Two simple experiments were undertaken using the viviparous ectoparasite, Gyrodactylus bullatarudis, and laboratory-reared guppies, Poecilia reticulata, whereby detailed records of the number and temporal sequence of all births and the age at death were obtained for flukes on isolated guppies. Gyrodactylus bullatarudis has an average fecundity of 1·68 offspring during its expected life-span of 4·20 days. The instantaneous birth rate is independent of generation but dependent on age. The first offspring is born approximately 1 day after the birth of the parent and subsequent offspring are born at 2–2·5 day intervals. The average instantancous birth rate (under given experimental conditions) is 0.43/parasite/day. The death rate increases exponentially with the age of the fluke and has an average value of 0.24/parasite/day. A simple deterministic model incorporating age structure and an age-dependent death rate was found to provide a good fit to the observed exponential increase in G. bullatarudis numbers. The variability observed in this host–parasite system was found to be largely a function of chance, as shown by Monte Carlo simulations of stochastic birth–death processes.

The fine structure and histochemistry of the envelopes surrounding the mature oncosphere of Hymenolepis diminuta have been investigated by transmission electron microscopy and light microscope observations of JB-4 embedded material. The fate of these envelopes during the hatching sequence in vitro has been followed by phase-contrast and scanning electron microscopy. Hatching is considered to comprise 4 stages. Stage 1 involves the mechanical breakage and removal of the shell and the outer cytoplasmic layer of the inner envelope; Stage 2 the activation of the oncosphere and the swelling of the gelatinous layer of the inner envelope; and Stage 3 the digestion and rupture of the embryophore. This is accomplished both by secretions from the penetration gland and by the action of external digestive enzymes, together with hook activity. Trypsin is more effective than amylase in digesting the embryophore. Stage 4 involves the enzymatic weakening of the gelatinous layer which helps the oncosphere to tear itself free with its hooks. Amylase is more effective than trypsin in attacking the gelatinous layer. On emergence from the gelatinous layer, the oncosphere is still enveloped by the ‘oncospheral membrane’, although this covering is soon lost. Once activated, the oncosphere is capable of completing the hatching sequence by itself, without the addition of enzymes. This process is a lengthy one, however, taking up to 2 h, and the expenditure of its glandular and energy reserves makes successful gut penetration by the oncosphere unlikely.

Clun forest sheep, worm-free from birth, were given a single dose of 20000 infective larvae of Haemonchus contortus. The total number of eggs/day in the faeces was recorded for 21 infections and data on population size, sex ratio and individual worm development were collected from 76 sheep. The female to male ratio was 1·28±0·07 (s.E.). The relation between increase in worm size and uterine egg content was linear. The number of eggs present in the uteri was found to be an accurate measure of eggs passed. It was shown that the daily faecal egg output is related to total parasite weight and is not a measure of the number of individuals present.

A rapid technique for the extraction of helminth eggs from vegetation and water is presented. The technique includes a period of continuous action centrifugation and gives recovery efficiencies of 84 % and 96 % from water and vegetation respectively.

Morphological and cytochemical aspects of the host response to almost mature exo-erythrocytic forms (EEF) of Plasmodium berghei in rat hepatocytes were studied by electron microscopy. Young stages (< 47 h) never evoked a local reaction. Two types of nearly mature EEF (47–51 h) could be distinguished, normal (EEF type I) and those that were the target of infiltrating cells (EEF type II). The latter were in a stage of early or advanced degeneration and generally exhibited increased electron density, especially in the contents of their peripheral vacuoles. Neither type of EEF exhibited detectable enzyme activity, although host cell enzymes, such as peroxidase and 5-nucleotidase, were demonstrable. However, the infected liver cell appeared permeable to ruthenium red whereas non-infected hepatocytes were not. When signs of degeneration were present, as shown by the increasing density of the cytoplasm, loss of enzyme activities occurred. The encompassing mononuclear cells were identified as true monocytes, non-monocyte-derived and monocyte-derived macrophages by their endogenous peroxidase activity. Macrophage filopodia penetrated and cleaved both hepatocyte and parasite cytoplasms. Subsequently, digestion and clearing of the remnants took place. This study clearly demonstrated that a proportion of the intra-hepatocytic EEF was destroyed by macrophages before they were able to mature completely and release their merozoites.

Promastigotes of Leishmania braziliensis panamensis absorbed the purines adenine, hypoxanthine, adenosine and inosine by a combination of diffusion and mediated components. When the uptake rates for these substrates were corrected for diffusion and compared, the purine bases adenine and hypoxanthine were transported at a significantly slower rate than the purine nucleosides adenosine and inosine. Competitive interactions among those purines tested confirmed the presence of mediated and diffusion components and suggested that three transport loci may be operating (Fig. 6). The first transport locus, designated Locus 1, transported inosine, Locus 2, the purine bases hypoxanthine and adenine and Locus 3, adenosine. In addition, adenine and hypoxanthine inhibited the uptake of one another competitively. A comparison of Ki values derived from double reciprocal plots of labelled hypoxanthine and adenine uptake in the presence of the unlabelled substrates as inhibitors suggested that adenine has a greater affinity for the transport locus.

Microfilariae of Onchocerca lienalis were obtained from the umbilical skin of naturally infected cattle and were injected into mice. Maximum numbers of microfilariae were recovered from the skin and ears of mice when using the subcutaneous route of inoculation. Microfilariae were distributed throughout the pinna of the ear but were concentrated towards the tip where histological sections showed them to be in the dermis and adipose tissue. Using the number of parasites recovered from the ears as an index of the intensity of infection it was found that inbred CBA/H T6T6 mice were one of the most susceptible of 11 strains of mice examined. No difference in susceptibility was found between male and female CBA mice of the same age, but marked differences were demonstrated between male CBA mice of different ages. After infection with 5000 microfilariae the recovery of parasites from the ears increased rapidly to a peak at day 35 when 10% of the inoculum was recovered, and thereafter declined up to day 242. Over a range of inoculation doses examined it was found that there was a direct, linear relationship between the number of microfilariae recovered from the ears and the number in the inoculated dose. CBA mice showed marked resistance to reinfection with microfilariae. Six days after challenge with a secondary infection recoveries of microfilariae from the ears were only 26% of the level in challenge controls and fell to 3% of the level of controls by day 35. It is concluded that the model of O. lienalis microfilariae in CBA mice shows considerable promise as a tool for research into immunological responses to skin-dwelling microfilariae, which are the principal cause of pathology in onchocerciasis.

At concentrations ranging from 0·1 to 10·0 mg/l, copper and zinc reduced both the longevity and infectivity of Echinoparyphium recurvatum cercariae. Concentrations of 10·0 mg/l copper and zinc in hard water reduced the time to 50% mortality of cercariae from 30·5 h to 8·5 h and 15·5 h respectively. Copper-induced effects upon cercarial infectivity were particularly severe and exposures of cercariae to 0·5 mg/l of this metal for as little as 15 min caused significant reductions in their ability to infect molluscan 2nd intermediate hosts. Water hardness had a marked influence on copper toxicity but had a much lower effect on the toxicity of zinc. Metal concentrations found to exert a profound influence on parasite transmission in the present laboratory-based study have been found to occur in natural, albeit polluted, freshwater habitats.

The level of immunity to a percutaneous cercarial challenge with Schistosoma mansoni was assayed 4–6 weeks after immunization of mice with highly irradiated (20 krad.) cercariae or schistosomula. When immunization and challenge occurred through the same skin site, resistance, particularly that which occurred in the skin, was greater than that observed when immunization and challenge occurred in different sites. The enhanced resistance is believed to be due to localized changes in the skin; 4 weeks after exposure to irradiated cercariae, abdominal skin is characterized by a thickened epidermis, changes in the ground substance and a cellular infiltration of the dermis. A convenient mouse model is described in which one or both ear pinnae are exposed to irradiated cercariae and a percutaneous challenge is given via the abdomen, thus eliminating the effects of local skin changes. In this model, the majority of the challenge infection which succumbs to the immune response appears to be killed in the skin.

The influence of the late components of complement (C) on percutaneous primary infections with Schistosoma mansoni was studied in inbred mice genetically deficient or intact in C5. Worm recoveries were diminished in C5-deficient male mice as compared to C5-intact animals. Twenty-four-day-old parasites were also shorter following growth in C5-deficient animals. At 7 weeks after infection, female schistosomes, but not male parasites, were shorter and fewer eggs/schistosome pair were deposited in the livers of C5-deficient as compared to C5-intact male mice. In addition to the presence of C5, the mouse sex was found to influence the outcome of an infection. Schistosomes showed a reduced infection rate and were relatively stunted after 24 days of growth in female mice of the two strains as compared to male mice of the corresponding strains. The results suggest that C5 plays no role in defence against a primary infection in mice.

The surface topography of the tegument of adult Schistosoma mekongi was studied by scanning electron microscopy (SEM). In comparison to other species of human schistosomes the male tegument lacks tubercles and, except in the gynecophoral canal, also lacks spines; instead the surface is composed chiefly of trabeculae of highly perforated ridges which give it the ‘spongy’ appearance. In addition, there are 3 kinds of papillae interspersed on the surface among the ridges. The first is a doughnut-shaped papilla with a central crater which is most abundant on the ventral surface of the anterior part, on the floor of the gynecophoral canal and on the dorsal-lateral aspect of the tail. The second is a pleomorphic papilla with irregular shape and size, which is scattered throughout the dorso-lateral aspect of the middle part of the body. The third type of papilla has a uniform hemispherical shape, possesses a cilium projecting from its apex and probably corresponds to the ‘sensory papilla’ found in other species. The tegument of the female differs from that of the male by having numerous short spines over the whole surface; however, the pleomorphic papillae are much fewer in number and the ridges are much less developed than those of the male tegument; complex trabeculae are absent.

Development of surface topography of cultured Schistosoma mansoni was followed in the cercariae, newly transformed and older schistosomula, and adults. Comparison with surface features of schistosomes from animal infections revealed no apparent differences in tegumental folding or morphogenesis, spines, sensory structures, or other components. Eggs produced by worm pairs grown in vitro were grossly abnormal, usually composed of loosely packed vitelline granules and lacking a shell.

We report the results of experimental work on (a) the influence of temperature on the age-dependent survival and infectivity of the mira-cidia of Schistosoma mansoni and (b) the relationship between snail age, snail size and susceptibility to infection. The death rate of miracidia declined exponentially with age where life-expectancy was maximal (approximately 16 h) at 15 °C. Infectivity also declined rapidly with larval age but, in contrast to larval survival, the rate of infection was at a maximum at 25 °C. Snail susceptibility was shown to be more closely correlated with host size rather than host age. Susceptibility declined exponentially with increased host size. Size-dependent susceptibility was shown to generate concave age-prevalence curves for infection within snail populations, where the maximum prevalence is generated in snails of intermediary age. Simple mathematical models are developed to aid estimation of larval survival and infection rates and experimental results are discussed in relation to the overall transmission success of the parasite from man to snail.

The relationship between infection success, the density of Schistosoma mansoni miracidia and the density of the snail host Biomphalaria glabrata was examined experimentally. Within laboratory infection arenas, the distribution of miracidial infections/snail was approximately uniform and the net rate of snail infection was directly positive binomial to miracidial density. In contrast, however, the rate of infection declined exponentially as snail density increased. Mathematical models are developed to aid in the estimation of infection rates and experimental results are discussed in the context of the transmission dynamics of schistosomiasis.

The paper examines the factors which generate various patterns of dispersion in the distribution of parasites within their host populations. Particular emphasis is placed on the role played by chance elements in the growth and decay of parasite populations and on the influence of different types of demographic processes. It is argued that observed distributions are dynamic, rather than static, entities generated by opposing forces, some acting to create over-dispersion and others acting to generate under-dispersion. Monte Carlo simulation experiments, based on probability models of the growth and decay of host and parasite populations, are used to study the dynamics of parasite dispersion. Attention is specifically focused on the role played by parasite-induced host mortality. It is shown that, for certain types of host–parasite associations, convex curves of mean parasite abundance in relation to age (age-intensity curves), concomitant with a decline in the degree of dispersion in the older age classes of hosts, may be evidence of the induction of host mortality by parasite infection. Empirical evidence is examined in light of this prediction. In general, however, simulation studies highlight the technical difficulties inherent in establishing clear evidence of parasite-induced host mortality from ecological studies of hosts and parasites in their natural habitats.

About 35 of Angiostrongylus cantonensis larvae became patent adults irrespective of the dose used to establish primary infections in laboratory rats. Worm growth and fecundity was depressed in infections from high compared with low doses of infective larvae. Rats infected with up to 50 A. cantonensis larvae survived infection better than rats given greater numbers of larvae. The biology of A. cantonensis surviving in laboratory rats from single infections with 50 larvae for over a year was studied. Stable worm numbers and fecundity were established 90 days after infection but the worms continued to grow until the experiment was terminated.

The coevolution of parasites and their hosts has both general biological interest and practical implications in agricultural, veterinary and medical fields. Surprisingly, most medical, parasitological and ecological texts dismiss the subject with unsupported statements to the effect that ‘successful’ parasite species evolve to be harmless to their hosts. Recently, however, several people have explored theoretical aspects of the population genetics of host-parasite associations; these authors conclude that such associations may be responsible for much of the genetic diversity found within natural populations, from blood group polymorphisms (Haldane, 1949) to protein polymorphisms in general (Clarke, 1975, 1976) and to histocompatibility systems (Duncan, Wakeland & Klein, 1980). It has also been argued that pathogens may constitute the selective force responsible for the evolution and maintenance of sexual reproduction in animal and plant species (Jaenike, 1978; Hamilton, 1980, 1981, 1982; Bremermann, 1980).