To the Editor: For most of the 3 decades since the first description of Kingella kingae, this gram-negative bacillus was considered a rare cause of human disease (1). Since the late 1980s, however, reports of infections by the organism in young children have increased in the United States, Western Europe, and Israel (2-6). The rapid emergence of K. kingae as an important cause of pediatric disease does not necessarily imply that the organism is truly a new pathogen. Better isolation techniques and awareness of the bacterium by microbiology laboratories may contribute to the apparent increase (4). Recent studies have demonstrated that primary isolation of K. kingae can be substantially improved by injection of synovial fluid and bone exudates into aerobic blood-culture bottles (4). Synovial fluid may inhibit the growth of K. kingae, and injection of the clinical specimen into a 50-mL volume of broth reduces the concentration of inhibitory factors, facilitating isolation of the organism (4).

In 1993, we reported results of routine use of blood-culture bottles for processing cultures of exudates (7) at the Soroka University Medical Center, Beer-Sheva, Israel. From 1988 to 1992, 25 children with invasive K. kingae infections, defined as isolation of the organism from blood or normally sterile body fluids, were identified in southern Israel. From 1994 to 1998, 33 additional patients, including 32 children and a 21-year-old adult, were detected in the same area. Twenty-four (63.6%) of the 33 patients were male. Eight (24.2%) cases were diagnosed between January and June and 25 (75.8%) between July and December. Median age of children was 13 months (mean ± SD: 15.0 ± 7.6 months; range 6 to 37 months).

The fact that all children in southern Israel are born and receive inpatient medical services at the Soroka University Medical Center allowed us to calculate the incidence of invasive pediatric K. kingae infections in this population. During the 6-year period, the average annual number of births was 10,860. The annual incidence of invasive K. kingae infections during the same period was 11.9 per 100,000 in children < 48 months of age, 19.2 per 100,000 in children <24 months of age, and 20.0 per 100,000 in infants <12 months of age.

When medical attention was sought, patients had been ill for a median of 3 days. Symptoms of upper respiratory tract infection were recorded in 12 (36.4%) children, stomatitis in 8 (24.2%), and diarrhea in 4 (12.1%). Occult bacteremia (positive blood culture with no obvious focal infection) was diagnosed in 16 children. In 15 children, K. kingae had invaded the bones. Septic arthritis was diagnosed in 11 children, involving the ankle in 4; the knee or wrist in 2 patients each; and the hip, shoulder, or elbow in one patient each. Osteomyelitis was diagnosed in two patients, affecting the femur in one and the tibia in the other. In two additional patients, both with fever and bacteremia, the location of the skeletal infection could not be determined. One limped and had tenderness over the femur, but X-rays and a Technecium₉₉-labeled bone scan showed no abnormalities. The other had pain in the heel but no fluid could be aspirated. Bacteremic tracheobronchitis occurred in one child, and endocarditis of the mitral valve was diagnosed in a 21-year-old woman who was receiving immunosuppressive therapy for systemic lupus erythematosus. All 33 patients were treated with ß-lactam drugs and recovered.

Injecting synovial fluid specimens into blood-culture bottles permitted the diagnosis of K. kingae in these patients and showed that this organism may be a common cause of invasive pediatric infections. The age distribution of the patients demonstrates that K. kingae is a pathogen of young children, especially those between the ages of 6 months and 2 years, among whom the incidence of invasive disease has remained stable since 1988. This age distribution of K. kingae infections parallels that for respiratory carriage of the organism. In a surveillance study among 48 children ages 6 to 42 months attending a day-care center in Israel, K. kingae was isolated from 109 (17.5%) of 624 throat cultures, and 34 children (70.8%) carried the organism at least once during an 11-month period (8). However, the organism was not detected in healthy infants ages 2 to 4 months attending a well-baby care clinic, which indicates some immunity to colonization and infection by K. kingae during the first months of life (8).

When the 1988 to 1993 surveillance data are added to those collected from 1994 to 1998, K. kingae infections show a significant seasonal pattern; 44 (75.9%) of 58 cases were diagnosed in the second half of the year (p = 0.007). This increase in K. kingae infections in winter has also been described in other respiratory pathogens. This finding, as well as the frequent detection of respiratory symptoms in children with invasive K. kingae infections, suggests that seasonal viral infections may facilitate the spread of K. kingae from the throat, to the bloodstream and bones. In a prospective study, K. kingae bacteremia was documented in 4 (13.7%) of 29 young children with culture-proven herpetic gingivostomatitis, confirming the role played by viral infections in the pathogenesis of infections caused by the organism (9).

With few exceptions, isolates of K. kingae remain susceptible to antibiotic drugs (10). Our results demonstrate that the prognosis of invasive K. kingae infections is generally good and patients respond promptly to appropriate antimicrobial therapy.

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