Outbreaks of Kingella kingae Infections in Daycare Centers Suggest Tissue Tropism of the Causative Strains

Background: Although Kingella kingae is recognized as an important pediatric pathogen, our knowledge of the virulence factors involved in the invasion of specific host's tissues is limited. Outbreaks of K kingae infections in daycare centers represent natural experiments in which a single virulent strain, introduced into a cohort of susceptible young children, causes multiple infections. If K kingae strains exhibit tissue tropism, the syndromes observed in a given cluster of cases would be relatively homogeneous. Methods: Clinical data of all the K kingae outbreaks known to date were gathered and analyzed. The clinical syndromes diagnosed in the affected attendees were classified as septic arthritis, osteomyelitis, tenosynovitis, soft tissue infection, bacteremia with no focal disease, endocarditis, and meningitis, and computed separately. To assess the similarity of the clinical syndromes detected within outbreaks, we used the Cramer V statistic, which is a measure of the association between 2 nominal variables and, for the purposes of the study, between the detected clinical syndromes and the outbreaks. Results: A total of 23 outbreaks involving 61 attendees were identified. The mean±SD attack rate in the affected classrooms was 15.8% ± 4.8%, and the K kingae colonization rate among the attendees was 54.8% ± 25.3%. Seventy-two separate foci of infection were diagnosed. Osteomyelitis and septic arthritis were the most common clinical syndromes and were diagnosed in 26 children each, followed by tenosynovitis in 4 children. The clinical syndromes diagnosed among attendees to the same classroom showed a statistically significant tendency to be similar (P =. 015). Conclusions: The distribution of clinical syndromes in clusters of K kingae infections differs from that of sporadic cases. The causative strains combine enhanced virulence and high transmissibility, and show tropism toward bones, joints, and tendon sheaths. This information can be used to identify virulence factors associated with invasion of these specific host tissues.