Strains of this organism have been generally named in the past according to the animal from which they were isolated (pasteurella boviseptica, pasteurella oviseptica, pasteurella aviseptica, etc.) but they are now regarded as members of the same species, pasteurella multocida, that differ in their parasitic adaptations to particular hosts.
Like the other GRAM NEGATIVE BACTERIA members of the pasteurella they are small, oval, Gram-negative that exhibit bipolar staining when treated with methylene blue. They are capsulated and non- motile; they grow under aerobic and anaerobic conditions on blood agar but do not grow on MacConkey medium.
Pasteurella multocida is extremely virulent to many species of birds and animals causing haemorrhagic septicaemia which is usually fatal. However, the organisms may be carried by apparently normal cattle, sheep, swine, dogs, cats and rats. Apart from cases resulting from animal bites reports of human pasteurella infections are comparatively rare. The organisms may sometimes be present as commensals in the respiratory tract and nasal sinuses of persons associated with animals. Clinical manifestations in man usually take one of three forms:-
(1)- A local abscess following an animal bite with cellulitis, adenitis, abscess formation and sometimes osteomyelitis.
(2) -Meningitis following head injury
(3)- Infections of the respiratory system such as pleurisy, pneumonia, empyema, bronchitis, bronchiectasis and nasal sinusitis in which, although not always the direct cause,pasteurella multocida probably contributes to the course, severity and duration of the illness. Cases of appendicitis from which pasteurella multocida was isolated in pure culture from the pus have also been reported. It appears that the organism becomes pathogenic to man only after trauma resulting from animal bites, operation, cranial fracture, and the like.
Identification is confirmed by isolating the organism on blood agar from swabs of dog or cat bite wounds, from cerebrospinal fluid in cases of meningitis and from the infective secretions in suppurative conditions of the respiratory system, followed by certain cultural and biochemical tests. Agglutinating antibodies against the infecting organism may be detected in the patient's serum.
Pasteurella multocida is moderately sensitive in vitro to penicillin, sensitive to sulphonamides, tetracycline and to streptomycin. Therapy may have to be continued for up to 8 weeks in cases of osteomyelitis following animal bites.
Epidemiology Infection following animal bites is a direct one, the organism passing directly to man in the animals' saliva. In those cases described under (2) and (3) above, the organisms, although no doubt derived from animals in the first place, may have existed as commensal in the upper respiratory passages until conditions were favorable to their spread and establishment at the site of infection.