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The Threshold of Parasite Density in Relation to Clinical Activity in Primary Infections with Plasmodium Vivax¹

A clinical reaction manifested by an elevation of temperature to 37.8°C., or higher, may be induced by P. vivax in densities in the peripheral blood of 10 per cmm. or fewer, regardless of whether the attack was induced by inoculation with trophozoites or sporozoites. Except during the few days subsequent to the onset, the duration of the clinical attack has never been observed to exceed the period in which the parasite density remained at 100 or more per cmm., and most commonly is shorter in duration. It is uncommon for a parasite density (P. vivax) in excess of 25,000 per cmm. to be observed, and exceptionally rare for it to attain 50,000. Trophozoite inoculations more commonly produce densities in excess of 25,000 than do those effected by sporozoites. Following trophozoite inoculation the maximum density noted is usually observed within 10 days of the first detection of parasites, while following sporozoite inoculation it occurs later. Counts in excess of 15,000 per cmm. are most often observed in attacks which will run for four weeks or longer, although attacks of such length may be experienced subsequent to sporozoite inoculation when the maximum density is not attained. Attacks initiated by trophozoite inoculation commonly terminate on higher parasite densities than do those initiated by sporozoites. Only rarely are densities of parasites of 500 or less per cmm. noted at the termination of an attack.

¹ The studies and observations on which this paper is based were conducted with the support and under the auspices of the International Health Division of The Rockefeller Foundation, in cooperation with the Florida State Board of Health and the Florida State Hospital.