I'm the CDC's top bird flu expert - this is why I'm losing sleep over the H5N1 outbreak tearing through US farms

America's top bird flu expert says the virus outbreak in cows suggests that the virus is one step closer to spreading to humans.

In an interview with STAT News, Dr Vivien Dugan — who also directs the CDC's flu division — insisted that the risk to the general public was still 'low'.

But she admitted that the repeated infections in cattle suggested the virus could become endemic in the species, raising the risk of an H5N1 spillover to humans — who have close contact with the animals.

She also warned officials were having trouble tracking infections because many farmers were not coming forward to get tested.

Dr Dugan warned: 'These viruses change. If this were to become seeded in cows and become a cow-endemic virus, it certainly would increase that risk to people.

'And certainly, our pandemic planning [operation] has been really thinking about this and trying to be as vigilant as we can...

'[The aim is] to try and understand not just current risk, but the future risk so that we're as prepared as we can be. But the risk could be quite big.' 

She added: 'It's probably why I'm not sleeping very much right now.'

Dr Dugan also warned that the CDC was struggling to track the virus and establish whether it had spread to people in other states.

After a person tested positive for the virus in Texas, it would be normal practice for the blood of their close contacts to be tested for antibodies against the virus — called serology testing — which would indicate whether the virus had spread.

But she said that this could not be carried out because the farmers in close contact with the case had refused to get tested.

It would also be common practice for the CDC to monitor those on farms with infected cattle who showed symptoms of illness.

But she said that, again, in many cases, health officials had 'not been as successful'  in getting them tested.

The low numbers of people checked for the disease — 25 tested and 100 monitored so far — appear to underscore these difficulties.

For comparison, among poultry workers who have been dealing with H5N1 for more than two years, the CDC has tracked more than 8,000 people for symptoms.

She also said that in some cases the agency was having trouble accessing states because they had to be invited in by local health departments, an invite that was not always forthcoming.

'We had a team pretty much ready to go six weeks ago when this really started to show up on our radar,' she added.  

'But this is not the average poultry-worker response situation. These are dairy farmers. 

'Some states have been able to contact workers directly and actively monitor. Others, of course, have not been as successful.'

Scientists are concerned about bird flu becoming established in a new species because this suggests it has acquired mutations enabling it to infect and spread between new animals - and people.

A total of 36 cattle herds across nine states have so far tested positive for bird flu, with fears raised that the virus may be spread through the air after testing detected the pathogen in the lungs of a cow.

The virus has also been found in their milk and in grocery store milks, cottage cheese and sour cream — in what could be possible transmission routes to humans.

But officials say that these pose no risk because the virus in these is 'not alive'. The products are treated with pasteurization, where they are rapidly heated and cooled, in order to kill any microbes within them.

Tests on ground beef, infant formula and powdered milk have, however, found no trace of the virus.

US officials have been repeatedly criticized for failing to share data publicly, including on whether pasteurized milk is still safe to drink.

Some scientists have said they will no longer consume milk until the FDA shares more data, although they marked the recent tests as 'reassuring'.