The first apparent case of ‘mad cow’ disease in Brazil has sent reverberations halfway around the world and left agricultural officials scrambling to reassure the public that the country’s prodigious volume of exported beef remains safe.
On 6 December, researchers at the Animal Health and Veterinary Laboratories Agency in Weybridge, UK, confirmed the occurrence of bovine spongiform encephalopathy (BSE) — commonly known as mad cow disease — in an animal that died two years ago in the southern state of Parana.
The cow, aged 13 at death, died one day after inspectors discovered the animal lying down with stiff limbs, a possible sign of BSE. Brazilian agricultural officials had sent samples from the animal for follow-up testing in the United Kingdom after earlier post-mortem tests yielded contradictory results.
In BSE, proteins called prions misfold and aggregate to form lesions in the brain, leading to a fatal brain-wasting condition in cattle. Humans exposed to BSE can develop a similar disorder called Creutzfeldt-Jacob Disease (CJD).
Secretary of agricultural defense Enio Marques called the case “an old and isolated occurrence that brings no risk to public or animal health in the country”.
Initial microscopic examination of the tissue in April 2011 revealed no signs of the disease, but subsequent biochemical tests had been delayed for several years by a case overload at a regional testing centre, according to an official government report. The animal was listed as a low priority for re-testing as it exceeded the age range most at risk for BSE. A second test in June 2012 — this time for BSE protein markers — yielded a positive result.
In response to the final test results, Japanese officials announced on 8 December the halting of beef imports from Brazil.
As the world’s second-largest exporter of beef (behind India), Brazil could face major losses if other nations follow Japan’s lead.
The US Department of Agriculture has requested “additional information” from Brazilian authorities, according to an e-mailed statement to Nature. “Once we have had an opportunity to review that information, we will work to determine our next steps,” US officials say.
Brazilian officials characterized the case as an isolated event, as the infected cow had been exclusively grass-fed and had not been given cattle feed containing rendered cow parts — a practice linked to ‘classical’ BSE cases that spread through the United Kingdom in the 1980s and 1990s. Cow-based cattle feed has since been banned in many countries, including the United Kingdom, Brazil and the United States.
Less is known about non-classical or ‘atypical’ BSE cases thought to arise from non-feed-based routes such as spontaneous mutations, especially in older animals.
“Every population of cattle should have atypical cases,” said Robert Rohwer, director of the Molecular Neurovirology Laboratory at the Veterans Affairs Medical Center in Baltimore, Maryland. “They arise spontaneously, and so it’s kind of a mark of the credibility of your surveillance programme if you’re finding those.”
Initial reports did not specify whether atypical BSE had been confirmed in Brazil by testing for particular prion protein subtypes, but Rohwer said the older age of the animal could be consistent with such a diagnosis.
Some evidence suggests that atypical BSE could be more virulent, although clinical examples are few and far between. (See ‘California BSE prion comes with a different twist‘.)
“It’s hard to say definitively, but if it did spread, I would think we would start to see it more,” said Linda Detwiler, a clinical professor at Mississippi State University’s College of Veterinary Medicine in Starkville. However, she added, “there’s not enough data in the world to know whether it is trending.”
UPDATE (13 December 2012): Russian agricultural authorities are considering suspension of beef imports from Brazil, a major source of beef for Russia.