Chapter 277. Pasteurella Multocida

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 gray or serosanguineous drainage. Infection in the subcutaneous space can result in abscess formation and regional lymphadenopathy. Infection also can present as tenosynovitis, septic arthritis, or osteomyelitis. These expressions of infection usually occur after cat bites, which tend to penetrate tissue spaces more deeply than do dog bites. Joint stiffness with cellulitis of the hand is a finding suggestive of tendon sheath involvement.

When P multocida infection is not related to an animal bite, the most common focus of infection is the respiratory tract. The clinical manifestations are those expected for children with exacerbations of chronic underlying conditions, such as bronchiectasis or chronic bronchitis. Several cases of pleural empyema and of lung abscess have occurred may occur.

Meningitis, with or without bacteremia, is the most common manifestation of invasive infection caused by P multocida. Most children diagnosed as having Pasteurella meningitis are younger than 1 year of age, and most have had contact with a pet within the household.1 Usually, these infections develop after animal contact, such as licking, that does not violate cutaneous barriers. The presenting features include lethargy, irritability, and fever, similar to bacterial meningitides.

P multocida urinary tract infection has occurred in children with obstructive uropathy, and peritonitis can occur in patients with chronic renal disease who are receiving peritoneal dialysis.5 The organism has been isolated from children with periappendiceal abscess or with peritonitis in association with appendicitis. It is not known whether these infections result from hematogenous spread or from ingestion of the organism. There are also case reports of other unusual manifestations of infection by due to P multocida such as include tonsillitis, endocarditis, brain abscess, or infection of a ventriculoperitoneal shunt.6-8 Brain abscess has resulted from a dog bite that penetrated a small child’s skull.

Diagnosis

Isolation of the organism from the drainage of skin lesions caused by bite wounds or from other sites of focal infection, such as joint fluid or aspirate of the subperiosteum, is diagnostic.9 Pleural fluid or sputum can yield the organism in patients with pulmonary infection. In disseminated infection, P multocida can be isolated from cultures of blood or, with meningeal involvement, from the cerebrospinal fluid.

P multocida is a small, nonmotile gram-negative rod that grows well on standard media, including blood, chocolate, and Mueller-Hinton agars. Colonies resemble those of enterococci on blood agar plates. If P multocida is suspected, laboratory personnel should be alerted so that appropriate confirmatory biochemical testing can be performed.

Laboratory differentiation from morphologically similar organisms, such as Haemophilus influenzae, is not difficult.

Treatment

Penicillin is the drug of choice for P multocida infection. Other effective oral agents include ampicillin, ampicillin-clavulanate, cefuroxime, cefpodoxime, doxycycline, and fluoroquinolones.9 Susceptibility testing should be performed, but β-lactamase–producing strains have been recovered only rarely.10 Because polymicrobial infection should be assumed after an animal bite, empirical therapy consisting of oral amoxicillin-clavulanate or intravenous ampicillin-sulbactam should be initiated in this setting while culture results are pending. Parenterally administered broad-spectrum cephalosporins such as cefotaxime and ceftriaxone have good activity and have been used successfully to treat invasive P multocida infection.

In children who are allergic to β-lactam agents, the optimal treatment for P multocida infection is problematic. Doxycycline is effective, but tetracyclines should be administered to children younger than 8 years of age only after assessment of the risk-to-benefit ratio. Azithromycin exhibits good activity against P multocida.11 A recommended oral alternative for empiric treatment of dog or cat bite wounds in penicillin-allergic children is an extended-spectrum cephalosporin or trimethoprim-sulfamethoxazole plus clindamycin.12 Trimethoprim-sulfamethoxazole is effective against P multocida as well as Staphylococcus aureus. Clindamycin is active against anaerobes, as well as streptococci and most S aureus. This combination can be employed parenterally for wounds of severity warranting hospitalization.

The usual duration of therapy is 7 to 10 days for focal soft-tissue infections. A longer treatment course can be required for septic arthritis and osteomyelitis. For pulmonary or disseminated infection, including meningitis, 10 to 14 days of therapy is usually required. Infected animal bite wounds should be adequately debrided. Infected collections of fluid should be drained, and devitalized tissue should be removed. Consultation with a specialist in hand surgery is appropriate for tendon sheath involvement. Preventive treatment for rabies (see Chapter 321) should be instituted only if clinical circumstances warrant.

Control

Educational interventions regarding appropriate interactions with domestic animals and limiting interactions with wild animals can help to prevent P multocida infections.