The first published finding and description of the oocysts/sporocysts of Sarcocystis sp. in the Tengmalm’s owl was made by Wiesner [6] in Europe. Other Sarcocystis using owls as definitive hosts are: S. espinosai (Espinosa, Sterner, Blixt et Cawthorn, 1988) Odening, 1997 in the northern saw-whet owl Aegolius acadicus (Gmelin, 1788) from the USA [22]; S. dispersa Černá et Sénaud, 1977 in the long-eared owl Asio otus (Linnaeus, 1758), barn owl Tyto alba (Scopoli, 1769), masked owl T. novaehollandiae (Stephens, 1826) and Ninox novaeseelandie (Gmelin, 1788) in the Czech Republic and Australia [23 − 27]; S. rauschorum Cawthorn, Gajadhar et Brooks, 1984 in the snowy owl Bubo scandiacus (reported as Nyctea scandiaca) (Linnaeus, 1758) from Canada [27]; S. scotti Levine et Tadros, 1980 (this species is considered invalid by [28]) and S. sebeki (Tadros et Laarman, 1976) Levine, 1978 both in the tawny owl Strix aluco Linnaeus, 1758 from Europe [29, 30]; and S. strixi in the barred owl Strix varia from USA [31]. Hoberg et al. [32] reported a coccidian (resembling Frenkelia or Sarcocystis) in the northern spotted owl Strix occidentalis caurina Xantus de Vesey, 1860 from the USA, although the proper identity of the parasite was undetermined. There also are reports of owls acting as intermediate hosts for S. falcatula Stiles, 1983 in the great-horned owl Bubo virginianus (Gmelin, 1788) from the USA [33] and S. otus Krone, Rudolphi et Jakob, 2000 (invalid species according to [28]) in As. otus from Germany [34]. Most of these were solely morphologically studied, while only sporocysts of S. dispersa in Tyto alba (18S rRNA) [35] and S. strixi (18S rRNA, 28S rRNA and cox1) [31] were morphological and molecularly characterized.
The size of fully sporulated sporocysts of Sarcocystis espinosai, S. rauschorum, S. sebeki and S. strixi are within the range for those of the new species (9.5 − 14.0 × 8.0 − 12.0 µm, 9.6 − 14.0 × 7.0 − 10.0 µm and 11.2 − 13.7 × 8.8 − 10.9 µm vs 11.85 − 13.52 × 7.77 − 9.25 µm), and partially with the sporocysts and oocysts of Sarcocystis sp. in S. o. caurina, S. dispersa and S. sebeki (11.0 − 12.0 × 5.0 − 6.0 µm, 11 − 14 × 8 − 11 µm and 10.0 × 14.0 µm vs 11.85 − 13.52 × 7.77 − 9.25 µm; 12.4 − 15.5 × 9.3 − 12.4 and 17 − 20 × 10 − 13 µm vs 16.34 − 16.96 × 11.47 − 12.09). Since the morphological parameters of oocysts/sporocysts are insufficient to distinguish species, the comparison of these Sarcocystis with the new species is unreliable. The sporocysts and oocysts of Sarcocystis sp. of Wiesner [6] were not described, so it is impossible to say if it belongs to S. funereus sp. nov., although they could be conspecific.
On the other hand, of those species molecularly characterized, as S. dispersa (18S rRNA) and S. strixi (18S rRNA, 28S rRNA and cox1), the first formed a different clade than that of S. funereus sp. nov., while the second grouped together with the new species in the three genes. Apparently, S. strixi and Sarcocystis sp. 5 are closely related (sister) to S. funereus sp. nov., but with genetic differences to be still considered as separated species. The ITS1 region is more sensitive to the genetic differences among Sarcocystis species (see [36 − 38]), while the 18S rRNA gene is now considered of limited taxonomic help [28]. Unfortunately, ITS1 region was not used previously in S. dispersa nor S. strixi, thus making their comparison with the new species limited. This is the first time that the ITS1 region was sequenced for a Sarcocystis from owls as definitive hosts and clearly revealed the differences among species.
The intermediate host of S. funereus sp. nov. is unknown, but apparently rodents (different species of mice and voles inhabiting the study area) (see [4, 39]) play that important role. Experimentally, Wiesner [6] observed that the bank vole M. glareolus acts as intermediate host and it could also be the potential host for the new species, while the northern saw-whet owl A. acadicus, a congeneric owl species from the USA, used the deer mice (Peromyscus maniculatus) (see [22]). According to Mikkola [1], König and Weick [2], there are more than 47 mammalian and 66 bird species used as preys by owls in Europe, which could act as intermediate hosts for S. funereus sp. nov. The most common small mammals used as prey by the Tengmalm’s owl are the bank vole, field vole (Microtus agrestis [Linnaeus, 1761]), sibling vole (Microtus levis [syn. M. rossiaemeridionalis] Miller, 1908) and harvest mouse (Micromys minutus [Pallas, 1771]); less commonly are shrews (genus Sorex) [4, 39]. In the study area, the main preys of Tengmalm’s owl are bank voles, field voles and sibling voles, whose abundances regularly fluctuate in high-amplitude (100 − 200-fold) three-year cycle [40 − 43]. Accordingly, the abundances of individual vole species vary strongly. The overall prey abundance could be 0.2–13.1 and 0.6–28.2 vole individuals per 100 trap-nights as revealed by regular long-term snap-trapping in the study area during spring and autumn, respectively, thus differing up to 65-fold times between different years/phases of the vole cycle [4, 10, 42].
It has been mentioned that species of Sarcocystis are more specific to their intermediate than definitive hosts, especially those using rodents as intermediate hosts (see [28]). In the case of M. glareolus, it has been found as intermediate host of several types and unnamed species of Sarcocystis from the Czech Republic [44], Baltic region [45] and Lithuania [46], as well as of S. clethrionomyelaphis Matuschka, 1986 in Germany, which uses canids, mustelids, snakes or birds of prey as definitive hosts (see [47, 48]). One of the forms from Lithuania showed similar features (dense hair-like projections on cyst wall) than that of Sarcocystis sp. described by Wiesner [6] (see [46]), thus corroborating the role of bank vole in the life cycle of the parasite.
The cause of death of Tengmalm’s owls was undetermined, but the occurrence of Sarcocystis in these birds should be monitored, since other taxa of this genus (S. falcatula, Sarcocystis sp. isolate from chicken) have been reported as causing encephalitis in free-ranging great owls (B. virginianus) and meningoencephalitis in chickens, respectively (see [33, 49]).
In the last decades, the integrative taxonomy by using morphological features and molecular analysis has uncovered the huge diversity of species in various groups of organisms, including protists [28]. Additionally, it particularly improved the recognition of the specificity of Sarcocystis in their intermediate and definitive hosts around the world. According to the high infection prevalence of the present study, the Tengmalm’s owl acted as natural definitive host for S. funereus sp. nov., thus representing the first host record in A. funereus and the nineth owl species with a Sarcocystis species. Apparently, S. funereus sp. nov. is specific to A. funereus, since the latter was experimentally infected with S. sinensis and sporocysts and oocysts were no found after some days of infection (see [8]). However, birds of prey might be infected by more than one Sarcocystis species, as S. halieti and S. lari in the white-tailed sea eagle (H. albicilla) (see [15]); therefore, more Tengmalm’s owls, other owl species and birds of prey should be examined to determine the presence of other species or forms.
Tengmalm’s owls are nomadic and the natal dispersal movements of those juveniles hatched in Finland could extend more than 1000 km [50, 51], while adult females show long distance breeding dispersal up to > 600 km in Finland [4, 50] and adult males are usually resident after their first breeding attempt [4]. They can also move over long distances and are widely distributed in North and Central Europe including the Italian Alps and Pyrenees in North Spain [1]. Therefore, it is highly probable that Tengmalm’s owls could spread S. funereus sp. nov. out of Finland to various other locations within their distribution range. For instance, during a long-term study of Tengmalm’s owl in the Czech Republic (years 2010–12 and 2015) a prevalence of 40% was found for a Sarcocystis sp. in 10 fledglings [52, 53]. Thus, these parasites seem to be present in that country, although the species identification should be confirmed to determine the real distribution of S. funereus sp. nov. However, Svobodová [54] examined two Tengmalm’s owls in the Czech Republic, which were negative to the presence of oocysts/sporocysts of Sarcocystis.
If we considered that family Strigidae comprises 223 species of owls reported around the world, more studies are needed to elucidate the parasite fauna and involvement of these birds in the life cycles of parasites. Thus, new findings will help in increasing the knowledge about this interesting group of predators, as well as their role as predators of rodents, which also act as intermediate hosts of several Sarcocystis.