Mad Cow Disease (cont.)

As of March 2011, 19 BSE cases in Canadian-born cattle have been identified, 18 in Canada and 1 in the U.S. Of these 19 cases, 13 were known to have been born after the implementation of the 1997 Canadian feed ban; 12 of these 13 were born after March 1, 1999. (See Figure above: BSE Cases in North America, by Year and Country of Death, 1993-03/2011). This latter date is particularly relevant to the U.S. because since a USDA rule went into effect on November 19, 2007, Canadian cattle born on or after March 1, 1999 have been legally imported into this country for any use. One of the 19 Canadian-born BSE cases was reported in an animal that was most likely born before or possibly very shortly after implementation of the 1997 feed ban. Based on the known or most likely year of birth, an average of 1.4 cases of BSE occurred among the group of animals born each year in Canada from 1991 through 2004. The highest reported number of cases by birth year in a single year, 3 BSE cases, occurred in 2000, 2001 and 2002. The most recently reported case extends the period of BSE transmission in Canada through at least the latter half of 2004.

There is increasing evidence that there are different strains of BSE: the typical BSE strain responsible for the outbreak in the United Kingdom and two atypical strains (H and L strains).

Typical BSE strain -- The BSE strain responsible for most of the BSE cases in Canada is the same classic or typical strain linked to the outbreak in the United Kingdom. It is known to be preventable through elimination of BSE contaminated feed and has been causally linked to vCJD in humans. This typical strain has not yet been identified in any U.S.-born cattle.

Atypical BSE strain -- In July 2007, the UK Spongiform Encephalopathy Advisory Committee (SEAC) suggested that atypical BSE may be a distinct strain of prion disease. Unlike typical BSE, cases of atypical BSE, according to SEAC, may have risen spontaneously (although transmission through feed or the environment cannot be ruled out). Recently reported French surveillance data support this theory that unlike typical BSE, atypical BSE appears to represent sporadic disease.

Both of the U.S.-born BSE cases and two of the 19 Canadian-born BSE cases were 10 years of age or older. Of these older North American cases, 3 were linked to an atypical BSE strain known as the H-type. The strain type for the fourth older North American case, a 13 year-old BSE-infected Canadian cow, has been identified as the L-type.

Source: MedicineNet.com
http://www.medicinenet.com/mad_cow_disease/article.htm