G. Webbe

Immunisation of sheep against Schistosoma mattheei using either irradiated cercariae or irradiated schistosomula.

Irradiated cercariae, irradiated schistosomula, or heterologous infections were used to vaccinate sheep against Schistosoma mattheei infection. In the first experiment four doses of 10(4) S. mattheei cercariae irradiated at 6Kr were administered to sheep by percutaneous infection at 4 week intervals. This induced a 74% reduction in a challenge infection compared to control sheep while only 13% protection was achieved in a third group of sheep immunised with normal cercariae of the heterologous parasite S. mansoni. No significant differences were seen in histopathology of the liver of any of the sheep but the pathological changes were more severe in the large and small intestines of sheep vaccinated with the heterologous parasite. In the second experiment with irradiated cercariae only one or two immunising exposures were used. The degree of protection in the adult worm load (9-11%) was not significant and no significant differences were noticed in the pathology of the vaccinated and control animals. In the third experiment four doses of irradiated organisms were used to vaccinate five groups of sheep: 3Kr or 6Kr cercariae were administered by percutaneous infection; 6Kr skin-transformed schistosomula were administered by intra muscular injection; the same 6Kr skin-transformed schistosomula were given by intravenous injection and 6Kr syringe transformed schistosomula were administered by intramuscular injection. The degree of protection (determined as the reduction in worm burden) achieved by these different procedures was respectively 72%, 61%, 77%, 56% and 78%. These results indicate the possibility of making a live vaccine against ovine schistosomiasis and show that effective immunisation is not dependent on the presence of a mature worm infection or on cercarial penetration of the skin by the immunising infection.

Immunisation of baboons against Schistosoma mansoni using irradiated S. mansoni cercariae and schistosomula and non-irradiated S. rodhaini cercariae.

In an attempt to develop a non-pathogenic procedure for immunising baboons against S. mansoni, groups of five baboons were exposed to three doses of 5000 6 Kr-irradiated S. mansoni cercariae or to similar numbers of normal S. rodhaini cercariae and challenged at week 15 with 500 normal S. mansoni cercariae. Faecal egg counts, worm and tissue egg counts, and histopathological examination, showed that neither of the immunising schedules had produced significant protection. In the second experiment baboons were injected by the intramuscular route with 31000 schistosomula of S. mansoni in three doses and the irradiation dose was reduced to near the minimum required for worm sterilisation (2-1--2-4 Kr). Challenge with 3500 normal cercariae of S. mansoni 21 weeks after the first immunising dose again showed no significant protection, although reductions of 20--30% were found in egg and worm counts resulting from the challenge. These results indicate that it may be difficult to develop an effective live vaccine for S. mansoni unless the antigenicity of the immunising larvae can be greatly increased.

A comparison of Sudanese and South African strains of Schistosoma haematobium.

The worm recoveries of two different strains of Schistosoma haematobium from male hamsters and their egg-laying capacities are studied. The infection rates obtained, the growth of worms and the distribution of eggs in hamster tissues are compared with those previously described for other strains of S. haematobium . Previous studies have shown that different geographical strains of Schistosoma haematobium have specific characteristics which reflect the infra-specific variations of this complex species (Webbe and James, 1971a, b; James and Webbe, 1973). The present paper records further observations on two strains of the parasite from the Sudan (Khartoum) and South Africa (Nelspruit).

A comparison of Egyptian and East African strains of Schistosoma haematobium.

The worm recoveries of two strains of S. haematobium from male hamsters and their egg laying capacities are studied. The infection rates obtained, the size of worms, and the number of testes and the distribution of eggs in hamster tissues are compared with those described for other strains of S. haematobium . It is emphasized that infra-specific variations in the host may also, in part, account for many of the differences that are attributed to different geographical strains of the parasite.

The effect of ultra-violet radiation on cercariae of Schistosoma mansoni and Schistosoma haematobium.

It was shown that ultra-violet radiation has a damaging effect on cercariae of Schistosoma mansoni and S. haematobium which are similarly influenced. Radiation of cercariae for intervals as short as 5–20 seconds markedly increased their mortality during penetration of host skin and inhibited migration of schistosomula in the lungs beyond days 3–4 post-infection. No mature adult worms were recovered from the liver and mesenteric veins of animals infected with irradiated cercariae. The practical significance of the cercaricidal property of ultra-violet radiation is discussed.

The significance of infra-specific variations of hosts and parasites in the epidemiology of helminths of medical importance.

The epidemiology of parasitic infections may be profoundly influenced by infra-specific variations in infectivity for either intermediate or definitive hosts. Such variations will decide the basic ecology of the host-parasite relationship, and variations in virulence and pathogenicity therefore determine different patterns of disease. While some of the evidence of infra-specific variations has been derived from experimental studies, much of it has been recorded by those concerned with investigations of the epidemiology and transmission of parasitic infections and the practical aspects of their control. The unequivocal demonstration of genetic heterogeneity of certain parasites of medical importance has therefore resulted in a better understanding of their epidemiology and, in some cases, is leading to greater precision in attempts to control them.

A comparative study of the death of schistosomula of Schistosoma haematobium and Schistosoma mansoni in the skin of mice and hamsters

This study shows that some cercariae of S. haematobium and S. mansoni die during penetration of mouse or hamster skin. Approximately 30-38% of cercariae of both species die in mouse skin and 14-16% die in hamster skin. The greater number of cercariae which die in the skin of mice seems to account for the higher yield of adult worms recovered in hamsters. Adult worm recoveries from animals infected with S. haematobium were, however, only about half the worm recoveries from hosts infected with S. mansoni.