Background
Septic arthritis represents an intra-articular infection caused by pyogenic bacteria, being rare in neonates. Delay in diagnosis or treatment may result in irreversible damage to the joint.

Case Presentation Summary
A newborn girl with 24 days of life, born at 40 weeks of gestation via eutocic birth, with negative maternal serologies performed in the 3rd trimester, and negative b group Streptococcus, researched in cultured rectal exudate after 35 weeks of gestation, was admitted in the emergency department due to decreased mobility of the left lower limb and intense crying when mobilized, without fever, vomiting or diuresis changes. Physical exam showed edema on the left knee, increased temperature and flushing, with the limb preferably in flexion and impaired range of movement. Blood workout showed 12,900/ul leukocytes (4360/ul neutrophils), C-reactive protein 29mg/L and procalcitonin 0.2ng/ml. Radiography of the knee and MRI showed no signs of osteomyelitis. With the suspicion of septic arthritis, an arthrotomy with drainage and lavage was performed and she started flucloxacillin (200mg/kg/day) and cefotaxime (200mg/kg/day). Joint fluid was positive for multisensitive group b Streptococcus and blood cultures were sterile. Antibiotics were replaced to ampicillin (200mg/kg/day) and it was maintained for 4 weeks with a positive clinical and analytical outcome.

Learning Points/Discussion
Septic arthritis in the neonate is a serious condition which could result in permanent dysfunction or deformity of the limbs. Staphylococcus aureus is the most frequent agent but others like group b Streptococcus can cause septic arthritis, particularly at this age. This case reminds that group B streptococcus can colonize birth canal and be negative when searched during pregnancy, so although rare, has to be considered in newborns.

ESPID 2018