The cull of 6,000 birds at a farm in east Yorkshire began this afternoon following an outbreak of bird flu.
A team of experts put the animals into crates before locking them in an airtight container where they were gassed. Their bodies will later be incinerated.
The cull will continue in the morning but it will be a few days before experts will know if it has been successful in stopping the spread of the disease.
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Teenagers, aged 13 and 14, were taken to hospital on Friday morning but passed away, South Yorkshire Police said.
Eleven people remain in hospital.