Next... Interstitial Pneumonia
Interstitial pneumonia is an uncommon but important cause of acute or chronic lower respiratory disease in horses and foals of unknown cause. Clinically it can present as respiratory difficulty, tachypnea, cyanotic mucous membranes, and pulmonary edema.
What causes interstitial pneumonia of horses?
| Cause is unknown, but thought to be from various agents, including infectious agents, ingested toxins, and immune-mediated factors. Some agents considered as possiblities include Rhodococcus equi, perilla ketone toxicosis, and inhalational toxins among others. |
Clinical signs: Adult horses may present with acute or chronic weight loss, progressive dyspnea, tachypnea, cough, nasal discharge, fever, tachycardia, cyanotic mucous membranes, crackles and wheezes on lung auscultation or, if there is severe pulmonary edema, absence of lung sounds.
Transmission: Unclear because agent is not identified in most cases.
Pathogenesis: Dr. Elizabeth Hall Bruce describes 4 phases of the pathogenesis of interstitial pneumonia: Phase I involves acute alveolitis with damage to capillary endothelial cells (edema!) and Type I pneumocytes in response to initial injury to lung parenchyma. In Phase II, inflammatory cells secrete mediators in response to acute damage, which leads to cellular and connective tissue changes (increase in connective tissue, fibroblasts, Type II pneumocytes). Phase III involves development of interstitial fibrosis which destroys the gas exchange mechanism of the lung. Finally, Phase IV is the end-stage, irreparably fibrotic lung.
Diagnosis: History, clinical signs, thoracic radiographs which show severe, diffuse, nodular interstitial disease. Serology and cytologic examination and culture of tracheal wash or bronchoalveolar lavage fluid may yield variable findings. Abdominocentesis and thoracocentesis may rule out neoplastic causes of pulmonary disease. A definitive diagnosis may be made by taking a transthoracic lung biopsy and looking for characteristic changes associated with different phases of the disease. For example, in Phase I, biopsy will show coagulation necrosis of alveolar walls, hyaline membrane formation, and focal hemorrhage and edema.
Treatment and Prevention: Usually unrewarding. Treat the underlying cause if it can be identified, supportive care, maintaining cool, dry, clean environment. Prevention is difficult as cause is not known; however, immunizations against viral respiratory agents, appropriate deworming program, and good nutrition can help maintain strong immunity to respiratory insults.