Foals, like other young animals, are especially susceptible to diseases. A review of necropsy cases over a one-year period at the University of Kentucky Livestock Disease Diagnostic Center was conducted to determine common pathologic diagnoses in the foals submitted for necropsy. For this study, all foals 1 day to 6 months of age were included. A total of 272 foals within this age range were submitted and examined in the one-year period. A variety of different pathologic diagnoses were made; however, by far the most common diagnosis was pneumonia.

There were 84 foals diagnosed with pneumonia, which represented 31% of all submitted foals. The cases were arbitrarily separated into age groups of 1 to 6 days, 1 to 4 weeks, and more than 1 to 6 months of age. Forty-two cases (51%) were less than 6 days of age, seven cases (9%) were 1 week to 4 weeks of age, and 33 cases (40%) were 1 to 6 months of age. In two cases the age was not indicated. These data show the first week of life to be a critical time for the development of pneumonia; however, pneumonia is also problematic in older foals. Fillies comprised 58% of the cases; colts, 42%. There were 71 Thoroughbreds (87%), four Standardbreds, two Quarter Horses, two Miniature Horses, one American Saddlebred, one Hanoverian, and one mixed-breed foal. The high percentage of Thoroughbreds is consistent with the horse population of the area. Thirty-eight of the 84 cases (45%) had other pathologic diagnoses in addition to pneumonia. Common additional conditions included enteritis, septicemia, and fractured ribs.

Pneumonia in this group of foals was commonly associated with bacterial infection. Other causes of foal pneumonia, such as viruses and parasites, were not diagnosed. Of the 84 pneumonia cases, bacteria were isolated from 40 cases, and 44 cases had either no growth (38 cases) or non-pathogenic bacteria (six cases). The foals often had been treated, and prior antibiotic therapy likely contributed to the inability to isolate bacteria even though there likely was an underlying bacterial etiology in many of these cases. The most commonly isolated bacterium from the cases of pneumonia was E. coli, which was cultured from 14 cases. E. coli was followed by Rhodococcus equi (13 cases), Klebsiella pneumoniae (8), Streptococcus zooepidemicus (7), Actinobacillus equuli (5), and Enterococcus spp. (5). A variety of other bacteria were isolated on rare occasion. Thirteen cases had multiple bacteria isolated from the lung. E. coli was the most common bacterium isolated when a mixed culture was obtained.

E. coli was isolated primarily from younger foals, with 71% of the isolates from foals less than 1 week old. Rhodococcus equi is typically associated with pneumonia in older foals, and in this group of cases all 13 were in foals over 1 month of age. Klebsiella pneumoniae was also recovered primarily from young foals, with seven out of eight cases in foals less than 1 week old. The Streptococcus zooepidemicus, Actinobacillus equuli, and Enterococcus spp. cases were more equally divided between both young and older foals