Sheep husbandry is a minor agricultural industry in Finland. The Finnish sheep population is small and the majority of the animals are old Finnish breeds, the most important of which is Finnish Landrace. In addition, a small proportion of other breeds exist, including Aland sheep, the Grey race sheep of Kainuu and Texel.
Our results provide the first data of PrP genotypes of these main landraces, both purebred and crossbred sheep, together with a small proportion of Texel sheep. The predominant allele in the Finnish breeds was ARQ (69.6–91.7%). ARQ is regarded as a wild type allele; it has also been found at a high frequency in old sheep breeds of Iceland [17
]. Like Icelandic sheep and native Austrian mountain sheep races, the Finnish Landrace is well adapted to harsh Nordic climate and aseasonal breeding. In addition, in these old breeds, only a few alleles dominate; we found altogether 13 genotypes in Finnish breeds, but nine of them were present in less than 10% of the sheep studied. Genotypes of Grey race sheep of Kainuu consisted of only ARQ and ARR alleles, the allelic distribution of Aland sheep was ARQ, AHQ and ARR, whereas Finnish Landrace sheep exhibited all five commonly observed alleles ARQ, ARR, VRQ, ARH and AHQ. In terms of assessing clinical scrapie risk with the National Scrapie Plan for Great Britain [23
] categorization, where R1 correspond to very low classical scrapie risk and R5 the highest classical scrapie risk, 71.6% of Finnish Landrace sheep, 76.8% of Aland sheep and 83.3% of Grey race sheep of Kainuu would fall into risk group R3. Compared with breeds in Great Britain, the high ARQ and notable AHQ allele frequency of Aland sheep corresponds to the allele distribution of Bluefaced Leicester sheep, whereas the Grey race sheep of Kainuu with an absent VRQ allele resembles Suffolk sheep, both of which are reported to be highly susceptible to classical scrapie [7
Our results indicate that Finnish sheep have genetically little resistance to classical scrapie. However, classical scrapie has never been detected in sheep in Finland despite intensive surveys. The likely explanation for this is non-existent or low infection pressure of classical scrapie due to Finland’s limited and strict import policy of live sheep intended to prevent the introduction of diseases. Prior to 1995, only a small number of sheep from Denmark and Sweden were imported [25
], and after 1995 only a few sheep have been imported to nine farms. All of the imported sheep after 1995 were from countries where the scrapie control programme is in place. The two farms that imported sheep from countries outside the scrapie programme are under strict control. Our results support the view that development of scrapie requires both susceptible genotype and sheep’s exposure to an infecting scrapie strain [8
In the intensive active surveillance for scrapie in sheep in Finland during 2002–2008, no classical scrapie was found, but five atypical scrapie cases were detected. It is noteworthy that we detected the first atypical scrapie case by using Prionics Western blot test until it became general knowledge that atypical scrapie is sensitive to the amount of proteinase K used by the method and could thus go unnoticed [20
]. Despite the higher concentrations of PrPSc in the cerebellum than in the obex [28
], in three out of five cases the diagnosis was made from the obex, as the cerebellum was not available.
The genotype was obtained from four out of five atypical cases. Atypical scrapie is most often associated to the ARQ allele, and contrary to classical scrapie, to the AHQ and ARR alleles. In addition, sheep with atypical scrapie have often been found to carry phenylalanine (F), instead of leucine (L), at codon 141, usually in ARQ homozygotes [13
]. In our study one of the five atypical scrapie sheep had the AFRQ genotype, although the AFRQ allele was rare in the studied sheep population (Tables and ). These results support the association of AFRQ with atypical scrapie. Furthermore, it has been reported that the genotypes of the youngest animals infected with Nor98 atypical scrapie were AFRQ/AFRQ, AFRQ/ALHQ or ALHQ/ALHQ, whereas atypical scrapie sheep with the ALRQ/ALRQ genotype were clearly older [13
]. The Finnish atypical scrapie sheep with the AFRQ genotype was slightly over two years old, the sheep with the ALHQ allele was four years old and the sheep with the ALRQ genotype was nine years old, but because all of them were fallen stock without any known clinical signs it cannot be concluded if the age of the Finnish cases is associated to the onset of atypical scrapie.
In addition to the known atypical scrapie-associated polymorphism L141F, some other previously characterized and novel polymorphisms linked to the ARQ allele were found in the Finnish sheep population. The most frequent polymorphisms present in Finnish Landrace were R151C and E224K (6.6 and 5.8% of sheep studied, respectively), both of which have been described before [17
]. Variation in ovine PrP gene is common, but an association of these polymorphisms with scrapie is mostly unknown [11
]. However, partial resistance with natural scrapie has been associated to R151C in one flock of Icelandic sheep [17
] and to M112T in Japanese Suffolk and Corriedale sheep [30
]. The importance of the relatively high frequency of R151C in Finnish Landrace is unknown. The dimorphism of E224K found in Finnish Landrace (5.8%) and Grey race sheep of Kainuu (18.8%) has recently been described in Polish sheep breeds [18
]. To our knowledge the study of Piestrzynska-Kajtoch et al. (2012) and this study are the only reports of E224K polymorphism so far. This might suggest that there would be a link between Finnish and Polish sheep, but this is yet to be determined. There has been no import of live sheep from Poland to Finland, but it cannot be excluded that some Finnish landrace sheep might have been exported to Poland from Finland or from other countries.
The epidemiology of atypical scrapie remains incompletely understood. The disease has been experimentally transmitted to ovinized transgenic mice and sheep with the genotype AHQ/AHQ [31
]. This indicates that the disease could be transmitted within a flock in natural conditions, although this is yet to be established. It has also been suggested that atypical scrapie could arise spontaneously its being common to find only one atypical case per flock [12
]. This suggestion is supported by the finding that atypical scrapie is usually detected throughout the country, and typically only a single positive sheep is identified in an affected flock [12
]. All five Finnish atypical cases were single cases in their flock, and no epidemiological connections existed between them. Four out of the five atypical scrapie cases were originally detected in the brainstem which was somewhat surprising because it is generally known that the highest concentration of PrPSc in atypical scrapie is detected in the cerebellum. However, it has been reported that in some cases of atypical scrapie PrPSc deposition can be detected in the obex region of the brainstem [32
]. The prevalence of atypical scrapie in Finnish sheep was similar to that in European sheep [12
]. The prevalence was slightly lower in the healthy sheep population than in fallen stock animals. This might be due to the mean age of fallen sheep being higher, as the incidence of atypical scrapie has been reported to be higher in older sheep [13