Molecular characterisation of Sarcocystis lutrae n. sp. and Toxoplasma gondii from the musculature of two Eurasian otters (Lutra lutra) in Norway

Abstract

Sarcocysts were detected in routinely processed histological sections of skeletal muscle, but not cardiac muscle, of two adult male otters (Lutra lutra; Mustelidae) from northern Norway following their post-mortem examination in 1999 and 2000. The sarcocysts were slender, spindle-shaped, up to 970 μm long and 35–70 μm in greatest diameter. The sarcocyst wall was thin (∼0.5 μm) and smooth with no visible protrusions. Portions of unfixed diaphragm of both animals were collected at the autopsies and kept frozen for about 14 years pending further examination. When the study was resumed in 2013, the thawed muscle samples were examined for sarcocysts under a stereo microscope, but none could be found. Genomic DNA was therefore extracted from a total of 36 small pieces of the diaphragm from both otters, and samples found to contain Sarcocystidae DNA were used selectively for PCR amplification and sequencing of the nuclear 18S and 28S ribosomal (r) RNA genes and internal transcribed spacer 1 (ITS1) region, as well as the mitochondrial cytochrome b (cytb) and cytochrome c oxidase subunit 1 (cox1) genes. Sequence comparisons revealed that both otters were infected by the same Sarcocystis sp. and that there was no genetic variation (100 % identity) among sequenced isolates at the 18S and 28S rRNA genes (six identical isolates at both loci) or at cox1 (13 identical isolates). PCR products comprising the ITS1 region, on the other hand, had to be cloned before sequencing due to intraspecific sequence variation. A total of 33 clones were sequenced, and the identities between them were 97.9–99.9 %. These sequences were most similar (93.7–96.0 % identity) to a sequence of Sarcocystis kalvikus from the wolverine in Canada, but the phylogenetic analyses placed all of them as a monophyletic sister group to S. kalvikus. Hence, they were considered to represent a novel species, which was named Sarcocystis lutrae. Sequence comparisons and phylogenetic analyses based on sequences of the 18S and 28S rRNA genes and cox1, for which little or no sequence data were available for S. kalvikus, revealed that S. lutrae otherwise was most closely related to various Sarcocystis spp. using birds or carnivores as intermediate hosts. The cox1 sequences of S. lutrae from the otters were identical to two sequences from an arctic fox, which in a previous study had been assigned to Sarcocystis arctica due to a high identity (99.4 %) with the latter species at this gene and a complete identity with S. arctica at three other loci when using the same DNA samples as templates for PCR reactions. Additional PCR amplifications and sequencing of cox1 (ten sequences) and the ITS1 region (four sequences) using four DNA samples from this fox as templates again generated cox1 sequences exclusively of S. lutrae, but ITS1 sequences of S. arctica, and thus confirmed that this arctic fox had acted as intermediate host for both S. arctica and S. lutrae. Based on the phylogenetic placement of S. lutrae, the geographical location of infected animals (otters, arctic fox) and the distribution of carnivores/raptors which may have interacted with them, the white-tailed eagle (Haliaeetus albicilla) seems to be a possible definitive host of S. lutrae. Some of the muscle samples from both otters were shown to harbour stages of Toxoplasma gondii through PCR amplification and sequencing of the entire ITS1 region (five isolates) and/or the partial cytb (eight isolates) and cox1 (one isolate). These sequences were identical to several previous sequences of T. gondii in GenBank. Thus, both otters had a dual infection with S. lutrae and T. gondii.