There have been no reports in the European Union of bird flu infecting a human or cow but research is ongoing on the virus's transmission, Reuters reported, citing an EU official on Thursday, one day after US officials confirmed that country's second human case.

Highly pathogenic avian influenza (HPAI), commonly called bird flu, has killed millions of birds around the globe in recent years. Its spread to humans and other mammal species, including US dairy cattle in March, is raising concerns that the virus could mutate into one easily transmissible between humans and spark a pandemic.

EU member states are required to inform the European Commission and other members if they detect bird flu or any other disease likely to create serious risk to animal or public health.

No member state has informed the commission about HPAI findings in humans or cattle on their territories, a European Commission spokesperson told Reuters.

An EU network of laboratories called DURABLE is studying how existing immunity might protect against an infection with bird flu viruses and how the viruses are transmitted between cattle, the spokesperson added.

The US government last month said cow-to-cow transmission was a factor in the spread of bird flu in dairy herds, but it still did not know how the virus was moving around.

France has been testing both humans and ruminant animals in recent months and all tests came back negative, a spokesperson for the French farm ministry said.

The infections of dairy cattle in the United States are with a type of H5N1 bird flu that has not been detected in Europe, the European Food Safety Agency told Reuters.

Even though expansion of the bird flu outbreak to mammals raises concern of a potential pandemic, both the US Centers for Disease Control and Prevention and France said the risk to the general public remains low.