Two types H type and L type were discovered as a result of better science and testing methods
The USDA announced on April 24 it had discovered its fourth case of BSE in a California dairy cow — the first animal to test positive in the U.S. since 2006.
Trade disruptions are not expected as a result of the latest American case. Within 12 hours of the announcement, some South Korean retailers were pulling U.S. beef from their shelves, although the Korean government did not cease trade.
John Clifford, chief veterinary officer in the U.S., stated that the 10-year-old dairy cow was euthanized after it was unable to stand. It was also announced that the animal in question had suffered from atypical BSE, and samples were being sent to Lethbridge and to the U.K. for further analysis.
The first U.S. BSE case came only seven months after Canada’s first indigenous BSE case in May of 2003. The first U.S. case is the only one to date in which the classic form of BSE (cBSE) was found. The other two American incidents occurred in Texas in 2004 and in Alabama in 2006, but both of those cases were atypical BSE H type.
Atypical BSE is distinctly different from cBSE, and there are two different atypical strains — H type and L type. Both were discovered after cBSE as a result of better science and testing methods. All types of BSE are the result of misfolded proteins called prions that replicate by causing healthy proteins to similarly misfold. The jury is still out as to how H type and L type are spread, but symptom onset and mortality occurs much later than with cBSE — the median age for atypical BSE presentation in cattle is 12. In cBSE, it is much younger, usually appearing in animals between four and six years old.
Two of Canada’s 18 BSE cases were atypical. The first was a 16-year-old beef cow from Manitoba with H type, and the second was a 13-year-old beef cow from Alberta with L type BSE.
The OIE, the scientific community and governments around the world treat all prions in the same manner as cBSE. Atypical prion disease is often referred to as sporadically occurring, but sporadic disease simply means the cause hasn’t been isolated or defined yet through research. Some research indicates that sporadic prion disease in both cattle and people may be the result of a genetic mutation.
H type and L type atypical BSE are quite different from one another. H type does not seem to be able to jump species as readily, whereas L type appears to be more infectious than classic BSE.
Although the USDA statement asserted that atypical BSE isn’t known to be spread through feed, it hasn’t yet been proven that it can’t be. In fact, primates have successfully been infected by the L-type prion through ingestion in experimental research.
Dr. Stefanie Czub, the manager of national and OIE reference laboratories for BSE, virology and pathology at the National Centre for Animal Disease and the CFIA, is currently conducting research at the Lethbridge facility on both atypical strains and cBSE. She says while the research around the world conducted on various animals and their reaction to prions is intriguing, it’s important to focus on more practical research.
“It’s fascinating and a really puzzling phenomenon for us scientists, but the more important experiment is of course by feeding, by oral challenge of cattle with these different types, and that’s what we have done in Lethbridge,” said Czub. “The cattle are now around 30 months of age and we haven’t seen anything so far.”
Czub’s work will help to discover whether atypical strains can be spread via feed in the same manner as cBSE. In the event that it can be, safeguards such as feed bans and SRM removal currently in place would prevent further spread.