I agree with Wyorp Rock. Penicillin does not treat most respiratory diseases. Tylan 50 can be given orally in chickens instead of injecting it. Dosage is 0.25 ml per pound given 3 times a day for 5 days. Your state vet may be helpful with testing, and Zoologix can send you materials to test yourself, but may be more expensive. Here is a link to contacting the state vet:
https://www.metzerfarms.com/poultry-labs.html
Here is the link for the respiratory panel testing:
https://www.zoologix.com/avian/Datasheets/PoultryRespiratoryPanel.htm
https://www.metzerfarms.com/poultry-labs.html
Here is the link for the respiratory panel testing:
https://www.zoologix.com/avian/Datasheets/PoultryRespiratoryPanel.htm
then added three to replace some along the way, not realizing that what the others had was so contagious and lost them all too. So discouraging. Next spring, chicks. I’m sorry! Money, time, energy all down the drain