Septic Arthritis in a 14-Month-Old Child

Jeanette M. Ramos, MD; Julie A. Ribes, MD, PhD


April 12, 2010

In This Article

Differential Diagnosis

Staphylococcus aureus is the most common pathogen implicated in septic arthritis in children, representing approximately 40% of cases.[1,2] Included in the differential are other gram-positive cocci, such as group A and group C streptococci and Streptococcus pneumoniae. Prior to routine vaccination, Haemophilus influenzae type B was a significant pathogen, but its frequency in joint infections has decreased in recent years. Other gram-negative organisms commonly found in joint infections in children include Neisseria species (Nmeningitidis and N gonorrhoeae), Enterobacteriaceae species, Kingella kingae, and Brucella melitensis. Seen in much lower frequency are other etiologic agents, including yeasts. In a significant proportion of cases (over 33% in most studies), no etiologic agent can be identified by routine culture methods.[1,2] A recent article describing polymerase chain reaction (PCR) for Kkingae demonstrated that K kingae DNA could be identified in almost half of the culture-negative joints, making this pathogen the most common organism identified in septic joints of children in that study.[3]

The Gram stain helped us to eliminate the gram-positive organisms, whereas the morphology of the gram-negative rods, growth rates, and growth media allowed us to eliminate Haemophilus influenzae (gram-negative coccobacillary forms, no growth on blood agar), Neisseria species (gram-negative diplococci), and Brucella melitensis (small, gram-negative coccobacilli) from the differential. The positive oxidase and lack of growth on MacConkey agar eliminated the Enterobacteriaceae from consideration. K kingae fit the Gram stain morphology, growth characteristics, and oxidase production. It is also the most likely etiologic agent in the patient's age group (under 2 years of age) when very sensitive methods like PCR are used to identify the infectious agent.[3]