Ovine Progressive Pneumonia – NAVLE Study Guide

Overview and Clinical Importance

Ovine Progressive Pneumonia (OPP), also known as Maedi-Visna (MV) outside North America, is one of the most economically important chronic diseases in sheep production worldwide. OPP is caused by a small ruminant lentivirus (SRLV) belonging to the family Retroviridae. The term "Maedi" is Icelandic for "dyspnea" or "labored breathing," while "Visna" means "wasting," referring to the neurologic form of the disease. The related disease in goats is called Caprine Arthritis Encephalitis (CAE). Cross-species transmission between sheep and goats is possible, making this virus relevant for mixed small ruminant operations.

The disease represents a significant economic burden due to decreased production, increased culling rates, poor lamb survival, and restricted international trade. In the United States, prevalence in some states reaches 60-80% of sheep flocks, with individual animal prevalence varying from 4-96% within infected flocks. OPP is found worldwide except in Australia, New Zealand, and Iceland (where it was eradicated in the 1940s-1950s).

Etiology

Viral Classification

The virus integrates its DNA into the host genome using reverse transcriptase, resulting in lifelong, persistent infection. Viral replication is restricted until monocytes mature into macrophages, which explains the long incubation period. The virus exhibits high genetic variability with multiple genotypes (A through E), which contributes to diagnostic challenges and immune evasion.