SUMMARY OF THE REPORT OF THE INVESTIGATION OF BOVINE SPONGIFORM ENCEPHALOPATHY (BSE) IN ALBERTA, CANADA

July 2, 2003

Background

On January 31, 2003, a cow in Northern Alberta was identified at slaughter as a downer (unable to walk). It had been sent to a provincially inspected abattoir, where the animal was condemned due to a post mortem finding of pneumonia. The head (which, along with the brain, comprises 2/3 of the infectivity of BSE) was removed and sent to the Alberta provincial laboratory for BSE testing under Canada's program of routine surveillance for this disease. No part of the cow entered the human food chain, the rest of the carcass having been sent to be rendered into animal feed. On May 16, 2003 the province made a preliminary diagnosis of BSE which was confirmed by CFIA's National Centre for Foreign Animal Disease on May 18^th and by the international reference laboratory in the United Kingdom (UK) on May 20^th.

The CFIA immediately started an epidemiological investigation, which has provided much detailed information about the case. We are now at the point where the investigation is sufficiently complete to summarize our findings. On the basis of the available information the CFIA is confident (with a 95% probability, as defined by the CFIA's chief investigator) that the animal was born in Canada and we have traced the sequence of farms through which the animal passed during its lifetime. There remains a 5% probability that the animal could have originated from an alternate line of procurement. This report describes the investigation, with particular emphasis on the risk factors relevant to the occurrence of BSE in this cow.

Identifying herds associated with the BSE-infected cow

The infected cow was between six and eight years old at the time of slaughter. It spent the last six months of its life in an 80-cow herd that had been established in 2001-2002 from two distinct lines of cattle. As part of the investigation, the CFIA used DNA testing to rule out several potential herds in which the cow may have been born. In addition, we have identified cattle that may have spent time in herds with the infected cow during its lifetime, including cattle that had moved out of those herds before the infected cow was identified. More than 2,700 animals were destroyed and more than 2,000 of these animals that were 24 months of age or older were tested for BSE, with negative results in all cases.

The cause of the BSE case in Canada

Consistent with scientific knowledge from the UK and Europe, in which BSE emerged and has been most prevalent, the most likely source of BSE for the infected cow would have been the consumption of feed containing meat and bonemeal (MBM) of ruminant origin contaminated with the BSE prion before the US and Canada implemented a feed ban in August 1997.

The ban meant that feed containing mammalian protein, with some exceptions, could not be fed to any ruminant species. Investigations undertaken by the CFIA revealed that some herds in which the infected cow resided had access to feed concentrates and/or high energy feed blocks which may have contained MBM prior to the feed ban. Inspection of feed mill records and compounding formulae confirmed that the incorporation of MBM in both products ceased in 1997.

As a result of the extensive integration of the cattle industries in Canada and the United States of America (US), the contaminated feed could have been manufactured in Canada or imported from the US. Historically, approximately fifty percent of the MBM used in Canadian feed mills was imported from the US. However, it is not possible to confirm if the supplements fed to the herds in which the cow resided were manufactured using MBM of Canadian or US origin.

The original source of the BSE prion in MBM is likely to have been from a limited number of cattle imported directly into either Canada or the US from the UK in the 1980s, before BSE was detected in that country. It is likely that some of these animals were slaughtered or died and entered the animal feed system prior to a ban on further importations from the UK in 1990.

Following the detection of BSE in an imported cow in Canada in 1993 all remaining imported animals in Canada, which had been under surveillance since the import ban in 1990, were slaughtered and incinerated. Similarly, once the situation with BSE became clear in the UK many, but not all, imported cattle were destroyed in the US. The rendering and feeding practices that existed at that time in both Canada and the US would have allowed BSE, if present, to cycle through cattle feed and potentially infect other cattle. Since the incubation period (the period from initial infection to the development of clinical signs) of BSE is prolonged (2-7 years) and very few potential cases of BSE are likely to have entered the feed chain prior to the 1997 feed ban, the number of animals subsequently infected with BSE is likely to have been extremely small. This view is supported by the report of the international team of experts, which noted that the feed ban would have been effective in limiting the spread and amplification of BSE.

While the infected cow, detected in Alberta, confirms that BSE is present in North America, the actual number of infected animals present in the cattle population is likely to be extremely low. Canada has conducted surveillance for BSE since 1992 and has generally met or exceeded international standards. There is no doubt that the steadily increasing intensity of surveillance has contributed to the probability of finding a case of BSE. This serves to illustrate how seriously Canada takes its international obligations for disease surveillance and reporting.

It is important to note that any further cases that might be identified are almost certainly indicative of exposure to BSE prior to the feed ban. In addition, the cumulative effect of the numerous stringent measures in place since 1990, as recognized by the international team of experts, and risk assessments conducted by other countries, ensures that the finding is not a precursor to a widespread outbreak. This is supported by the Harvard Risk Assessment and the Canadian self assessment of the risk of BSE being present in Canada which both indicate that, if BSE was present, the prevalence would have peaked or started to decline by the time it was detected.

Other possible causes of BSE have been proposed for a number of years including: spontaneous mutation of normal protein to a pathogenic (resistant) form of prion protein or exposure to prions associated with another transmissible spongiform encephalopathy, such as scrapie of sheep and goats or chronic wasting disease (CWD) of deer and elk. However, despite exhaustive investigations, the scientific evidence to date does not support any of these theories. Furthermore, it is important to note that the prion associated with the index case was characterized by molecular analysis at the international reference laboratory in the UK as BSE, not CWD.

Disposal of animal feed made from the BSE-infected cow

The remains of the BSE-infected cow were traced through their distribution into pet food and animal feed. As many as 1,800 farms (600 recipients of bulk feed and 1200 recipients of bagged feed) may have received animal feed containing MBM made from the infected cow. Inspections undertaken by CFIA confirmed that the renderer and the feed mills had very good records of compliance with the feed ban. Notwithstanding the fact that the feeding to cattle and other ruminants of products containing ruminant origin MBM is prohibited, the CFIA conducted on-farm investigations on a representative sample of (170) farms to evaluate the risk of ruminant exposure to the contaminated feed. Based on these investigations, the CFIA concluded that 99% of farms experienced no (96%) or incidental (3%) exposure to potentially contaminated feed. In 1% of farms, ruminants may exceptionally have been exposed to feed containing prohibited material. Three farms were quarantined and 63 cattle that may have eaten poultry feed were destroyed. Taking into account the very small probability of exposure of ruminants on the remaining farms, and impending enhancements to BSE risk management, the CFIA decided not to impose specific risk management measures on other farms. This decision was consistent with the recommendations of the international scientific experts who reviewed the CFIA's investigation.

Current Activities

The CFIA is refining approaches to surveillance and working with the Harvard Risk Assessment Group to re-evaluate the North American BSE exposure model. Trilateral meetings with NAFTA members are underway to refine the North American risk management strategy to take account of the most up-to-date thinking on BSE risk factors in North America.

The CFIA is actively reviewing policy adjustments in the areas of specified risk materials, surveillance, feed measures, and traceability and awareness programs, in consultation with federal, provincial, territorial, and industry representatives.