P. multocida is one of 11 Pasteurella species, most of which are associated more commonly with animal disease than with human disease. P. multocida includes three subspecies: multocida, septica, and gallicida. P. multocida is the most virulent species in animals. Most human Pasteurella infections are caused by organisms identified as P. multocida subspecies multocida and subspecies septica, followed by P. canis, P. stomatis, and P. dagmatis. P. multocida is indole and urease positive and is characterized by absence of hemolysis when grown on blood agar. It grows well on a variety of routine media that do not contain bile, including blood, chocolate, and Mueller-Hinton agars; it does not grow on the bile-containing MacConkey agar. On blood agar, its colonies resemble enterococci. Optimal growth occurs at 37°C under aerobic or facultatively anaerobic conditions.

P. multocida is a small, nonmotile, gram-negative rod that occurs singly, in pairs, or in short chains and may exhibit bipolar staining. These organisms may be confused microscopically with Haemophilus influenzae, Neisseria species, and Acinetobacter species.

The incidence of asymptomatic respiratory tract or oral cavity colonization with P. multocida approximates 70% to 90% in cats, 50% to 70% in dogs, 50% in pigs, and 15% in wild rats. Carriage has been documented in larger felines (e.g., lions, tigers, leopards, panthers, lynx) and in a variety of other animals, including horses, cattle, sheep, rabbits, and water buffalo.

Most P. multocida infections in humans are the result of direct inoculation by animal bites or scratches, and this pathogen has been implicated in 80% of infected cat bites and in 50% of infected dog bites. No seasonal or gender predilection for infection exists. P. multocida infections for which specific exposure to a bite cannot be elicited usually are the result of contact
with animal secretions. Occasionally, asymptomatic upper respiratory tract colonization may precede dissemination of infection in humans, and P. multocida has been isolated from asymptomatic persons who have frequent contact with animals. Close contact with the family cat during breast-feeding was the source for transmission through oropharyngeal colonization to an infant who developed meningitis and bacteremia caused by P. multocida. In some cases, no animal contact can be established. Although exquisitely susceptible to direct sunlight, the organism can survive in water or soil for approximately 3 weeks. Human-to-human transmission has not been documented, but an animal-soil-human route could account for some cases that cannot be linked directly to animals.

The pathogenesis of P. multocida infections may be understood by examining the several potential routes of inoculation. The most common way in which infection is established is direct implantation of organisms beneath the skin from an animal bite or scratch. Inoculation is likely to be deeper and to penetrate the periosteum or a joint space if the source is a cat bite (i.e., puncture wound injury) rather than a dog bite (i.e., laceration wound). The rapidly established and intensely painful local cellulitis that results may be attributed in part to the production by P. multocida of neuraminidase and endotoxin. The discharge from wounds is gray and serosanguineous. Localized infections are characterized by an infiltration of neutrophils that may manifest as abscess formation, osteomyelitis, septic arthritis, or tenosynovitis.

Nasopharyngeal colonization with P. multocida may occur before respiratory tract infection develops. Invasive infection occurs almost exclusively in the setting of underlying respiratory tract disease, such as chronic bronchitis or bronchiectasis.