Adult stage

Redescription

(Based on specimens from intestines of S. araneus from Artybash village, Turochakskiy District, Altai Republic, Russia; figs 1 and 2.) Small-sized tapeworm (fig. 1). Gravid specimens 1.2–1.5 mm (1.4 mm, n = 11) long. Strobila flat, consisting of 35–50 (38, n = 11) proglottides: 6–9 juveniles (with primordia of male gonads), 6–9 with mature testes (with primordia of female gonads), 6–9 with hermaphroditic (with testes and female gonads), 8–15 postmature and 2–4 gravid. Strobilation gradual. Proglottides acraspedote, transversely elongate, in process of maturation increase somewhat in length (fig. 2a). Development and maturation of strobila follows pattern of typical protandry: primordia and maturation of male gonads appearing before primordia and maturation of female gonads. Female gonads appear in proglottides with mature testes and an almost formed male copulative apparatus. In the first female proglottis, only rudiment of the aporal testis remaining, internal seminal vesicle and seminal receptacle empty. Cirrus sac persisting in pregravid and gravid proglottides. In the last two female proglottides, young vesicular uterus distinct.

Fig. 1. General view of Urocystis prolifer Villot, Reference Stammer1880.

Fig. 2. Urocystis prolifer: (a) whole strobila; (b) rostellar apparatus complex; (c) hook; (d) egg. Abbreviations: Ro, rostellum; Rh, rhynchus; H, hooks; Gm, glandular matrix.

Scolex small, conical, 150–170 × 160–180 (161 × 167, n = 11), with rostrum (fig. 2a). Suckers subspherical, 80–83 × 80–84, with well-developed musculature, widely spaced and shifted to corners of scolex. Rostellar apparatus complex. Rhynchus small, 58–68 long, 55–57 wide, with well-developed own musculature consisting of circular and retractor muscle system. Rhynchus retractors extending from its top, divided into two bundles: one fixed at top of rostellar pouch, second at its equator. Surface of rhynchus corrugated (fig. 2b).

Rostellum subspherical, 30–35 × 35–40 (32 × 38, n = 8), with invagination, deeply immersed in rostellar pouch. Rostellar hooks 120–130 in number, very small, arranged in single row, 4–6 (5, n = 25) long (fig. 2c). Each hook associated with muscle bundle. Muscle fibres from several rostellar hooks merge, thus forming 14–16 retractors. Rostellar pouch voluminous, 80–83 × 100–140 (81 × 128, n = 8), reaching posterior margins of suckers. Ten bundles of retractor muscles extending from bottom of rostellar pouch and passing into thin layer of longitudinal musculature of strobila in neck region. Basal part of rostellar pouch filled with glandular matrix (fig. 2b). Neck clearly distinct from scolex.

Two pairs of osmoregulatory canals without transverse anastomoses. Ventral osmoregulatory canals 5–7 (5, n = 8) in diameter, dorsal canals 2–3 (3, n = 8) in diameter. Genital pores unilateral, dextral, opened in middle or somewhat anterior to middle of proglottis margin. Genital atrium simple, cylindrical, 8–16 deep (14, n = 8), surrounded by large glandular cells.

Male mature proglottides 15–20 × 135–150 (15 × 142, n = 11). Testes two, 15–18 × 20–28 (16 × 26, n = 11), oval, in one row, one poral and one antiporal, situated symmetrically in median field of proglottis (fig. 3a). Testes of neighbouring male proglottides adjacent. Cirrus sac cigar-shaped, elongate, thin-walled, 13–15 × 55–62 (13 × 57, n = 11), crossing poral osmoregulatory canals; not reaching midline of proglottis (fig. 3a). Cirrus short, 20−22 (22, n = 11), armed with small spines, covering only its proximal part (fig. 3b). Internal seminal vesicle ovoid, elongated, 10–12 × 15–25 (10 × 26, n = 11). External seminal vesicle 15–18 × 23–25 (16 × 26, n = 11), connected to cirrus sac by long and curved duct.

Fig. 3. Urocystis prolifer: (a) strobila fragment with male and female mature proglottids; (b) copulatory apparatus.

Hermaphroditic mature proglottides 15–20 × 150–175 (16 × 168, n = 11). Ovary sacciform, transversely elongate, 15–19 × 58–63 (16 × 61, n = 11), in centre of proglottis. Vitellarium compact, subspherical, 11–13 × 15–18 (11 × 16, n = 11), aporal and dorsal to ovary. Vagina thin-walled, opening ventral to cirrus sac. Copulatory part of vagina 25–28 (26, n = 11). Seminal receptacle pear-shaped, large, 15–16 × 25–26 (16 × 26, n = 11) (fig. 3a).

Young uterus vesicular. Gravid uterus sac-like, occupying entire median field, does not cross osmoregulatory canals (fig. 2a), containing 12–20 (13, n = 10) eggs. Eggs large, 45–48 × 56–60 (46 × 56, n = 11), with a sclerotized outer embryonic membrane (fig. 2d). Embryonated eggs lie freely in uterine cavity, not contacting closely with uterine epithelium, scattered one by one through breaks in uterine wall.

Amended generic diagnosis (modified after Czaplinski & Vaucher, Reference Czaplinski, Vaucher, Khalil, Jones and Bray1994). Proglottides much broader than long. Scolex with retractile, armed rostellum. Hooks numerous (more than 100), very small. Testes two or three, in transverse row, more or less overlapping female gonads. Ovary sacciform, transverse, in centre of proglottis. Vitellarium compact, subspherical, aporal to ovary. Cirrus sac not reaching middle of proglottis. Cirrus armed with small spines. Uterus sacciform, containing few eggs. Type-species U. prolifer Villot, Reference Stammer1880.

Distribution of the species in the Palaearctic

The monotypic genus Urocystis is included in many checklists of helminths of shrews from the western to eastern areas of the Palaearctic. The species has been recorded mostly in the taiga and forest zones (in temperate broadleaf deciduous and mixed forests) (fig. 4) (Vaucher, Reference Spassky and Andrejko1971; Prokopič & Matsaberidze, Reference Prokopic1972; Genov, Reference Genov1984; Sato et al., Reference Ryšavý and Prokopič1988; Anikanova et al., Reference Anikanova, Bespyatova, Bugmyrin and Yeshko2001; Kornienko, Reference Korneva, Kornienko and Jones2001; Irzhavsky & Gulyaev, Reference Haukisalmi, Hardman, Foronda, Feliu, Laakkonen, Niemimaa, Lehtonen and Henttonen2002; Dokuchaev et al., Reference Dokuchaev, Melnikova, Gulyaev and Chernyagina2003; Ribas et al., Reference Quentin and Beaucournu2003; Skolka et al., Reference Sheykina and Zhigileva2004; Binkiene, Reference Binkiene2006; Kornienko et al., Reference Kornienko and Ishigenova2008; Zubova et al., Reference Yushkov2008a, Reference Zarnowskib; Binkiene et al., Reference Binkiene, Kontrimavichus and Hoberg2011). Despite Vaucher's statement that U. prolifer is absent from Scandinavia (Vaucher, Reference Spassky and Andrejko1971), there are records of it in southern Finland (on the coast of the Gulf of Finland) (Haukisalmi et al., Reference Haukisalmi2010; Haukisalmi, Reference Haukisalmi2015). In addition, we have found this species in Sweden and Finland (Västernorrland County, Sweden, 62°45′N, 17°52′E; northern Savo, Finland 62°53′N, 27°40′E). According to our data and data in the literature, this species has not been recorded in the tundra and arid zones and some forest zone regions of the Palaearctic (north-east of the East European Plain, Russian Far East (Chukotka Peninsula), eastern Siberia (Yakutia and Tuva), south of the Lesser Caucasus (Armenia) and northern Kazakhstan) (Yushkov, Reference Villot1995; Movsessian et al., Reference Mariaux, Tkach, Vasileva, Caira and Jensen2006; Kirillov et al., Reference Joyeux2017; Kornienko et al., Reference Kornienko, Zubova, Gulyaev, Dokuchaev, Galaktionov and Dobrovolskij2018; Sheykina & Zhigileva, Reference Sadovskaja, Leonov, Mamaev and Oshmarin2018). The lack of records in these regions is likely due to the absence of intermediate hosts of the cestode (diplopods). Novikov (Reference Movsessian, Chubarian and Nikogosian1995) found three specimens of U. prolifer in S. isodon caught in the Selemdzha River basin (Magadan Oblast, Russian Far East). However, the coordinates of the cestodes (60°18′N) provided by the author do not correspond to the location of this river. The site of this record requires clarification.

Fig. 4. Localities of Urocystis prolifer in the Palaearctic. Own data are indicated by circles, literature data by triangles (see chapter “Distribution of the species in the Palaearctic”).

The cestode U. prolifer is characterized by not only an extensive geographical range but also wide host specificity. Definitive hosts of U. prolifer include S. araneus, S. isodon, S. minutus, S. caecutiens, S. gracillimus, S. daphaеnodon, S. roboratus, S. satunini, S. raddei, S. volnuchini, S. minutissimus, S. tundrensis, S. unguiculatus and Sorex shinto Thomas, 1905.

The characteristics of cestode infection (prevalence, abundance and intensity) vary significantly among different species of shrews in different parts of the Palaearctic. In a study of S. araneus in Bulgaria, no more than 5% of the shrews were infected by this tapeworm (Genov, Reference Genov1984). In a study in the territory of Belarus, only 2% of the studied common shrews were found to host U. prolifer (Shimalov, Reference Sato, Kamiya and Ohbayashi2012). In a study in Lithuania, approximately 20% of the examined common and pygmy shrews were infected with U. prolifer (Binkiene, Reference Binkiene2006). In our investigations in the European part of Russia and West and East Siberia, approximately a third of the studied shrews were infected with U. prolifer; in mainland and insular part of the Russian Far East, the prevalence of the cestode is lower. Despite the large regional variation in the prevalence of infection with this cestode, the intensity of infection is high across the study regions (table 2). There is some difference in infection levels between different species of shrews. In the Caucasus, S. raddei has the highest infection rate with U. prolifer. In Siberia (both western and eastern), all shrews species (except tundra shrew in the western, and pygmy shrew in eastern Siberia) have high level of infection with U. prolifer: the prevalence, abundance and intensity. At the same time, S. caecutiens has the highest infection rate with U. prolifer in all of the researched regions (table 2). About 30% of the common shrews from the European part of Russia and Siberia are infected with cestode. The prevalence of U. prolifer in S. minutus is low: single specimens of pygmy shrew are infected. In mainland and insular part of the Far East, the prevalence of U. prolifer is no higher than 20%. The lowest infection with U. prolifer is detected in S. daphaenodon, S. gracillimus and S. minutissimus. Some specimens of S. roboratus, S. minutissimus and S. minutus caught in the Khabarovsk Kraj are not infected with U. prolifer. Among the Japanese shrews, only the long-clawed shrew is infected with U. prolifer (table 2).

Table 2. Infection in shrews of the genus Sorex with Urocystis prolifer.

P, prevalence; SE, standard error; MA, mean abundance; I, intensity range; MI, mean intensity.

Larvogenesis of cysticercoid of U. prolifer in millipedes J. ghilarovi

Megalospheres lying in the body cavity of millipedes were found on the ninth DPI. The megalosphere is a morula-like structure covered with a thin fibrillar membrane (fig. 5a), reaching 52 in diameter.

Fig. 5. Postembryonic development of Urocystis prolifer: (a) megalosphere; (b) elongation and differentiation; (c) the formation of primary blastogens, leading to the formation of a saccular colony (d); (e, f) formation of a primary lacuna in secondary blastogens. Scale bars: (a) 20 μm; (b, c) 50 μm; (d–f) 30 μm.

In the following six days (10th–16th DPI), larvae were at the stage of elongation. From the megalosphere, a saccular maternal individual (blastomere) is formed (fig. 5b) with a primary lacuna, which increases in proportion to the growth of the larva and reaches 57 in length. The length of the elongated larva is 71–75.

On the 22nd DPI, metacestodes at stages of budding of primary blastogens were found. The maternal individual begins to bud off primary blastogens (fig. 5c), and a saccular colony of larvae appears (fig. 5d). The maximum colony length is 800 and the width ranges from 38 to 100. Each colony contains several dozen spherical and elongated larvae with transverse constrictions. Young blastogens of the colony do not have primary lacuna. During the period of growth and formation of the primary lacuna, daughter buds detach from the wall of the maternal individual (fig. 5e, f) and are capable of a new cycle of budding (fig. 6a). Blastogens with a primary lacuna reach 11–17 in diameter (fig. 5e).

Fig. 6. Postembryonic development of Urocystis prolifer: (a) secondary blastogens; (b) detached secondary blastogen at the stage of early morphogenesis of the scolex; (c) formation of suckers and rostellar apparatus; (d) morphogenesis of the scolex in undetached secondary blastogens; (e) late morphogenesis of the scolex and primordium of rostellar hooks; (f) fully developed urocyst. Scale bars: (a–f) 40 μm.

The stage of budding of secondary blastogens was found on 24th–25th DPI. This blastogen has narrow tail-like outgrowth connecting it to the maternal individual during the early stages of morphogenesis (fig. 6b). Subsequently, the secondary blastogen detaches from the maternal individual and develops separately.

On the 26th–28th DPI, metacestodes were at the stage of elongation and early morphogenesis of the scolex. At this stage, the larva measures 38–40 in width by 54–56 in length, and the size of the primordium of the scolex is 17–18 × 16–18 (fig. 6b). Subsequently, suckers are formed and rostellar hooks start their development on the rhynchus (fig. 6c). The scolexogenesis of secondary blastogens can occur in unseparated (fig. 6d) and separated individuals (fig. 6c). Thus, at least two asexual larval generations are observed in the ontogeny of U. prolifer. Characteristic features of this stage of larvogenesis are the absence of the primary lacuna and cercomer and the small size of the rudiments of the strobila and cysts (fig. 6c, d). There is no formation of excretory atrium.

At the stage of late morphogenesis of the scolex (29th–30th DPI), the measurements of larvae increase to 69–71 × 69–75; suckers are 25–33 × 24–28 (fig. 6e). Rhynchus, 25–30 wide in 24–26 long, is retracted before the encysting of the larva (fig. 6e). Encystation occurs without invagination of the scolex; the primordium of the cyst actively advances on the scolex. The two-layered cyst wall of the encysted larva is very thin and mobile. The cyst of the definitive urocyst lacks anterior and posterior obturator valves (fig. 6f). In addition, excretory bodies are completely absent (a unique feature among metacestodes).

The fully developed cysticercoids measure 54–80 × 46–75 (fig. 6f).