Scientists test a cow carcass for BSEScientists test a cow carcass for BSE © Burger/Phanie/REX/Shutterstock

Exposure to old feed contaminated with bovine spongiform encephalopathy (BSE) is the most likely explanation for the continuing cases of the disease in cattle, EU scientists have concluded.

Incidence rates are now low. Out of 73 million cattle tested between 2005 and 2015, just 1,259 were found to be positive for the disease, spread across 11 countries.

And of these, just 60 were found to have been born after the EU introduced a total ban on meat and bonemeal in animal feed in 2001.

See also: ‘Isolated’ BSE case found in Irish dairy cow

The EU commission asked the European Food Safety Authority (Efsa) to investigate these cases and their conclusion is contaminated feed is the most likely source of infection, given the infectious agent has the ability to remain active for many years.

“Cattle may have been exposed to contaminated feed because the BSE infectious agent was present where feed was stored or handled,” said a statement.

“A second possibility is contaminated feed ingredients may have been imported from non-EU countries.”

Other explanations

The scientists also looked at other plausible sources of infection, including maternal transmission, environmental factors and “iatrogenic” causes, whereby the agent could have been introduced via some other medical intervention.

But poor data collection, human recall and documentation made it hard to draw any conclusion other than that feed-borne exposure is the most likely cause.

This conforms with previous, much larger investigations in the UK and Ireland at the height of the BSE crisis, which concluded that feed was a “significant risk factor”.

The fact there were geographical “clusters” of BSE cases in the UK in the 1990s also supported this conclusion, though data also indicated some possible maternal transmission or environmental contamination.

Insufficient testing

The Efsa report also questions why many member states have had no cases of BSE since the 2001 feed ban and suggested this may be due to insufficient testing.

It also said there were deficiencies in the implementation of feed controls in some member states in the early days, which meant contaminated feed was still in circulation for some time after the feed ban came in.

BSE – the facts

  • BSE was first diagnosed in the UK in 1986, with affected cattle losing physical control, then dying
  • Between 1986 and 1998 there were more than 180,000 cases diagnosed
  • A further 4.4m British cattle were culled as part of the eradication policy
  • The most likely cause was BSE existing in feed as meat and bonemeal, with the agent surviving at very high rendering temperatures
  • The link with new variant CJD in humans was confirmed in March 1996
  • Since then, there have been more than 200 human cases, mostly in the UK
  • The most likely cause was contaminated brain and spinal cord ending up in processed beef
  • This lead to the ban on all specific risk material in the food chain
  • British beef exports were banned for 10 years, with France the last EU country to lift the ban in May 2006
  • Only cattle aged under 24 months were initially allowed to be exported
  • An EU-wide ban on meat and bonemeal in feed was introduced in 2001
  • Since then, the number of BSE cases has dropped from 554 in 2005 to just two in 2015