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Workers set up spray stations April 18, 2022, outside of Kreider Farms in Mount Joy, Pa., where avian influenza was detected.

Two people have tested positive for avian influenza after working on an infected farm in England.

Neither worker had any symptoms and later tested negative, the United Kingdom government said May 16.

When poultry workers show no signs of illness, it’s difficult to tell if they are truly infected or if virus particles simply contaminated their nose and throat while at work.

One of the workers is likely to have only contamination, and investigators aren’t sure about the other one.

Only one American has tested positive for avian influenza since the outbreak hit the U.S. in February 2022. Researchers couldn’t tell if the Colorado farm depopulation worker had actually been infected.

Both the U.S. and U.K. say the disease remains a low risk to the general public. The disease has spread to a number of wild animal species.

USDA began poultry vaccination trials in April and, in the best case, could have a vaccine available in 18 to 24 months. Vaccinated poultry faces a tough road to acceptance from trading partners.

USDA says plans to vaccinate endangered California condors should not affect exports.

From May 1 to 22, no commercial flocks and only four backyard flocks had tested positive for avian influenza in the United States, a period of relative quiet in an outbreak that has killed almost 59 million birds.

Avian influenza continues to threaten poultry of all kinds. For background on the disease, and numbers to call to report an outbreak, go here.