Complement pathway activity in serum from patients with classical dengue fever.

Complement activity in 125 cases of classical dengue fever was examined through the measurement of haemolytic activity. During the first 3 d of fever, the classical complement pathway activity (CCPA) was not altered in 109 cases. After 4 d of fever, 9 of 16 patients in the viraemic period had CCPA decreased by 45% (995 +/- 119 units/ml) and serum complement component C4 decreased by 40% (10.4 +/- 0.9 mg/dl). The alternative complement pathway activity was not affected in any case tested throughout both viraemic and convalescent stages. Both CCPA and C4 persistently decreased in 3 of these 9 patients at the convalescent stage. A decrease in serum C3 was also observed in these 3 patients only, and circulating immune complexes (CIC) levels were particularly high in these 3 patients. These results indicate that there is little evidence of complement activation on days 1-3 of viraemia but that complement activation may occur subsequently. It is concluded that both CIC and other unknown factors not related to CIC may contribute to complement activation in some cases (9/125) of classical dengue fever.