Experimental and field studies on some biological characteristics of Plasmodium vivax isolated from tropical and temperate zones of China.

OBJECTIVE

To clarify some biological behaviors of Plasmodium vivax including the periodical infectivity of gametocytes for mosquitoes, the prepatent period, the incubation period, the relapse patterns and their relationship.

METHODS

Laboratory reared Anopheles sinensis were fed through fetal membrane on blood from patients naturally infected with vivax malaria. After feeding at different hours, the engorged mosquitoes kept in the insectarium were dissected 5-7 days later. The percentage of mosquitoes infected and the mean number of oocysts per gut were taken as criteria of the viability of the gametocytes. The healthy volunteers artificially infected by mosquito bite or quantitative inoculation of sporozoites were treated with 1.5 g chloroquine base alone before and after the first and subsequent onset of fever to detect the natural course of disease in human body.

RESULTS

The infection in mosquitoes can readily be divided into upward, peak, downward and non-infective period. The infectivity reached a peak at interschizogony period and fell to low level at the time when schizogony occurred. The short incubation period was frequently observed in persons bitten by 10 infected mosquitoes or 10 000 sporozoites inoculated and the long incubation period was constantly recorded in cases receiving 100 sporozoites. When one mosquito was allowed to feed on volunteers or 1 000 sporozoites were injected, the incubation period may be short or prolonged.

CONCLUSIONS

The 48-hour cycle in the infectivity of gametocyte for mosquitoes was demonstrated and the peak of oocyst production preceded the peak of parasitaemia by 1 to 3 days. Prompt radical treatment is extra important. Since the incubation period is variable depending on the number of sporozoites inoculated and the tropical versus temperate zone malaria is not all-inclusive, the classification of tertian malaria might be reconsidered.