A 24-hour-old Hackney pony filly developed signs of weakness, depressi
on and a poor suck reflex, with harsh lung sounds over both fields, an
d a 48-hour-old Arabian colt from a normal birth which had sucked vigo
rously developed loose stools and became depressed, weak and anorectic
. Both foals had serum IgG concentrations greater than 800 mg/dl, but
each had a severe neutropenia with a left shift, and blood cultures fr
om both of them yielded Actintobacillus suis. The A suis isolates had
different antimicrobial susceptibility patterns and, in the case of th
e Arabian, the isolate was resistant to commonly used broad spectrum a
ntimicrobial agents.