• An acute bacterial infection of a joint.
• Peak incidence in children is younger than 2 years of age.
• Risk factors:
  • history of trauma
  • preceding URI
  • immunodeficiency
  • hemoglobinopathy
  • Diabetes.
• Age is the most important determinant of cause.
  • In all age groups, S aureus is the primary organism accounting for more than 50% of cases.
  • Among neonates, enteric gram-negative organisms and group B Streptococcus are the most frequent causes.
  • Group A Streptococcus, S pneumoniae, and K kingae are common causes in children younger than 5 years old.
• Blood culture, joint fluid aspiration and analysis, gram stain, and culture of fluid is recommended.
• In pyogenic arthritis, the joint fluid is usually cloudy and has a leukocyte count of at least 50 x 10000/mcL, with a predominance of polymorphonuclear cells, low glucose concentrations, and high protein values.
• Treatment involves a combination of parenteral antibiotics, surgical drainage, and decompression of the affected joint.
• All children who have pyogenic arthritis of the hip or shoulder require prompt open surgical drainage and irrigation to prevent permanent joint damage as the increased intra-articular pressure can compromise blood flow resulting in avascular necrosis of the femoral or humeral head and predisposing the patient to dislocations.
• Open surgical drainage of other joints usually is not required.

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Epstein-Barr virus (EBV)-associated infectious mononucleosis (IM) 

Most commonly presenting clinical findings: fever, fatigue, exudative pharyngitis, lymphadenopathy, and hepatosplenomegaly. 

Self-limited illness that lasts an average of 2 - 3 weeks. 

Treatment is primarily supportive.  Use of ampicillin, amoxicillin, or penicillin during the acute phase not indicated and may result in the development of a morbilliform rash, which studies have suggested may occur in more than 50% of the cases.  Antiviral therapy is not recommended. Splenic rupture occurs in about 1 - 2:1000 cases.  Therefore, avoidance of activities that increase the risk for injury is recommended until splenomegaly has resolved. 

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