Abstract
Neonatal interstitial lung disease occurs within the first 3 wk of life and includes any disease process that affects alveolar septa and, in some species, interlobular septa. Postmortem diagnosis of neonatal interstitial lung disease is challenging because of our incomplete understanding of normal postnatal lung development, especially in altricial species such as dogs and cats, which are born with morphologically and physiologically immature lungs. Most altricial species are born with lungs in the saccular stage of development and continue development to the alveolar stage in the postnatal period. I address normal postnatal lung development in dogs and cats, structural immaturity of the lung, neonatal respiratory distress syndrome, and sepsis. Histologically, neonatal lungs are easily mistaken for infectious pneumonia or structural immaturity based on their thick alveolar septa and hypercellularity. However, by determining the stage of lung development and considering factors such as gestational age, birth weight, and age at death, the degree of lung development may be entirely appropriate for that animal. Neonatal respiratory distress syndrome is a clinical term for inadequate gas exchange. The underlying cause is surfactant dysfunction, which can be primary or secondary. Mature surfactant is essential for neonatal survival but is extremely difficult to assess in a postmortem lung. Sepsis is the leading cause of death in canine and feline neonates; histologic lesions are often subtle given the acute nature of the disease.