Dr. Murai Éva szerk.: Parasitologia Hungarica 20. (Budapest, 1987)

MATERIALS AND METHODS The voles were kept in pairs (one female and one male) in boxes for mice in the laboratory. The offspring were weaned at about four weeks of age. In all comparative infection experiments 4-8 growing voles, of at least 25 gramm body weigth and of the same litter, were used. The voles were fed on rat diet, occasionally corn and cereals, and substantial amounts of green forage (alfalfa, stalk of green peas, field kale and carrot). Drinking water was supplied ad libitum. A total of 184 voles of both sexes, including 6 litters (A, B, C, D, E and F), and 120 albino mice were used. The mean body weight of the voles and mice was 2 5 and 20 gramm, respectively. The Texas strain of T. equiperdum was used (PACHANIAN, 1963). The strain was maintained in mice and, occasionally, in rats and horses. Mice in death agony were bled, their blood was collected in 3. 3% sodium citrate, diluted in saline, and 0. 5 ml aJiquotes were used for intraperitoneal (i.p.) infection. In each experiment we inoculated mice and litters of voles simultaneously with the same dilution of blood. Thus, from the time of the mice's agony the trypanosomal counts inoculated could be assessed (KEMENES and HORVÁTH, 1986). The number of trypanosomes of the inoculum was determined fromthe red blood cell/trypanosome ratio of the citrate-treated blood. The spleen of the trypanosome-infected and killed (and sometimes that of the dying) voles was weighed and compared to that of their uninfected litter mates. The enlarged spleens were subjected to the usual histopathological examinations. Protection experiments were done in mice using the plasma (antibody-containing serum) of the convalescent voles. The mice were inoculated intraperitoneally with 0.5 ml aliquotes of the 1: 2 dilutions of vole plasma and infected subcutaneously with T. equiperdum at the same time. The convalescent voles' immunity to T. equiperdum was checked by challenge infections. Mice, susceptible voles (control) and voles that had recovered 2 weeks and 3 months before, respectively, were infected intraperitoneally with 5 x 10

6 trypanosomes. On days 5 and 6 after the challenge all voles were killed by bleeding. Their immunity to T. equiperdum was evaluated on the basis of enlargement and histological picture of the spleen and the presence of trypanosomes in the blood. RESULTS Voles experimentally infected with the medium dose consistently survived the infection. Contrarily, mice that had received the same dose died of parasitaemia on postinfection (PI) days 3-5. Most of the infected voles appeared healthy throughout. Occasionally they exhibited a slight lack of appetite, shaggy haircoat and slow movement. In voles infected with the medium dose and killed on PI days 4-5 (i. e. on days when mice infected in the same way died) there was a 5- to 7-fold enlargement of the spleen (Fig. 1) and a few trypanosomes were present in the blood. At the same time, in the carcasses of mice dying of the infection, enlargement of the spleen was only 2- to 3-fold and the heart blood contained almost as many trypanosomes as red blood cells. The blood of convalescent voles contained trypanosomes demonstrable by mouse inoculation up to 2 weeks. Subsequently trypanosomes occurred in the blood only exceptionally and after one month not at all. The spleen of voles killed 3 months after infection was of nearly normal size and their serum was devoid of trypanosomal antibodies (Table 1). The infected voles usually became trypanosome-free after one month. Trypanosomes persisted for more than one month only in a few voles inoculated during pregnancy. E. g. an infected female delivered two litters within a short time, reared her young, and conceived again. On PI day 102 she was still positive for infection by