Sneezing can be from irritation of the nostrils by inhaling a piece of feed, or poor ventilation, viruses, such as infectious bronchitis which lasts around a month, and in more serious respiratory diseases. Since you are not seeing the bubbles/ foam in eyes or nasal drainage that may be mycoplasma gallisepticum (MG,) I would look at her nostrils for stuck feed, mucus, and something that might be irritating her. Many chickens sneeze when they are eating. A sneeze every few minutes could be IB virus, mentioned above. Antibiotics would not treat that. Since you have used Denagard before, have you treated MG in a flock member before? If it resolved to Denagard before, that would lean toward MG. Have you added any new members to the flock recently? As long as there is just sneezing alone, I would probably just watch her. If she gets bubbles in her eyes, then use the Denagard to treat for MG. IB virus normally spreads rapidly through the flock over several weeks and lasts about a month.
