* Officials say no threat to humans, did not enter food chain
* Major foreign buyers say will continue to import
* Chicago live cattle futures edge back after plunge
By Charlie Dunmore and Theopolis Waters
BRUSSELS/CHICAGO, April 25 (Reuters) - Major markets for U.S. beef from Canada to Japan stayed open to imports on Wednesday after the first U.S. discovery of mad cow disease in six years on assurances that rigorous surveillance had safeguarded the food system.
U.S. live cattle futures rode back into the black after posting their biggest drop in seven months the previous day.
On reporting the discovery of a California dairy cow infected with bovine spongiform encephalopathy (BSE) on Tuesday, U.S. authorities quickly told consumers and importers around the world there was no danger the meat would enter the food chain.
Mexico, Korea, Japan, Canada and the European Union said they would continue to import U.S. beef, although two major South Korean retailers halted sales.
Benchmark June live cattle futures on the Chicago Mercantile Exchange were up 0.9 cent at 112.45 cents per lb at 1424 GMT, still near their lowest in ning months after tumbling by the daily 3-cent trading limit on Tuesday.
Samples from the infected cow have been sent to laboratories in Canada and Britain for final confirmation, Paris-based World Organization for Animal Health (OIE) said in a statement, adding that the case was unlikely to affect the current USDA "controlled risk" categorisation for mad cow disease.
"According to USDA statements, the steps taken so far are consistent with OIE standards," it added.
Russia's health watchdog said it could consider restrictions on U.S. imports but that it was waiting for more information on the outbreak and the planned U.S. response before taking a decision.
Korean retailer Lotte Mart, a unit of Lotte Shopping Co. , said it had suspended sales due to what it said was "customer concerns", as did Home Plus, a unit of Britain's Tesco PLC.
Three previous cases of mad cow disease were confirmed in the Unites States between 2003 and 2006. Memories were still sharp of the first case in 2003, which caused a $3 billion drop in U.S. exports. It took until 2011 before those exports fully recovered.
Experts said the latest case was "atypical", meaning it was a rare occurrence in which a cow contracts the disease spontaneously, rather than through the feed supply.
They said the dairy cow had not been eaten by other animals and there was no risk of the disease being spread and estimated the chance of an animal spontaneously contracting the disease at about one in a million.
The USDA is still tracing the exact life of the infected animal, and the carcass of the cow is under quarantine and will be destroyed.
The cow was found at a rendering plant, which processes diseased or sick animals into mainly non-edible products for use in products such as soap or glue.
Mexico, which buys more U.S. beef than any other country, said it has no plans to halt imports and that it would maintain the same regimen of inspections for trade across the border.
The Canadian Food Inspection Agency said trade would not be affected in one of the top importers of U.S. beef.
The case also will not affect negtiations on Japan's possible membership in a U.S.-led Pacific trade pact, Tokyo said on Wednesday. Japan already only allows imports of U.S. and Canadian beef from cattle aged 20 months or less.
Vietnam, which suspended U.S. beef imports between December 2003 and September 2011, also said it had not changed its policy on U.S. beef in response to the latest news.
In the U.S. domestic market, companies are still smarting from the fallout over a ground beef filler that critics called "pink slime", made from scraps of beef sprayed with ammonia gas to kill bacteria. The product was pulled from grocery store shelves and forced one producer to idle several factories and another to file for bankruptcy.