Abstract:
Growth in serum of Pasteurella multocida and related species in chicken, turkey, duck and pig sera
were compared, and selected serum-resistant and serum-sensitive strains were inoculated into
18-week-old layers. Eighty-seven field strains of Pasteurella spp. and nine reference strains
representing different clones defined by restriction endonuclease analysis ( REA) profiles were used in
the study. Serum activity was measured by changes in the optical density ( OD) of the serum after
inoculation and incubation at 41°C for chicken, turkey and duck serum and 39°C for pig serum. Serum
activity was measured by comparison with previously determined serum-resistant ( P-1059) and serum-
sensitive ( CU vaccine ) strains, and classified into highly serum-resistant, moderately serum-resistant
and serum-sensitive. Strains of the same REA type were found to have identical growth curves and the
same maximum OD values when tested in serum from the same host species. Turkey serum was shown
to be less inhibitory to a wide range of P. multocida strains than chicken, duck and pig sera. Serum-
resistant strains were demonstrated among avian as well as mammalian strains. Among the avian
strains, the proportion of serum-resistant strains was higher in outbreak strains than in strains from
apparently healthy carriers. Removal of the capsule from selected strains by hyaluronidase treatment
failed to change the serum activity. The most severe lesions in experimentally infected chickens were
produced by a serum-resistant strain; however, lesions were also found in chickens infected by serum-
sensitive strains, indicating the involvement of multiple factors in the virulence of P. multocida. Further
investigations on serum resistance are indicated in order to relate other host and bacterial factors
responsible for the development of fowl cholera.
