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Fact sheet – Atypical bovine spongiform encephalopathy (atypical BSE)

What is atypical BSE

Atypical BSE is one form of BSE. It is different from classical BSE, which was responsible for the BSE epidemic that started in the United Kingdom in 1986. 

Both the geographical distribution of atypical BSE cases, even in countries where no cases of classical BSE have been reported, and the fact that the disease mostly occurs in old animals, support the assumption that this extremely rare disease develops spontaneously (for example without an apparent cause) in any cattle population. The rate of reported atypical cases is not affected by the control measures implemented to eliminate the transmission of classical BSE and this provides evidence that it occurs spontaneously.

Atypical BSE, just like classical BSE, is a disease that affects the nervous system of cattle and is always fatal, progressive and does not respond to treatment. It belongs to a group of diseases that affect the nervous system of humans and animals called transmissible spongiform encephalopathies. There is no treatment or vaccine currently available for the disease.

There are two types of atypical BSE: H type and L type. The biological properties and the biochemical characteristics of the prion proteins that cause the disease are different than those of classical BSE.

Atypical BSE is known to be present in old animals. In the 125 atypical BSE cases with known age reported worldwide from 2001 to 2021, the average age at detection was 12 years (ranging from 5.5 to 18.5 years).

Atypical BSE in Canada

BSE has been a reportable disease in Canada since 1990. Canada is required to notify the World Organisation for Animal Health (WOAH; founded as Office International des Épizooties (OIE)) of any occurrence of a confirmed BSE case (either classical or atypical).

There have been three cases of atypical BSE reported in Canada:

  • the first one was diagnosed in June 2006 in a beef cow born in 1989
  • the second one in January 2007 in a beef cow born in 1994
  • the third one in December 2021 in a beef cow born in 2013

Like all the atypical BSE cases reported worldwide, the cases in Canada were not linked to a common infectious source (such as prion-contaminated feed).

Clinical signs of atypical BSE

Cattle with atypical BSE will likely have some of the same clinical signs as cattle with classical BSE.

A bovine with a nervous form of atypical BSE resembling classical BSE would show signs such as over-reactivity to external stimuli, unexpected startle responses, and incoordination. In contrast, a bovine with a dull form of atypical BSE would show signs such as dullness accompanied by a low head carriage and compulsive behaviour (licking, chewing, pacing in circles).

The disease usually progresses over the course of a few weeks to several months to the final stages of the disease (inability to stand, coma and death).

As these signs are not particularly distinctive of BSE, it is expected that individual animals displaying clinical signs suggestive of BSE would be observed in all cattle populations.

Transmission of atypical BSE

Research has confirmed that L-type BSE may be transmitted orally to calves. In light of this evidence, and the assumption that atypical BSE can develop spontaneously in any cattle population (in really low numbers), it is reasonable to conclude that atypical BSE could be transmitted if cattle were to be fed contaminated feed. Therefore, the same prevention measures are in place for both classical and atypical BSE.

There is no evidence at this time that atypical BSE could be linked to cases of Creutzfeldt-Jakob disease in humans.

Diagnosis of atypical BSE

There is no test to diagnose any form of BSE in live animals, although a tentative diagnosis may be made based on clinical signs. Diagnosis can only be confirmed by laboratory examination of the animal's brain after its death.

Laboratory tests must be performed to distinguish between the classical and atypical BSE, and between the two types of atypical BSE.

Food safety and animal health in Canada

Canada, as well as many other countries, has taken precautions to prevent the spread of BSE. The BSE enhanced surveillance program in place has been able to detect cases of atypical BSE, which is a very rare disease. This is an indicator of the quality of such program and the veterinary services in Canada.