The impact of bird flu on dairy cows is significant, with a 25% reduction in milk yield in herds affected by the disease.

The Irish Farmers Journal spoke to a dairy farmer from Michigan whose herd had bird flu, who said that the disease is much more prevalent than official figures suggest.

He said he knew of 15 farms in the area that are affected, even though the official tally was far lower than that.

Ill health

During the peak of the outbreak, milk production on the farm decreased by 25% and about 25% of the cows were presenting signs of ill health.

“These were mostly the older and higher producing cows. They were off feed and milk production crashed. Some of these cows were milking 80lbs of milk per day (35l/cow/day) but dropped to just 20lbs per day (9l/cow/day).

“We pumped them with fluids and gave them aspirin [anti-inflammatory drugs] and they eventually came around but milk yield on the farm is still 10% behind where it should be,” he said.

It is thought that the main source of infection in dairy cows is in the udder and that the disease spreads from cow to cow during the milking process.

The disease was first observed in the wild bird population and then spread to poultry farms.

Wild birds are suspected of spreading the disease on to dairy farms and the rate of infection is increasing.

According to the Centre for Disease Control (CDC) the first confirmed case was in a dairy herd in Texas on 25 March.

By the end of April the disease had been confirmed in dairy cows in six states, including Michigan, Idaho and New Mexico.

As of this week, there are a total of 67 confirmed outbreaks in dairy herds across nine states.

The CDC monitors people exposed to infected cattle for 10 days after exposure and of 350 people monitored so far, two have tested positive for the highly contagious avian influenza a (H5N1) strain.

One of these cases was in Texas and the other in Michigan. Symptoms in humans are said to be mild, with conjunctivitis the only symptom in one case.

Beef cattle have so far been unaffected but the USDA has confirmed this week that viral particles of the disease were found in muscle tissue samples of a condemned dairy cow.