Previous studies using radioactive miracidia have shown that a number of non-host snails and bivalves, interposed as ‘decoys’ in linear test channels, may interfere with the capacity of Fasciola hepatica miracidia to infect Lymnaea truncatula. Applying similar experimental principles, the role of several other potential interferents have been analysed in the present study. Daphnia pulex (Cladocera) and larvae of Corethra sp. (Diptera) exercised significant interfering effects by protecting ‘target’ snails from infection. Evidence suggested that this effect was a result of their normal predatory behaviour. Other organisms including Herpobdella testacea and Helobdella stagnalis (Hirudinea), Acellus aquaticus (Isopoda), Planaria lugubris (Turbellaria) and L. Truncatula egg clusters failed to interfere with miracidial host-finding. Nor did P. lugubris and L. truncatula ‘conditioned water’ interfere with the capacity of the miracidia to infect their host snail.

A comparison was made of the ultrastructure, development and antigenic nature of the surfaces and of the viability of three types of schistosomula of Schistosoma mansoni: schistosomula formed afrer cercariae had penetrated isolated skin (SS), schistosomula produced after mechanical separation of cercarial tails from bodies (MS), and schistosomula transformed from cercariae after incubation in fresh rat serum (RS).

Within 2 h of transformation, the surface membrane of all three types of schistosomula had changed from trilaminate to heptalaminate structures and SS and MS had lost their cercarial glycocalyx. Initially a dense amorphous material was demonstrated on the surfaces of RS, which was thought to be the result of an interaction between a factor in rat serum and the glycocalyx: this material was greatly reduced within 2 h of transformation. The pre-acetabular glands of SS were emptied while those of MS and RS retained their contents. Immunofluorescent studies showed that all schistosomula bound serum from mice immune to S. mansoni, but the binding was stronger with MS and RS. The mixed agglutination reaction demonstrated the presence of human A and B blood group-like antigenic determinants on approximately 30% of 3 h old SS; these determinants were not detected on MS or RS. In vitro, the development of MS and RS was similar to SS; the first schistosomula reached the ‘gut-closed’ stage by day 10; 50–70% of SS reached this stage by day 12, in contrast to only 25–50% of MS and RS. Between 28 and 45% of all schistosomula developed to maturity when injected intravenously into mice.

It was concluded that the two types of artificially prepared schistosomula fultil the main criteria of transformation from cercaria to schistosomulum. Further, it is suggested that MS are the most appropriate source of material for immunochemical and physiological studies.

A Babesia-free batch of laboratory reared Boophilus decoloratus ticks was infected with a Kenyan strain of Babesia bigemina by feeding them on a steer infected by inoculation of a blood stabilate. The engorged female ticks showed developing stages of B. bigemina in their haemolymph and subsequently their progeny transmitted the parasite to a susceptible splenectomized steer.

Attempts were made to produce stabilates from pre-fed larvae and nymphs derived from infected batches of B. decoloratus. Only the stabilates derived from the nymphs pre-fed either on cattle or rabbits produced B. bigemina infection when inoculated intravenously into susceptible cattle.

The nature of antigens present on the surface coat of Trypanosoma brucei brucei (serotype AnTat–1) trypanosomes has been investigated by light and electron microscopy using indirect immunofluorescence and immunoenzymatic labelling techniques. The use of a monospecific anticomponent V serum has shown that the variant-specific antigen, component V, is the major constituent of the surface coat. Antigens of heterologous serotype, of culture form trypanosomes, of host serum proteins or host red blood cells were not demonstrated on the surface coat.

On parasitization with larval Brugia pahangi the infected flight muscle fibres of ‘resistant’ Anopheles labranchiae atroparvus undergo the following ultrastructural changes. The fibres become almost totally devoid of glycogen, their sarcoplasmic reticulum becomes elongate and closely associated with muscle fibrils. These fibrils degenerate and vesicles appear both within the degenerate fibril and within elements of the sarcoplasmic reticulum. Vesicles accumulate around the worm and degenerate to a uniform mass which eventually becomes melanized from its inner edge (next to the parasite) outwards.

The infected flight muscle fibres of both ‘resistant’ Aedes aegypti and ‘susceptible’ Aedes togoi are almost totally devoid of glycogen granules, but show no other ultrastructural change from the uninfected state.

Five normal rhesus monkeys were infected with Plasmodium knowlesi sporozoites (A-strain); two developed rapidly fatal malaria and three chrinic relapsing infections. Vaccination with P. knowlesi (W-strain) merozoites (unmodified or formol-treated and freeze-dried) in Freund's complete adjuvant (FCA) did not inhibit pre-erythrocytic parasite development after challenge with A-strain sporozoites. However, the subsequent blood-stage infection was terminated in nine out of ten vaccinated monkeys even though the challenge strain was different form that used for vaccination. The degree of parasitaemia (0·01–0·70 %) and brevity of infection (1–12 days) in six animals vaccinated with untreated merzoites was similar to that observed after direct challenge with blood-stage parasites. Monkeys were equally resistant to sporozoite challenge given as the post-vaccination infection or administered 6 months after blood challenge. These results are discussed in relation to the development of a human malaria vaccine.

An initial observation converning the failure of [3H]thymidine at high specific activity to be incorporated into the DNA of Crithidia fasciculata for more than a brief intitial period has been correlated with the presence at high specific activity in the organism of thymidine phosphorylase activity with an equilibrium in the direction of catabolism. This enzyme degrades thymidine to thymine which is not utilized by the organism. The enzyme has also been shown to be present in a number of other trypanosomatids, including the culture forms of Trypanosoma cruzi, where the specific activity was nearly as high as that in C. fasciculata.

Evidence is presented that in C. fasciculata, the culture forms of T. cruzi and possibly other species of trypanosomatid, the thymidine phosphorylase, together with a thymidylate phosphatase, forms a catabolic pathway which degrades thymine nucleotides to thymine, which is then excreated. About 60% of the thymine nucleotides made by organisms appear to be matabolized through the pathway, suggesting that their synthesis is not subject to completely effective regulatory control.

The humoral immune response of plaice to infestations of selected metacercariae was studied. Precipitating antibodies to both Cryptocotyle lingua and Rhipidocotyle johnstonei were demonstrated using agar geldiffusion techniques. Immunochemical investigations of these antibodies indicated they were macroglobulins, resembling immunaglobulin M(IgM) of mammals. The rate and magnitude of antibody production by the host was determined by the ambient temperature

The response of intestinal host-cell nuclei of calves infected with Eimeria zuernii 6 and 8 days post-infection was examined using Feulgen-DNA microspectrophotometry. The results show that nuclear hypertrophy is dissociated from DNA replication. In the light of previous work (Fernando, Pasternak, Barrell & Stockdale, 1974) it is surmised that the specificity of infection of cells by E. zuernii is not stringent, with the major target being non-proliferative cells. At most, 20% of the first-generation schizonts develop within cells that would have had proliferative potential and as a result some non-scheduled DNA synthesis occurs.

Experiments are described which support the view that, in Eimeria maxima, drug resistance factors are transferred at zygote formation. The phenomenon occurred between infections which were initiated by the inoculation of merozoites. It also took place between variants of the same, but not between different species. Transference of drug resistance wsa not affected by the treatment of the chicks with acriflavine.

Wistar-Furth strain male rats and CFW strain male mice were immunized against Trichinella spiralis using an antegenic fraction derived from a cell-free homogenate of mature muscle larvae.

In rats, animlas immunized with 250 μg of antigen harboured singnificantly fewer (135000) muscle larvae 30 days after oral challenge than controls (231000). Furhter 7-day-old adult worms derived from immunized rats shed 48% fewer (P < 0·001) newborn larvae over a 24 h period in vitro than adult worms from non-immunized animals.

Mice were injected with either 10 or 100 μg of antigen. In comparison with non-immunized controls, mice immunized with 100μg of antigen harboured significantly fewer adult worms at days 7 and 9 after oral challenge infection, while female worms recovered from immune mice on days 6–10 after challenge shed fewer newborn larvae in vitro. Finally, mice immunized with 100μg of antigen harboured significantly fewer (10391) muscle larvae at 30 days after challenge than did controls (47750).

Immunization of mice with 10μ of antigen did not induce a statistically significant reduction in adult worms at either days 7 or 9 after challenge (P < 0·5). However, adult female worms from mice receiving 10μg of antigen still shed fewer larvae than did adults from control mice (P < 0·05). Mice immunized with 10μg of antigen harboured significantly fewer (13700) recoverable muscle larvae than did controls at 30 days after challenge (39000).