The World Health Organization (WHO) has said a man’s death in Mexico was caused by a strain of bird flu called H5N2 that has never before been found in a human, writes The Guardian.

The WHO said it wasn’t clear how the person became infected. “Although the source of exposure to the virus in this case is currently unknown, A(H5N2) viruses have been reported in poultry in Mexico”, it said in a statement.

Scientists are on alert for changes in the virus that could signal that bird flu is adapting to spread more easily among humans. But the UN agency said said the current risk of the bird flu virus to the general population in Mexico is low.

The 59-year-old man, who had been hospitalised in Mexico City, died on 24 April after developing a fever, shortness of breath, diarrhoea, nausea and general discomfort, the WHO said. Mexico’s health ministry added in a statement that there had so far been no evidence of person-to-person transmission of bird flu in the case of the man who died, and that he had several prior health conditions. All people who had contact with him have tested negative, it said.

In March, Mexico’s government reported an outbreak of A(H5N2) in an isolated family unit in the country’s western Michoacan state, but said at the time it did not represent a risk to distant commercial farms, nor to human health. After the April death, Mexican authorities confirmed the presence of the virus and reported the case to the WHO, the agency said.

There had been 3 poultry outbreaks of H5N2 in nearby parts of Mexico in March, but authorities haven’t been able to find a connection. Scientists said the case in Mexico is unrelated to the outbreak of a different strain of bird flu – H5N1 – in the United States that has so far infected three dairy farm workers.