-Susceptible animal exposed.
-Incubation is the period (time) from the first replication of the disease causing biological agent until sufficient compromise of the target organ(s) occurs causing loss of function of the target organ(s). For most viral BRD this averages three days. The secondary bacterial BRD this may be three to five days behind the initial viral infection.
-Inflammation occurs in stages. Early, the body diverts white blood cells and blood in to the affected area typically causing swelling of tissue, both cells and spaces between cells. As the inflammation continues, loss of function of the affected tissue occurs. The late stage of inflammation is involved in the body trying to clean up, remove, or repair/reconstruct the damaged tissue. The late stage of inflammation is the first stage of recovery.
-Recovery begins in the last stage of inflammation. In the first recovery stage, the body begins building defense against future infections and cleaning up the damage tissue. This stage requires seven to ten days. While improving future defense and tissue continues in the next stages of recovery, the focus becomes tissue repair and beginnings of the return of function. These stages may require 14 to 21 days. In the last stage of repair, the focus is on remodeling of damaged tissues and compensation for any function loss. The final stage may not begin until the third or forth week of disease recovery and will typically last of about four weeks, but if the damage was severe it may continue for the life of the animal.
-Expect a complete recovery to apparent normal lung function and growth from BRD events from which the animal recovers without treatment or responds well to appropriate treatment. Early intervention (by both the animal and the caretaker) is critical for this outcome.
-Expect decrease response to therapy, subsequent growth (gain and efficiency) in BRD events that either get a head start because of inadequate animal response or delayed therapy. This is the common outcome in put-together cattle (commingling and time), especially in high-stressed younger stock.
Source: Dr. David Smith, University of Nebraska - Lincoln