The mature dairy cow that became Canada's 10th case of mad cow disease was probably infected by commercial feed that it received after weaning, says a report by the Canadian Food Inspection Agency released Wednesday.
The Holstein, which was destroyed earlier this year, had spent its entire life on the same dairy farm in B.C.'s Fraser Valley.
It appeared to be lame a few weeks before calving, and after calving became unsteady, eventually losing its ability to walk at all.
Tests confirmed on May 2 that it was infected with bovine spongiform encephalopathy (BSE), more commonly known as mad cow disease.
No part of the carcass entered the human or animal food chain.
The most likely cause was commercial feed, which the cow would have eaten only during its first year, that got cross-contaminated with prohibited materials -- rendered products from other ruminant animals.
The most likely source was cross-contamination of the heifer feed either at the feed mill or during transportation.
The CFIA investigated 155 of the 66-month-old cow's herdmates, and their feed.
"Only cattle in the feed cohort were implicated in the investigation," said Dr. Connie Argue, an epidemiologist and scientific advisor to the CFIA.
Of those cattle, five of the animals have since been slaughtered for unrelated reasons. Another 87 had already been slaughtered -and of those, five had tested negative for BSE. There were 23 animals that could not be traced, and the remaining 41 live animals have been quarantined and will be killed in the next few months.
Their carcasses will be incinerated at the CFIA's facility in Alberta, says Argue.
"Those animals will not be allowed to enter the food or feed chain," she said.
On May 22, 2007, Canada was officially categorized under the OIE's science-based system as a controlled BSE risk country. This status recognizes the effectiveness of Canada's surveillance, and eradication measures, and acknowledges the work done by all levels of government, the cattle industry, veterinarians and ranchers to effectively manage and eventually eradicate BSE in Canada, said the CFIAreport.
The feed ban implemented in 1997 -- no longer allowing ruminant animal products to be fed to other ruminants -- is effectively keeping BSE out of Canada's food system.
"The detection of BSE in a few animals born after the 1997 feed ban is not unexpected and does not indicate a failure of those measures,"says the report.
Additional regulations to enhance Canada's feed ban were enacted on July 12, 2007. The most important change is the removal of specified risk materials -- brains, eyes, tonsils and other select tissues from older cattle, even bone dust generated when their spines are split open -- from all animal feeds, pet food and fertilizer.
As to the safety of the human food chain, Argue says the "specified risk materials" have been removed from the food chain since 2003.
"(This legislation) will remove 99 per cent of infectivity from rendered products," say Argue. "We have already taken appropriate measures to prevent that from leading to additional cases of BSE."