A five year-old Quarter Horse mare presented to Cornell University for a recurrent fever of five months. She had been treated with multiple antibiotics and anti-inflammatories without significant improvement. On presentation, she was in respiratory distress with decreased lung sounds ventrally. Blood work revealed hyperfibrinogenemia, hyperproteinemia, anemia, and leukocytosis. Ultrasound imaging revealed severe fibrinous pleural effusion. The main differential diagnoses were a thoracic neoplasia or severe pleuropneumonia. Intranasal oxygen was administered and a chest tube was placed. Due to poor prognosis for recovery the mare was euthanized. Necropsy revealed hemorrhagic pleuritis with numerous adhesions and minimal pulmonary disease. Body wall abscesses positive for Streptococcus zooepidemicus also present. The most common cause of equine pleural effusion is pleuropneumonia secondary to shipping or choke and aspiration. Postmortem evaluation indicated that the mare instead suffered from primary pleuritis likely secondary to a penetrating injury that was not appreciated at initial insult, which is unusual.