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8-29-02
Dr. Sam Holland provided me some details on the new CWD case in South Dakota. As a reminder, CWD was first found in South Dakota in 1997 in 7 elk game farms. They all traced back to one source in Rapid City, considered the source herd. In the previous year (1996) CWD was found in Saskatchewan game farms which traced back to South Dakota. These traceback connections are from the movement of animals. This newest farm that just came up CWD positive was adjacent to one of the 1997 positive farms but had not received animals from any known affected herd.
The newly affected herd had fence-line contact with the CWD-positive herd for only 2 months in 1998. Separation was achieved and maintained thereafter. This herd was then quarantined under SD regulations for a 5 year period. However the regulations permitted early release from quarantine for adjacent herds if no disease was found and risk assessment suggested low probability of infection. That assessment was conducted after 3 years (late 2001) and no positives had been found. The quarantine was lifted but with the understanding that no animals be moved from the farm for one more year.
Now, 6 months later (mid 2002) a 3 year old positive bull shows up. Dr. Holland thinks there may have been other, at least one, missed positive animal previously though all mortalities were required to be examined and they monitored the inventory of the herd. He believes, and I agree, that a 5 year surveillance for fenceline contact herds is likely adequate to catch transmission that can occur by direct contact. But he also pointed out the possibility that the agent could have been transmitted by environmental exposure. The route for environmental contamination of this adjacent herd isn't clear but could be drainage or other mechanisms such as scavengers that might move pieces of tissue. Dr. Holland also indicated that you can't rule out a spontaneous case of CWD, although I think this would be quite unlikely.
Thomas J. Roffe, PhD, DVM
Northern Rocky Mountain Science Center
USGS-BRD
FWP Bldg, 1400 S. 19th Ave.
Bozeman, MT
T: 406-994-5789
F: 406-994-4090
Cell: 406-539-4955
U.S. Department of the Interior | U.S. Geological Survey
URL: http://biology.usgs.gov/cro/CWD8-29-02.htm
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tlwilliams@usgs.gov
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