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."

Sources of hepatitis B virus infection in hospital environment.

Investigations on the incidence of hepatitis B antigen (HB Ag)-carriers among 1466 patients of 11 various hospitals and departments showed that sources of hepatitis infection were more numerous in hospitals for children than in those for adults. The high-risk departments were those of dialysis (5 out of 12 patients), clinical medicine (6.6%), and obstetrics and gynecology, for adults, and those of neuropsychiatry (17.9%) and clinical medicine (11.2%)for children . The highest incidence of HB Ag-carriers among the medico-sanitary personnel of 5 hospitals (436 sera) was recorded in a hospital of obstetrics and gynecology.