Journal of Helmhuhology (1975) 49, 153—159
The effect of ultra-violet radiation on cercariae of Schistosoma mansoni and Schistosoma haematobium* A. M. GHANDOUR** and G. WEBBE London School of Hygiene and Tropical Medicine, Winches Farm Field Station, St. Albans
ABSTRACT 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.
Radiation of different wavelengths causes severe damage to a wide range of living organisms. This damage may be associated with changes in normal physiological processes and, at high levels of radiation, with cytological interference and eventual death. Ultraviolet radiation is similar in its action on living organisms to X-rays or gamma radiation, and different stages of helminths are adversely influenced by it. The effects of U.V. radiation on eggs of helminths have been studied by several workers (Nolf, 1932; Jones et al.t 1940; Jones and Hollaender, 1942). Stowens (1942) observed that exposure to U.V. radiation interfered with the normal development of Trichinella spiralis larvae in mice, and that female worms which developed after small doses of radiation were sterile. Keeling (1960) showed that larvae of Nippostrongylus muris were completely immobilised when exposed to U.V. radiation at a distance of 0*5 cm for four minutes. Larvae which received smaller doses of radiation and which appeared normal were incapable of reaching maturity in rats. Schistosome cercariae were found to be very sensitive to U.V. radiation (Krakower, 1940; Tomberg and Lagrange, 1952). Standen and Fuller (1959) have demonstrated that exposure of cercariae of S. mansoni to U.V. radiation resulted in inhibition of their development to the adult stage in mice exposed to them. No similar studies were conducted for S. haematobium. The present work was designed to find the sites and identify possible mechanisms of failure of irradiated cercariae of S. mansoni and S. haematobium during their migration pathways in the definitive host. MATERIALS AND METHODS A Puerto Rican strain of S. mansoni was used in the present study and was maintained in Biomphalaria glabrata (Say) and white mice of the T.O. strain. S. haematobium from the Sudan was supplied by Dr. M. A. Amin and was cycled in Bulinus (Bulinus) truncatus (Audouin) and Syrian hamsters. *Part of a Ph.D. thesis accepted by the University of London **Present address: Department of Zoology, Faculty of Science, University ofKIiartoum, Kliartoum, Sudan 153
A. M. GHANDOUR and G. WEBBE
A Hanovia U.V. lamp (with a mercury-vapour U-shaped bulb) emitting wavelengths in the region of 2537 A was used to irradiate freshly shed cercariae of S. mansoni and S. haematobium. Before use, the lamp was switched on for 10-15 minutes in order to ensure its maximum output (Stiff, 1971). The distance of the lamp from the cercarial suspension was controlled by an adjustable head and kept constant at 16*5 cm. The cercariae were irradiated by placing them directly under the lamp and exposures were timed by a stopwatch. The cercarial suspension never exceeded 10 ml in volume since U.V. radiation does not penetrate well in liquid media. Four experiments were conducted and each was repeated: (a) First, it was decided to investigate the effects of radiation on the viability and behaviour of cercariae of S. mansoni and £. haematobium. Embryo dishes, containing 80-100 cercariae in 5 ml of suspension, were exposed to radiation for 5-60 seconds and subsequently observed for some time. At all exposures an embryo dish containing normal non-irradiated cercariae was set up as a control. (b) The second experiment was designed to compare the death of irradiated (5-20 sec.) and normal non-irradiated cercariae during penetration of host skin. Schistosomula were recovered from host skin (after exposure to 1000-2000 cercariae/animal for 15 minutes), as described by Clegg and Smithers (1968) and by Ghandour and Webbe (1973). (c) The migration of irradiated (10 sees.) and non-irradiated cercariae to the lungs of infected animals was followed by making lung recoveries of schistosomula on days 3, 5, 7 and 10 for S. mansoni and on days 4, 6, 9 and 11 for S. haematobium, and by histological examinations. Lung recoveries were made, as described by Clegg (1965) from groups of 20 animals each infected with 300-500 cercariae. (d) A combined experiment was set up to compare the mortality of irradiated (10 sees.) and non-irradiated cercariae, during penetration of host skin, with the recovery of adult worms from the liver and portal system. Adult worm recovery was estimated from groups of 20 animals each infected with 80-100 cercariae as described by Smithers and Terry (1965).
RESULTS Effects of radiation on the viability and behaviour of cercariae: Cercariae of S. mansoni and S. haematobium were severely damaged following 30-60 second exposures to radiation. They immediately became sluggish and gathered in the centre of the dishes, moving very slowly. Eversion of the ventral sucker, loss of the tail and complete cessation of nephridial movements soon occurred (1-3 minutes), indicating death of the cercariae. However, exposures at 5-20 sees, seemed to have no apparent effect on them. Death of cercariae during penetration oj host skin: The results of this experiment are shown in Figs, la and 2a. The mortality of S. mansoni cercariae during penetration of mouse skin corresponded to that obtained by Ghandour and Webbe (1973), when normal nonirradiated cercariae were used (30-4%). Mortalities increased however to 42-4%, 50-1%, 58-6% and 68-4% when the cercariae were exposed to ultra-violet radiation for 5, 10, 15 and 20 sees, respectively. There is a significant difference between the mortalities of normal non-irradiated cercariae and those of irradiated cercariae observed at all exposures (p 10*7% and 3*1%, while none were recovered from the irradiated group. Comparison of death of cercariae in the skin with the recovery of adult worms: Analysis of the results of this experiment (Figs, lc and 2c) shows that no adult worms were recovered from animals infected with irradiated cercariae of S. mansoni or S. haematobium. This was partly due to the increased mortality of irradiated cercariae of both species (524% and 28*9% respectively) in host skin during penetration. The mean percentage recoveries of adult worms from animals infected with non-irradiated cercariae of S. mansoni and S. haematobium were quite normal: up to 33-9% and 16*9% respectively were noted. Those recoveries correlate well with the levels of mortality of cercariae in the skin (30*5% and 16*5% respectively) and correspond to results obtained in previous experiments using S. mansoni in mice (Ghandour and Webbe, 1973). Ultra-violet radiation affected the ability of S. mansoni cercariae to penetrate host skin: only 2% of normal non-irradiated cercariae did not penetrate while up to 10% of irradiated cercariae failed to penetrate host skin. A similar result was obtained with S. haematobium: 3% and 11% of non-irradiated and irradiated cercariae respectively failed to penetrate host skin. •T
DISCUSSION It is considered that this study clearly establishes the inimical effects of ultra-violet radiation on cercariae of S. mansoni and S. haematobium. Most of the damage to cercariae became apparent during penetration of host skin. The levels of mortality of non-irradiated cercariae of S. mansoni (304%) and S. haematobium (19*3 %) were quite normal and similar to results obtained in other experiments (Ghandour and Webbe, 1973). The mortality of irradiated cercariae however steadily increased until high levels of 684% (S. mansoni) and 52*8% (S. haematobium) at 20-secs. exposures were reached. The subsequent lung migration of irradiated schistosomula did not proceed beyond days three to four postinfection. Most of the schistosomula that passed undamaged through the skin apparently died en route to the lungs. This was revealed by the fact that, in animals infected with irradiated cercariae, no living schistosomula were detected by either the direct recovery technique or by histological means. In contrast, the lung migration of non-irradiated 157
A. M. GHANDOUR and G. WEBBE
schistosomula of S. mansoni was normal in all respects and confirmed results obtained by Lichtenberg and Sadun (1963) and Clegg (1965) who found that migration of S. mansoni in the lungs of mice followed a general pattern whereby peak concentrations were attained on day seven post-infection with smaller recoveries on either side of the peak. Sher et al. (1973) obtained similar results with S. mansoni except that the peak concentration was on days four to six post-infection, depending on the strain of mice used. The lung migration of normal non-irradiated schistosomula of S. haematobium followed the same pattern as that of 5. mansoni but took longer to attain the peak concentration, on day nine post-infection. The results of this study show that irradiation of cercariae of S. mansoni and S., haematobium for intervals as short as 10 sees, inhibited their development to the adultrstage. These results confirm the observations of Standen and Fuller (1959) on S. mansoni cercariae. In general, the intensity of U.V. radiation used in the present study seems to have damaging effects on cercariae of S. mansoni and S. haematobium similar to those produced by high doses of other kinds of radiation. Hsu and Hsu (1963) and Lichtenberg and Sadun (1963) showed that exposure of cercariae of S. mansoni to 48,000 r from X-ray and gamma ray sources, respectively, prevented them from migrating from the skin, where they eventually died. The activity of U.V. radiation against viruses, bacteria and fungi reaches a maximum at about 2650 A (Hollaender, 1955). This wavelength is also at a peak of the absorption spectrum for nucleic acids. Keeling (I960) suggested that wavelengths of 2650 A and higher are effective through their action on the nuclear components which are vital in the duplication of chromosal structures during growth. Ellis and Wells (1941) found that U.V. radiation inhibits the action of many enzymes including those concerned with oxidation and digestion. It is possible that schistosome cercariae may be similarly affected by U.V. radiation thus explaining their failure to develop to maturity in mice. The cercaricidal action of natural U.V. radiation in waterbodies might reduce cercarial contamination in surface waters but is likely to have little effect at depths greater than 15 cm since radiation penetrates only the top layers of still clear water and is quickly absorbed in turbid conditions (Prah, 1973; Ph.D. Thesis of London University). U.V. radiation may be of value in attenuation of helminth larvae for their use in eliciting immune responses in experimental models. ACKNOWLEDGEMENTS We arc very grateful to Drs. C. James and M. G. Taylor for reading the draft manuscript and for useful discussion. We should like to thank the technical staff at Winches Farm Field Station, particularly Mr. E. J. Blackie and Mrs. N. Robson, for their help. REFERENCES CLEGG, J. A. (1965) In vitro cultivation of Schistosoma mansoni. Experimental Parasltology, 16, 133-147. CLEGG, J. A. and SMITHERS, S. R. (1968) Death of schistosome cercariae during penetration of the skin. II. Penetration of mammalian skin by Schistosoma mansoni. Parasitology, 58, 111-128. ELLIS, C. and WELLS, A. A. (1941) TJie chemical action of ultra-violet rays. Rcinhold Publishing Co., New York. CHANDOUR, A. M. and WEBBE, G. (1973) A study of the death of Schistosoma mansoni cercariae during penetration of mammalian host skin: the effects of the ages of the cercariae and the host. InternationalJournal for Parasitology, 3, 789-794. HOLLAENDER, A. E. (1955) Radiation Biology, Vol. II. Ultra-violet and related radiations. New York, McGraw-Hill. 158
Effec£of ultra-violet radiation on schistosome cercariae HSU, H. F. and HSU, S. Y. (1963) Histopathology in albino mice and rhesus monkeys infected with irradiated cercariae of Schistosoma mansoni. Zeitschrlft fur Tropenmedizin laid Parasitologie, 14, 240-261. JONES, M. F., JACOBS, L. and HOLLAENDER, A. (1940) The effects of monochromatic ultra-violet irradiation on eggs of the nematode Enterobius vermicularis. II. Sublethal effects. Journal of Parasitology, 26,435-445. JONES, M. F and HOLLAENDER, A. (1942) The effect of long ultra-violet and near visible radiation on the eggs of the nematodes Enterobius vermicularis and Ascaris lumbricoides. Journal ofParasitology, 28,17-18 (suppl.). KEELING, J. E. D. (1960) The effects of ultra-violet radiation on Nippostrongylus muris. I. Irradiation of infective larvae: lethal and sub-lethal effects. Annals of Tropical Medicine and Parasitology, 54, 182-191. KRAKOWER, C. A. (1940) Some observations on the effects of physical and chemical agents on the cercariae of Schistosoma mansoni. Puerto Rico. Journal of Public Health and Tropical Medicine, 16,26-44. LICHTENBERG, F. and SADUN, E. H. (1963). Parasite migration and host reaction in mice exposed to irradiated cercariae of Schistosoma mansoni. Experimental Parasitology, 13, 256-265. NOLF, L. O. (1932) Experimental studies on certain factors influencing the development and viability of ova of the human trichuris as compared with those of the human ascaris. American Journal of Hygiene, 16, 288-322. SHER, A., HEREZ, H. and MACKENZIE, P. E. (1973) The recovery of young schistosomes from the lungs: a new and rapid assay for immunity. Parasitology, 67. SMITHERS, S. R. and TERRY, R. J. (1965) The infection of laboratory hosts with cercariae of Schistosoma mansoni and the recovery of the adult worms. Parasitology, 55, 695-700. STANDEN, O. D. and FULLER, K. A. (1959) Ultra-violet irradiation of the cercariae of Schistosoma mansoni. Inhibition of development to the adult stage. Transactions of the Royal Society of Tropical Medicine and Hygiene, 53, 372-379. STIFF, F. R. (1971) The disinfection of industrial and potable water supplies using ultra-violet irradiation. Chemistry and Industry, 116-120. STOWENS, D. (1942) The effect of ultra-violet irradiation on Trichinella spiralis. American Journal of Hygiene, 36, 264-268. TOMBERG, V. and LAGRANGE, E. (1952) Study of the action of some physical agents on planorbids and on the cercariae of bilharzia mansoni. Amiales de la Sociitd Beige da Midecine Troplcale, 32, 501-511. Accepted 13 June 1975
159
The effect of ultra-violet radiation on cercariae of Schistosoma mansoni and Schistosoma haematobium* A. M. GHANDOUR** and G. WEBBE London School of Hygiene and Tropical Medicine, Winches Farm Field Station, St. Albans
ABSTRACT 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.
Radiation of different wavelengths causes severe damage to a wide range of living organisms. This damage may be associated with changes in normal physiological processes and, at high levels of radiation, with cytological interference and eventual death. Ultraviolet radiation is similar in its action on living organisms to X-rays or gamma radiation, and different stages of helminths are adversely influenced by it. The effects of U.V. radiation on eggs of helminths have been studied by several workers (Nolf, 1932; Jones et al.t 1940; Jones and Hollaender, 1942). Stowens (1942) observed that exposure to U.V. radiation interfered with the normal development of Trichinella spiralis larvae in mice, and that female worms which developed after small doses of radiation were sterile. Keeling (1960) showed that larvae of Nippostrongylus muris were completely immobilised when exposed to U.V. radiation at a distance of 0*5 cm for four minutes. Larvae which received smaller doses of radiation and which appeared normal were incapable of reaching maturity in rats. Schistosome cercariae were found to be very sensitive to U.V. radiation (Krakower, 1940; Tomberg and Lagrange, 1952). Standen and Fuller (1959) have demonstrated that exposure of cercariae of S. mansoni to U.V. radiation resulted in inhibition of their development to the adult stage in mice exposed to them. No similar studies were conducted for S. haematobium. The present work was designed to find the sites and identify possible mechanisms of failure of irradiated cercariae of S. mansoni and S. haematobium during their migration pathways in the definitive host. MATERIALS AND METHODS A Puerto Rican strain of S. mansoni was used in the present study and was maintained in Biomphalaria glabrata (Say) and white mice of the T.O. strain. S. haematobium from the Sudan was supplied by Dr. M. A. Amin and was cycled in Bulinus (Bulinus) truncatus (Audouin) and Syrian hamsters. *Part of a Ph.D. thesis accepted by the University of London **Present address: Department of Zoology, Faculty of Science, University ofKIiartoum, Kliartoum, Sudan 153
A. M. GHANDOUR and G. WEBBE
A Hanovia U.V. lamp (with a mercury-vapour U-shaped bulb) emitting wavelengths in the region of 2537 A was used to irradiate freshly shed cercariae of S. mansoni and S. haematobium. Before use, the lamp was switched on for 10-15 minutes in order to ensure its maximum output (Stiff, 1971). The distance of the lamp from the cercarial suspension was controlled by an adjustable head and kept constant at 16*5 cm. The cercariae were irradiated by placing them directly under the lamp and exposures were timed by a stopwatch. The cercarial suspension never exceeded 10 ml in volume since U.V. radiation does not penetrate well in liquid media. Four experiments were conducted and each was repeated: (a) First, it was decided to investigate the effects of radiation on the viability and behaviour of cercariae of S. mansoni and £. haematobium. Embryo dishes, containing 80-100 cercariae in 5 ml of suspension, were exposed to radiation for 5-60 seconds and subsequently observed for some time. At all exposures an embryo dish containing normal non-irradiated cercariae was set up as a control. (b) The second experiment was designed to compare the death of irradiated (5-20 sec.) and normal non-irradiated cercariae during penetration of host skin. Schistosomula were recovered from host skin (after exposure to 1000-2000 cercariae/animal for 15 minutes), as described by Clegg and Smithers (1968) and by Ghandour and Webbe (1973). (c) The migration of irradiated (10 sees.) and non-irradiated cercariae to the lungs of infected animals was followed by making lung recoveries of schistosomula on days 3, 5, 7 and 10 for S. mansoni and on days 4, 6, 9 and 11 for S. haematobium, and by histological examinations. Lung recoveries were made, as described by Clegg (1965) from groups of 20 animals each infected with 300-500 cercariae. (d) A combined experiment was set up to compare the mortality of irradiated (10 sees.) and non-irradiated cercariae, during penetration of host skin, with the recovery of adult worms from the liver and portal system. Adult worm recovery was estimated from groups of 20 animals each infected with 80-100 cercariae as described by Smithers and Terry (1965).
RESULTS Effects of radiation on the viability and behaviour of cercariae: Cercariae of S. mansoni and S. haematobium were severely damaged following 30-60 second exposures to radiation. They immediately became sluggish and gathered in the centre of the dishes, moving very slowly. Eversion of the ventral sucker, loss of the tail and complete cessation of nephridial movements soon occurred (1-3 minutes), indicating death of the cercariae. However, exposures at 5-20 sees, seemed to have no apparent effect on them. Death of cercariae during penetration oj host skin: The results of this experiment are shown in Figs, la and 2a. The mortality of S. mansoni cercariae during penetration of mouse skin corresponded to that obtained by Ghandour and Webbe (1973), when normal nonirradiated cercariae were used (30-4%). Mortalities increased however to 42-4%, 50-1%, 58-6% and 68-4% when the cercariae were exposed to ultra-violet radiation for 5, 10, 15 and 20 sees, respectively. There is a significant difference between the mortalities of normal non-irradiated cercariae and those of irradiated cercariae observed at all exposures (p 10*7% and 3*1%, while none were recovered from the irradiated group. Comparison of death of cercariae in the skin with the recovery of adult worms: Analysis of the results of this experiment (Figs, lc and 2c) shows that no adult worms were recovered from animals infected with irradiated cercariae of S. mansoni or S. haematobium. This was partly due to the increased mortality of irradiated cercariae of both species (524% and 28*9% respectively) in host skin during penetration. The mean percentage recoveries of adult worms from animals infected with non-irradiated cercariae of S. mansoni and S. haematobium were quite normal: up to 33-9% and 16*9% respectively were noted. Those recoveries correlate well with the levels of mortality of cercariae in the skin (30*5% and 16*5% respectively) and correspond to results obtained in previous experiments using S. mansoni in mice (Ghandour and Webbe, 1973). Ultra-violet radiation affected the ability of S. mansoni cercariae to penetrate host skin: only 2% of normal non-irradiated cercariae did not penetrate while up to 10% of irradiated cercariae failed to penetrate host skin. A similar result was obtained with S. haematobium: 3% and 11% of non-irradiated and irradiated cercariae respectively failed to penetrate host skin. •T
DISCUSSION It is considered that this study clearly establishes the inimical effects of ultra-violet radiation on cercariae of S. mansoni and S. haematobium. Most of the damage to cercariae became apparent during penetration of host skin. The levels of mortality of non-irradiated cercariae of S. mansoni (304%) and S. haematobium (19*3 %) were quite normal and similar to results obtained in other experiments (Ghandour and Webbe, 1973). The mortality of irradiated cercariae however steadily increased until high levels of 684% (S. mansoni) and 52*8% (S. haematobium) at 20-secs. exposures were reached. The subsequent lung migration of irradiated schistosomula did not proceed beyond days three to four postinfection. Most of the schistosomula that passed undamaged through the skin apparently died en route to the lungs. This was revealed by the fact that, in animals infected with irradiated cercariae, no living schistosomula were detected by either the direct recovery technique or by histological means. In contrast, the lung migration of non-irradiated 157
A. M. GHANDOUR and G. WEBBE
schistosomula of S. mansoni was normal in all respects and confirmed results obtained by Lichtenberg and Sadun (1963) and Clegg (1965) who found that migration of S. mansoni in the lungs of mice followed a general pattern whereby peak concentrations were attained on day seven post-infection with smaller recoveries on either side of the peak. Sher et al. (1973) obtained similar results with S. mansoni except that the peak concentration was on days four to six post-infection, depending on the strain of mice used. The lung migration of normal non-irradiated schistosomula of S. haematobium followed the same pattern as that of 5. mansoni but took longer to attain the peak concentration, on day nine post-infection. The results of this study show that irradiation of cercariae of S. mansoni and S., haematobium for intervals as short as 10 sees, inhibited their development to the adultrstage. These results confirm the observations of Standen and Fuller (1959) on S. mansoni cercariae. In general, the intensity of U.V. radiation used in the present study seems to have damaging effects on cercariae of S. mansoni and S. haematobium similar to those produced by high doses of other kinds of radiation. Hsu and Hsu (1963) and Lichtenberg and Sadun (1963) showed that exposure of cercariae of S. mansoni to 48,000 r from X-ray and gamma ray sources, respectively, prevented them from migrating from the skin, where they eventually died. The activity of U.V. radiation against viruses, bacteria and fungi reaches a maximum at about 2650 A (Hollaender, 1955). This wavelength is also at a peak of the absorption spectrum for nucleic acids. Keeling (I960) suggested that wavelengths of 2650 A and higher are effective through their action on the nuclear components which are vital in the duplication of chromosal structures during growth. Ellis and Wells (1941) found that U.V. radiation inhibits the action of many enzymes including those concerned with oxidation and digestion. It is possible that schistosome cercariae may be similarly affected by U.V. radiation thus explaining their failure to develop to maturity in mice. The cercaricidal action of natural U.V. radiation in waterbodies might reduce cercarial contamination in surface waters but is likely to have little effect at depths greater than 15 cm since radiation penetrates only the top layers of still clear water and is quickly absorbed in turbid conditions (Prah, 1973; Ph.D. Thesis of London University). U.V. radiation may be of value in attenuation of helminth larvae for their use in eliciting immune responses in experimental models. ACKNOWLEDGEMENTS We arc very grateful to Drs. C. James and M. G. Taylor for reading the draft manuscript and for useful discussion. We should like to thank the technical staff at Winches Farm Field Station, particularly Mr. E. J. Blackie and Mrs. N. Robson, for their help. REFERENCES CLEGG, J. A. (1965) In vitro cultivation of Schistosoma mansoni. Experimental Parasltology, 16, 133-147. CLEGG, J. A. and SMITHERS, S. R. (1968) Death of schistosome cercariae during penetration of the skin. II. Penetration of mammalian skin by Schistosoma mansoni. Parasitology, 58, 111-128. ELLIS, C. and WELLS, A. A. (1941) TJie chemical action of ultra-violet rays. Rcinhold Publishing Co., New York. CHANDOUR, A. M. and WEBBE, G. (1973) A study of the death of Schistosoma mansoni cercariae during penetration of mammalian host skin: the effects of the ages of the cercariae and the host. InternationalJournal for Parasitology, 3, 789-794. HOLLAENDER, A. E. (1955) Radiation Biology, Vol. II. Ultra-violet and related radiations. New York, McGraw-Hill. 158
Effec£of ultra-violet radiation on schistosome cercariae HSU, H. F. and HSU, S. Y. (1963) Histopathology in albino mice and rhesus monkeys infected with irradiated cercariae of Schistosoma mansoni. Zeitschrlft fur Tropenmedizin laid Parasitologie, 14, 240-261. JONES, M. F., JACOBS, L. and HOLLAENDER, A. (1940) The effects of monochromatic ultra-violet irradiation on eggs of the nematode Enterobius vermicularis. II. Sublethal effects. Journal of Parasitology, 26,435-445. JONES, M. F and HOLLAENDER, A. (1942) The effect of long ultra-violet and near visible radiation on the eggs of the nematodes Enterobius vermicularis and Ascaris lumbricoides. Journal ofParasitology, 28,17-18 (suppl.). KEELING, J. E. D. (1960) The effects of ultra-violet radiation on Nippostrongylus muris. I. Irradiation of infective larvae: lethal and sub-lethal effects. Annals of Tropical Medicine and Parasitology, 54, 182-191. KRAKOWER, C. A. (1940) Some observations on the effects of physical and chemical agents on the cercariae of Schistosoma mansoni. Puerto Rico. Journal of Public Health and Tropical Medicine, 16,26-44. LICHTENBERG, F. and SADUN, E. H. (1963). Parasite migration and host reaction in mice exposed to irradiated cercariae of Schistosoma mansoni. Experimental Parasitology, 13, 256-265. NOLF, L. O. (1932) Experimental studies on certain factors influencing the development and viability of ova of the human trichuris as compared with those of the human ascaris. American Journal of Hygiene, 16, 288-322. SHER, A., HEREZ, H. and MACKENZIE, P. E. (1973) The recovery of young schistosomes from the lungs: a new and rapid assay for immunity. Parasitology, 67. SMITHERS, S. R. and TERRY, R. J. (1965) The infection of laboratory hosts with cercariae of Schistosoma mansoni and the recovery of the adult worms. Parasitology, 55, 695-700. STANDEN, O. D. and FULLER, K. A. (1959) Ultra-violet irradiation of the cercariae of Schistosoma mansoni. Inhibition of development to the adult stage. Transactions of the Royal Society of Tropical Medicine and Hygiene, 53, 372-379. STIFF, F. R. (1971) The disinfection of industrial and potable water supplies using ultra-violet irradiation. Chemistry and Industry, 116-120. STOWENS, D. (1942) The effect of ultra-violet irradiation on Trichinella spiralis. American Journal of Hygiene, 36, 264-268. TOMBERG, V. and LAGRANGE, E. (1952) Study of the action of some physical agents on planorbids and on the cercariae of bilharzia mansoni. Amiales de la Sociitd Beige da Midecine Troplcale, 32, 501-511. Accepted 13 June 1975
159
