Annals of Tropical Medicine & Parasitology, Vol. 97, No. 6, 587–592 (2003) Paroxysm serum from a case of Plasmodium vivax malaria inhibits the maturation of P. falciparum schizonts in vitro Y. NAGAO*, P. CHAVALITSHEWINKOON-PETMITR*, H. NOEDL ² , S. THONGRUNGKIAT*, S. KRUDSOOD*, Y. SUKTHANA*, M. NACHER  , P. WILAIRATANA* and S. LOOAREESUWAN* *Faculty of Tropical Medicine, Mahidol University, 420/6 Rajvithi Road, Bangkok, 10400, Thailand ² Department of Specic Prophylaxis and Tropical Medicine, Institute of Pathophysiology, University of Vienna, Kinderspitalgasse 15, A-1095 Vienna, Austria  INSERM U511, Centre Hospitalo-Universitaire Pitie ´-Salpe ˆtrie `re, 91 Boulevard de l’Ho ˆpital, 75643 Cedex 13, Paris, France Received 16 December 2002, 21 May 2003 In concurrent infections in vivo, the blood stages of Plasmodium vivax suppress those of Plasmodium falciparum. To see if the paroxysm (i.e. the periodic febrile episode) of P. vivax infection contributes to this suppression, sera from a P. vivax-infected volunteer were added to cultures of whole blood taken from cases of P. falciparum malaria. The crude ‘rate’ of schizont generation from the ring forms, measured as the percentage of all asexual parasites that were schizonts after incubation for 24 h, was similar whether the cultures contained serum samples collected during paroxysms or those collected, from the same volunteer, at other times (19.1% v. 18.9%; P =0.842). After a random-eVect linear regression was used to adjust for disparities between the P. falciparum isolates, however, the degree of schizont maturation, measured as the mean number of nuclei per schizont, was signicantly lower for the cultures with ‘paroxysm serum’ than for those with ‘non-paroxysm serum’ (4.8 v. 5.3; P =0.002). The proportion of schizonts considered mature was also signicantly lower when ‘paroxysm serum’ was used (3.7% v. 6.3%: P =0.03). This appears to be the rst in-vitro study in which sera collected during a paroxysm of P. vivax have been shown to inhibit the maturation of P. falciparum schizonts. The role of this mechanism in intra- and inter-specic competition is discussed. Plasmodium falciparum and P. vivax, the species et al., 1987; Pukrittayakamee et al., 1994). In mixed infections, the blood stages of P. vivax which cause most human malaria in the Asia–Pacic region, appear to suppress each often outnumber those of P. falciparum even though, in mono-infections, P. falciparum other when present in the same individual at the same time (Boyd and Kitchen, 1938; parasitaemias are generally higher than those of P. vivax. Human infection with Maitland et al., 1996; Luxemburger et al., 1997). In hospitalized patients, for example, P. vivax is characterized by the presence of periodic ‘paroxysms’, with acute clinical mani- the appearance of one species often follows the disappearance of the other (Looareesuwan festations dominated by fever. A paroxysm occurs when there is a synchronised rupture of the schizont-infected erythrocytes. Glyco- Reprint requests to: Y. Nagao, Department of sylphosphatidylinositol is probably released Immunoregulation, Research Institute for Microbial as the schizonts rupture (Bate et al., Diseases, Osaka University, 3-1 Yamadaoka, Suita, Osaka 1992; Schoeld et al., 1993), triggering the 565-0871, Japan. E-mail: nagaoy@biken.osaka-u.ac.jp; fax: +81 6 6875 5233. generation of non-specic immune eVectors, © 2003 The Liverpool School of Tropical Medicine DOI: 10.1179/000349803225001409