The spotlight is shifting to atypical BSE cases
Ten years after delivering the shocking news in May, 2003, of the first confirmed case of bovine spongiform encephalopathy (BSE) in a Canadian-born cow, Dr. Stefanie Czub has some very encouraging news. So far, there have been no new cases in Canadian cattle born after the enhanced feed ban came into effect in 2007, indicating that all of the country’s control measures seem to be working.
It is now generally accepted that the classical form of BSE is caused by a malformed host protein known as a protease-resistant prion protein, which is transmitted to other cattle in contaminated rendered feed products.
There were only 18 confirmed BSE cases worldwide last year and no new cases have been confirmed in Canada since February, 2011. This compares to peak occurrences of 37,280 new cases in 1997 in the United Kingdom alone. In Canada, 18 cases have been confirmed since 2003, with 12 of those detected in the surveillance years 2006, 2007 and 2008.
“Concern about BSE, at least classical BSE, is spiralling down,” says Czub, who heads the Canadian Food Inspection Agency’s (CFIA) National Reference Lab and World Reference Lab for BSE in Lethbridge, Alta. “There are still some unsolved issues we haven’t answered, such as are only young animals susceptible to infection and why only a small number of animals in a herd are affected and not all, but the focus now worldwide is on atypical BSE.”
Atypical BSE is so named because the prions behave differently in the Western blot test used for BSE diagnosis, she explains. There are two types of the atypical form: the high type and the low type. They can be identified in the lab because they have higher or lower masses of a specific form of the prion compared to classical BSE and will show up at slightly different locations higher or lower compared to the classical form in the Western blot.
Atypical BSE has been confirmed in 67 cattle worldwide, including countries that have never had a confirmed case of classical BSE, such as Sweden and the U.S. Unlike classical BSE, which is mainly detected in cattle beyond a 60-month incubation period, atypical BSE has so far been found only in older cows. Canada has had two positives for atypical BSE in cows aged 14 and 18 years, while all three of the BSE cases in U.S.-born cattle have been the atypical form in cattle aged 10 to 12 years.
An article on the U.S. Centers for Disease Control and Prevention website describes the occurrence of atypical BSE in France, where testing of every animal at slaughter or death on the farm is mandatory. The first atypical case there was identified in 2000. Slightly more than 17 million adult cattle were tested between 2001 to 2007. Of the 645 BSE positive cases, 13 were the atypical form. This work showed sporadic occurrence with one or two atypical BSE cases among the positives in animals born in the same birth year from 1986 through 1997.
Research on atypical BSE is currently in progress in Germany, Italy and at the CFIA lab in Lethbridge, where Czub is studying how this form occurs and how it is transmitted.
When cattle were infected with atypical BSE directly into the brain, clinical signs became apparent within 11 to 12 months and the animals continued to go downhill, dying of the disease five to six months later. The same experiment with classical BSE caused the disease 16 to 18 months after infection and its progression was slower with swings back and forth, but once the symptoms became permanent, it was only a matter of two to three months before death.
The animals infected with atypical BSE were examined once a month to monitor their response to light, noise and touch on the hocks, face and neck, compared to that of normal cows. All of the animals were allowed to calm down in the chute before testing. Those with low-type BSE were super-excited in the chute and barn, showing signs of severe anxiety, but calmed down once returned to their pens. The response of the animals with high-type BSE was the exact opposite, Czub says. They were lethargic and actually went down in the chute, requiring help to get up. Once helped out into their pens they were fine.
The next step is to determine whether cattle can acquire atypical BSE from ingestion and whether only young animals can get infected, as has been the commonly held theory. As of June, it had been 60 months since researchers fed atypical BSE infectious prions to 13-month-old animals and none have yet shown any clinical signs of the disease.
The lab will also be studying the distribution of atypical BSE in the carcasses compared to that of classical BSE to determine whether the specified risk materials for classical BSE are applicable for atypical BSE. So far, this research shows that distribution in the brain is fairly similar, but examinations of other tissues are still in progress.
Czub says it’s highly unlikely that there will be a live test for BSE anytime soon. As far as a blood test goes, it has yet to be determined whether the infectious prion protein is ever in the blood. If so, it would only be early in the infection, at which time the animal would not have any clinical signs of the disease that would lead a producer to have the animal examined or a blood test taken.
The reason transmissible spongiform encephalopathy in other species, such as chronic wasting disease in elk and scrapie in sheep, can be detected in saliva, urine or feces is that the infectious prions affect so many tissues, not only the brain, she explains. BSE, on the other hand, can only be detected in the brain, spinal cord and some areas of the gut, making it difficult if not impossible to obtain appropriate samples for a BSE prion-specific live test. Development of any potential live test in itself would be a complicated procedure that would need to be reproduced, validated and compared to present-day tests.
In recapping 10 years of BSE research, Czub says definitely a lot more is known about the disease and there is much more expertise in Canada, but many unanswered questions and lots of work remains.
“As far as the classical type, the numbers are really encouraging. As for atypical BSE, we need to be vigilant and cautious as we know it transmits easily in non-human primates by injection into the brain and by feeding, has a much shorter incubation time than classical BSE, and we want to be sure we have the necessary safeguards in place to protect both consumers and the national herd.”