Parasitemia in Rats Inoculated with Varying Numbers of Plasmodium berghei

Abstract

Variations in size of inoculum over a 100-fold range did not significantly affect the mean parasitemia, phagocytic activity, spleen, liver, or lung weight in rats infected with Plasmodium berghei for 7 days. Hematocrit did regress with size of inoculum. Large inocula stimulated phagocytosis more than small inocula on the 3rd day of infection but not on the 4th day. The failure of parasitemia to regress with size of inoculum is attributed to the fact that only a small fraction of the cells, the reticulocytes, are suitable for the development of the parasites and possibly to the greater phagocytic activity that occurred before the 4th day in rats receiving the larger inocula. The prepatent period of blood-induced malaria is inversely proportional to the logarithm of the size of the inoculum, according to numerous authors, including Taylor et al. (1951) who studied Plasmodium gallinaceum in chickens, Fabiani et al. (1951) who studied Plasmodium berghei in rats and mice, and Wellde et al. (1966) and Warhurst and Folwell (1968) who studied P. berghei in mice. As a corollary, larger inocula usually produce higher parasitemia at any time up to the peak of parasitemia of the animals receiving the larger inoculum. We have tested the NYU-2 strain of P. berghei in rats and found the 7th-day parasitemia to be independent of the size of inoculum in the range of 2.1 X 105 to 2.1 X 107 parasitized cells. This unexpected invariance has not been previously reported although many studies on P. berghei in rats have been published (Galliard and Lapierre, 1951; Jones et al., 1951; Mercado and Coatney, 1951; Black, 1951; Hsu and Geiman, 1952; Hughes and Tatum, 1956; and others). MATERIALS AND METHODS The NYU-2 strain of P. berghei had been maintained in rats for 2 years. Male rats of the WistarPurdue strain, 55 days old, were infected by intravenous inoculation with parasitized blood diluted with unparasitized, defibrinated rat blood to give inocula of 0.2 ml containing 2.1 X 105, 2.1 X 106, or 2.1 X 107 parasitized cells. Uninfected controls received 0.2 ml of unparasitized blood. Blood films were fixed with methyl alcohol and stained with Giemsa's stain. Parasitized cell counts were made to a precision of 10 to 15% (standard error). The Received for publication 18 May 1970. * Supported in part by PHS grants AI-06629 and GM-00274. number of parasites in 50 parasitized cells was also determined. Hematocrits were determined by the capillary tube method. Spleen, liver, and lungs were weighed on a torsion balance to the nearest 0.01 g. Phagocytic activity was determined by the method of Halpern et al. (1953) by measuring the disappearance of Pelikan Special Biological Ink (John Henschel and Co., 195 Marine St., Farmingdale, L. I., New York), injected intravenously at a dose of 16 mg/100 g. At 1-min intervals seven 0.01-ml samples of blood were taken from the cut tip of the tail, placed in 4 ml of 0.1% sodium carbonate solution, and mixed. Optical density was measured with a Beckman model B spectrophotometer at 675 mA.