Type C Botulism in Turkeys: Determination of the Median Toxic Dose

The median toxic dose (TD50) of type C botulinurn in turkeys was determined using an up-and- down method for toxicologic testing. Birds were dosed intravenously with 10, 20, or 40 mouse lethal dose per kilogram body weight (MLD/kg) of type C botulism toxin and observed for 5 days. Administration of toxin resulted in acute death, posterior paresis, or no effect, depending upon the dose. The TD 50 causing posterior paresis was calculated as 25.75 MLD/kg (confidence interval = 17.08-38.82 MLD/kg). Posterior paresis was identified as the principal sign associated with sublethal botulism toxicosis in turkeys. The resultant posterior paresis was similar to the clinical syndrome observed in an unsolved field investigation. Extreme rear limb weakness in a commercial turkey flock associated with high mortality was investigated. 14 Botulism toxicosis was highly suspected as the cause of paresis in this flock; however, numerous attempts to identify botulism toxin in serum, liver, and cecal contents using the mouse bioassay procedure 9 were unsuccessful. An up-and-down method for toxicologic testing 3,4 was utilized to determine if a posterior paresis like that observed in the field investigation could be reproduced in turkeys using type C botulinurn and, if so, to determine the median toxic dose (TD 50 ).