The study of regulatory peptides has its origins in the classical work of Bayliss & Starling (1902). Their pioneering work on the presence of a factor in intestinal extracts which, when injected into the bloodstream of experimental animals, elicited pancreatic secretion, led to the genesis of the concept of the hormone, i.e. a chemical messenger which is released from one part of the body in response to a stimulus to travel in the bloodstream to a distant target tissue where it would elicit a physiological response appropriate to the original stimulus. In keeping with accepted scientific tradition, this concept had its critics. Pavlov, who had been studying secretory stimulation from a different perspective, concluded from his work on salivation in dogs, that this was mediated via neural pathways. With hindsight, and the benefits of knowledge obtained from nearly a century of scientific research, we now know that these pioneers were in actual fact studying different aspects of the same process and that both theories were complementary. In fact, it is becoming increasingly difficult to ascribe secretory control to either circulating or neuronal factors as both appear to be intimately involved in regulation.

The effect of temperature (19–36 °C) and snail host density (0.014–10 snails/1) on the snail-finding capacity of Echinostoma caproni cercariae is described. The initial swimming speed increased whereas the length of the infective period decreased with increasing temperature. The combined effect resulted in the E. caproni cercarial snail-finding capacity being temperature independent in the range 19 to 36 °C at a snail density of 0·014 snails/1. A moderate temperature dependency was, however, seen at a lower snail density. The cercarial snail-finding capacity was snail-host density dependent in the density range 0.014 to 1 snail/1. The findings from this study show that a relatively low and biologically realistic snail host density must be used in experimental studies if realistic estimates of the dynamics of cercarial transmission are to be obtained.

Bloodstream forms of Trypanosoma congolense (2 clones: ILNat3·1 and IL3000, and 4 stocks: IL2079, IL2466, IL3266 and CP-81) were continuously cultivated in vitro at 34–36 °C in the absence of feeder cell layers, using HMI-93 medium which was modified from Iscove's modified Dulbecco's MEM (Flow Laboratories, Irvine, Scotland). The modification was done by supplementing the medium with 0·05 mM bathocuproine sulphonate, 1·5 mM L-cysteine, 0·5 mM hypoxanthine, 0·12 mM 2-mercaptoethanol, 1 mM sodium pyruvate, 0·16 mM thymidine, 20% (v/v) heat-inactivated young goat serum and 5% (v/v) Serum Plus™ (Hazleton Biologics, Lenaxa, KS, USA). Trypanosomes obtained from two different sources were used to initiate primary cultures: (1) metacyclic forms which were produced in vitro at 27 °C, and (2) bloodstream forms obtained from Balb/c mice which had been infected with the bloodstream forms transformed in vitro from the metacyclic forms. Metacyclic forms placed in 25 cm2 T-type (T-25) flasks rapidly attached to the bottom of the flasks and transformed to bloodstream forms during the initial 24 h and continued to proliferate. The bloodstream forms isolated from the infected mouse blood by means of diethylaminoethyl cellulose (DE52) column chromatography also continued to proliferate in the flasks. Cultures were maintained by replacing the medium every 24 h. Every 4–5 days, the attached bloodstream forms were resuspended in fresh medium by gentle pipetting and then were subcultured. The method was further simplified by initiating primary cultures directly with 10 μl of the tail blood of infected mice in 24-well culture plates and then by subcultivating either in wells or in T-25 flasks. The shortest population doubling time, 9 h, was achieved by seeding subcultures with 106 bloodstream forms/ml. The bloodstream forms propagated in this system were morphologically similar to those seen in infected mouse blood, they were covered with a surface coat as examined by electron microscopy and they were infective to mice.

Subadult and adult specimens of the pond snail Lymnaea stagnalis were infected with the schistosome Trichobilharzia ocellata. Egg production and growth of the snails were monitored over an 8-week period post-infection (p.i.). Snail haemolymph was collected and analysed for the presence of schistosomin, a neuropeptide which antagonizes the action of the snails' female gonadotropic hormones. Snails infected as subadults showed an increase in fecundity during the first 4 weeks p.i. compared with non-infected controls. The possibility is discussed that this increase is caused by an accelerated maturation of the female sex organs due to elevated levels of Dorsal Body Hormone, a female gonadotropic hormone. No difference in fecundity was found between snails infected as adults and control snails during the first 4 weeks p.i. Snails infected as subadults and as adults showed a decrease in fecundity from week 5 p.i. and onwards. This decrease coincided with the appearance of schistosomin in the haemolymph of the snails and with that of differentiating cercariae in the daughter sporocysts. Cercariae are probably involved in the induction of schistosomin release from the snails' CNS into the haemolymph. Snails infected as subadults or as adults grew at approximately the same rate as uninfected snails.

Nine clones were derived from a drug-resistant Trypanosoma congolense stock (IL 2856) and characterized in mice for their sensitivity to isometamidium chloride and diminazene aceturate. All clones were derived from the stock without drug selection and expressed high levels of resistance to isometamidium chloride (50% curative dose [CD50] values ranging from 1·5 to 5·1 mg/kg) and intermediate to high levels of resistance to diminazene aceturate (CD50 values ranging from 5·1 to 21·0 mg/kg). By contrast, the isometamidium chloride and diminazene aceturate CD50 values for a drug-sensitive clone, T. congolense IL 1180, were 0·018 mg/kg and 2·3 mg/kg, respectively. For both drugs, there appeared to be significantly different levels in expression of drug resistance amongst the 9 clones derived from IL 2856. Isoenzyme analysis of 7 enzymes showed that all 9 clones expressed the same electrophoretic variants. Thus, all 9 clones were identical for these phenotypic markers. The clone which expressed the highest level of resistance to isometamidium in mice (IL 3270) was transmitted to Boran cattle via the bite of infected Glossina morsitans centralis. IL 3270 produced an infection rate in tsetse of 5·0%. The resulting infections in cattle were shown to be resistant to intramuscular treatment with 2·0 mg/kg isometamidium chloride and 14·0 mg/kg diminazene aceturate. This contrasts with doses of 0·25 mg/kg isometamidium chloride or 3·5 mg/kg diminazene aceturate which are deemed sufficient to cure fully sensitive infections. Finally, 9 clones (subclones) were derived from IL 3270 and characterized in mice for their sensitivity to isometamidium chloride. Seven of the subclones expressed a significantly lower level of resistance to isometamidium than the parental clone and amongst the subclones there was significant variation in resistance. Thus, expression of a high level of resistance to isometamidium appears to be unstable in the rodent host and at least a component of the genetic determinant(s) for this drug-resistant phenotype is (are) likely to be unstable.

Most of the successful anti-nematode drugs currently available affect the nematode locomotory system. Their success is due to their interactions with molecules associated with the main neuro-transmitters of the motor nervous system, acetylcholine and GABA. These drugs tend to have a relatively broad spectrum of action, affecting a wide variety of nematodes, presumably because nematode motor nervous systems are conservative in their use of these transmitters.

During 1988 and 1989, the mesenteric veins of 901 cattle were examined for the presence of schistosomes at the Kandy slaughterhouse (Sri Lanka). The overall prevalence of infection was 31·2%. Animals younger than 2 years were less infected (21·3%) than those older than 5 years (47·9%). Based on the number of paired worms counted, three intensities of infection were recognized: low (1–20 pairs), moderate (21–100 pairs) and heavy (> 100 pairs). Intensities increased with the age of the animals but remained low (average 10 worm pairs). The worm burden increased by approximately 20% for each step in age group. The number of miracidia/100 g faeces was measured in 85 animals of all age groups and intensities of infection; 77% of the samples contained less than 100 miracidia. Miracidia counts decreased with age; moderately and heavily infected animals in each age group had the highest and the lowest counts, respectively. This may be due to a host immune response. The results raise questions on the sensitivity of faecal egg counts as a diagnostic method for visceral schistosomiasis in cattle.

DNA sequences homologous to the mouse intracisternal A particle and endogenous type C retrovirus were detected in the DNAs of Schistosoma japonicum adults and S. mansoni eggs. Furthermore, other kinds of repetitive sequences in the host genome such as mouse type 1 Alu sequence (B1), mouse type 2 Alu sequence (B2) and mo-2 sequence, a mouse mini-satellite, were also detected in the DNAs from adults and eggs of S. japonicum and eggs of S. mansoni. Almost all of the sequences described above were absent in the DNAs of S. mansoni adults. The DNA fingerprints of schistosomes, using the mo-2 sequence, were indistinguishable from each other and resembled those of their murine hosts. Moreover, the mo-2 sequence was hypermethylated in the DNAs of schistosomes and its amount was variable in them. These facts indicate that host-related sequences are actually present in schistosomes and that the mo-2 repetitive sequence exists probably in extra-chromosome.

Threshold density levels were investigated experimentally in rainbow trout, Oncorhynchus mykiss, for the acanthocephalan Pomphorhynchus laevis. Fifty small fish 120–150 mm long were infected with 50 cystacanths each every 4 weeks for 16 weeks, and samples of up to 10 fish examined at 1, 5, 9, 13 and 17 weeks after the original infection. The mean intestinal length was found to have increased significantly over this period, from a mean of 125 to 173 mm and the mean number of parasites recovered increased after each interval of 4 weeks, and significantly from 19·30 to 86·40 per fish over the 17 weeks. The distribution of the parasites in the alimentary tract of O. mykiss was unaltered, indicating that the favourable region for establishment had remained unchanged. The failure to detect any manifestations of competition is discussed, and the relevance of conventional approaches to experimental investigations of both intra- and interspecific competition in fish was queried, and contrasted to the situation in birds and mammals.

The present study shows an in vivo role for interleukin-2 in the formation of hepatic granulomatous inflammation in murine Schistosoma japonicum infection. Mice which had been administered an inhibitor to interleukin-2 function during acute infection were noted to have reduced cell-mediated immune responses to ConA and S. japonicum soluble egg antigen (SEA). Lymphoid tissue from these treated mice also have reduced numbers of cells which could be activated by ConA to produce interleukin-2 mRNA as shown by in situ hybridization studies. Mice with less activated lymphocytes had, on the average, a 70% reduction in the granulomatous inflammation surrounding eggs deposited in hepatic tissues of acutely infected mice. The data demonstrate that IL-2 is a key factor in the generation of granulomatous inflammation in S. japonicum infection and suggest that a potential anti-pathology vaccine could be generated based on limiting the presence of IL-2 generated during acute infection.

Although structurally simple, the nervous systems of parasitic helminths are biochemically complex. As well as the classical transmitters, the nervous systems of helminths contain a range of regulatory peptides and so far some 29 different peptides have been described. The only group of compounds thought to be transmitters in free-living organisms which have not yet been shown to be transmitters in helminths are the purines. There is evidence in helminths, as in vertebrates, for the co-localization, in the same cell, of classical transmitters and peptidergic molecules and for the non-neuronal occurrence of some regulatory peptides. Co-localization would effectively increase the number of transmitter types, since compounds could be released in different combinations. How such a selective release might be achieved is unknown. Different regulatory peptides are localized in different parts of the helminth nervous system, but it is quite probable that more than one peptide can occur in the same cell. Whether the relative proportions of the different peptides change under different physiological or developmental conditions is not known. The classical transmitters similarly show a differential distribution. It is presumed that the regulatory peptides in helminths, as in other organisms, are synthesized by post-translational modification of a high molecular weight precursor protein and that the peptides are not recycled but are removed from their site of action by specific peptidases. The latter are located near the receptor sites and may be membrane-bound. In vertebrates the precursor molecules usually contain several different peptides with different biological functions and processing of the precursor often shows tissue-specific variations.

The cercaria of Acanthostomum brauni penetrates the skin of its fish host in response to a combination of two chemical signals from the fish skin surface: free fatty acids and a macro-molecular mucus component. The latter seems to be a protein, as the penetration-stimulating activity of fish skin surface mucus is eliminated by digestion with proteinase, but not by digestion with glycosidases, nor by a removal of glycosaminoglycans. These penetration-stimulating host signals differ from the glycoproteins that stimulate the attachment of A. brauni cercariae to the host and also from the macro-molecular fish host signals which have been found to stimulate the attachment and penetration by Opisthorchis viverrini cercariae.

The relationship of survival and reproduction of Gyrodactylus salaris Malmberg on the Atlantic salmon (Salmo salar) to water temperature (2·5–19·0 °C), was studied on the basis of temporal sequence of births and age at death of individual parasites on isolated salmon, and of infrapopulation growth on isolated and grouped salmon. Mean life-span of the parasite was negatively correlated with water temperature: 33·7 days at 2·5 °C and 4·5 days at 19·0 °C. The average number of offspring per parasite peaked between 6·5 and 13·0 °C, and was approximately 2·4 at these two temperatures. Both the period between the successive births of the offspring (max 4) and the estimated generation time were negatively correlated with temperature. The innate capacity for increase (rm) was positively correlated with temperature: from 0·02 (/parasite/day) at 2·5 °C to 0·22 (/parasite/day) at 19·0 °C. Growth of the infrapopulations was positively correlated with water temperature and was higher on isolated fish than on grouped fish, though less than the potential parasite population growth estimated from rm. In the infrapopulations the mean intensity of parasites continued to increase throughout all the experiments on both isolated fish and on grouped fish.

Muscle cells from adult male Schistosoma mansoni have been isolated using a combination of papain digestions and mechanical dissociation procedures. The muscle fibres isolated in this way lacked nuclei but they did contract and relax in response to high [K+], a response which was blocked in the presence of Co2+. From this we conclude that the isolation procedure yields viable muscle fibres useful for physiological studies. Patch-clamp recordings taken from the isolated fibres show a variety of discrete ionic conductances. In inside-out patches one prominent channel was a Ca2+-activated K+ channel with a conductance of 195 pS and a selectivity greater than 10:1 for K+ over Na+, Cs+ or NH4+. Percentage open time was dependent on [Ca2+] at the intracellular face. With [Ca2+] at 1 μM or greater percentage open time was > 95%; at 0.1 μM it was < 2%. No voltage sensitivity could be detected in the voltage range from –50 to –10 mV membrane potential. Ba2+ (10 mM), but neither tetraethylammonium nor 3,4-diaminopyridine blocked the channel from the intracellular face. This Ca2+-activated K+ channel in the muscle membrane of this acoelomate animal is similar in most respects to the maxi-K+ channels which have been described in a variety of cells from more highly evolved animals.

Muscle-phase Trichinella larvae depress the immune response of mice to the phosphorylcholine (PC)-bearing Trichinella antigen FCp without affecting responses to other PC-bearing or non-PC antigens. The depressive activity is independent of antigen dose and Trichinella species and, in adoptive cell transfer experiments with lethally irradiated recipient mice, depended on the state of the recipient (infected recipients had a depressed response even a month after their encysted larvae had been killed and regardless of whether the donor had been exposed to FCp) but not on the state of the transferred cells. We conclude that lymphocytes are not permanently altered by the depressive action, that the agent responsible persists in the host at least a month after the death of the encysted Trichinella larvae, and that the alteration does not eliminate lymphocyte immunological memory.

The infectivity of wild and laboratory strains of Heligmosomoides polygyrus (Nematospiroides dubius) in laboratory mice and in three species of wild British rodent was compared. Wild strains, of the subspecies H. p. polygyrus, were isolated from wild caught Apodemus sylvaticus. Only very low-level infections of the wild strains became established in laboratory mice. Similar worm burdens of the laboratory strain became established in laboratory mice and A. sylvaticus, although infections in A. sylvaticus were more short lived. Cortisone treatment of hosts increased the establishment and survival of the heterologous worm strain to that of the homologous strain. In contrast, neither strain of parasite established in Clethrionomys glareolus or Microtus agrestis, and cortisone treatment of C. glareolus did not increase establishment. Infection of laboratory mice with the wild-strain parasite induced significant immunity to a challenge infection with the laboratory strain.

It has been shown that in intact cercariae of Schistosoma mansoni in water, both head and tail had an identical, aerobic energy metabolism. As long as the environment was water, glucose was mainly degraded to carbon dioxide by both head and tail whether or not these two were still connected to each other. Transfer of intact cercariae into a simple salt medium supplemented with glucose resulted in a very rapid transition towards a more anaerobic energy metabolism: the production of lactate and pyruvate increased, whereas the production of carbon dioxide remained more or less constant. A concomitant rise in temperature to 37°C was not essential for this biochemical transition, but made it more pronounced. Experiments on isolated cercarial bodies and tails in a transforming medium demonstrated that the tails oxidized glucose to carbon dioxide, whereas bodies produced mainly pyruvate and lactate. The results showed that the metabolic transition towards a more anaerobic energy metabolism occurred only in the head and not in the tail of the cercariae. Loss of the tail was shown not to be a pre-requisite for this transition, nor did it by itself trigger a metabolic switch in the resulting cercarial body.

High molecular weight proteinases of Trichomonas vaginalis (with apparent Mr values 142 and > 220 kDa) and Tritrichomonas mobilensis (Mr 67, 86, 104 and 120 kDa), optimally active at pH 8, were analysed in gelatin-containing polyacrylamide gels. All of these proteinases were resistant to serine-, aspartic- as well as cysteine proteinase inhibitors. Both proteolytic bands in T. vaginalis and two proteinases in T. mobilensis (67 and 104 kDa) were inhibited by EDTA and EGTA suggesting that they belong to the metallo-proteinase class. The 67 kDa proteinase of T. mobilensis was inhibited also by o-phenanthroline. The other two bands of T. mobilensis (86, 120 kDa) were not classified to any proteinase group since they appeared to be resistant to the chelating agents tested in this study.