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Risk Assessment on Bovine Spongiform Encephalopathy in Cattle in Canada
Part A: Evaluation of Risk Factors

Table of contents | List of tables | List of figures | List of appendices | Acronyms | Acknowledgments
Main page | Part A | Part B | Part C

1. INTRODUCTION

Bovine spongiform encephalopathy (BSE) is an invariably fatal neurological disease of cattle that was first diagnosed in the United Kingdom in 1986. It is one of a group of slowly progressive neurodegenerative disorders collectively known as the transmissible spongiform encephalopathies. Although the causative agent of BSE has not been fully characterized, the protein-only or prion theory has emerged to dominate the scientific literature, describing the only disease-specific macromolecule consistently isolated in BSE-affected animals. The prion theory assigns infectivity to a structurally modified form of the normal cell-membrane-associated prion protein, which then has the capacity to promote the conversion of further normal prion molecules to the same, abnormal form. The accumulation of these abnormal isoforms within the affected cells interferes with normal cell function and contributes to the characteristic spongiform changes, eventually resulting in cell death. The abnormal, disease-associated prion protein is resistant to heat, ultraviolet and ionizing radiation and an extensive range of common chemical disinfectants.

Since the disease was first described in the U.K., it has been diagnosed in 21 other countries, affecting more than 180,000 animals. There is general agreement that the most significant source of infection is the feeding of supplemental proteins containing the BSE agent to susceptible cattle. There is no evidence that BSE is transmitted via bovine embryos or semen, and if there is maternal transmission it occurs at such an extremely low rate that it would be insufficient to begin or maintain an epidemic. Within scientific circles debate continues surrounding other possible modes of transmission. BSE has a long incubation period, averaging five to six years, and there is currently no existing ante-mortem test for the disease.

Effective BSE prevention and/or control involves evaluation of risk factors, implementation of appropriate risk management measures (based on the outcome of the risk factor evaluation), and an effective surveillance program. The Canadian Food Inspection Agency (CFIA) has completed the following risk assessment on BSE in cattle in Canada to evaluate these factors. This comprehensive report details the findings and is presented in three parts. Part A evaluates the risk factors for BSE in Canada, Part B describes BSE surveillance in Canada, and Part C is a risk estimation of BSE in Canada.

At the time of the 1993 diagnosis of bovine spongiform encephalopathy in an imported cow in Canada, BSE was an obscure disease of British cattle with no apparent links to human health. Nevertheless, Canada made the disease reportable in 1990 and a surveillance program involving U.K. imports was also established. As a result, the Canadian response was rapid and comprehensive, and no other case of BSE has been diagnosed in Canada.

The chronology of measures taken by the Canadian government to address the risks associated with BSE is outlined in Appendix 1. It demonstrates that, as new scientific information has become available, Canada has taken action to prevent the entry and establishment of BSE in this country. It should be noted that in some cases Canada has met with opposition from other countries with respect to some of the measures implemented. For example, Canada’s decision in 1997 to accept cattle and cattle products only from countries Canada had assessed free of BSE was contested by several countries. Given the subsequent spread of BSE within Europe and beyond, this policy has served Canada well. It should also be noted that from the very first appearance of BSE, CFIA officials have maintained close contact, both officially and unofficially, with researchers and government officials in the U.K. and Europe. Hence, the CFIA has been well informed throughout the emergence of this disease.

A 1994 risk assessment determined that the likelihood of BSE arising under Canadian conditions was extremely low, given the differences in key risk factors between Canada and the U.K. (Kellar 2002). The occurrence of BSE in Canada would, therefore, be dependent upon the importation of infected animals and contaminated livestock products and the subsequent amplification of infectivity through the livestock feeds system.

The objective of Part A is to evaluate the risk factors for BSE in Canada. The main focus is on imports of ruminants and ruminant by-products and controls on imports, rendering and livestock feeds. Information on the structure and demographics of ruminant sectors and veterinary legislative authorities provides a contextual framework.

2. ASSUMPTIONS AND METHODS

This evaluation considers the following internationally recognized risk factors:

• importation of livestock and products from BSE-infected countries;
• rendering and livestock feed controls and practices.

2.1. Assumptions

• The only source for BSE in Canada is the importation of ruminants or ruminant products. A number of theories on the origin of BSE have been proposed, including spontaneous occurrence; however, this assessment follows the guidelines of the Office International des Épizooties (OIE) and focusses on importations.
• The main route of transmission of BSE is through feed.
• The presence of scrapie and chronic wasting disease (CWD) in Canada does not alter the risk of BSE emerging in Canada. Although one possible explanation as to the source of BSE in the United Kingdom implicates the inclusion of scrapie-contaminated MBM in cattle feed, this situation was assessed as highly unlikely to occur in Canada given the low numbers of sheep and the absence of other key risk factors (Kellar 2002). Regardless, information on all livestock prion diseases has been included for completeness in the Appendices, in response to international standards as published by the OIE.
• Bovine embryos, collected and processed according to protocols established by the International Embryo Transfer Society, do not pose a BSE risk. (IETS 2001; OIE 2002).

The risk assessment also uses assumptions compiled by Health Canada in co-operation with the Canadian Food Inspection Agency (CFIA) (see Appendix 2), with the exception of the oral ID50, where 10,000 ID50s (SSC 1999) was used instead.

2.2. Methods

• Figure 1 details the risk pathways for BSE. This provides the framework for the evaluation.
• Information on other TSEs is included.
• This evaluation is much more exhaustive than Canada’s evaluations of other countries for BSE because detailed information and original files were available and adequate to perform a more critical analysis. Accepting that Canada has implemented controls according to (and, in some cases, in addition to) internationally recognized standards, this critical analysis is expected to be useful in the fine-tuning of Canadian BSE control programs.
• The time period used in this risk assessment for the potential introduction of BSE is from 1979 onward for imports from the U.K. and other BSE-infected countries.

Figure 1 (Click on thumbnail to see a larger image.)

3. DEMOGRAPHICS AND INDUSTRY CHARACTERISTICS

Agriculture and agri-food is one of the most important sectors of Canada’s economy. It is the second-largest manufacturing sector, the source of one in seven Canadian jobs, and is valued at $130 billion. Rapid growth in exports (to the current level of $23 billion) over the past several years has been largely attributed to Canada’s reputation as a producer of safe, high-quality food. Canada is committed to maintaining this reputation, as demonstrated by the central role that food safety plays in the Canadian agricultural policy framework and in the food safety objectives of the CFIA (www.inspection.gc.ca), Health Canada (www.hc-sc.gc.ca), and Agriculture and Agri-Food Canada (AAFC) (www.agr.gc.ca).

3.1. Population and Distribution of Dairy and Beef Cattle

The population breakdown of cattle in Canada is found in Table 1. The cattle population has remained largely stable over the last several years at 14.6 million head in July 2001, compared with 14.7 in July 1995 and 14.9 in July 1997. The cattle population decreases by about 2 million head during the winter months, as animals are culled to reduce feeding costs. Of the total cattle population in 2001, 2.2 million were dairy cattle and 12.4 million were beef. The beef industry generally follows an 11-year cycle of growth followed by contraction in response to market supply-and-demand pressures. The dairy industry has a well-established domestic demand and limited exports. Hence, the population of dairy cattle has remained relatively stable, increasing slightly in response to small increases in the Canadian population.

Figures 2 and 3 show the geographic distribution of cattle populations across Canada. The dairy population is proportional to the human population in each province, and as such is concentrated in the central Canadian provinces of Quebec and Ontario. Beef production is concentrated in the western provinces of Alberta and Saskatchewan.

Table 1: Total Cattle Inventory, 2000–2001

Figure 2: Distribution of Cattle in Canada

Figure 3

The distribution of slaughter and age at slaughter for Canadian cattle is found in Table 2. The bulk of the Canadian cattle slaughter (96%) occurs in federally inspected plants, which are the only establishments eligible to transport meat products across provincial and international borders. Less than 5% of the slaughter takes place in provincially inspected plants (Table 3), and of this 5%, the provinces with the largest cattle populations, Alberta, Ontario and Quebec, require that all cattle be inspected prior to slaughter (80% of the dairy population and over 60% of the total cow population are located in these three provinces). There is virtually no on-farm slaughter occurring in Canada. See Section 6.1 for details on the slaughter inspection system.

Over 70% of the beef slaughter in federally inspected plants takes place in Alberta. In federally inspected plants, 84% of the slaughter is made up of young animals (steers and heifers, mostly 18 months of age), with the balance consisting of cull cows and bulls. Provincially inspected plants are generally small in size, therefore tending to slaughter cattle from small farms and receiving a greater proportion of lower quality cull animals. Approximately 1.1 million young beef animals and 174,000 cull cows and bulls were exported to the U.S. for slaughter in 2000.

Table 2: Slaughter Cattle - Number Slaughtered in Federally Inspected Establishments in Canada

Table 3: Slaughter Cattle - Number Slaughtered in Provincially Inspected Establishments in Canada

3.2. The Canadian Dairy Sector

The dairy sector is important to Canada, with total net farm cash receipts of $4.1 billion in 2000. Dairy products shipped from approximately 281 processing plants were valued at $9.8 billion, accounting for 14.3% of all processing sales in the food and beverage industry in Canada. There are nearly 26,000 people working on dairy farms, and almost 20,500 other workers are employed at the primary processing level.

About 81% of Canada's dairy farms are located in Ontario and Quebec, where the bulk of the Canadian population resides, with 14% in the western provinces and 5% in the Atlantic provinces.

For the year 2000–2001, there were 1.16 million cows in Canada on 19,411 dairy farms. The primary breeds in Canada are Holstein (comprising more than 91% of Canadian dairy herds), Ayrshire, Jersey, brown Swiss, Canadienne, Guernsey, and milking shorthorn. For each breed, a breed association has been established under the Animal Pedigree Act.

Based on official records for all breeds in 2000, the average production of cows enrolled on official milk recording programs was 9,152 kg of milk in 305 days of lactation. Protein and butterfat levels averaged 3.23% and 3.70% respectively. Canada has one of the highest average levels of milk production in the world. This success is due, in large part, to a well-organized national approach to dairy cattle improvement and genetic evaluations.

With an average herd size of 56 milking cows, most dairy farms in Canada are small, although the trend over the past several years is towards larger but fewer farms. Most producers raise their own replacement heifers.

Calving occurs year-round on dairy farms to ensure a constant supply of fluid milk. Calves are removed from their dams shortly after birth and fed on milk replacers. Male calves and cull heifer calves are generally sold to veal or beef operations. Replacement heifer calves are fed a grain and forage-based ration to ensure rapid growth and early onset of puberty.

Most farms produce much of their own forage and grains. Supplements are also provided to cows depending on their level and stage of production. Bypass protein is important during early lactation. Given the high productivity of Canadian dairy cattle, close management of nutrition is critical to maintaining good health.

On average, dairy cows are culled at about five years of age (Bouchard 2002). In Quebec dairy herds, about 30% of milking cows in all herds are over five years of age (purebred cows are kept longer than average). Turnover in the milking herd is estimated at 30% per year.

Generally, on farms that raise their own replacement heifers, it is more profitable to retain cows that are producing well than to cull them at an early age. Hence, one of the criteria by which bulls kept for artificial insemination are assessed is the longevity of their daughters. The longevity index is set at 3, meaning cows are expected on average to be kept for three lactations (Dagenais 2002).

Canada supplies more than 20% of dairy genetics to the world in the form of high-quality dairy cattle, embryos and semen. Canadian dairy cattle are exported to more than 50 countries. Exports of Canadian dairy genetic material were valued at more than $128 million annually prior to the BSE-related restrictions put in place by the EU in October 2001. In 2000, major export markets were the U.S., the U.K., Spain, Germany, Japan, Brazil, Australia, Iran, Italy, and Mexico.

3.3. The Canadian Beef Sector

Beef production contributed over $6.6 billion to farm cash receipts in 2000, with Canadian beef and veal packers processing over 3.4 million head of cattle. Total beef production has increased by 78% since 1970 to its current level of 1.51 million tonnes. Companies across Canada continue to invest capital for processing capacity expansion and product quality improvements.

Exports have become increasingly important, given that beef consumption in Canada has remained stable. Over 50% of Canada’s production of cattle and beef is currently exported. Key markets are the U.S. (accounting for approximately 70% of total exports), Mexico, and Asia.

The major beef breeds in Canada are Hereford, Angus, Charolais, Simmental and Limousin. The industry is concentrated in regions with natural feed and land-base advantages for beef cattle production. Some 70% of beef cattle are located in the foothills and grasslands of Alberta and Saskatchewan.

Cow-calf ranching and cattle feedlot finishing are two distinct areas of specialization in Canadian beef production.

There are 4.8 million beef breeding cows and heifers in Canada (January 1, 2001). Total beef production per cow has increased from approximately 170 kg in 1972 to approximately 277 kg in 2000.

Most cow-calf ranches in Canada breed their cows in June and July, with calves born in March and April of the following year. This means that the young calves, almost all of which are raised outdoors, are not subjected to cold winter weather. The calves graze with their mothers on pastures and grasslands throughout the spring, summer, and fall seasons.

The average weight of calves at weaning in the fall (October or November) is about 250 kg, but weights can range from 160 to 320 kg depending on age at weaning, the genetic background of the calf, and grass conditions during the summer grazing season. The lighter calves (160–225 kg) are typically left on pasture for an extra 120–150 days before they enter backgrounding and high-energy feeding programs for slaughter between 18 and 24 months of age. The medium-weight calves (225–275 kg) at weaning are normally placed on a lower-energy backgrounding feeding program before being placed on a high-energy grain-feeding program for slaughter between 14 and 18 months of age. The heavier calves (275–320 kilograms) are normally placed on a high-energy grain-feeding program after weaning for up to 225 days, and are ready for slaughter between 12 and 14 months of age.

Feed rations are mostly based on barley in Western Canada's grain-feeding operations, while corn and barley is fed in Central and Eastern Canada.

Backgrounding is the process of feeding high-forage (alfalfa hay and straw) feeds to increase the weight of smaller calves up to 350 kg. At least one half of the calves produced in Canada each year are backgrounded before they start on a high-energy feedlot finishing program. After weaning, light calves are fed forages and grain through the winter to gain weight at 680 g to 1 kg per day. In the spring, the smaller of these calves remain on pasture or are put into feedlots to gain weight at a rate of about 1.2–1.4 kg per day. The larger calves move into feedlots and are fed high-energy feed rations.

Along with the trend to larger and more specialized cow-calf ranches, the Canadian cattle industry has evolved towards more specialization in the grain feeding of slaughter cattle. Feedlots range in size from a few hundred head capacity to very modern operations feeding over 40,000 animals at one time. Historically, most cattle were fed in small feedlots on diversified farms that also grew feed grains and wheat for human consumption. Since the land and water resources and climate in Canada are very suitable to cattle feeding, many feedlots have become larger and more highly mechanized over the past fifteen years to specialize in cattle feeding. It is estimated that over 70% of the cattle grain-fed in Canada are produced in feedlots with capacities over 1,000 head. This produces uniform and high-quality beef products.

In 2000, Canadian feedlots finished 2.7 million steers and heifers for slaughter in Canada. Another 780,000 were fed in Canada for slaughter in the U.S.

Feedlot owners purchase calves or feeder cattle from either cow-calf ranches or backgrounding operations. Only a small portion of the calves produced in Canada are fed to slaughter weights by the original owner of the ranch on which they were born.

In the feedlot industry, there are two basic types of feeding systems, the system used depending on the weight of the animals when they are placed on the finishing program. A multi-stage feeding system is used for those steers and heifers that enter the feedlot at lighter weights. These cattle are started on a higher forage-lower grain feed ration to initially gain weight at about 1 kg per day. They are fed at this level for a few weeks, after which the proportion of grain in the feed ration is gradually increased to 85–90%. Heavier feeder cattle begin at these high-percentage-grain feed rations. The average live weight at slaughter for steers is about 590 kg, while the average weight for heifers is about 550 kg.

In recent years, Canada has exported approximately 1 million head of live cattle to the U.S. annually. Canada has also begun importing more cattle from the U.S. for feeding and/or processing in Canada. The total number of cattle imported by Canada rose from 91,000 in 1998 to 233,000 in 1999 and to over 260,000 head in 2000.

3.4. SUMMARY - DEMOGRAPHICS AND STRUCTURE

• Food safety is a key commitment of three major federal government organizations dealing with agriculture and agri-food, food inspection, and public health.

• The dairy and beef sectors are important to Canada, contributing tens of thousands of jobs and about $11 billion in farm cash receipts to a vigorous and growing agriculture and food sector.

• The dairy and beef sectors are modern and specialized. Canadian dairy cattle are high-producing, and their genetic material is in demand in many countries of the world. Canadian beef exports are expanding, a good part of this based on Canada’s reputation for delivering a safe, high-quality product.

• The dairy population is concentrated in central Canada where most of the Canadian population resides. On average, dairy cows are retained for at least three lactations before being culled at about five years of age, and would therefore be old enough to show clinical signs of BSE if the disease were present in Canada.

• Beef production is concentrated in Western Canada. Most beef animals are slaughtered at 18 months of age and would introduce little infectivity into the food or rendering system if BSE were present in Canada. Beef cows are generally forage fed over winter and put out to pasture with their new calves in the spring and are less likely than dairy cattle to be exposed to high-protein meals.

Introduction | Assumptions and methods | Demographics and industry characteristics | Legislative authority
and veterinary infrastructure | Import | Slaughter and disposition | Rendering | Feed | References