SAO PAULO, May 22 (Reuters) - Brazil on Monday declared a state of animal health emergency for 180 days in response to the country's first ever detection of the highly pathogenic avian influenza virus in wild birds, in a document signed by Agriculture Minister Carlos Favaro.

Infection by the H5N1 subtype of avian flu in wild birds does not trigger trade bans, based on guidelines of the World Organization for Animal Health. However, a case of bird flu on a farm usually results in the entire flock being killed and can trigger trade restrictions from importing countries.

Brazil, the world's biggest chicken meat exporter with $9.7 billion in sales last year, has so far confirmed eight cases of the H5N1 in wild birds, including seven in Espirito Santo state and one in Rio de Janeiro state.

The country's agriculture ministry said later on Monday it has created an emergency operations center to coordinate, plan and evaluate "national actions related to avian influenza."

Though Brazil's main meat producing states are in the south, the government is on alert after the confirmed cases, as avian flu in wild birds has been followed by transmission to commercial flocks in some countries.

Shares in Brazil-based BRF SA (BRFS3.SA), the world's biggest chicken exporter, were up 3.6% before the government announcement and ended the day 0.5% lower.

Over the weekend, the Health Ministry said samples of 33 suspected cases of avian influenza in humans in Espirito Santo, where Brazil confirmed the first cases in wild birds last week, came back negative for the H5N1 subtype.