Congenital polyalveolar lobe in three foals.

Three cases of congenital polyalveolar lobe (pulmonary hamartoma) were diagnosed in female Thoroughbred foals. Foal 1 was born at full term but died shortly afterwards. Foal 2 was aborted at the seventh month of gestation. Parturition was induced at the tenth month of gestation in foal 3 because it developed hydrops of the amnion and ascites. In all three foals, the polyalveolar lobe occurred on the right side and affected the entire right lung. In each case, the right lung formed a tumour-like mass, and expanded into the left chest cavity. The lung masses were pink to dark red and spongy to rubbery, with marked lobular patterns on the pleural and cut surfaces. The left lung was compressed and small. The right and left lungs weighed 0.9-6.3 kg and 80 g-0.3 kg, respectively. Microscopically, the polyalveolar lobe consisted of normal alveoli, bronchioli and blood vessels, but the alveolus:artery ratio was greater than normal. In the polyalveolar lobe of two foals the numbers of alveoli per artery were 65.2 and 52.5; in contrast, the corresponding values for three control lungs were 26.9, 26.5 and 27.6. Chronic passive congestion with generalized oedema was observed in foals 2 and 3.