Cytology of synovial effusions in epidemic polyarthritis.

Examination of synovial fluid in eight patients with epidemic polyarthritis following Ross River virus infection showed cell counts ranging from 1,500 to 13,800 per mm3. Stained smears were notable for a paucity of neutrophils and high proportions of monocytes and vacuolated macrophages, which were further characterised by light and electron microscopy. Ross River virus antigen was detected by specific immunofluorescence in monocytes and macrophages of four cases early in the course of the illness, but intact virus was not identified by electron microscopy or cell culture. No cell-associated C3 component of complement or immunoglobulin of IgM or IgG class was detected in any cell-type. These findings indicate that cell counts and simple smears may be useful in the early diagnosis of suspected epidemic polyarthritis, and provide further information pertinent to the pathogenesis of this disease.