Histologically lymphocytes and macrophages are accumulated around blood vessels, and degeneration of nervous tissue is seen too. Since the virus is carried in macrophages, which are found in the tissue and fluids of the sheep, bodily secretions are sources of virus transmission. Secretions of the udder and lungs are thought to be the main source of virus transmission. Transmission from ewe to lamb via colostrum and milk has been documented. In addition, the longer the lamb stays with an infected ewe the greater the risk that the lamb will become infected.
Infected ewes can also be the source of infection for lambs produced by uninfected ewes.
In adults, the transmission of disease is thought to occur mainly through respiratory secretions, especially in confinement situations. Other bodily secretions such as blood, semen, and urine are not yet proven to transmit the disease, but should still be considered possibilities. Diagnosis Diagnosis of OPP can be difficult. Most sheep do not show any clinical signs, and those that do show clinical signs that are similar to other diseases. Therefore clinical signs alone are not often used alone to diagnose disease.