(Adds details of cases, context on China ban)

RIO DE JANEIRO, Nov 11 (Reuters) - Brazilian authorities are investigating two possible cases of mad cow disease in humans in Rio de Janeiro state, a municipal health secretary said in a statement on Thursday, raising a red flag for meatpackers that already halted beef exports to China.

On Sept. 4 Brazil confirmed two cases of what it called "atypical" mad cow disease in animals, triggering a suspension

of beef exports to China under a standing bilateral agreement.

At the time, Brazil's Agriculture Ministry stressed that the two cases identified in meat plants in the states of Mato Grosso and Minas Gerais had generated spontaneously and were not related to contaminated feed, as in classic mad cow disease.

On Thursday, municipal health authorities in the city of Rio said federal biomedical institute Fiocruz had flagged two cases of "prion disease," which can occur spontaneously in elderly patients or by eating contaminated beef in younger populations.

The municipal health authorities said the two cases identified in residents of the Rio suburbs had been referred to state health authorities, without giving the patients' ages.

The new cases could further delay a Chinese decision on lifting the Brazilian beef ban, which has stranded

dozens of shipments, rerouted many others and weighed on beef exports from Latin America's largest country.

Brazil's Agriculture Ministry, along with major Brazilian beef exporters JBS SA JBSS3.SA , Minerva SA BEEF3.SA and Marfrig Global Foods SA MRFG3.SA did not immediately respond to requests for comment.

(Reporting by Rodrigo Viga Writing by Ana Mano Editing by Brad Haynes, Kirsten Donovan)

((ana.mano@thomsonreuters.com; Tel: +55-11-5644-7704; Mob: +55-119-4470-4529; Reuters Messaging: ana.mano.thomsonreuters.com@reuters.net))