PATHOGENICITY OF PASTEURELLA-MULTOCIDA - ITS VARIABLE NATURE DEMONSTRATED BY IN-VIVO PASSAGES

Pasteurella multocida serotype 3,4 was isolated from a dead turkey, an d the variable nature of its pathogenicity was demonstrated after in v ivo passages. The original isolate was encapsulated, and its mean infe ctious dose (ID50) was higher than 10(8.2) colony-forming units (CFU). To increase virulence, the organism was passaged intravenously in tur keys. After five passages, the encapsulated organism caused 67% mortal ity with a 10(2) CFU dose, and 50% of the contact control birds also d ied. A non-encapsulated variant that developed from the original isola te resulted in no mortality, even at a dose as high as 10(9) CFU. Afte r four intratracheal passages, however, the virulence of the non-encap sulated variant increased (ID50 approximately 10(6) CFU), despite no a pparent change in its morphological characteristics. These results sug gest that both encapsulated and non-encapsulated forms of P. multocida can increase their pathogenicity by bird-to-bird transmission in a sh ort period of time.