflects an environmental component (temperature or humidity) in the origins and drivers that serve to maintain the balanced polymorphism for structurally discrete males among species that characterize these primarily northern or high-latitude genera (Hoberg and Abrams, 2008; Hoberg, Polley et al., 2008).

Globally, ostertagiine faunas and more generally those of gastrointestinal nematodes in ungulates are defined by an intricate mosaic structure composed of endemic and introduced species (Hoberg et al., 1999; Hoberg, Polley et al., 2008). Among ostertagiines, there are 15 genera in the global fauna, with 5 being endemic to Africa (Africanastrongylus, Hamulonema, Longistrongylus, Pseudomarshallagia, and Robustostrongylus); although generic-level endemism is high, there are relatively few species limited solely to Africa (Hoberg, Abrams, and Ezenwa, 2008). There are 26 species partitioned in 10 genera that occur in African mammals and, among these, 22 species in 8 genera are endemic (Hoberg and Abrams, 2008; Hoberg, Abrams, and Ezenwa, 2008). Among those occurring in free-ranging ungulates, most genera are monotypic, but highly discrete morphologically, including Africanastrongylus (1 species), Hamulonema (2), Longistrongylus (8), Pseudomarshallagia (1), and Robustostrongylus (1). Also, 9 endemic species are known among Ostertagia (5), Cervicaprastrongylus (2), and Hyostrongylus (2), and, for the latter 2 genera, distributions encompass some non-ungulate hosts such as lagomorphs and primates, respectively (Hoberg, Abrams, and Ezenwa, 2008). In contrast, non-endemics, primarily associated with introduced and domestic ungulates, represent species that are geographically widespread around the globe, including Hyostrongylus (1), Marshallagia (1), Ostertagia (1), and Teladorsagia (1). The center of diversity for the ostertagiines is outside of the African region, although this continent has been significant in the radiation of this nematode fauna (Daubney, 1933; Hoberg, Abrams, and Ezenwa, 2008).

Endemism in the contemporary fauna may be an indicator of the temporal duration and degree of isolation of ostertagiines and other gastrointestinal nematodes among African ungulates (Hoberg et al., 2004). Among the Cooperiinae, another related subfamily among the trichostrongyloids, there are both a high number of endemic genera (5 of 8, including Cooperioides Daubney, 1933; Impalaia Mönnig, 1923; Megacooperia Khalil and Gibbons, 1976; Minutostrongylus Le Roux, 1936; and Ortleppstrongylus Durette-Desset, 1970) and some taxa, particularly Cooperia Ranson, 1907, that contain numerous endemic species primarily associated with bovids as hosts (Gibbons 1978, 1981b; Gibbons and Khalil, 1982; Khalil and Gibbons, 1976; Durette-Desset, 1983; Hoberg and Lichtenfels, 1994; Durette-Desset et al., 1999). Haemonchinae (specifically Ashworthius, Haemonchus, and Leiperiatus Sandground, 1930) diversified in large part in association with ungulates in Africa (Gibbons, 1979; Hoberg et al., 2004) and collectively with Cooperiinae must be regarded as a strongly tropical-adapted fauna (Hoberg, Abrams, and Ezenwa, 2008).

Parallel radiations in Africa are postulated for some ostertagiines, cooperines, and haemonchines, although it is not clear whether periods of diversification across these disparate nematode taxa and their artiodactyl hosts were congruent on spatial and temporal scales (Hoberg et al., 2004; Marcot, 2007; Hoberg, Abrams, and Ezenwa, 2008). Diversification in Haemonchus was associated with successive waves or episodes of biotic expansion by ungulates from Eurasia into Africa and sequential host switching by nematodes extending since the Miocene (Hoberg et al., 2004). Radiation among parasite groups coincided with periods of ecological perturbation and variation in climate which influenced ecological structure for artiodactyl faunas in Africa during the late Tertiary (Vrba, 1985, 1995; Vrba and Schaller, 2000). Host switching beyond ungulates is also seen among the Cooperiinae (in Bathyergidae for Ortleppstrongylus; in Lagomorpha for some Impalaia) and also among Ostertagiinae (Cervicaprastrongylus in Lagomorpha; Hyostrongylus in Pongidae) in Africa. Such distributions are indicative of colonization events involving mammalian groups occupying habitat and in sympatry with artiodactyls, and ecological structure promoting host switching (Hoberg and Brooks, 2008).

Geographic and host colonization are emerging as primary mechanisms involved in diversification of complex host–parasite assemblages (Hoberg and Brooks, 2008). Ecological perturbation driven by episodic variation in climate was an important determinant for biotic expansion, isolation, and development of faunal associations across sub-Saharan Africa since the Miocene. These processes that have functioned in evolutionary time continue to be manifested in shallow ecological time where a breakdown in ecological isolation leads to expansion of invasive species and emergence of disease in contemporary systems involving both free-ranging and domestic ungulates (Hoberg, 1997; Brooks and Hoberg, 2006, 2007). Tropical-adapted faunas, including haemonchines, cooperiines, and some ostertagiines have been introduced or translocated with ungulate hosts around the world. In temperate to boreal latitudes, current regimes for global warming and alteration in precipitation are predicted to be associated with novel patterns of host and geographic associations for this fauna, potentially driving the emergence of disease (Hoberg, Polley et al., 2008). Thus, knowledge of biodiversity extending from species to population structure is critical, particularly when explored in the context of historical determinants that may either limit or serve to promote expansion by an array of pathogens into new geographic settings and host groups.