Humoral immune responses against filarial parasitic infection were studied in 62 cases of acute filarial disease presenting with filarial fever with adenolymphangitis, in a community where Bancroftian filariasis was endemic, during and about one month before and after the febrile' episode. Their total leucocyte and differential peripheral blood cell counts and anti-streptolysin 0 titre were determined and compared. Polymorphonuclear cellular responses and anti-streptolysin 0 titre did not show any significant alteration during and after fever. Three of 53 previously amicrofilaraemic subjects (9 of whom were initially microfilaraemic) had microfilaria in their circulation during fever, with a significant increase in their geometric mean microfilaria count. Titres of specific immunoglobulin (Ig) G and IgG4 antibody to Wuchereria bancrofti microfilarial excretory/secretory antigens (measured by enzyme-linked immunosorbent assay) decreased significantly during the fever and the lower levels were maintained one month after fever. The mean circulating immune complex level increased significantly during fever, and a significant percentage of cases demonstrated circulating filarial antigen during fever, which declined after one month, suggesting the release of filarial antigen into the circulation during fever which bound to antibodies to form immune complexes. These observations do not support the suggestion that bacterial infection is the aetiology of filarial fever. It is postulated that antigens released from parasites into the circulation during parturition by adult worms may evoke an allergic response in the host, causing periodic febrile episodes.
