Streptococcus, mycoplasma, pasteurella, chlamydia and other bacteria can be opportunistic secondary invaders. However, she adds, stress can hinder immunity, leaving calves vulnerable to pneumonia, at any age. Some viruses can make calves pretty sick by themselves, even without bacterial complications, says Cope. But often, after a viral pneumonia gets started, bacterial invasion occurs, creating more severe infection. Secondary bacterial pneumonia will usually respond to antibiotic treatment, but relapses are common if viral infection is extensive.
Viruses tend to reduce resistance of the mucous membranes, allowing bacteria to invade the tissues.
They constantly move dirt and bacteria out of the windpipe into the back of the throat where they can be safely and easily removed from the body. When viruses damage these cilia, they can no longer defend the lungs. The main bacterium to take advantage of the situation is Pasteurella haemolytica, says Cope. Since these bacteria can always be found in the nose and sinuses of normal, healthy cattle, they are always available to move down the windpipe after damage to the cilia has occurred. Once lung damage exceeds 15 percent of lung capacity, the calf will not grow properly even if it does survive.