Osmolarity and volatile fatty acid content of feces from horses with chronic diarrhea.

The concentrations of acetate (C2), propionate (C3), isobutyrate (iC4), butyrate (nC4), isovalerate (iC5), and valerate (nC5) were measured in the feces of 16 clinically normal horses and 44 horses with chronic diarrhea of at least 1-month's duration. The diarrheal horses were categorized diagnostically as: (1) no discernible clinical abnormalities other than diarrhea and in some cases, weight loss (open); (2) clinical evidence of strongyle larval migrans; and (3) Salmonella organisms recovered from the feces. Osmolarity of the feces of 14 of the normal and 15 of the sick horses also was measured. The mean concentration of C2 was significantly less (P < 0.05) in feces from horses of all diagnostic categories, whereas there was a significantly increased isobutyrate concentration (P < 0.01) in feces from horses in the open category. When individual volatile fatty acids (VFA) were expressed as percentage of total, only isobutyrate in feces from horses in the open category was significantly different (P < 0.01) from normal and was increased. Some horses were monitored sequentially for a number of weeks. Fecal VFA concentrations showed considerable variability, and a transient decrease of isobutyrate concentration was not necessarily coincidental with cessation of diarrhea. Horses responding to empirical therapy had a more stable reestablishment of a normal fecal VFA distribution. Correlation between total fecal VFA concentration and osmolarity was significant (P < 0.05) for diarrheal horses, but was not significant for normal horses.