Pasteurella multocida osteomyelitis: An unusual case presentation

P a steurella multocida is a Gram-negative nonsporulating bacillus which is a commensal organism in dogs, cats, fowl, swine, rats, sheep, cattle and horses (1). In these animals it may cause hemorrhagic septicemia, chronic pulmonary infiltrates and several other animal diseases and may thereby have significant economic consequences in the animal industries. Human infection with P multocida is rare, but may occur following a dog or cat bite, scratch or other contact (2-4). Such an infection is characterized by local wound infection, cellulitis and suppuration (2). Infections are usually associated with leukocytosis. Bacteremia with fever, chills and ade-nitis may occur, and meningitis, brain abscess, pyogenic arthritis, endocarditis and pyelonephritis have been described as infrequent complications. P multocida has also been isolated from the nasopharynx of asymptomatic individuals who have been colonized with the organism as a result of exposure to animals; it has also been isolated from the sputum of asymptomatic animal workers with bronchiectasis (5). The pres-ent case of P multocida infection illustrates an unusual presentation and osseus involvement. HP VON SCHROEDER, RS BELL. Pasteurella multocida osteomyelitis: An unusual case presentation. Can J Infect Dis 1996;7(2):137-139. A healthy male farm employee developed an unusual infection caused by Pasteurella multocida .Atypicalfeaturesincludedthechronicnatureoftheinfection,thedevelopmentofosteomyelitisofthetibiawithout direct animal inoculation, and lack of fever and leukocytosis. Radiographic appearance of P multocida osteomyelitis may be the result of osteoclast activation and can be confused with musculoskeletal tumour. P multocida infection re-quiresahighdegreeofsuspicion,andshouldbeconsideredincasesoffarm-oranimal-relatedinjuriesevenifthereisnohistoryofdirectanimalcontact.


CASE PRESENTATION
A 35-year-old male blacksmith employed at a horse farm was admitted to hospital with a painful mass on his left shin. He had fallen off a tractor at work five months previously and sustained an abrasion to his shin. There was no direct contact between the abrasion and any farm or domestic animal (including his two dogs), animal saliva or his own saliva. The abrasion resolved quickly and he was completely asymptomatic until three months later, when he developed a painful, slightly erythematous mass in the region. Radiographs and a computed tomography scan ( Figure 1) revealed a soft tissue mass and diffuse permeative changes in the tibial diaphysis. Technetium bone scan showed intense uptake in both dynamic and pool phases, which extended into the inner cortex of the tibia at the affected site. Gallium scan also showed high activity indicative of an inflammatory process. He was treated with analgesics and did not receive any empirical antibiotics. Two months later he was admitted to hospital for biopsy; differential diagnosis included bone sarcoma and infection. On admission, the patient was afebrile and appeared reasonably well. Ambulation caused severe pain necessitating use of a cane. A 3x3 cm firm lesion was noted over the tibia. Chest xray was normal, there was no lymphadenopathy and his white blood cell count was 7.2x10 9 /L with a normal differential. At biopsy, purulent and suppurative tissue was noted. Aerobic and anaerobic cultures taken at the time of biopsy both revealed P multocida. The organism was sensitive to penicillin, ampicillin, cefazolin, ceftazidime, ceftriaxone, ciprofloxacin, co-trimoxazole, gentamicin and tobramycin.
Following the biopsy, the patient received intravenous cloxacillin for two days until cultures were available; therapy was then changed to intravenous ceftriaxone for six days, followed by oral ciprofloxacin (750 mg twice daily) for two weeks. He was then maintained on amoxicillin (500 mg three times daily) for five months. Follow-up at one month revealed only residual symptoms and improvement of radiographic findings ( Figure 2). At seven months the patient was symptom-free with the exception of a mild throbbing sensation associated with changes in the weather.

DISCUSSION
The family Pasteurellaceae contains Gram-negative, facultatively anaerobic and fermentative bacteria of the genera Pasteurella, Haemophilus and Actinobacillus. Over 30 different species of the genus Pasteurella have been identified by phenotypic and genetic techniques. In addition to being commensal flora in animals, P multocida is one of the most prominent pathogens in domestic animals (1) and is a leading cause of animal bite wound infection in humans. However, as in the case presented, not all P multocida infections occur after animal bites and transmission may occur with minimal animal contact (3,4). Furthermore, the organism may be transmitted via aerosols and fomites (6), and has been isolated in 19 different body specimens, most often from the sputum (48.5%), but also from the urine (7). This raises an unresolved question of mechanism of infection in the case presented. Was the wound inoculated with the organism at the time of the origi-  nal injury (which would imply a quiescent phase for three months) or did the infection occur months later at the time of presentation by an opportunistic organism upon encounter with devitalized tissue? It is also possible that the wound was licked by a domestic animal or, if the patient was an asymptomatic carrier, he may have inoculated himself with saliva on his hand. However, the patient denied any direct animal contact or contact of the wound with his own saliva.
Due to the ubiquitous nature of P multocida in animals, infection with this organism probably occurs more frequently than is usually appreciated. A high index of suspicion and early diagnosis, especially in immunocompromised patients, are necessary because the disease is potentially life threatening (8). The infection is readily treatable and, as demonstrated by sensitivity testing in the case presented, it is susceptible to many antibiotics. However, P multocida is typically not sensitive to cloxacillin, some first-generation cephalosporins, erythromycin, clindamycin and vancomycin, which are frequently used to treat soft tissue and bone infections. Furthermore, an infection following animal bite or other animal contact or farm-related injury will initially require a flexible antibiotic approach due to the nonspecific clinical presentation and wide variety of possible pathogens. P multocida infections do not have any unique characteristics and may resemble staphylococcal, streptococcal, cat-scratch fever or tularemia infections (9). Although empirical antibiotic ther-apy may be the initial step, once culture and sensitivity results are available, specific therapy should be started. Less expensive antibiotic regimens, such as intravenous penicillin or ampicillin followed by oral amoxicillin, may be effective against P multocida infections provided appropriate antibiotic levels can be attained in the target tissues.
Orthopedic infections caused by P multocida following animal-related injuries include septic arthritis (10,11), tenosynovitis (12) and infection of total joint replacements (13)(14)(15). Osteomyelitis following penetrating inoculation from a dog or cat bite is classically described (10); however, osteomyelitis without such direct inoculation, as in our case, has not been previously reported. The permeative radiographic changes may have been a result of bone resorption due to the activation of osteoclast activity by the P multocida toxin. Osteoclast stimulation has been described in vitro (16,17) and provides a novel potential mechanism for bacterial migration and virulence.
The bone resorption and soft tissue mass represent the unusual features by which this case was originally diagnosed as a tumour. Presentation several months after a trivial abrasion in a healthy individual with no systemic symptoms further illustrate the atypical features of this case. This case demonstrates that P multocida infection should be considered in cases of farm-or animal-related injuries even if there is no history of direct animal contact.