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INVESTIGATION OF THE SECOND CASE OF BOVINE SPONGIFORM ENCEPHALOPATHY (BSE) IN CANADA

Background

On December 17, 2004, a cow in Northern Alberta was identified as a downer (unable to walk) and was euthanized and sampled by a private veterinarian under Canada's National BSE Surveillance Program.  Brain samples from the animal were sent to the Alberta provincial laboratory and screened for BSE using a Bio-Rad rapid test.  Testing produced a reaction on December 27, 2004 and again on December 28, 2004.  Brain samples were then sent to the Canadian Science Centre for Human and Animal Health in Winnipeg where BSE was confirmed on January 2, 2005 using the immunohistochemistry procedure for BSE.  No part of the cow entered the human food or animal feed chains, and the carcass was secured at the index premises before being transported to the Canadian Food Inspection Agency's Animal Diseases Research Institute in Lethbridge, Alberta for incineration.

The Agency quickly located the farm of origin and immediately started an epidemiological investigation on the following three lines of inquiry:

• 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 infected Holstein cow was just over eight-years-old at the time of death on the index farm, having been born October 5, 1996.  The animal left its farm of origin by sale through a livestock auction in June 1999 and was subsequently purchased from a livestock dealer in March 2000, prior to arriving on the index farm.  Before being euthanized and sampled for BSE testing, the animal had been sick for some time and had experienced posterior paralysis for a few days before the veterinarian was called.  It previously had problems delivering its last calf in 2004. 

The investigation revealed that the cow had two calves during the previous two years, one born in 2003 and the other in 2004.  Both calves were determined to have died of causes unrelated to BSE.  An investigation was also undertaken to locate the birth cohorts of the affected animal. 

The size of the birth cohort was determined to be 135 animals.  The trace-out investigation identified nine living birth cohorts which were subsequently euthanized, sampled and tested negative for BSE.  These animals were disposed of through incineration and did not enter the human food or the animal feed chains.  Because birth cohort cattle would be seven- to nine-years-old today, most had previously died or been slaughtered.  Finding more than one case of BSE in a birth cohort is rare.  This has been consistently shown internationally, even in the United Kingdom during the height of their BSE epidemic.  It has also been demonstrated in all investigations completed to date in North America.  The remaining 126 animals were traced as follows:

• five animals had died of causes unrelated to BSE on the farm of origin;
• 110 had died elsewhere or had been slaughtered;
• six animals were confirmed to have been exported to the United States for slaughter;
• four animals were untraceable because of missing records;
• one animal had previously entered the National BSE Surveillance Program in November 2004 and tested negative for BSE.

Feed Investigation

A full on-farm investigation into feed purchases and feeding practices was undertaken at the farm of origin.  The investigation revealed that the index animal was exposed to a dairy ration containing meat and bone meal between early April and mid-May 1997.  The animal was further exposed to meat and bone meal in the latter part of September 1997 through a heifer ration manufactured in March 1997.  Fifteen per cent of the meat and bone meal used in both feed rations was derived from ruminant material, which was permitted under the regulations then in effect.

Investigation Overview

While this second case of BSE is unwelcome, it was not entirely unexpected.  The first case, detected in May 2003, indicated that Canada had a low, previously undetected incidence of BSE.  Therefore, increased testing of older animals, particularly those with signs of neurological disease, as was the case with this animal, was bound to lead to the detection of one or more additional cases of BSE.  However, it is reassuring that this is only the first case of BSE detected amongst more than 23,000 tests conducted in 2004, confirming that the level of BSE in Canada is indeed extremely low.

The investigation revealed that this animal was most likely exposed to a low level of BSE infectivity through consumption of feed containing ruminant meat and bone meal during its first year of life.  The suspect lots of feed were manufactured at a time when the use of meat and bone meal in cattle rations was still legal, i.e. before the use of such materials was prohibited in 1997.  Given the direct relationship between level of exposure and length of incubation period, the age of this animal confirms that the amount of BSE infectivity present in the feed consumed in 1997 was most likely very small.  Finally, the detection of this case attests to the strength and integrity of Canada's National BSE Surveillance Program.

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