Mad Cow Had Atypical Strain

CANADA - On December 9, 2007 a cow located on a commercial beef farm in East Central Alberta was destroyed following a confirmed infection of mad cow disease. This was the 11th case of BSE to have been identified in Canada since tests began, but this week it became clear that the case wasn't an ordinary one.
calendar icon 18 April 2008
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It has been determined that Canada’s eleventh case of BSE was from a less prevalent, atypical strain of BSE which has also been reported in Europe.

This is the second case of BSE in Canada that has involved an atypical strain. A common feature of atypical BSE cases is that the affected animals are of an advanced age at the time of diagnosis (for example, both of Canada’s atypical cases involved cattle that were over 13 years of age at the time BSE was confirmed). This is in contrast to Canada’s classical BSE cases where the average age has been approximately 6 years.

Officials were cited as saying the nearly 14-year-old Hereford likely contracted mad cow disease from feed early in its life, since it was born before 1997 when Canada imposed a ban in on mixing protein from rendered cattle or other ruminants in feed. The cow lived its entire life on the Alberta farm where it was born and no part of the animal ever entered the human or animal food supply. It was Canada's 11th home-grown case of BSE since 2003.

The History

A private practitioner first sampled the animal on December 9, 2007 under Canada's National BSE Surveillance Program. On December 11, 2007, brain samples were received by the National BSE Reference Laboratory in Lethbridge, AB, where they were screened for BSE using a Prionics-check priostrip test.

The result of this preliminary test did not rule out BSE. In accordance with the prescribed testing protocol, the test was repeated and produced a second reaction that day. Additional rapid screening tests conducted at the National BSE Reference Laboratory, were positive on December 12, 2007 and December 13, 2007, respectively.

The Scrapie Associated Fibril Immunoblot was positive on December 14, 2007 and on December 17, 2007, the immunohistochemistry procedure was positive. The carcass was secured at the sampling site, and was subsequently transferred to the CFIA Laboratory in Lethbridge for incineration. No part of the carcass entered the human food supply or animal feed chain.

The CFIA immediately initiated an epidemiological investigation based on the most recent recommended BSE guidelines of the World Organisation for Animal Health, referred to as OIE. Specifically, the CFIA followed the recommended guidelines for a country with controlled BSE risk status and investigated:

  • the feed cohort, comprising all cattle which, during their first year of life, were reared with the BSE cases during their first year of life, and which investigation showed consumed the same potentially contaminated feed during that period, or
  • the birth cohort, comprising all cattle born in the same herd as, and within 12 months of the birth of, the BSE cases, if the above cannot be identified.

The Investigation

The positive animal was an unregistered Hereford cow born on March 15, 1994 and was 165 months of age at the time of death. The age of the animal complicated the investigation because it exceeded the normal information retention period of the commercial enterprises involved (for example, auction records are normally kept for seven years). However, farm records were extensive and indicated the case animal was born, raised and had spent its entire life on the same farm. The producer reported that the cow had been ill for approximately three and a half months and that prior to death was lame and had an abnormal posture.

The producer determined that the animal should be destroyed, and a private veterinary practitioner attended the premises to determine if the animal met the inclusion criteria of Canada's National BSE Surveillance Program. Since the inclusion criteria of Canada’s National BSE Surveillance Program were met, arrangements were made to forward appropriate samples for laboratory evaluation. A post-mortem examination was conducted, abdominal adhesions were observed, and a presumptive diagnosis of peritonitis was made by the submitting practitioner.

The case premises is a beef cow-calf operation. The operation has both a purebred and commercial component. The birth cohort was determined to comprise 357 animals, which, along with the case animal, were born on the birth farm. This includes male and female animals born from March 15, 1993 to March 15, 1995. Both sexes were included as they had access to the same commercially prepared feeds and may have been exposed to the same potentially contaminated feed as the case animal. During the time period of interest, no animals one year of age or less were purchased. All replacements were sourced from the producer’s own calf crop. Therefore, there were no additional feed cohorts.

The trace-out investigation of the cohort located eight live animals on the premises and in one other herd. The eight animals have been humanely destroyed, and their carcasses were disposed of along with the case animal’s by incineration, in accordance with OIE recommendations.

Because the cohort cattle would now be 13-15 years of age, most had previously been slaughtered or died of natural causes. The following is the disposition of the remaining 349 animals:

  • 81 animals were traced and confirmed to have died or been slaughtered,
  • 254 animals were traced and presumed to have died or been slaughtered,
  • 14 animals were determined to be untraceable due to incomplete records.

The investigation revealed that the case animal had two calves born within the previous two years. The CFIA no longer requires the destruction of calves of BSE positive cows born within 24 months of the development of clinical signs, in accordance with the current Bovine Spongiform Encephalopathy Chapter of the OIE Terrestrial Animal Health Code (2007). However, the CFIA continues to trace calves born to a positive female in respect of the current export certification requirements of some importing countries. The 2006 progeny was confirmed to have been slaughtered and the 2007 progeny was located on the case farm and was humanely destroyed to ensure Canada’s continued compliance with current export certification requirements. Its carcass was incinerated at the CFIA Lethbridge Laboratory.

The Feed Investigation

The feed investigation yielded limited records specific to the animal’s first year of life. A probable feeding regime was identified through recollection of standard feeding practices and an index of feed products used was developed based on invoices available for the period when the animal was 10 - 20 months of age. These records provided the basis for determining the types of products used and the feed suppliers with which a business relationship existed.

Review of the information identified feeding practices consistent with an operation of this type. The case animal was reported to have remained with its dam from time of birth through to weaning at approximately eight months of age. Prior to weaning, the animal was kept on community pasture and had access to mineral feed products. At the time of weaning, the animal was removed from pasture and fed farm-harvested forage as well as a farm-mixed ration comprised of farm-grown grains, commercial protein supplement, and/or mineral feed products. Feeding of this type of product continued through to approximately 13 months of age.

Other feed products identified on the farm but not directly linked to the subject animal included milk replacer and commercially prepared complete feed. Commercial feed products (minerals, milk replacers, protein supplements, complete rations) were purchased from a variety of suppliers and represented all identified businesses within the trading area.

The period of interest of this feed investigation, pre-dates the implementation of Canada’s initial ruminant feed ban in August of 1997. The age of the animal further limited the investigation as the time period exceeded normal information retention periods. As a result trace back inspections at the suppliers and manufacturers of commercial feeds distributed to the birth farm did not yield specific distribution records or mixing formulas for the time frame of interest. Three of four possible manufacturers supplying a protein supplement likely fed to the animal could have included meat and bone meal (MBM) as an ingredient in its formulation.

One of these manufacturers was able to confirm usage of meat and bone meal in supplements and confirm a source of MBM to be one common to previous BSE investigations. This information is consistent with the commercial feed industry’s practice of using of meat and bone meal as a source of protein in livestock feeds at the time (prior to the 1997 Feed Ban).

A review of the common feeding and manufacturing practices indicates probable exposure to infectious material through a commercial feed supplement containing meat and bone meal.

Investigation Overview

Canada’s eleventh case of BSE has been attributed to a less prevalent, atypical strain of BSE which has also been reported in Europe. This is the second case of BSE in Canada that has involved an atypical strain. A common feature of atypical BSE cases is that the affected animals are of an advanced age at the time of diagnosis (for example, both of Canada’s atypical cases involved cattle that were over 13 years of age at the time BSE was confirmed). This is in contrast to Canada’s classical BSE cases where the average age has been approximately 6 years.

The identification of these atypical strains of BSE is a reflection of an increased global awareness of the potential for multiple strains of the BSE agent to exist, continuous advancements in diagnostic test methods and is a direct result of the enhanced BSE surveillance activities occurring worldwide.

Since the confirmation of BSE in a native-born animal in May 2003, Canada has significantly increased its targeted testing of cattle in high-risk categories advocated by the OIE. This effort is directed at determining the level of BSE in Canada, while monitoring the effectiveness of the suite of risk-mitigating measures in place. Canada’s National BSE Surveillance Program continues to demonstrate an extremely low level of BSE in Canada, with 11 positive animals detected.

With respect to BSE, the safety of beef produced in Canada is assured by public health measures enacted in 2003, following the first detection of BSE in a native-born animal in Canada. The removal of specified risk material (SRM)-those tissues that have been demonstrated to have the potential to harbour BSE infectivity-from all animals slaughtered for human consumption is the most effective single measure to protect consumers in Canada and importing countries from exposure to BSE infectivity in meat products.

As demonstrated by the surveillance system, the feed ban implemented in 1997 is effectively preventing the amplification of BSE in Canada’s feed system. Additional regulations to enhance Canada’s feed ban were enacted on July 12, 2007. The most important change is the removal of SRM from all animal feeds, pet food and fertilizer. The enhancement will significantly accelerate progress toward eradicating BSE from the national cattle herd by preventing more than 99 per cent of potential BSE infectivity from entering the Canadian feed system. These measures are effectively minimizing the risk of transmitting BSE.

On May 22, 2007, Canada was officially categorized under the OIE’s science-based system as a controlled BSE risk country. This status clearly recognizes the effectiveness of Canada’s surveillance, mitigation 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.

Further Reading

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