The symptoms of uncomplicated rhinotracheitis usually peak by days 3 to 5, and patients are asymptomatic by days 7 to 10. Transmission electron microscopy shows that serous cells have short microvilli and mucous granules.
Ciliated cells have longer microvilli interspersed between cilia.
A third cell type, the basal cell, is also present. Basal cells normally cover about one third of the basement membrane. Normally, the mucous film produced by serous cells lining the airways traps bacteria, soot, and other particles, including virus, and cilia push the film and its entrapped debris into the posterior pharynx, where it is swallowed. This is called the tracheal toilet. Three days after influenza infection, serous and ciliated cells are desquamated, and the basal cells have now spread out to cover the entire basement membrane.