Pleuritis in a Thoroughbred Horse
History. At your practice in California, you are asked to examine a 3-year-old Thoroughbred stallion, which had arrived the previous day by truck from a racetrack in New York. On arrival the horse had appeared depressed.
This morning the stallion refused to eat and drink and is breathing rapidly. The owner reports that in New York the horse was at a racetrack where there was much through-traffic of young horses, many of which were coughing.Clinical Examination. On arrival at the farm, you meet the anxious owner, who leads you to a stall where the Thoroughbred is standing with its elbows slightly abducted, its head lowered, its nostrils flared, and an anxious look in its eye. The grain and hay from the mornings feed are untouched. The horse has a respiratory rate of 65 (normal, 12-20). On further questioning, the owner reports that the horse looked much as it does now when it arrived from New York, but he thought it was just tired from the truck ride. The trucker reported that the horse drank little when it was offered water on its 3-day trip across country and had only nibbled at its hay. It was in the truck with four other younger horses. The condition of these horses is unknown.
You examine the horse and find that it is febrile, and its pulse rate as well as its respiratory rate is greatly elevated. The horse becomes anxious when approached and particularly when hands are laid and pressed on the thoracic cage. The horse’s mucous membranes are a dull red. Auscultation of the abdomen reveals minimal gastrointestinal sounds, and there is no evidence of feces in the stall. You listen to the respiratory system and note louder, harsher sounds than normal in the trachea and in the dorsal part of the lung bilaterally. However, the ventral part of the lung is notably silent bilaterally.
You elect to take thoracic radiographs and notice that there is a fluid accumulation in the ventral half of the thorax, obscuring the cardiac shadow and much of the lung.
In the dorsal part of the thorax, the lung tissue has a number of radiographic densities that have a fluffy appearance, suggesting they are in the alveolar spaces. With the assistance of ultrasound, a teat cannula is placed in the right pleural cavity to drain the pleural fluid, which is foul smelling and purulent; 10 L are removed. This is repeated on the left side, with similar results. Repeat radiographs then reveal that the alveolar densities extend into the ventral part of the thorax.Bacterial cultures of the pleural fluid grow an anaerobic organism (Bacteroides fragilis), which is probably responsible for the foul smell of the pleural fluid. A complete blood cell count reveals a decreased number of circulating neutrophils and a large number of immature forms of neutrophils (bands). This is an indication that the neutrophil resources of the body are being depleted and the bone marrow is putting out immature forms. Presumably, the neutrophils are being sequestered within the lung and pleural cavity.
Comment. The history and clinical findings in this horse are fairly typical of a case of pleuropneumonia. In New York the horse was exposed to other animals that were coughing, probably as a result of a viral infection, such as equine influenza or equine rhinopneumonitis. Respiratory viruses generally impair the defense mechanisms of the lung in two ways. First, they denude the tracheobronchial epithelium of cilia and therefore reduce mucociliary clearance of the airways. Second, they impair macrophage function. This combination of events results in the deposition of bacteria in the lung and failure of the lung to remove them by either the ciliary system or the macrophages. As a result, the bacteria multiply. The stress of shipping makes the situation even worse. Stress probably resulted in the release of corticosteroids from the adrenal gland, and this further suppressed the defense mechanisms of the lung. Failure to drink leads to dehydration, which can make mucus more difficult to clear.
Keeping the horse’s head elevated during transport also impairs mucociliary function. As a result of these events, the horse acquired a bacterial infection of the lung, which resulted in the migration of large numbers of neutrophils into the alveoli. This resulted in the fluffy densities on the radiograph.When the infection spread to the pleural cavity, neutrophils migrated to this region as well. The release of neutrophil products designed to kill bacteria caused extensive damage to the membranes of the alveolar epithelium, pulmonary capillaries, and pleural capillaries. The protein that leaked into the alveolar spaces, interstitium of the lung, and pleural cavity raised the osmotic pressure within these regions. This resulted in the movement of fluid from the vascular space into the alveolar spaces, interstitium, and pleural cavity. Within the pleural cavity, the fluid accumulates ventrally because of gravity, and it is probably this accumulation of fluid that results in the absence of lung sounds in the ventral part of the thorax.
Treatment. Chest tubes are placed in the pleural cavity bilaterally so that the fluid can be drained repeatedly. The horse is given high levels of antibiotics and a prostaglandin synthetase inhibitor, flunixin meglumine, which should reduce the inflammation and make the horse more comfortable. However, in view of the degree of alveolar involvement, the presence of an anaerobic organism, and the large amount of fluid in the pleural cavity, the prognosis for this horse is not good.