There has been slight increases in milk production in OPP infected ewes a few days after lambing, but this transient slight increase is usually attributed to a slight decrease in udder swelling. The arthritis is caused by a thickening and mineralization of the joint capsule and the subsequent bone and cartilage deterioration. The longer this inflammation persists the greater the damage to the joint and the greater the deterioration of the joint. Again, this arthritic change is preempted by in filtration of lymphocytes and macrophages into the joint. Central nervous system involvement is the least frequent form of OPP seen in the United States.
Early signs are muscle quivering and imbalance seen in the rear legs.
This slowly progresses to total paralysis of the hind legs, which makes the sheep unable to stand. 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.