Epidemiology of Ovine Scrapie in Japan

During the period 1984 to 1987, postmortem brain tissues were obtained in 23 samples of pathologically confirmed ovine scrapie in Japan. Although most of them were collected from Hokkaido, some brain tissues originated from Yamagata, Fukushima and Kanagawa. Occasional incidences of temporal and spatial clustering on anon-familial basis have been reported in the past, each case of which included a small number of sheep centers in Hokkaido and Fukushima. This geographical and temporal clustering, however, did not apply to the present cases with a few exceptions. Ninety percent of those incidences indicated a familial association. A majority of the diseased sheep were from Suffork strain, while three cases were from Corrie dale strain. It is very likely that sheep-to-sheep transmission of scrapie has taken place in Obihiro, Hokkaido, Japan. Discipline: Animal health Additional keywords: purion, spongiform encephalopathy, unconventional virus Although ovine scrapie has been studied much more intensively than any other unconventional virus, the mechanism of its spreading under natural conditions is not identified yet. It might spread from naturally infected sheep to un-infccted sheep, though such a lateral transmission has never been reported so far in sheep or goats experimentally infected. It appears to pass from ewes to lambs, even without suckling; the contact of the lamb with the infected ewe at birth appears to be effective, because the placenta itself is highly infectious>. Transplacental infection during the perinatal period rather than oral, nasal, optic, or cutaneous infections might be a possible route of transmission, though not confirmed2>. The first appearance of scrapie agent in young lambs naturally infected occurred at the age of IO to 14 months in tonsils, suprapharyngeal and mesenteric lymph nodes and intestine, suggesting infections tak ing place in an alimentary tract. Older sheep were infected only after a long contact with diseased animals. Incidences of the disease took place in pastures previously occupied by scrapie sheep3>. Although scrapie has a widespread distribution in Europe, Asia and America >, few reports are available for Japanese cases. The present paper reviews outbreaks of ovine scrapie in northern Japan and discusses possible lateral transmission of the disease. Since the first outbreak of scrapie in Obihiro of Hokkaido, Japan in 1984•>, postmortem brain tissues have been attained in more than 20 cases of pathologically confirmed scrapie. Clinical records on more than 10 cases have been made ava ilable. The prevalence of scrapie has varied markedly in time and place throughout northern Japan. Since attention was drawn to the syndrome of ovine scrapie, particularly its lateral transmission in Obihiro area, more frequent diagnoses have been undertaken than before by various an imal health stat ions in Hokkaido'>. During the period 1984 to 1986, approximately 50 cases of incidences were reported in Hokkaido. There were several cases outside Hokkaido: i.e . one scrapie-like disease incidence in Yamagata in 1983, two pathologically confirmed cases in Yamagata and Fukushima both in 1984, and scrapie-like disease cases in Iwate, Ibaraki and Chiba in 1984. In 1987, three incidences were informed in Kanagawa: the presence of scrapie was pat hologically confirmed in one of them through brain tissue inspections (Fig. I). In regard to the duration of ovine scrapie disease, it is a 310 6-month period on a n average. It was earlier reported that occasional incidences of temporal and spatial clustering had taken place on a non-familial basis in a few sheep centers in Hokkaido and Fukushima. This geographical and temporal clustering, however, did not apply to a majority of the cases as mentioned above. It was recognized that 900Jo of those incidences were 217 familial. The diseased sheep were mostly from Suffork strain, and three cases were from Corriedale strain. It is quite probable that sheep-to-sheep transmission of scrapie took place in Obihiro, Hokkaido, since approximately 50 cases in Suffork and three cases in Corriedale originated from the sheep in Obihiro or those transferred from there9>. Narnral scrapie in sheep is pathologically characterized by the presence of vacuolated neurons and extensive astrogliosis. In the cases as memioned above, the presence of amyloid plaques is rare. A fl : Affected sheep pathologically confirmed ~: Affected sheep informed in personal communicat ions Fig. I. Distribution of scrapie-affected sheep by prefecture, 1984 to 1987