Ovine pulmonary adenocarcinoma is a contagious lung cancer of sheep caused by a retrovirus that causes progressive respiratory failure and significant welfare harm.
OPA causes a prolonged, progressive welfare deterioration. Affected sheep slowly lose lung capacity over weeks to months, becoming increasingly breathless and debilitated. Weight loss, weakness, and eventual drowning in their own lung secretions characterize the terminal phase. There is no therapeutic intervention — only early culling prevents prolonged suffering. The challenge for farmers is that early OPA is difficult to distinguish from other respiratory conditions. The "wheelbarrow test" — raising the hindquarters to drain nasal fluid — identifies cases that have reached the fluid-producing stage. Biosecurity to prevent introduction of infected sheep is the primary preventive welfare measure.