Dr. Kassai Tibor szerk.: Parasitologia Hungarica 2. (Budapest, 1969)

Changea in sex ratio

. Aa T ah le 2 shows,

in the early • stage of infection the number of femalea in the population slightly exceeded that of males regardless of the size of infection. This ratio underwent characteristic shift in favour of males after the tenth day. In groups exposed to light infections (25-500 larvae), the development of male preponderance was gradual, reaching 66-80 per cent by days 25-35. Groups infected with 25, 50, 100, 200 and 500 larvae exhibited no marked difference in the rate or extent of change in the sex ratio of the worm burden. However, in groups infected with 1200 and 2000 larvae, concomitantly with the sudden decrease of worm numbers the sex ratio underwent a considerable shift by the twentieth day, and by days 25-35 the proportion of males in the population approached 94-96 per cent. The sex ratios of the Nematode populations found within the several intestinal sections were not essentially different. However with larger burdens it was usual to find more females in the second than in other intestinal sections. Discussion Changes in the sex ratio of Nippostrongylus populations are considered to be a result of immunological reactions evoked in the host (6). The following observations comprise the main evidence in support of this view. 1. Alteration in the sex ratio of the population, and the associated decrease of egg output coincide with the onset of the primary immune response. 2. The immunogenic effect of female Nippostrongylus worms is considerably stronger than that of males (8). 3. If the Nematodes are transferred on days 10-13 into the small intestine of a clean host,egg output is continued and worm sur-