A strain of Plasmodium vivax characterized by prolonged incubation: the effect of numbers of sporozoites on the length of the prepatent period.

The North Korean strain of Plasmodium vivax was characterized by its extraordinarily prolonged incubation period in certain circumstances. It was clearly demonstrated by quantitative observations that the phenomenon may be elicited by the inoculation of small numbers of sporozoites. After the intradermal inoculation of 10 or 100 sporozoites, the incubation period was delayed for periods varying between 262 and 628 days; after u,000 sporozoites, with one exception, the same delay occurred; after 100,000 sporozoites, the incubation period was always of normal duration (13--16 days). Two laboratory-acquired infections in workers who had taken a prophylactic drug showed incubation periods of 315 and 329 days, respectively. Various theories for the phenomenon of prolonged prepatent periods are examined, and the most satisfactory one is based on the presumed existence of two populations of sporozoites in P. vivax. In temperate strains, sporozoites requiring long prepatent periods (LPP) for development are present in great excess over a much smaller proportion of sporozoites characterized by short prepatent periods (SPP); thus small doses will elicit the phenomenon, though doses of over 1,000 sporozoites will mask the effect as the few SPP sporozoites will produce an infection with a normal (i.e. short) prepatent period. In tropical strains, the relative proportions are different, perhaps in equal numbers, and even in small doses some SPP sporozoites will be present and normal prepatent periods should ensure whatever the dosage."

Parasitological and clinical investigations on infections with the VS Romanian strain of Plasmodium malariae transmitted by Anopheles labranchiae atroparvus.

In order to contribute to knowledge of the sporogonic cycle of malaria parasites, 3 batches of Anopheles labranchiae atroparvus were infected with the VS Romanian strain of Plasmodium malariae by feeding them on 2 patients undergoing malariotherapy in Romania. Of the 310 infected mosquitos 59 were dissected for ookinetes, oocysts and sporozoites, and the rest were used for further clinical trials in England. Mature ookinetes were observed 18 hours after infection, and sporozoites were first observed in the salivary glands 19 days after infection.One of the main characteristics of the VS strain of P. malariae observed in Romania was the relatively large number of gametocytes and subsequent heavy infections it could produce. However, gametocytes were found only in the heart blood of 1 of the 4 subjects infected in England (on autopsy after accidental death); post mortem examination of the liver of the same subject failed to show exo-erythrocytic schizonts. None of the other 3 subjects showed gametocytes or infected mosquitos. On the basis of clinical symptoms, the incubation period of P. malariae in these subjects was found to be 18-19 days.