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

Background

On January 4 , 2005, a beef cow in Innisfail, Alberta was euthanized and sampled by a private veterinarian under Canada's National BSE Surveillance Program. Brain samples from the animal were sent to an Alberta provincial laboratory where they were screened for BSE using a Bio-Rad rapid test and produced a reaction on January 6, 2005 and again on January 7, 2005. Brain samples were then sent to the Canadian Science Centre for Human and Animal Health in Winnipeg where BSE was confirmed, using the immunohistochemistry procedure for BSE on January 11, 2005. No part of the cow entered the human food or feed chain. The carcass was secured from the index premises and transferred to the CFIA Lethbridge Laboratory where it was subsequently incinerated.

The CFIA immediately initiated an epidemiological investigation along three lines of inquiry, based on World Animal Health Organization (OIE) guidelines, namely:

• calves born to the affected cow during the two years prior to the onset of clinical signs;
• the birth cohort (cattle born on the farm of origin within 12 months before and 12 months after the birth of the affected animal); and
• feed to which the animal may have been exposed early in its life.

Animal Investigation

The affected Charolais beef cow was just under seven years old at the time of death, having been born on March 21, 1998. The animal remained on the farm of birth during its entire life. The cow had separated from the herd, experienced loss of condition and eventually hind-limb dysfunction that the owner attributed to injury. A private veterinary practitioner was consulted and the animal was euthanized and sampled for BSE testing.

The investigation revealed that the animal had two progeny born within the previous two years, one of which was confirmed to have been slaughtered. The other has been euthanized and incinerated at the CFIA Lethbridge Laboratory. This animal was not tested for BSE because it was less than one year of age.

The birth cohort was determined to comprise 349 animals. The trace-out investigation of the birth cohort located 41 live animals that were subsequently euthanized, sampled and tested negative for BSE. These animals were disposed of by incineration. Because birth cohort cattle would be five-to-seven years old today, most had previously been slaughtered or had died of natural causes. The other 308 animals were traced as follows:

• 273 animals were confirmed to be dead or slaughtered in Canada
• 32 animals had died on the farm of origin
• three animals were deemed untraceable because of inadequate records.

Feed Investigation

A thorough investigation into feed purchases, feeding practices, manufacturing processes and documentation was undertaken at the farm of origin, feed manufacturers and retailers. The investigation revealed that the index animal was exposed to four commercial feed sources (calf ration, creep feed, and two mineral supplements) during its early development that may have been the source of infection. Although these four feed sources should not have contained ruminant meat and bone meal (MBM), the possibility that one or more of them may have been contaminated cannot be ruled out. The feed manufacturers were handling ruminant MBM for the manufacture of non-ruminant feeds during the time-frame of interest. These feed sources were likely manufactured a short time after the feed ban was implemented, however, as historical production records were not available, manufacturing dates could not be confirmed.

Investigation Overview

Consistent with risk assessments and knowledge derived from previous investigations, this most recent case of BSE was not entirely unexpected. The first case, detected in May, 2003, indicated that Canada had a low, previously undetected incidence of BSE. Since that time, Canada's national surveillance program, which targets cattle most likely to be affected by BSE, has tested more than 30,000 animals. With this testing intensity and focus, the sporadic detection of a small number of additional cases was anticipated. Both the age and number of animals identified through intensified surveillance testing continues to suggest that the level of BSE in Canada is low and declining. The fact that the feed ban was introduced almost six years prior to the detection of BSE in a native-born animal in May, 2003, has been extremely important in preventing amplification and limiting the spread of BSE. These cases also demonstrate the integrity of Canada's surveillance system and the commitment of Canadian cattle producers and veterinarians to responsibly and pro-actively report animals for testing.

This investigation identified that certain feed materials, likely manufactured a short time after the implementation of Canada's feed ban, may have been contaminated. This finding is consistent with the experience of all countries with BSE which have implemented feed bans. As with any major policy that requires restructuring of operations, some time may have been required for the feed ban to be implemented completely and uniformly. Renderers, feed manufacturers, retailers, distributors and producers were required to develop and implement new processes into their operations. These processes included sequencing and flushing systems in feed mills manufacturing a variety of feed sources, as well as new label requirements and enhanced record keeping generally. As these changes were being developed, implemented and refined, it is possible that some ruminant feed produced shortly after the feed ban became contaminated with prohibited materials.

The feed ban is an important animal health measure whose primary objective is to curtail the spread of BSE in a cattle population, and that is why, in 1997, Canada implemented this measure, in advance of the detection of any native cases. The degree of effectiveness of the feed ban can influence the length of time it could take to completely eliminate the disease from a cattle herd population. It is clear that Canada's feed ban has been effective enough to limit the occurrence of BSE in Canada to an extremely low level and lead to elimination of the disease over time. The results of Canada's surveillance program to date bear this out, based on the small number of cases found and the age of the affected animals. Given that the incubation period of BSE is influenced by the level of infectivity to which an animal is exposed (the higher the exposure, the shorter the incubation period, and vice versa), the older age of the affected animals indicates that the level of contamination in the feed source would have been very low, even before the feed ban was implemented. Notwithstanding short delays in the implementation of the feed ban in 1997, there is a strong basis to believe that the feed ban, as designed and delivered, is doing its job. Proposed enhancements to the ban would serve to further shorten the time required to achieve complete elimination of BSE from Canada.

While the feed ban is an important BSE animal health measure, the detection of this animal, born after its introduction, does not impact on the safety of meat currently being produced in Canada. Following the initial detection of BSE, the Government of Canada moved quickly to implement the most effective public health measure that a BSE-affected country could take by requiring the removal of specified risk materials (SRM) from all cattle slaughtered in Canada. Removal of SRM is verified by inspection staff of the CFIA and provincial and territorial counterparts. This science-based measure ensures that consumers in Canada and in importing countries are effectively protected from exposure to BSE infectivity in meat products produced in Canada.

With respect to birth cohorts slaughtered prior to the July 2003 implementation of the requirement to remove SRM from the food chain, a number of factors contribute to the very low risk associated with meat from these animals. These include the fact that the majority of animals slaughtered for beef consumption in Canada are between 18-22 months of age and are, therefore, considerably less likely to develop infective levels of the disease, that all animals in the federal system are subjected to ante and post mortem inspection, that the "within herd" incidence of BSE is a rare event which has been reconfirmed by the recent Canadian experience in which no additional BSE positive animals have been found upon tracing and testing, and finally, that the highest level of prions are located in the brain, spinal cord and eyes, all of which are not generally included in the diets of the majority of Canadians.