The cercaria and sporocyst of Bucephaloides gracilescens are described from Abra alba (Wood). Observations were made on the behaviour of the cercaria and the mechanism of release from the first intermediate host. The metacercaria was obtained experimentally for the first time using Ciliata mustela (L.), a species of Gadidae from rock pools, as second intermediate host. It has not previously been recorded from this fish. Unsuccessful attempts were made to infect species of fishes from three other families, namely, Bothidae, Pleuronectidae and Gobiidae, confirming the high degree of host specificity of the metacercaria to Gadidae. The metacercaria, its development and effect on the host are briefly discussed. It was linked with the adult on the basis of comparative morphology and ecology of the hosts.

Previously immunized rats of different ages were exposed either to one large, or several small challenge infections with the nematode, Nippostrongylus brasiliensis. The number of secondary worms harboured by each of these rats was then determined on the fourteenth day after exposure to challenge. It was found that worms adapted more efficiently to host immunity when both (a) the rats were less than 11 weeks old when challenged and (b) the challenge was given in small repeated doses over a period of weeks. The age of the rats on exposure to the immunizing primary infection, however, had little or no affect per se on the subsequent establishment of immune-adapted worms. It is tentatively suggested that the failure of worms to adapt effectively to the acquired immunity of older rats resulted from the acquisition by these rats of a fully competent cellular immune response.

The malaria parasites, Plasmodium berghei yoelii and P. vinckei chabaudi, run comparable courses of infection in mice. When mice were treated with betamethasone infections with P. v. chabaudi were enhanced while those of P. b. yoelii were depressed. Antibody levels in betamethasone-treated mice infected with either P. b. yoelii or P. v. chabaudi were depressed to similar extents. The differences in parasitaemias in the two different infections are attributed to the suppression of reticulocyte production by betamethasone. P. b. yoelii preferentially invades reticulocytes and the patterns of parasitaemia follow the level of reticulocytes in the blood. P. v. chabaudi has no preference for reticulocytes and the pattern of parasitaemia is influenced by the depressed immunological response of the host.

Most successful parasitoids are able to avoid the encapsulation reaction of their host. A similar avoidance was observed with particles of a weakly acidic cation exchanger which was not encapsulated following its injection into the haemocoele of an insect. The ionic nature of the surface of the acidic cation exchanger and the two species of parasitoid eggs appeared to be similar. A weakly basic anion exchanger was readily encapsulated following injections into the haemocoele of a host. The encapsulation of the basic anion exchanger was prevented by coating the surface with fluid from the calyx region of the female parasitoid ovary. The importance of the fluid from the calyx region of the parasitoid and the ionic nature of the surface in avoiding the encapsulation reaction of the hosts haemocytes is discussed.

Immunity to Eimeria maxima, judged by oocyst production resulting from a low challenge inoculum, was evident when the challenge was given as early as 3 days after the immunizing dose of 250 or 500 oocysts. Immunity was complete, i.e. no oocysts were produced, to challenge at 7 days after an initial inoculum of 500 oocysts, or at 14 days after 250 oocysts. Judged by the above criteria 250 oocysts of E. tenella did not induce immunity to challenge given at 3, 5, 7 or 14 days. The findings are discussed.

The growth rates of the intramolluscan stages of the parasite Fasciola hepatica were determined at three temperatures 16°, 20° and 25°C. A graphical relationship of growth rate with respect to temperature was produced and a growth array was calculated from this. Meteorological data were summarized in the form of a temperature array. These arrays were then used in conjunction with a simple equation to simulate parasite growth within the snail host. A digital computer was employed to solve iteratively the simulation equation, initially by checking its application against experimentally determined results. It was then employed with the meteorological data to simulate growth with respect to environmental temperatures and the results obtained were compared with those derived from field collections. Estimates were also made of the duration of the shortest life-cycle under field conditions.

The tegumentary ultrastructure of Leucochloridiomorpha constantiae metacercariae and adults has been described. A filamentous glycocalyx invests the tegument of metacercariae and large numbers of biconcave, disk-shaped secretion vesicles are found in its outer zone, cytoplasmic bridges and cytons. Mitochondria within the outer tegumentary zone of metacercariae are restricted to its basal half. The transformation of metacercariae to adults involves the loss of the glycocalyx, a decrease in the number of disk-shaped secretion vesicles, a more uniform distribution of mitochondria through the outer tegumentary zone, an apparent degeneration of cytons, a thinning of the fibrous basal lamina and a decrease in the number of visible cytoplasmic bridges traversing it. The significance of these events for a metacercaria which does not encyst within the molluscan host is discussed. In addition, crystalline, spine-like inclusions in the outer tegumentary zone of adult L. constantiae are described and their possible function discussed.

A substituted azauracil derivative has been shown to have marked anticoccidial activity. Chickens given large challenge doses of Eimeria tenella were protected from mortality by a dietary level of 0·25 ppm drug, while oocyst output was controlled by 10 ppm. Under conditions of continuous medication using 15 ppm of the azauracil derivative the development of E. tenella was arrested at the multinucleate first generation schizont stage; in the majority of parasites merozoite differentiation was inhibited. Experiments using restricted medication indicated that, with E. tenella in the chick, the drug was coccidiocidal rather than coccidiostatic, and that treatment for as little as one day could control the infection. Medication started during the second asexual cycle again resulted in inhibition at the multinucleate schizont stage just before merozoite differentiation. Mortality in birds inoculated with E. brunetti was controlled by 1 ppm azauracil in the diet and oocyst output by 2.5 ppm; unlike the situation with E. tenella, the drug had no influence on the first two asexual cycles of reproduction. There was some inhibition of the third schizogony cycle, but the main effect of the drug in the case of E. brunetti was found to be against sexual reproduction. Effects similar to those noted in vivo were seen with E. tenella and E. brunetti growing in tissue culture. Occasional schizonts of E. tenella under the influence of drug were able to produce a peripheral array of merozoites surrounding a multinucleate residual mass. In contrast to the situation in vivo, with drawal of drug from tissue cultures infected with E. tenella allowed the further development of some inhibited parasites. E. necatrix responded to the azauracil derivative in a manner similar to E. tenella.

An outbreak of the Microsporidian parasite Thelohania contejeani is reported in a population of Austropotamobius pallipes in Northumberland, U.K. Since less than 10% of all crayfish caught were infected it is concluded that T. contejeani has no serious effect on the population studied. The mode of transmission of the parasite from host to host is discussed.

The host–parasite relationship and life-cycle of T. contejeani were studied using histological and electron microscope techniques. Spores were observed in some detail, and diplocarya, sporont and pansporoblast stages were identified within the general spore mass that invades host muscle tissue. Using serial sections the sporoblast ‘diplocaryon’ was shown to be a single nucleus with a deep cleft. T. contejeani was found to occur mainly in striated muscle, although it was found in cell bodies in the central nervous system.

Experiments to study the mortality rate of the acanthocephalan Pomphorhynchus laevis in goldfish are described. Infections with different population densities of parasite were given to fish on one occasion only and on several occasions with different time intervals between infection.

The parasites established and grew in the experimental hosts at rates comparable to those in natural hosts on all occasions. The proportion establishing did not decrease with increasing population size and bore no relationship to the presence of an existing infection. The greatest loss of parasites occurred during the period of establishment. Thereafter, mortality was gradual, and the proportion of the population lost was not related to either the density of infection or to the intervals between infections. No evidence of reduced growth rate was found at the higher population densities.

It is concluded that parasite mortality does not function as a feedback control and so is relatively unimportant in regulating the population size of fish acanthocephalans. Both field and laboratory evidence indicate that this is controlled by factors influencing recruitment. The equation of mortality and recruitment rates observed in many field populations must therefore be a matter of chance, and the implications of this with respect to the stability of fish-acanthocephalan systems is discussed.