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SUMMARYThe parameters involved in the induction of resistance against Schistosoma mansoni by injection of irradiated, artificially transformed schistosomula were studied in mice. Single intramuscular injections of 500 schistosomula exposed to radiation doses in the range 2·3 to 160 krad. resulted in significant protection (in the range 20 to 50% as assessed by reduced worm burdens) against a challenge infection administered at intervals from 3 to 24 weeks post-vaccination. However, schistosomula irradiated with 20 krad. consistently resulted in better protection than those exposed to either higher or lower radiation doses despite the persistence of stunted adults from the infections irradiated with 2·3 krad. Vaccination with 40 krad. schistosomula resulted in significant protection in terms of reduced worm and tissue egg burdens and increased survival following lethal challenge. Varying the number of irradiated schistosomula, the frequency and route of their administration, the site of challenge and the strain of host all failed to enhance the level of resistance. However, percutaneously applied, irradiated cercariae were found to be more effective in stimulating resistance (60%) than intramuscularly injected, irradiated schistosomula (40%).

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1. O. gutturosa infections are of widespread occurrence in South-East England. 58% of 1583 cervical ligaments from milk cattle slaughtered in an abattoir at Reading, Berkshire, showed the presence of adult worms. A more detailed study of 50 cattle from a small abattoir near St. Albans in Hertfordshire showed 42 (84%) infected.2. The adult worms were found either in the connective tissue adjoining the nuchal ligament or between the spleen and rumen. There was no evidence of an increase in the parasite density with age of the cattle suggesting a strong immunity to superinfection.3. Irrespective of the situation of the adult worms the microfilariae of O. gutturosa appear to have a directional mechanism and were always concentrated in the skin around the umbilicus. This is an adaptation to transmission by Simulium ornatum which bites preferentially in the umbilical region.4. There were no marked pathological changes associated with the parasites and no evidence of clinical disease attributable to infection with O. gutturosa.

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1. At least twenty-two species of filarial worms are known to develop in mosquitoes. Filarial larvae of non-human origin must often be present in mosquitoes dissected during filariasis surveys yet the larvae seen are commonly assumed to be of human origin.2. On Pate island off the Kenya coast where bancroftian filariasis is prevalent, several distinct species of infective larvae were found in mosquitoes both in the houses and in the bush. Seven species of filarial worms were found in the local animals and a collection of known infective larvae was made by feeding mosquitoes on these different infections. A study of the material in this “reference collection” showed that many of the larvae in “wild” mosquitoes could be identified with confidence.3. A technique has been developed for the dissection of preserved and stained mosquitoes; this method has many advantages and can be used in routine filariasis surveys.4. The characters which were most useful in differentiating the infective larvae were: the length; the position of the anus; and the shape of the caudal extremity. Eight species of infective larvae were available for study, these are described and illustrated: W. bancrofti, W. malayi, W. pahangi, W. patei, D. corynodes, D. repens, D. immitis and S. equina. The literature dealing with a further fourteen species is reviewed.5. A simple key has been produced to help identify the species of infective larvae found in mosquitoes.6. On the Kenya coast W. bancrofti infective larvae have been found only in C. fatigans, A. gambiae and A. funestus. Infective larvae of animal origin which might easily be mistaken for those of W. bancrofti have been found in Ae. pembaensis, Ae. aegypti, M. uniformis and M. africanus.7. Formulae for transmission indices based on infection rates in insect vectors are of little value unless there is confidence in specific identification.8. A study of the infective larvae in Simulium neavei from an onchocerciasis area in Uganda has shown that not all the larvae are of human origin. A closer examination of the morphology of infective larvae in the vectors of filarial parasites of man in other areas will almost certainly show that the interpretation of infection rates may be complicated by the presence of infections of nonhuman origin.9. Feeding experiments were carried out with Aedes pembaensis and Ac. aegypti on a donkey infected with Setaria equina; both of these mosquitoes proved to be suitable vectors of this filarial worm.