Pathogenicity of Pasteurella multocida: its variable nature demonstrated by in vivo passages

Pasteurella multocida serotype 3,4 was isolated from a dead turkey, and the variable nature of its pathogenicity was demonstrated after in vivo passages. The original isolate was encapsulated, and its mean infectious dose (ID50) was higher than 10(8.2) colony-forming units (CFU). To increase virulence, the organism was passaged intravenously in turkeys. After five passages, the encapsulated organism caused 67% mortality with a 10(2) CFU dose, and 50% of the contact control birds also died. A non-encapsulated variant that developed from the original isolate resulted in no mortality, even at a dose as high as 10(9) CFU. After four intratracheal passages, however, the virulence of the non-encapsulated variant increased (ID50 approximately 10(6) CFU), despite no apparent change in its morphological characteristics. These results suggest that both encapsulated and non-encapsulated forms of P. multocida can increase their pathogenicity by bird-to-bird transmission in a short period of time.