Parasites are more fecund than free-living relatives. The traditional explanation of this is that parasites have to compensate for massive mortality in the transmission phase of their life cycles, but there are neo-Darwinian problems with this interpretation. Similarly, parasites invest more resources in reproduction than free-living relatives but often live longer as adults, and yet negative correlations are expected between fecundity and longevity. These patterns and paradoxes are discussed within the context of a general life-cycle theory. The theory is also used to address questions concerning the influence of age-specific mortality on life-cycle patterns, the trade-off between gamete size and numbers, and the relative merits of gametic and non-gametic reproduction. Wherever possible, the theory is related to facts about parasites.

The results of an epidemiological study involving the estimation of worm counts of gastro-intestinal nematodes of lambs, during the period December 1978 to November 1979, in the Baghdad area are reported. Ostertagia spp. and Trichostrongylus spp. were the most abundant nematodes in the weaned lambs used in this study. Ostertagia spp. infection was present throughout the period of observation, the level of infection being the highest during early summer (May–June). Trichostrongylus spp. infection, however, reached a peak during July–August. In the tracer lambs the maximum population of Ostertagia spp. was found during February–March, while that of Trichostrongylus spp. could be observed only after mid-June. Other nematodes encountered were Haemonchus contortus and Trichuris sp. but they were only present in small numbers. The proportion of inhibited larvae of Ostertagia spp. was markedly high during the dry summer months. Little inhibition was noticed during autumn and the early winter months. It appears that if the seasonal inhibition of Ostertagia spp. in Iraq is brought about by an environmental stimulus acting upon pre-parasitic larval stages, that stimulus cannot be chilling or falling temperatures, as observed in temperate northern zones.

The synergistic relationship between host nutrition and the effects of Nippostrongylus was investigated experimentally in rats fed a diet containing 1% (w/w) protein. Rats were infected with 10 doses of N. brasiliensis varying from 0.35 to 99 larvae/g and their daily food intake and weekly body mass were compared with those of uninfected rats fed the same diet for a period of 9 weeks. The deterioration of the uninfected rats observed as a result of protein-energy malnutrition was found to be accelerated in a dose-dependent manner in the rats infected with N. brasiliensis. Analysis of food intake and body mass measurements demonstrated that this acceleration was related to a decrease in food intake and indicated a likely alteration in nutrient utilization, both of which were established by the end of the first week of the infection. Remission from the combined effects of malnutrition and infection was achieved by offering the rats a high-protein diet (20% w/w), whereupon rehabilitation was found to be complete within a period of 9 days. The implications of the results and the possible mechanisms involved in their generation are discussed in relation to the frequent coexistence of helminthiasis and malnutrition.

The fine structure of the nervous system in the scolex and neck region of Hymenolepis nana has been investigated by transmission electron microscopy. A description of the gross neuroanatomy in these regions of the worm is presented. The ganglia, commissures and nerve cords consist of an incomplete cortex of nerve cell bodies, and a core of nerve fibres. A delimiting sheath or capsule is absent. The nerve cell bodies contain a single nucleus with a single nucleolus, mitochondria, many ribosomes, Golgi complexes and vesicles formed within the Golgi cisternae. Numerous sub-surface cisternae are present beneath the outer plasma membrane of the nerve cell body, and the inner surfaces of these cisternae are studded with ribosomes. Some of the cisternae run tangentially into the cytoplasm of the perikaryon, particularly in the vicinity of the Golgi complexes; both sides of these cisternae are studded with ribosomes. From each neuronal perikaryon arise one or more neurites that contain neurotubules, mitochondria, ribosomes and electron-lucent or dense-cored vesicles. Five types of vesicle have been distinguished on the basis of their size and content. The neurites are unmyelinated and form synapses in the neuropile; the synapses possess features typical of those where mechanical strength is of importance. Three types of sensory receptors have been observed in H. nana, two ciliated and one non-ciliated; the latter typically form double or triple nerve endings which terminate within the tegument.

The development, survival and infectivity of larvae of Nippostrongylus brasiliensis at 10 and 21 °C have been studied. In cultures developed for 1 weekat21 °C but stored at either 10 or 21 °C the mean recovery of larvae in 1·5 h Baermann sediment (46% and 42% of total eggs, respectively) and their infectivity (66% and 70% day 8 p.i. recovery, respectively) at 4 weeks of cultivation were similar. However, at 20 weeks of cultivation only a few larvae survived storage at 21 °C (1·4%) and the survivors showed very low infectivity (4·1%), whereas larvae stored at 10 °C retained most of their survival and infectivity rates (41% and 44%, respectively). In cultures developed and stored at 10 °C a lower mean proportion (17%) of the eggs was recovered as larvae at 4 weeks of cultivation. The mean infectivity rate of these larvae increased from 4 to 12 weeks of cultivation (18 % and 42 %, respectively) but was reduced at 20 weeks of cultivation (27 %). An increased proportion of adult female worms was recovered from rats infected with larvae from cultures showing poor development or survival and the fecundity of the adult females decreased in all groups as the storage time of the cultures was increased. Eggs from low fecundity adult worms showed normal development of larvae. The relation between survival, temperature in the habitat and the geographical distribution of the larvae of N. brasiliensis is discussed.

Plasma etching has been used to strip away the cortical layers of the cuticle of adult Nippostrongylus brasiliensis to reveal the struts, with their supporting fibres, which are found in the fluid-filled middle layer of the cuticle, and the basal fibre layers. The etched specimens were studied by means of scanning electron microscopy. The results support earlier work, obtained by transmission electron microscopy, on the cuticle of this nematode. Plasma etching has been shown to have potential in studying the structure of nematodes.

The behaviour of mice exposed to graded doses of Trichinella spiralis larvae was monitored in an open-field situation. Ambulatory and exploratory activity declined significantly as the infections progressed, whereas the time spent inactive increased concomitantly. The advent of these behavioural changes coincided with the expected release of larvae by the adult worms and showed no clear signs of abating as late as 90 days post-inoculation. The levels of activity were negatively correlated with the inoculating dose. Such behavioural changes may render the host more vulnerable to predation and thus enhance parasite transmission.

Trichinella spiralis infections influenced the establishment and maintenance of behavioural dominance among outbred male mice. Infected animals assumed a subordinate status when challenged by normal or more lightly infected conspecifics both in an unfamiliar test arena and in their established home cages. These effects were demonstrable during the acute and the chronic phases of infection. The significance of this phenomenon to the survival of the mouse host and the transmission of the parasite is discussed.

Glycosidase activities in the adults and juveniles of the lung fluke Paragonimus ohirai and P. westermani adults were demonstrated histo-chemically. For comparative studies, histochemical examination was also made on the adults of the liver fluke Fasciola hepatica. The enzymes examined were N-acetyl-β-glucosaminidase (EC 3.2.1.30), β-glucuronidase (EC 3.2.1.31), β-galactosidase (EC 3.2.1.23), α-glucosidase (EC 3. 2. 1. 20) and β-glucosidase (EC 3.2.1. 21). The distribution of β-glucosaminidase was similar in juveniles and adults. Strong reaction sites for the enzyme were the caecal brush border, tegument, sub-tegumental cells and testes. In contrast, no staining reaction occurred in the caeca of F. hepatica, although the tegument and parenchymal cells were weakly stained, β-glucuronidase activity was associated only with the luminal surface of the caeca in the juveniles. However, luminal contents also appeared stained and this might suggest that the activity in the caeca is not endogenous. β-galactosidase was localized in the caeca, sub-tegumental cells and tegument in both juveniles and adults. No reaction occurred for the other two enzymes, α- and β-glucosidase.

Males of Nematodirus battus have a body wall composed of a longitudinally ridged cuticle made up of 8 layers, a hypodermis and a layer of longitudinal muscle cells which are divided into 4 sectors by the hypodermal cords. There is a characteristic cephalic inflation of the cuticle in both sexes. The structure of the body wall in the anterior half of the female is similar to that of the male. In the posterior half of the female the cuticle is not ridged and has 7 layers; the longitudinal muscles are reduced in number and the lateral hypodermal cords increasingly extend dorsally and ventrally along the length of the posterior part of the nematode and come to occupy most of the lateral sectors of the body. The possible significance of these modifications in the structure of the female nematode are discussed.

The development of acquired resistance in opisthorchiasis was studied in hamsters experimentally infected with Opisthorchis viverrini. The induction of protective immunity was attempted by first exposing adult female golden Syrian hamsters to 1,2 or 3 doses of infective metacercariae obtained from naturally infected cyprinoid fishes and then reinfecting them with 80 metacercariae. In other experiments, animals that were infected with 50 metacercariae were treated with praziquantel prior to being rechallenged in order to eliminate the flukes that had developed from the first infection. The effect of long-term chronic infections was also studied. Faecal egg counts were determined at weekly intervals from 4–5 weeks onwards. The animals were killed 2–3 months after the last infection for worm recovery, and terminal faecal egg output/g faeces/worm was calculated. The data showed that prior infection of animals with O. viverrini did not induce significant protective immunity against reinfection by the same parasite. Lack of protection was also noted in animals reinfected several times with small doses of metacercariae. However, under certain circumstances, prior infection could result in a significant reduction in the faecal egg output due to subsequent infection.

The tegument of the metacercaria of Gorgoderina vitelliloba contains S1, S2, S3 and S4 secretory bodies. Between the parenchymal cells 2 types of tegumental cyton occur. Type 1 contains S1 and S3 secretory bodies, whilst S2 and S4 bodies are limited to Type 2. Type 1 cytons and S1 secretory bodies disappear at excystment. Post-metacercariae contain a third type of tegumental cyton in which a new secretory body, S5, occurs. This cyton type disintegrates 1–5 days after infection. The maintenance and development of the outer anucleate layer after this period is the responsibility of Type 2 cytons. Type 2 cytons and S2 and S4 secretory bodies have also been found within the cercarial body. When juvenile flukes migrate from the kidney to the bladder of their frog definitive host 21 days after infection, the outer anucleate layer increases in electron density. Within the bladder, however, the tegument of the adult fluke appears normal. The distribution of mitochondria within the outer anucleate layer changes during the fluke's migration. The mitochondria, which have a random distribution in the metacercaria, are located adjacent to the outer and basal plasmalemmas in the early kidney stages. In late kidney stages and bladder stages mitochondria occur only against the former membrane. The mitochondria increase in size in the adult fluke.

Individual females of Gyrinicola batrachiensis produce 2 types of eggs: thin-shelled auto-infective eggs are produced in the ventral horn of the reproductive tract and thick-shelled eggs (transmission agents) are produced in the dorsal horn. Fine structure of oogenesis and egg-shell formation in the 2 horns of the reproductive tract were studied and compared. Early stages of oogenesis were similar in both horns but mature oocytes differed considerably. Those in the dorsal horn were larger than those in the ventral horn; they contained large numbers of lipid droplets, peripheral patches of glycogen and several types of cytoplasmic granules presumably acting as yolk or playing a role in shell formation. Mature oocytes in the ventral horn contained large amounts of glycogen, relatively few lipid droplets and large multivesicular bodies. Four shell layers formed around ova in the dorsal horn: a vitelline layer, a lipid layer, a chitinous layer and an outer protein coat similar to that described in other oxyurids. Only the vitelline layer formed around thin-shelled eggs. Thick-shelled eggs did not embryonate in utero but thin-shelled eggs nearest the vagina contained larvae. The first moult in eggs of G. batrachiensis was described in a previous communication and it is suggested here that the thin fibrous layer loosely applied to the cuticle of infective larvae in thin-shelled eggs is the moulted 2nd-stage cuticle.

A method of interrupting the life-cycle of the human blood fluke Schistosoma by increasing the proportion of genetically insusceptible intermediate host snails in natural populations was first proposed nearly 25 years ago. The method assumes that insusceptible snails will be at a selective advantage over susceptible snails when the schistosome parasite is present, and therefore natural selection will act to increase the proportion of alleles for insusceptibility. A major objection to the proposed technique is ‘If insusceptible snails are at a selective advantage, then why are they not predominant in natural populations that transmit disease?’ One explanation of this paradox is that insusceptibility may be associated with a disadvantageous character or a physiological defect. This study tests this hypothesis by measuring the relative reproductive success of susceptible and insusceptible snails under controlled conditions. Results indicate that insusceptible (unsuitable) snails are negatively affected in the presence of either susceptible snails or schistosome parasites. Furthermore, in the presence of both susceptible snails and schistosome parasites, insusceptible snails are selectively disadvantaged compared to susceptible snails. These results obtained under laboratory-controlled conditions suggest a plausible answer as to why insusceptible snails are not predominant in natural populations that transmit disease.

Rats and mice were infected with either normal metacercariae or metacercariae γ-irradiated at 3 krad. or 4 krad. Flukes were recovered at various times after infection and their teguments examined using a transmission electron microscope. In normal flukes, the secretory granules TO, Tl and T2 were all seen during tegumental development. The teguments of flukes from mice developed faster than the corresponding teguments in rats. TO granules were present from day 0 to day 10 post-infection (p.i.) in mouse flukes and from day 0 to day 14 p.i. in rat flukes. Tl granules first appeared in mouse flukes by day 4 p.i. but not until day 8 p.i. in rat flukes. T2 granules were seen in mouse flukes 2 days p.i. but not before 6 days p.i. in rat flukes. γ-Irradiation at 4 krad prevented normal tegumental development in flukes from both rats and mice. TO granules were present at all times in flukes from either host. Tl granules were produced in mouse flukes but their appearance was delayed until day 6 p.i. No significant production of T2 granules occurred in flukes from either host. Parasite survival was also affected by γ-irradiation and none of the flukes reached maturity. Flukes from rats died between 10 and 21 days p.i and flukes from mice died between 14 and 28 days p.i. γ-Irradiation of metacercariae at 3 krad. had an extremely variable effect on subsequent tegumental development in both rats and mice. Some flukes developed normally, some showed development associated with γ-irradiation at 4 krad, whilst some showed intermediate development.

Some unique features of monogenean biology have emerged from studies on 2 polystomatids which infect spadefoot toads in Arizona, USA: Pseudodiplorchis americanus in Scaphiopus couchii and Neodiplorchis scaphiopodis in S. multiplicatus and S. bombifrons. Transmission is confined to 1–3 nights each summer when the desert-adapted toads enter water to spawn. Encapsulated larvae complete development in utero and hatch within seconds of deposition; their body length (up to 600 μm) is 2–4 times greater than that of most other oncomiracidia, and their swimming life (over 48 h at 25–27 °C) is twice that usual amongst monogeneans. The ciliated oncomiracidia invade the nostrils, a target usually above the water surface and requiring migration over the exposed skin; remarkably they tolerate drying for up to 1 h (at 32 °C, 45% rel. hum.). The larvae migrate via the buccal cavity and glottis to the lungs where juvenile development occurs until early autumn. Migration from the lungs to the urinary bladder (by an as yet undetermined route) is followed by maturation and reproduction during host hibernation, and developing embryos accumulate in preparation for a period of transmission more restricted than that of any other monogenean. No other monogenean is known to exploit the lungs nor to inhabit, successively, anterior and posterior sites of the same adult host individual.

The life-cycles of two closely related cephalobaenid pentastomids, Raillietiella gehyrae and Raillietiella frenatus, which utilize geckos as definitive hosts and cockroaches as intermediate hosts, have been investigated in detail. Early development in the fat-body of cockroaches involves 2 moults to an infective, 3rd-stage larva which appears from 42–44 days post-infection. Complete development in geckos involves a further 5 moults in the case of males and 6 for females. Males mature precociously and copulation is a once-in-a-lifetime event which occurs around day 80 post-infection when both sexes are the same size but the uterus of the female is undeveloped. Sperm, stored in the spermathecae, is used to fertilize oocytes which slowly accumulate in the developing saccate uterus. Patency commences when the uterus carries approximately 4000–5500 eggs but only 25–36 % of these contain fully developed primary larvae. Since only mature eggs are deposited, we postulate that the vagina (?) of the female must be equipped with a selective filter that allows through large eggs but retains smaller, immature eggs. Thus the only limit on fecundity is the total number of sperms in the spermathecae and this is precisely the same factor that constrains egg production in the advanced order Porocephalida.

It was found that interference contrast microscopy gave good morphological differentiation of the living and intact salivary glands from adult ixodid ticks, Rhipicephalus appendiculatus and Amblyomrna variegatum, removed by dissection. In many cases the individual cell types of the acini could be recognized. When the salivary glands of infected adult ticks were examined Theileria parasites were detected. Theileria parva, T. taurotragi and T. mutans could be detected in the salivary glands after they reached the mature secondary sporoblast stage achieved even in unfed ticks. Parasites were detected only in type III acini. The morphology of tertiary sporoblasts and sporozoites of these Theileria spp. was clearly recognized in feeding ticks. Sporozoites of T. parva proved to be infective to cattle and to peripheral blood leucocytes of cattle in vitro after interference contrast examination. Interference contrast examination of living salivary glands was used for selection of salivary glands with high infections of T. parva sporozoites and the sporozoites could be separated in large numbers from these salivary glands. Using this technique it appeared possible to clone Theileria parasites from salivary glands with 1 infected acinus.