A revised checklist of Cooperia nematodes (Trichostrogyloidea), common parasites of wild and domestic ruminants

Gastrointestinal nematodes of the genus Cooperia Ransom, 1907, belong to the most important group parasitizing ruminants, trichostrongylids (Amarante et al., 2014). Up to now, more than 30 species were proven within this genus (Skrjabin et al., 1954; Yamaguti, 1961; Rees, 2018; Raműnke et al., 2018). However, the detailed review of taxonomic and systematic studies discovered new information in validity of some taxa. A correct species designation is important from both, the theoretical and practical point of view since mixed gastrointestinal nematode infections are common in domestic ruminants and various parasite species may differ significantly in their fertility and pathogenicity (Amarante et al., 2014). Shared infections among wild and domestic ruminants also occur, and co-infections with other trichostrongylid parasites, such as Ostertagia spp. or Haemonchus contortus, are common (Vlassoff & McKenna, 1994).

Nematodes of the genus Cooperia parasitize the small intestines in a wide spectrum of domestic and wild ruminants. Several species have a worldwide distribution as well as broad ruminant host spectrum. Meanwhile, others have been described only in a single host species. In fact, Cooperia spp. are considered less pathogenic than the other gastrointestinal roundworms of cattle and sheep. On the other hand high worm burden may substantially reduce the production in host animals where the infection has been associated with their inappetence and insufficient weight gain (Raműnke et al., 2018).

The abundance of various species of the genus Cooperia can be different depending on climate conditions and geographic location. In general, a higher temperature and a humid environment are more favourable for these parasites (Gibbons, 1981). On average, a richer species spectrum was referred in Africa than in Europe or the North America. The infectivity of Cooperia spp. in cattle and goats reaches nearly 40 % in subtropical or tropical regions (Coelho et al., 2012; Mahmuda et al., 2012; Kulišić et al., 2013; Radavelli et al., 2014). In livestock, Cooperia spp. have recently become the most prevalent parasites in the United States. This is partially due to the widespread use of veterinary drugs, macrocyclic lactones what resulted in reduced drug effectiveness (Stromberg et al. 2012). The trichostrongylid roundworms including Cooperia spp., have acquired a multi-species anthelmintic resistance what was, for instance, experimentally demonstrated on cattle in the western parts of the United States (Gasbarre et al. 2009; Edmonds et al. 2010).

Nematodes of the genus Cooperia are monoxenous parasites with a direct life cycle where pre-parasitic larval phase is completely free-living. Eggs, produced by females located in host intestine, are passed through host faeces and hatch in the “faecal pat”. The first stage larvae (L₁) feed on soil and faecal bacteria. Than, two subsequent moults to L₂ and L₃ are completed between 24 and 36 hours (Ciordia & Bizzel, 1963; Kotrlá et al., 1984). The temperature range for larval development is 5 °C and 33 °C (Knapp-Lawitzke et al., 2016). The L₃ larvae do not feed and are enclosed by a sheath composed of the retained L₂ cuticle. They migrate from the “faecal pat” to the grass, where they develop within 1 to 6 weeks (depending on the time of year) and become infectious to the host (Fiel et al., 2012). Infective larvae can survive for up to one year, until they are swallowed by the ruminant host (Kotrlá et al., 1984). Then the L₃ larvae release from the sheath, move into mucosa of small intestine, and undergo the third and fourth moults to L₄ and L₅ larvae. Within 2-3 weeks, the L₅ larvae develop into sexually mature adult males or females (Jennings et al., 1966; Leland, 1967; Kotrlá et al., 1984). The fertilised females then begin producing eggs and entire cycle repeats.

Trichostrongylids, including Cooperia spp., have developed an exclusive strategy to arrest their development in the host via hypobiosis during conditions that are unfavourable for the free-living stages (Michel et al., 1974; Armour & Duncan, 1987). For instance, large number of arrested L₄ larvae of Cooperia spp. may persist in a host digestive tract for up to six months (Kotrlá et al., 1984). Thus, outbreaks of this parasitic disease can occur at times when natural infection would not arise (Armour & Duncan, 1987; Eysker, 1993).

The revision of the current species spectrum is based on the fundamental taxonomic works of Skrjabin et al. (1954) and Yamaguti (1961) and subsequent publications about the taxonomy and systematics of the Cooperia spp. The data included in various current electronic databases were also considered and scrutinised.

This article does not contain any studies with human participants or animals by any of the authors.

According to the most relevant systematics of Nematoda (de Ley & Blaxter, 2002), the genus Cooperia belongs to the class Chromadorea, order Rhabditida, suborder Strongylida, superfamily Trichostrongyloidea, and the family Trichostrongylidae. Based on morphology and DNA analyses, the family Trichostrongylidae has been recognised as monophyletic, comprising of six subfamilies (Durette-Desset, 1974; Lichtenfels et al., 1997). Of these subfamilies, primarily Ostertagiinae, Haemonchinae, Trichostrongylinae and Cooperiinae include economically important and taxonomically problematic genera (Wyrobisz et al., 2016).

The genus Cooperia is a well-defined group of species that are characterised by a typical anterior cephalic cuticular swelling (vesicle), transverse cuticular striations in the oesophageal region, a pair of typical spicules, a lyre-shaped dorsal ray within the male bursa, and the special morphology of cuticular ridges system - so called synlope system (Lichtenfels et al., 1997). Both males and females are relatively small, measuring from 5.0 to14.8 mm (Skrjabin et al., 1954). The buccal capsule is reduced. Three lips and small papillae surround the mouth (phasmids are present). The oesophagus possesses the usual cylindrical shape and it is slightly thickened distally. The excretory pore is situated near the end of oesophagus (Durette-Desset, 1974). Cuticular ridges (striations) longitudinally stripe most of the outer surface of the body; this body surface type was named synlope by Durette-Desset (1969). The striations assist in the locomotion of the worm or in its attachment to the host stomach or intestinal mucosa. The number and pattern of the cuticular ridges may serve as additional diagnostic characteristics.

Males have a large bursa with two lateral lobes and a distinctly demarcated dorsal lobe, typical for the genus Cooperia. Each lateral lobe contains six bursal rays: ventral rays are widely separated; postero-lateral ray is separated from the laterals. The dorsal ray is lyre-shaped. Two identical and relatively short spicules up to 420 μm long are well sclerotized, having a rounded end and a longitudinal line pattern. They display a distinct wing-like expansion in the middle region and often bear ridges. Although several morphotypes might exist, the morphology of spicules is species specific. There is no gubernaculum (Skrjabin et al., 1954; Walker & Becklund, 1968; Gibbons, 1981).

Females are didelphic and the excretory pore is situated not far from the anterior end. The vulva lies in the posterior quarter of the body and the ovijector is well developed. Vulvar flaps are limited to the immediate vicinity of the vulva and they expand only slightly from the surrounding cuticular ridges. The longitudinal striations are interrupted in the vulvar region. The tail is sharply pointed and lacks spine. The eggs are thin-shelled and vary from ovoid to elongated shape (Skrjabin et al., 1954; Durette-Desset, 1974; Gibbons, 1981).

The comprehensive lists of the Cooperia species were published by Skrjabin et al. (1954) and Yamaguti (1961). The first inventory contained 22 species while the second one 21 species. The differences were that Skrjabin et al. (1954) suppressed C. okapiae as a synonym of C. okapi while Yamaguti (1961) suppressed C. fieldingi and C. bisonis as synonyms of C. punctata and C. oncophora, respectively. This information is marked in Table 1. Later on, several other species were originally described, re-described, synonymised with congeners, or designated as species inquirendae (Isenstein, 1971a; Lichtenfels, 1977; Gibbons, 1981; Newton et al., 1998;Ramünke et al., 2018).

For instance, Gibbons published in 1981 a merit work revising African species of the genus Cooperia. Out of 20 species reported from Africa, 14 were redescribed, four (C. borgesi, C, hippotragusi, C. minor, and C. reduncai), were considered as species inquirendae, and two (C. africana and C. surnabada) were discussed as synonyms of valid species C. punctata and C. oncophora (see Table 1).

The species C. surnabada has had adaptable story and now is considered a synonym of C. oncophora. This proposal was first published by Isenstein (1971a) on the basis of mating experiments and high morphological similarities between both species. Later, Lichtenfels (1977) tried to show that those two species are valid on the basis of different cuticular ridge patterns (synlope). Nevertheless, the invalidity of C. surnabada was finally accepted by Gibbons (1981), Humbert & Cabaret (1995), Lichtenfels et al. (1997), Newton et al. (1998), and Raműnke et al. (2018) (Table 1).

Cooperia mcmasteri was initially synonymized with C. surnabada by Karamendin (1967) and Lichtenfels (1977) and then later, with C. oncophora (Ramünke et al., 2018).

Two other morphologically similar species are C. spatulata and C. punctata (Table 1). Walker & Becklund (1968) thoroughly compared their morphology and revealed slight differences in the shape of the concavity and ventral flange of the spicule. They also found overlapping in other features, such as size and the spicule length. The authors therefore confirmed a very high level of similarity between the congeners. Using molecular analysis, Ramünke et al. (2018) recently found out extreme similarities between the two species and suggested C. spatulata as a synonym of C. punctata. This was also confirmed by the latest analysis performed by Sun et al. (2020) (Table 1).

Cooperia asamati represents a special issue of the controversial species. The species name C. asamatiSpiridonov, 1985 is currently reported in several Web databases (Fauna Europaea; YRMNG; BioLib.cz, Nederlands Soortenregister ID: 136135; Global Biodiversity Information - Facility ID: 7034224; YRMNG ID: 11474162 - Rees 2018). The main European zoological index, Fauna Europaea (de Jong, 2014), refers the occurrence of C. asamati exclusively from the Netherlands despite the facts that no further data are available. Moreover, the original description of C. asamati does not exist at all. The Spiridonov paper (1985) described the rhabditid nematode species Angiostoma asamati n. sp. from slugs in Kyrgyzstan, and the species name of A. asamati was dedicated a Kyrgyz’s colleague (S.E. Spiridonov, personal communication). This nematode has never been re-described or transferred into another genus. Therefore, we consider C. asamati as nomen nudum.

Overall, 19 species of the genus Cooperia are currently recognized as valid (Table 1). Four of these, the C. curticei, C. oncophora, C. pectinata and C. punctata occur nearly worldwide, remaining taxa have been found on selected continents (Table 1). Perhaps the most thorough research was carried out in Africa (Gibbons, 1981; Boomker, 1982; Boomker & Taylor, 2004) but many regions of the world have not been sufficiently explored. In Europe, only C. curticei, C. oncophora, C. pectinata, and C. punctata represent the valid taxa.

Due to polymorphism and/or inter-species hybridization which may occur within the members of the genus (Wyrobisz et al., 2016) species boundaries of currently deemed valid Cooperia spp. may be rather unclear. For instance, the experimental evidence of successful interspecific cross-breeding between C. oncophora and C. pectinata was provided by Isenstein (1971b) where the hybrid generation of nematodes had morphological characters from both purebred parental species. This phenomenon is particularly important since mixed infections comprising several Cooperia species may occur in ruminant hosts (Giudici et al., 1999).

In conclusion it is evident that the true species spectrum can be objectivized solely with modern integrative approaches. As yet, only several molecular studies have been applied to the Cooperia spp. The first molecular characteristics of C. oncophora were accomplished by van der Veer & de Vries (2003) and van der Veer et al. (2003). In addition, various genes were sequenced from C. punctata and C. oncophora in North and South America (Avramenko et al., 2015, 2017). Amarante et al. (2014) developed the rDNA marker specific for C. curticei. Recent study of Ramünke et al. (2018) provided molecular data about two mitochondrial and two nuclear genes that allow diagnosis of four common Cooperia spp. commonly infecting cattle. Currently, C. oncophora, as a model species, is being investigated for transcriptomic data, and the complete genome sequencing is expected to be completed at the Washington University Genome Sequencing Center (Borloo et al., 2013; Heizer et al., 2013; https://en.wikipedia.org/wiki/ Cooperia_oncophora). The latest comparative analysis of mitochondrial DNA of Chinese Cooperia sp. showed probable existence of a new species. However, further integrative studies involving classical taxonomy, population genetics and probably cytogenetics are urgently needed to accomplish.