China and South Africa informed Brazil on Thursday that they were suspending imports of beef from the world's biggest exporter of the meat, following a case of atypical BSE that was confirmed last week, Brazilian agriculture ministry officials said.
Including Japan, which suspended imports on Monday, three countries have now restricted purchases of beef from Brazil while seeking details about the death of an elderly cow in 2010 which never actually developed the disease.
None of these countries are significant buyers of Brazilian beef. Brazil's top customer, Russia, has so far imposed no such restrictions, though it said on Thursday that it was weighing its options.
Brazil has launched a diplomatic offensive to clarify the details of the case of suspected atypical BSE, which it has been at pains to differentiate from regular BSE, which is usually caused by contaminated feed.
Atypical BSE can arise in elderly cattle due to a spontaneous genetic mutation that causes it to begin producing distorted proteins called prions. The proteins can trigger BSE, which eventually destroys the animal's nervous system, and it is believed humans ingesting beef from a stricken animal can contract a fatal form of the disease.
The 13-year-old cow in southern Brazil tested positive for prions, a result confirmed by the World Organization for Animal Health (OIE) last week. But it died of other causes in 2010 and never actually developed the disease.
The animal was buried on the farm where it had been used for breeding purposes and never entered the food chain.
Outbreaks of mad cow disease in Europe, North America and Japan in the past decade, following an epidemic in Great Britain in the late 1980s, prompted some importers to embargo shipments and roiled the industry on several occasions.
In April, the United States reported a case of atypical BSE in an animal which never entered the food chain, but the country escaped a backlash from importers.
The Brazilian agriculture ministry's secretary for animal and plant health, Enio Pereira, told Reuters this week that much of the two-year delay between the cow's death and confirmation of prions in its tissue was caused by a logistical anomaly at the laboratory.
-- Peter Murphy writes for Reuters from Sao Paulo, Brazil.