In both primary and secondary HLH, Epstein Barr virus is the most commonly identified inciting infection, although cases associated with cytomegalovirus, human herpes virus 8, influenza, parvovirus, enterovirus and human immunodeficiency virus have also been described [11] . In secondary HLH, associated malignancies include neuroblastoma, Non-Hodgkin's lymphoma and Langerhans' cell histiocytosis. Macrophage activation syndrome (MAS) can be considered a form of secondary HLH syndrome associated with autoimmune disease. MAS complicates an estimated 10% of Systemic Juvenile Idiopathic Arthritis (sJIA) cases [12] , and approximately 1-5% of Systemic Lupus Erythematosus (SLE) cases [13] . Clinical features of MAS similar to primary HLH include high unremitting fever, hepatosplenomegaly, hepatic dysfunction, lymphadenopathy, encephalopathy, cytopenia, elevated ferritin and coagulopathy [12, 14] . Laboratory similarities include depressed natural killer (NK) cell cytotoxic function, elevated soluble IL-2 receptor levels and soluble CD163. In its early stages, MAS can be a diagnostic challenge due to the overlap of symptoms with the underlying autoimmune disease. Factors potentially differentiating HLH from MAS include: 1.) change from quotidian to persistent fever pattern; 2.) sudden change from cytosis to cytopenia; 3.) coagulopathy; and 4.) decreasing ESR. Preliminary diagnostic criteria have been examined for MAS in sJIA, which may help to improve diagnosis of this condition in these patients [13, 15] .