We are seeing a lot of this lately and it may be related to how the birds are being raised at the new locations. Chilling, lack of feed and water, over handling etc.
I would suggest a combination of Gallimycin and Oxytet 343 mixed together in the drinking water for 10 days. This combination has been working well for similar cases.
Peter Brown Aka The Chicken Doctor
Link to buy http://www.firststatevetsupply.com/