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."

Prepatent periods of a tropical strain of Plasmodium vivax after inoculations of tenfold dilutions of sporozoites.

Eleven patients requiring malaria therapy were inoculated intradermally with graded doses of sporozoites of the Chesson strain of Plasmodium vivax. Estimated doses of 10 sporozoites were given to four patients, of 100 to three patients, of 1,000 to two patients, and of 10,000 to two patients. Parasitaemia was detected in all patients 12 to 17 days after inoculation; fever began on the 14th to 19th days. The results of the trials are compared with earlier work on a temperate strain of P. vivax in which patients given small doses of sporozoites exhibited long prepatent periods of 257 days or more. It is concluded that the differences in the two strains can be explained by the assumption that, in varying proportions, all strains of P. vivax produce two types of sporozoites, one eliciting short prepatent periods (Type I), and the other lying dormant or developing slowly to give rise to long prepatent periods (Type II). The latter type greatly predominates in temperate strains, but not in tropical strains; at high dilutions, therefore, pure suspensions of Type II sporozoites of temperate strains can be prepared. It is thought that relapses of P. vivax are in reality a dalyed parasitaemia arising from Type II sporozoites.

A strain of Plasmodium vivax characterized by prolonged incubation: morphological and biological characteristics.

Numerous strains of P. vivax, distinguishable chiefly by their biological characteristics, are known to exist. Two main varieties are recognized: the so-called temperate and tropical strains. The most extreme example of the former-designated by Nikolaev as P. vivax hibernans-constantly exhibited an extremely long incubation period. The strain is no longer in existence and no type material has been preserved. In its place, a North Korean strain with a generally long incubation period has been studied and compared with the well-known tropical Madagascar strain, which frequently but not constantly has a short incubation period. The data presented here concern the behaviour of various strains from the USSR and the morphological characteristics of the North Korean and Madagascar strains. Splenectomized chimpanzees were used as the host of these parasites, particularly in regard to exoerythrocytic schizogony. Attempts were also made, by late biopsies of the liver of the apes, to elucidate the prolonged latency of the North Korean strain. Although there was no evidence of specifically dormant forms, it is probable that certain sporozoites fail to develop in the normal time and that they are reactivated by an unknown factor a year or more after inoculation.

Imported malaria in the United Kingdom.

Over 2,000 cases of imported malaria have been confirmed by blood examination. Ninety percent. of cases from tropical Africa were infected with P. falciparum. Most of the patients were Caucasians and had primary infections. All developed fever within a month after arrival and most of them within two weeks of arrival. In some patients malaria parasites were seen in routine blood films.Developing forms of P. falciparum were always present in the peripheral blood of patients suffering from a primary attack which was not diagnosed or treated until a week or more after the onset of fever.All deaths investigated were caused by P. falciparum and were primary infections.In not one of the P. falciparum infections did the victim continue taking prophylactic drugs for more than a few days after leaving the endemic area. Had drugs been continued for one month probably not a single overt case of P. falciparum would have occurred.A primary attack of P. falciparum malaria is seldom, if ever, classical in that the fever is never tertian and may resemble clinically many other diseases.Children in boarding-schools returning from the tropics should be supplied with prophylactic tablets and instructions to the matron. If there is an epidemic of a fever any students who have recently returned from the tropics should have a blood film examined for malaria.The risk of contracting malaria among drug addicts is considerable, especially with P. falciparum.