Last week the Canadian Food Inspection Agency (CFIA) announced that a case of bovine spongiform encephalopathy (BSE), more commonly known as Mad Cow Disease, was found in an Alberta beef cow

BSE belongs to a group of diseases called transmissible spongiform encephalopathies (TSEs) that are not caused by bacteria, viruses, fungi or parasites, but rather by abnormally-shaped proteins call prions.  Prions are extremely difficult to destroy, and they can’t be killed using antimicrobial drugs because they aren’t actually microbes.  Exposure to the prions, most commonly through ingestion, can lead to spread of disease.  That’s why Canada banned the use of most animal proteins (specifically from most other mammals) from use in cattle feed back in 1997, in order to decrease the risk of BSE spreading if it ever got into the Canadian cattle population.  In 2003 an enhanced feed ban was introduced after the first case of BSE was found in a Canadian cow.  The ban prohibits "specified risk materials" (SRM) from cattle from entering the human food chain AND from being used in animal feeds.  The SRM includes all the tissues where prions would most likely be found, such as the brain and spinal cord, in animals over 30 months of age.

Most TSEs seem to be relatively species specific, but there is still a lot we don’t know about them.  Unfortunately, there is strong evidence that the prion that causes BSE can cause disease in people, called variant Creutzfeldt-Jacob disease (vCJD).  The "variant" differentiates this disease from sporadic or familial CJD, a rare human disease that has been recognized since the 1920s.  Just over 200 cases of vCJD have been diagnosed since it was first detected in 1996, most of which occurred in Great Britain.  There is currently no evidence that these prions can cause disease is dogs or horses, but they do appear to be the causative agent of a similar disease in cats (feline spongiform encephalopathy (FSE)).  Given that these prions have crossed at least two species barriers (people and cats), the possibility that they could affect other species as well cannot be dismissed.

What will be the impact of this single case in Alberta?  Hopefully not much.  The World Organization for Animal Health considers Canada a controlled BSE risk country, and one case won’t change that status.  Canada has an extensive surveillance system through which more than 30 000 cattle are tested every year for BSE, and this is the first case detected in 4 years.  This case was in fact a good example of the surveillance system in action – the case was detected before any part of the cow (not just the SRM) was allowed to enter the food chain.  The Canadian food supply is still very safe, as is the animal food supply.  The worrisome part of this case is that the cow was born in Alberta in 2009, well after the enhanced feed ban was put in place.  So the question is, how did the cow get exposed to the prions?  Cases can rarely also occur sporadically in cattle (as for classical CJD in people), could this have been one of those?  The incubation period for BSE is typically years, so the investigation is focusing a lot on the farm of origin, not just the farm where the cow last resided. This is where the ability to trace animal movement and movement of animal products becomes so important, as they are in so many disease investigations.

  • Actually we cannot know which species a prion will affect. Sometimes the species barrier is crossed in unexpected ways. Cats and other felines definately can be infected by the bse prion producing the feline spongiform encephalopathy.
    Mad cow disease can come from sporadic cases also not related to contaminated feed (the classical or c type), called the h and l type, usually in older animals. Most probably cases appear time to time in the cattle population like the sporadic cjd in humans, and one such case spread to cause the well-known bse epidemic. Without the monitoring brought by this epidemic, we would not know about the rarer sporadic bovine subtypes, wich even now don’t get much attention.
    There is though the possibility of transmition to humans, as l type prions have been experimentally transmitted to monkeys with disease pathology more similar to sporadic human cjd rather than the variant type. Thus some human sporadic cases might come from food as well. But given the rarity of the disease, the sometimes extremely long incubation periods which will make tracing impossible, and the high cost of monitoring compared to the cases, I don’t believe much funding will be directed to such research.