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

Parasit. Hung. 2. 45-54. 1969 The Influence of Size and Duration of Infection on Changes in the Sex Ratio of Nippostrongylus brasiliensis Populations in Rats Dr. Ottó SEY Department of Zoology, Teachers' College, Pécs It is well established that in rats harbouring an old infection of Nippostrongylus brasiliensis the worm population consists largely or exclusively of male' worms in contrast to the essentially equal number of male and female worms (1:1 sex ratio) occurring in recently acquired infections (l, 2, 3, 4, 10). This shift in sex ratio, associated with ageing of the Nematode population is a characteristic feature of N. brasiliensis infection and is considered to be the outcome of an immune reaction evoked in the host (6). The aim of the present experiment was to investigate the effect of intensity of infection on changes in the sex ratio of Nippostrongylus populations in adult rats. Materials and Methods Culture of infective larvae, worm egg counts and determination of worm numbers followed the procedures used by KASSAI and AITKEN (6). White rats weighing 120-170 g were used. They were narcotized with chloroform and the infective larvae injected subcutaneously in 0.2 ml physiological salt solution. Groups of 30 rats (15 males and 15 females) were infected with 25, 50,