PLASMODIUM-BERGHEI - INFECTIVITY OF MICE TO ANOPHELES-STEPHENSI MOSQUITOS

The infectivity of P. berghei-infected TO mice to mosquitoes declines rapidly 2 to 5 days after blood inoculation, in spite of rising number s of gametocytes in the blood. This pattern is typical of many malaria infections and various factors, particularly specific and nonspecific immune responses, have previously been implicated in the decline. Her e we report that (1) simple physiological changes in the mouse blood n amely, falling pH and bicarbonate levels induced by high parasitaemias , are responsible for the sustained inhibition of infectivity; (2) the inhibition is reversible in vivo by the addition of sodium bicarbonat e alone; (3) the inhibition occurs at the point of exflagellation; (4) contrary to previous observations (Kawamoto et al. 1992), exflagellat ion in P. berghei, like that in P. gallinaceum (Bishop and McConnachie 1956; Nijhout and Carter 1978; Nijhout 1979) and P. falciparum (Ogwan 'g et al. 1993), is dependent on extracellular bicarbonate; and (5) in duction of exflagellation by a mosquito factor is bicarbonate dependen t. These new observations are critical to the design and interpretatio n of experiments on other transmission blocking phenomena. (C) 1996 Ac ademic Press, Inc.