광우병 인자의 소-인간 종간장벽에 대한 미 농무부 동식물검역청의 자료

일반적으로 종간 장벽은 1(감소가 전혀 나타나지 않음)에서 1000 사이의 범위에 있는 인자(因子)에 따라 전염의 효능성을 감소시키는 것으로 추정된다. 종간장벽이 1(즉, 효과적인 종간장벽이 전혀 없다는 말이다) 이라는 가정은 최악의 사례가 되는 시나리오로 간주된다. 소에게서 인간에게로 광우병 전염에 대한 종간장벽에 대한 의견 중에는 EU 과학전문가위원회가 1에서 10만 사이의 전염 효능 범위에 대한 종간 장벽의 영향을 가능성 있는 변수로 평가한 것이 있다. 이 범위는 훗날 10에서 10만 사이로 업데이트 되었다. 그러나 EU 과학전문가위원회는 현재까지 알려진 과학지식의 범위 내에서 소와 인간사이의 광우병 종간장벽의 참값을 측정하는 것은 불가능하다는 결론을 내렸다.
 
(이 내용은 EU과학전문가위원회의 업데이트된 종간장벽 내용을 약간 왜곡했다고 판단됩니다. 왜냐하면 EU과학전문가위원회는 훗날(2000년을 가리킵니다) 종간장벽에 관한 내용을 업데이트하면서 과학적 데이터가 서로 상충되고 불확실성이 존재할 경우 종간장벽을 1로 간주하는 가정을 반드시 포함시켜야 한다고 결론에서 명시적으로 밝혔기 때문입니다.)

제목 : Evaluation of the Potential for Bovine Spongiform Encephalopathy in the United States
(Harvard Center for Risk Analysis Harvard School of Public Health, November 26, 2001)
(전문은 첨부파일 참조)

출처 : 미 농무부 동식물검역청 자료
http://www.aphis.usda.gov/newsroom/hot_issues/bse/background/documents/mainreporttext.pdf
 
2.1.2 The Species Barrier (7쪽~8쪽)

Interspecies transmission of TSEs is mitigated by a so called “species barrier”. This
barrier represents the decreased efficiency with which TSEs are passed from one animal to a
second animal of a different species, compared with the efficiency with which the TSE is passed among animals of the same species. That is, a much greater amount of infective material is necessary to infect an animal from a different species than is needed to pass the disease to an animal of the same species. The species barrier is also associated with an increase in the disease’s incubation period (i.e., the delay between exposure to the agent resulting in infection and the manifestation of disease). In some instances the species barrier seems to confer complete resistance to transmission. It is at least conceptually possible that an animal failing to develop the disease following cross species challenge would become infected if administered a sufficiently large dose of infectivity, or would manifest clinical signs of disease if it somehow lived longer than the incubation period associated with the species barrier (Hill et al., 2000).
 
Although transmission of a TSE from one species to another may be less efficient than
the transmission within the same species, once it occurs, the TSE may become “adapted” to the new host. Because it has adapted to the new species, it can be transmitted more efficiently among members of that species, and the incubation period becomes shorter and less variable. For example, when scrapie is transmitted experimentally from one species to another, the incubation period is usually longer in the first passage than that seen in subsequent passages within the new species (Dickinson et al., 1976).

The species barrier probably reflects some combination of factors including differences
between the donor’s and recipient PrP. Scrapie studies conducted in mice, rats, and hamsters demonstrate the presence of a species barrier. These findings include pathogenesis differences between the first and subsequent passages in the new species, and how rapidly the transmitted strain replicates in the new host (Kimberlin et al., 1987, Kimberlin and Walker, 1989), and others.

The response of some TSEs exhibits heterogeneity within a species, a characteristic that
appears to be due to the existence of different strains of the agent. Strains are distinguished  by highly replicable differences in the incubation period, neuropathology, and host range (Fraser and Dickinson, 1968, Bruce et al., 1989). CJD, scrapie, TME, and CWD show strain diversity, while BSE appears to be a single, stable strain (Bruce et al., 1994, Bruce et al., 1997). vCJD (i.e., the new form of CJD related to exposure to the BSE agent) does not demonstrate morphologic strains (Will et al., 1996, Hill et al., 1997, Bruce et al., 1997, Scott et al., 1999).

Recipient characteristics also affect the efficiency with which TSEs are transmitted
across species. Some species, such as rabbits or chickens, do not develop disease when
challenged with specific TSEs, while other species do. It has been postulated that the similarities between the PrP structure between the donor and the recipient explain the differences in transmission efficiency (Priola et al., 1994, Raymond et al., 2000).
 
Because the presence of a TSE agent is often assessed by inoculating a test species (e.g.,
mice) with the suspect material, the species barrier compromises the sensitivity of these
bioassays. Cattle-to-cattle transmission of BSE by the intracerebral route is known to be 1,000 times more efficient than cattle-to-mouse transmission by the same route (MAFF, 2000b). It is often assumed that the species barrier decreases transmission efficiency by a factor of between 1(no decrease) and 1,000 (Det Norske Veritas, 1997). The assumption that the species barrier is 1(i.e., that there is effectively no species barrier) is considered to be a worst-case scenario. In an opinion on the species barrier for transmission of BSE from cattle to humans, the EU Scientific Steering Committee suggested that plausible values for the impact of the species barrier on transmission efficiency range from between 1 and 100,000 (SSC, 1999a). This range was later updated to between 10 and 100,000 (SSC, 2000b). However, the committee concluded that it is impossible to estimate the true value for BSE species barrier between cattle and humans within an order of magnitude given current knowledge (SSC, 2000b).