Dr. Murai Éva - Gubányi András szerk.: Parasitologia Hungarica 31. (Budapest, 1998)

Figure 2 shows that the SI of P. c, adami varied from 0.57 to 0.62, the SI of P. v. vinckei from 0.49 to 0.62, the SI of P. berghei from 0.42 to 0.48 and the SI of P. y. killicki from 0.20 to 0.29. The SI of the four Plasmodium species were significantly different when compared by si 1 T • P. c, adami o P. v. vinckei • P. berghei a p. y, killicki TIME (hrt) 0 —1 1 1 1 1 1 1 1 1— 0 6 12 18 24 30 36 42 48 Fig 2. Follow-up of the mean synchronicity index (SI) of 4 batches of mice (n = 5) infected with P. c. adami, P v. vinckei, P berghei and P. y. killicki evaluated at 6-hour intervals over 42-48 hours. Student's t-test (p < 0.01). Thus, the more resistant the strain was to chloroquine, the more asynchronous it was. As chloroquine hits preferentially the mid-term trophozoite (Cambie et al. 1991 ), the drug sensitivity of a strain will depend on the parasitic stages present at the time of treatment. Thus, it is not surprising that species which develop synchronously in the blood such as P. chabaudi and P. vinckei are more sensitive than asynchronous species like/? berghei and P. yoelii. This was confirmed by the following experiments: when P. y. nigeriensis, the most drug-resistant of the rodent strains was synchronized by the Percoll-glucose method (Beaute-Lafitte 1994a) it appeared that the mid-term trophozoite had in fact a susceptibility equivalent to that of P. v. petteri ox P. c. chabaudi. When//? c. chabaudi was desynchronized by the delayed inoculation of blood kept at 4 °C, it became less susceptible to the drug (Tahar et al. 1995). In conclusion, the drug resistance of Plasmodium is a complex phenomenon, and very likely multifactorial. In rodents, an essential factor is the chronobiology of the strain which depends on the number of free merozoites in the organism. The so-called natural drug resistance is the association of asynchrony and merozoite resistance. ACKNOWLEDGEMENTS We are very thankful to Prof. A. G. Chabaud and Miss V. Chamlian for kindly reviewing the manuscript. REFERENCES Beaute-Lafitte, A., Altemayer-Caillard, V, Chabaud, A. G. and Landau, I. (1994a): Plasmodium yoelii nigeriensis: biological mechanism of resistance to chloroquine. —Parasite 1: 227-233. Beaute-Lafitte, A., Altemayer-Caillard, V., Gonnet-Gonzalez, F., Ramiaramana, L., Chabaud, A. G. and Landau, I. (1994b): The chemosensitivity of the rodent malarias-relationships with the biology of merozoites. —Int. J. Parasitol. 24: 981-986.