Investigations into Sarcocystis of the bird order Anseriformes are not abundant in Eurasia, so the objective of our work was to determine the occurrence of Sarcocystis in these birds and to analyse the morphology of cyst walls and merozoites under the light microscope.

In the period 1997–2003, 342 dead birds that had been accidentally caught in fishing nets or shot were investigated in Lithuania. The largest part of the material was collected in the vicinity of the Baltic Sea and the Curonian Bay. Neck muscles of 227 birds, breast muscles of 20 birds and muscles of the neck, breast and leg of 95 birds were examined. Sections of muscles of the size of an oat grain were stained with methylene blue (1:500), lightened with 1.5% acetic acid and pressed in a glass compressor. The morphometric analysis of cyst walls and merozoites was carried out in native preparations in a physiological solution after the cysts had been isolated from the muscle fibres. Cysts were differentiated on the basis of the wall thickness, the length of protrusions and the morphometric analysis of merozoites. Infection intensity (II) for 322 birds was determined by counting cysts in 28 sections of neck muscles. Investigations were made under the light microscope.

Having examined 342 birds of 20 species, we detected Sarcocystis cysts in 100 (29.2±2.5%) individuals belonging to 15 species (Table 1). The highest prevalence of infection (65.8±5.6%) was determined in the white-fronted goose (Anser albifrons). The age of the birds could have influenced this index because all geese were examined during the spring period, that is, when they were adult. The highest II was determined in the white-fronted goose and the long-tailed duck (Clangula hyemalis) (66 and 53 cysts, respectively) (Table 1).

Table 1 Investigation results of Sarcocystis of the bird order Anseriformes
Full size table

One type of macrocysts and four types of microcysts were determined.

Macrocysts were found only in the neck muscles of the mallard (Anas platyrhynchos). They were of a white colour, could be well seen with the naked eye, were 1.5 mm×4.0 mm in size and resembled a grain of rice. The wall of the cyst was of a complicated structure and reached 5.7 μm. Merozoites were straight and 12.1–15.5 μm long (n=29). This type is thought to be of the species S. rileyi.

Microcysts were temporarily called type I, type II, type III and type IV. Type I cysts had a wall reaching 1.2 μm, without clear protrusions on the surface. Merozoites were banana-shaped and 6.1–8.8 μm long (n=98). Cysts of this type were found in ten species of birds (Table 1). Type II was distinguished by palisade-like protrusions (up to 1.5 μm) on the outer surface of the primary wall. Merozoites were slightly bent, broader at one end, and 11.9–16.1 μm long (n=81). Cysts of this type were found in four species of birds (Table 1). Type III cysts had a wall with teat- or finger-like protrusions (up to 2.4 μm), which may be bent either to one or the other side in the preparation. The groups of “teats” formed sometimes resembled a molar. There were gaps between the protrusions. Merozoites were straight and 10.0–13.8 μm long (n=102). Cysts of this type were found in the geese of all species examined and in the mallard (Table 1). The wall of type IV cysts was wavy, reaching 1.0 μm. Merozoites were banana-shaped and 6.3–8.1 μm long (n=35). The appearance of the cyst of this type depends to a large extent on the method of preparation—when the cover glass was pressed more strongly the height of the waves decreased significantly. Such cysts were found in the ducks of two species only (Table 1). Sometimes it is very difficult to distinguish the walls of cysts of type I and type IV at the level of light microscopy, whereas differentiation of cysts of type II and type III is possible. We suppose that types of microcysts represent at least four different Sarcocystis species.

Investigations into Sarcocystis of the birds of order Anseriformes are worth carrying on because life cycles of most species have not been elucidated and studies of the ultrastructure of cysts, as well as DNA, necessary for final differentiation of species, have not been made yet.