Studies on immunity to reinfection with Schistosoma mattheei in sheep and cattle

Bull. Org. mond. Sant1 1974, 50, 566-568Bull. Wid Hitha Org. Studies on immunity to reinfection with Schistosoma mattheei in sheep and cattle J. M. PRESTON I & G. WERE 2 Abstract Few experiments have been reported on immunity to reinfection in domestic stock. The degree of resist- ance to reinfection with S. mattheei was studied in cattle and sheep. On the basis of the results, the only evidence of acquired immunity in calves and sheep is a reduction in the length of adult worms. Although Fujinami (1) noted that calves entering for the first time an area in which Schistosomajapon- icum was endemic were much more severely affected than indigenous stock, few studies have since been reported on immunity to reinfection in domestic stock. Massoud & Nelson (2) studied Ornithobil- harzia turkestanicum and S. bovis in calves. With the S. bovis system there was no reduction in the return of adult worms to immunized animals, but some immune response was evidenced by decreased egg densities in the tissues, whereas, in calves re- infected with 0. turkestanicum, the adult-worm load of the challenge infection was reduced by 12.4% Lawrence (3), who studied the occurrence of S. mat- theei in cattle, found that the egg count in infected calves (to which 5000-45 000 cercariae had been administered) rose sharply to a peak at 10-11 weeks and then declined to a low level, which was main- tained for 75 weeks in some animals. The decline in the egg count was considered to be due mainly to the suppression of egg laying in the host. How- ever, the proportion of the parasites administered that was recovered at slaughter showed a highly significant decrease between the 8th and 78th weeks after infection. Animals subjected to reinfection (15 000-21 000 cercariae) showed little or no increase in faecal egg counts. The present study concerns the degree of resistance to reinfection with S. mattheei in cattle and sheep. 1 Veterinary Services Division, Veterinary Research Laboratory, P.O. Kabete, Kenya. ' Reader in Medical Helminthology, London School of Hygiene and Tropical Medicine, Gower Street, London, England. Materials and methods Twelve 5-month-old Dorset Horn lambs were divided into 3 groups (A, B, & C) of 4 animals matched according to weight. Twelve Jersey calves aged 31/24½/2 months were similarly divided into 3 groups. A South African strain of S. mattheei maintained in Bulinus africanus africanus was used for infection. The lambs and calves of groups A and B were percutaneously exposed to 500 cercariae for 30 min; 14 weeks later, the animals of groups B and C were exposed to 5 000 cercariae. The animals were autopsied between the 10th and 11th weeks after challenge. Adult worms were col- lected by perfusion and by dissection. The liver and the small and large intestines were weighed, and a 300-g sample of each was retained for digestion in 5% potassium hydroxide solution according to the method of Cheever (4). The eggs were counted in Sedgwick Rafter chambers. The value of P was calculated by means of the t test of Students. Results S. mattheei eggs were detected in the faeces of all the animals between the 6th and 7th weeks after infection. There was no apparent difference from the clinical point of view between the challenge con- trol animals (C) and those that had been infected previously (B). However, the sheep of groups B and C were more severely affected than the calves of the corresponding groups, being duller and more anaemic, as well as exhibiting greater weight loss. The. recoveries of adult worms and egg counts shown in Table 1 indicate that pre-exposure to 500 cercariae provided no significant protection against challenge infection in either species. When 100 gravid S. mattheei females, recovered from each animal, were measured, it was seen that their mean 3226 -566- BRIEF COMMUNICATIONS Table 1. S. mattheei worm recoveries, mean worm size, and tissue egg counts in immunized and control animals Sheep Cattle mean no. of mean no. of mean length of mean no. of mean no. of mean length of Group a worms per eggs per worms worms per eggs per worms animal animal (mm ± SD) b animal animal (mm ± SD) A 65 199156 15.3 1.1 86 195210 17.0 0.86 B 2134 2 231 722 12.7 0.9 c 2470 3 647 548 13.7 2.0d C 2 207 2 504775 16.6 1.6 2288 3632007 17.2 2.0 % --6.1 -17.5 + 4.0 -4.7 change (N.S.) (N.S.) (N.S.) (N.S.) a b c d A = immunizing exposure (500 S. mattheeicercariae); B = immunizing and challenge exposure (500 + 5 000 S. mattheei cercariae); C = challenge exposure (5 000 S. mattheei cercariae). Mean length of 100 gravid female worms from each of the 4 animals in two groups. P < 0.02. P < 0.05. lengths in the immunized sheep and calves were significantly smaller than in the challenge control group (P< 0.02 and P< 0.05, respectively). The proportions of adult worms recovered from the challenge exposure of 5 000 cercariae were the same in the sheep and in the calves. Discussion On the basis of these results, the only evidence that immunity might have been acquired in the calves or sheep was the reduced length of adult worms. This is in contrast to the results obtained in the S. bovis and 0. turkestanicum systems, in which 1 000 cercariae were used for the immunizing ex- posure. However, Lawrence (3) considered that the low egg production of reinfecting S. mattheei ob- served in cattle was due to immunological suppres- sion of egg laying in the host since there was no apparent reduction in the establishment of adult parasites. An interesting feature is that previous exposure to S. mansoni cercariae stimulated a more marked immunity to S. mattheei in sheep and cattle than did the homologous parasite in the present study. Immunizing with S. mansoni, Hussein et al. (5) demonstrated a reduction in the S. mattheei worm load of 40.1-94.4%. On the other hand, Preston et al. (6) found that, in sheep, although there was little alteration in the worm load, the density of eggs in the tissues was reduced by 11.2-83.3%. Similar results were obtained by Massoud and Nelson (2), who found that the immunization of calves with S. haematobium resulted in a 42% reduc- tion of S. bovis challenge, whereas no such decrease occurred after immunization with S. bovis. Taylor (7) exposed sheep to either 10 000 normal S. mansoni cercariae or 10 000 S. mattheei cercariae irradiated at 6000 rad. At 16 weeks these sheep, together with 5 control animals, were challenged with 5 000 normal S. mattheei cercariae and were killed and autopsied 9-11 weeks later. No eggs were seen in the faeces of sheep exposed to irradiated cercariae, and S. mattheei worm burdens and tissue egg counts from the challenge were significantly reduced in this group. In experiments with heterologous parasites (6), large numbers of cercariae were used for immuniza- tion (10 000-30 000) and, although the worm returns were very low, these parasites, which primarily infest man, developed to the adult stage. However, baboons have been shown (7) to develop partial resistance to S. mansoni following exposure to large numbers of S. rodhaini cercariae, when no infections with adult worms were detected and no eggs were recovered. In the other experiment with homologous S. mattheei, large numbers of cercariae were used for the initial infections. These did not subsequently prevent or reduce the establishment of adult parasites from challenge infections, but apparently resulted in reduced egg laying. The relatively small numbers of cercariae used for initial exposure in the present 567 568 BRIEF COMMUNICATIONS study provided no significant protection against challenge infections, and produced only a dwarfing effect in adult worms. It appears, therefore, that the number of cercariae used in the immunizing exposure may be more important than the number of adult worms that develop. ACKNOWLEDGEMENTS The authors are grateful to Dr C. James and to the technical and animal-house staff of Winches Farm Field Station, St. Albans, Herts, England, for their assistance in the course of the work. The investigation was sup- ported by the Overseas Development Administration and by the Medical Research Council, Great Britain. REFERENCES 1. FunJNAMI, A. China med. J., 31: 81 (1917). 2. MASSOUD, J. & NELSON, G. S. Bull. Wld Hlth Org., 47: 591-600 (1972). 3. LAWRENCE, J. A. Res. Vet. Sci., 14: 400-402 (1973). 4. CHEEVER, A. W. Amer. J. trop. Med. Hyg., 17: 38-60 (1968). 5. HUSSEIN, M. F. ET AL. Bull. Wld Hlth Org., 42: 745-749 (1970). 6. PRESTON, J. M. ET AL. Bull. Wid Hith Org., 47: 587-590 (1972). 7. TAYLOR, M. G. In: The use of nuclear techniques in research and control of helminth infections. Proceed- ings of an IAEA panel meeting, Nairobi, Kenya, 26-30 November 1973. Vienna, IAEA, in press.