Chapter 277

Pasteurella multocida, formerly known as Pasteurella septica, was renamed P multocida or “killer of many” because it affects many different animal species.1 The first report of human infection caused by P multocida was published in 1919.2 The primary importance of the organism in pediatrics is in animal bites in which P multocida, alone or in concert with other organisms, is the most common infecting organism.

### Epidemiology

There are several species in the genus Pasteurella. The most common human pathogen is P multocida, but infection also can be caused by one of the related species such as P canis or P dagmatis. There are 3 subspecies of P multocida: multocida, septica, and gallicida. More than one half of human infections are caused by P multocida subspecies multocida. Defining the subspecies aids in epidemiologic investigation, but is not necessary in the usual clinical setting.

The organism is found as a component of the oral flora of 70% to 90% of cats and at least 25% to 50% of dogs. Other animals, including rabbits, rats, or pigs, also can harbor the organism in respiratory tract or oral secretions. Carriage of P multocida by humans is uncommon, but commensal carriage can occur as a consequence of frequent animal contact.3 The usual mode of transmission is direct inoculation from the bite or scratch of a colonized animal. P multocida has been implicated as causal in 50% of dog and 80% of cat bite wound infections.

### Pathophysiology

The pathogenesis of infection caused by P multocida is dependent upon the portal of entry of the organism. The three major clinical expressions of disease are focal infection, respiratory tract infection, and invasive infection.4 Focal infection is initiated by direct inoculation of the organism into the subcutaneous tissue, bone, or joint space after a cat scratch or bite or a dog bite. Inoculation is likely to be deeper after a cat bite than a dog bite. The organism produces endotoxin, which may promote the inflammatory reaction that is observed, often within hours, after inoculation.

Respiratory tract infection occurs as the result of inhalation of P multocida. Animal-to-human but not human-to-human spread has been documented. The organism has low pathogenicity in the respiratory tract, and infection has been documented almost exclusively in the setting of altered host resistance from disease processes such as bronchiectasis or chronic bronchitis.

Invasive infection occurs when hematogenous dissemination complicates primary soft tissue or pulmonary infection. Bacteremic infection is a particular risk for children with hepatic dysfunction and reduced efficiency of reticuloendothelial clearance mechanisms.

### Clinical Manifestations

Focal infection, usually manifested as cellulitis, develops rapidly after inoculation of P multocida. The average time of onset of erythema, swelling, and pain is within 24 hours after an animal bite or scratch. Infections due to P multocida characteristically develop watery ...

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