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 the1997 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
All (3) of the U.S.-born BSE cases and two of the 19 Canadian-born BSE cases were 10 years of age or older and caused by atypical BSE strains. Of these 5 older North American cases, 3 were linked to an atypical BSE strain known as the H-type. The strain type for the other two older North American cases, a 13-year-old BSE-infected Canadian cow and a 10-year-old BSE-infected US cow, have been identified as the L-type.
For additional data on this topic: http://www.cdc.gov/eid/content/14/2/298.htm

As of October 26, 2009, a regulation issued by FDA in April 2009 came into effect establishing an enhanced BSE-related feed ban in the United States. This enhanced ban will further harmonize BSE feed control measures in the U.S. with those in Canada (see below). In addition, FDA continues to enforce its important 1997 mammalian-to-ruminant feed ban through its BSE inspection and BSE feed testing programs.
As of July 12, 2007, an enhanced BSE-related feed ban  came into effect in Canada. CFIA established this ban to more effectively prevent and quickly eliminate BSE from Canada. The enhanced ban prohibits most proteins, including potentially BSE infectious tissues known as “specified risk materials” (SRM) from all animal feeds, pet foods, and fertilizers, not just from cattle feed as required by the ban instituted in 1997. The 1997 feed ban in Canada was similar to the feed ban instituted in the United States that same year. As recently reported by CFIA, removing SRM from the entire animal feed system addresses risks associated with the potential contamination of cattle feed during production, distribution, storage, and use. Applying the same measure to pet food and fertilizer materials addresses the possible exposure of cattle and other susceptible animals to these products. CFIA expects that with this new ban, BSE should be eliminated from the Canadian cattle herd by about the year 2017.
The Canadian-born cow confirmed to be infected with BSE in 2010 illustrates the difficulty in determining the effectiveness of previously instituted feed bans to prevent BSE transmissions. The initial feed bans established in both the United States and Canada were instituted in 1997. After an assessment by USDA and its Canadian counterparts, the Canadian feed ban was judged to be fully effectiveness as of March 1999. However, largely because of recognized limitations of this ban and the ban established in the United States, new, enhanced feed bans went into effect in Canada, July 12, 2007, and in the U.S., October 26, 2009. While USDA has confirmed no U.S.-born cattle as having a classic form of BSE, Canadian cattle born after March 1999 have been legally imported into the United States for any purpose since November 19, 2007.

Based on World Organization of Animal Health (OIE) standards for BSE surveillance, the reported national prevalence rates of BSE in North American cattle, particularly in animals born in the United States, is very low, and therefore, difficult to measure accurately. In September 2007, the USDA published updated results of the two statistical models used by Harvard University investigators to estimate the prevalence of BSE in Canada. The results incorporated the 11 Canadian-born animals with BSE that had been reported at that time. A key advantage of these models is that they provide statistical confidence limits that measure some of the uncertainty associated with expected estimates. To view the results of the model, called BSurvE, that is most comparable to the observed surveillance data, see page 15 of the document titled "Response to peer review of risk assessment of Bovine Spongiform Encephalopathy (BSE) risks associated with the importation of certain commodities from BSE minimal risk regions (Canada)"   (PDF – 261 KB). This model estimated that the true prevalence of BSE in Canada has been 90% likely to be between 18-fold and 48-fold higher than the previously published best estimate of the prevalence of BSE in the United States (3.0 to 8.0 cases per million in Canada compared to a best estimate of 0.167 cases per million in the United States [reference #3]). The previously published best estimate of Canada’s BSE prevalence in 2006 using the BSurveE model was 23-fold higher than that of the United States and is the estimate of the BSE prevalence in Canada that continues to be used in the Harvard Risk Assessments’ "worst case" analyses when evaluating the risk of imported Canadian cattle causing BSE to spread among US animals