Calves exposed to overwhelming challenge with infectious agents, or calves with inadequate nutrition, may not be able to resist respiratory infection even if they have been vaccinated and receive adequate colostrum.

Additionally, poor air quality can overwhelm respiratory defenses. High counts of bacteria in the air in calf barns, which are an indicator of poor ventilation, are related to increased risk of calf respiratory disease, says Amelia Woolums, DVM, MVSc, Dipl. ACVIM, Dipl. ACVM, University of Georgia.

Dust and other particulates and irritating gases or fumes such as ammonia from urine, chlorine from bleach treatment of the environment, etc., can cause dysfunction of the mucociliary apparatus that keeps viruses and bacteria out of the lower respiratory tract.

“These insults can also cause dysfunction of alveolar macrophages, which are the most important component of defense in the lowest part of the lung,” Woolums explains.

“If the mucociliary apparatus and alveolar macrophages are not functioning normally, then viruses and bacteria that would normally be restricted to the upper respiratory tract can advance into the lower respiratory tract and cause pneumonia.”

Bad weather is often anecdotally related to respiratory disease, says Woolums, but the link has not consistently been found. However, specific instances of extreme weather could serve as a stressor which, when compounded with other stressors mentioned above, could contribute to occurrence of respiratory disease in certain cases.

Read more from Woolums about neonatal calves and BRD in Bovine Veterinarian.