Previously it has been shown that eggs of the skin-parasitic monogenean Entobdella soleae, maintained free of host contamination at constant temperature (12 °C) and exposed to alternating 12 h periods of light and darkness, hatch during the first few hours of each period of illumination as a result of endogenous processes in the larva in conjunction with the illumination cycle. Washings, prepared by immersing the parasite's host (the common sole, Solea solea) for 1 h in seawater (just sufficient to cover the body), enhance this ‘morning’ hatching. Furthermore, the application of host washings during the latter half of the period of illumination or during the dark period also stimulates hatching. Experiments have shown that the hatching factor is present in sole skin mucus, is not destroyed by boiling for 5 min or by freezing and is produced by small soles measuring 4–5 cm in length as well as by larger soles (15–21 cm long).

When eggs are incubated at about 12°C in the absence of fish washings, hatching begins about 30 days after laying, but there is evidence that some of the larvae inside their shells are fully developed and capable of hatching in response to host hatching factor 1–3 days earlier.

When fish washings are added to eggs which range in age from 25 to more than 30 days (at 12 °C) many eggs are stimulated to hatch (in addition to ‘morning’ hatching as a result of endogenous rhythmical processes and the illumination cycle) and others fail to hatch. These remaining unhatched eggs, in the absence of further treatment with host hatching factor, will complete their development and subsequently will hatch during the ‘morning’ hours in response to the illumination cycle. Further contact with host hatching factor is likely to stimulate the hatching of some of these remaining eggs.

The host hatching factor is not specific to S. solea; washings from plaice, dab, halibut, whiting and ray induce hatching in E. soleae. In experiments in which oncomiracidia were offered scales from dab and sole, most of the attached larvae were found on sole skin, irrespective of whether the hatching stimulant had been provided by dab or by sole.

The relationship between the behaviour of the common sole and hatching phenomena in the skin parasite is discussed; hatching rhythms and the use of host hatching factors adapt the parasite to take advantage of most opportunities to infect the host.

The number of episodes of diseases caused by bacterial contamination of food has shown a real increase by about threefold in the last decade in the U.K. The numbers for 1992 are estimated to be 2 million. The causes are multifactorial and complex, and 4 representative pathogens are reviewed. The main increase in diseases due to salmonella has been caused by Salmonella enteritidis, especially from eggs. The commonest bacterial food pathogen is Campylobacter, which causes an illness with specific season peaks in May and June. This may be related to the activities of birds and mammals. Both these bacteria cause common diseases that are rarely fatal. In contrast, Listeria and E. coli are ubiquitous but rarely produce disease; however, the consequences of any such illness are often dire. Procedures and techniques are available for the control of most of these diseases, but society does not seem determined to implement them.

Tsetse flies were infected with 5 different variable antigen types (VATs) or with a mixture of VATs of the AnTAR 1 serodeme of Trypanosoma brucei. Metacyclic forms from the salivary glands of infected flies were used to initiate infections in mice. Immunofluorescence and trypanolysis reactions employing 24 monospecific antisera were used to analyse the VATs present in the mice following cyclical transmission. Regardless of the VAT used to infect tsetse flies, the first VATs detectable in the bloodstream were those previously identified as metacyclic VATs (M-VATs). These were present until at least 5 days after infection, at which time lytic antibodies against at least 2 of the M-VATs were detectable in the blood of infected mice. In mice immunosuppressed by X-irradiation the M-VATs were detectable in the bloodstream for longer periods, but the percentage of the population labelled with anti-metacyclic sera showed a decrease on day 5 as in non-irradiated animals. The VAT ingested by the tsetse was always detectable early during the first parasitaemia following cyclical transmission and was usually the first VAT detected after the M-VATs. Neutralization of selected M-VATs before infecting mice resulted in elimination of the neutralized M-VAT from the first parasitaemia but had no effect on the expression of other VATs in the early infection.

A parasite can be considered to be the device of a nucleic acid which allows it to exploit the gene products of other nucleic acid–the host organisms. In this view, all parasites are ‘molecular parasites’. But it is interesting to restrict our attention to nucleic acids which do not encode organisms, as these live in a purely molecular world which lacks emergent features such as fangs and ovipositors. Viruses and transposons are molecular parasites in this sense. Most viral nucleic acids do code for some proteins, such as replicases and the protein shell in which they travel between their cellular oases. Some, however, do not even have a shell and code for nothing at all–these are the ‘viroids’ (Reisner & Gross, 1985), the smallest parasites in the world.

The encapsulation of Trichinella spiralis larvae in host striated muscle was studied.

Capsule formation starts at 13 days post-infection. Between days 20–50 p.i. the largest increase in capsule formation was observed. The capsule is exclusively, or almost exclusively, formed from and by the infected muscle fibre, as might be concluded from the presence of the sarcolemmal basal lamina surrounding the capsule substance during the initial stages of capsule formation. A collagen nature of the capsule cannot be excluded.

It is well established that pregnant women are at an increased risk of Plasmodium falciparum infection when compared to non-pregnant individuals and limited epidemiological data suggest Plasmodium vivax risk also increases with pregnancy. The risk of P. falciparum declines with successive pregnancies due to the acquisition of immunity to pregnancy-specific P. falciparum variants. However, despite similar declines in P. vivax risk with successive pregnancies, there is a paucity of evidence P. vivax-specific immunity. Cross-species immunity, as well as immunological and physiological changes that occur during pregnancy may influence the susceptibility to both P. vivax and P. falciparum. The period following delivery, the postpartum period, is relatively understudied and available epidemiological data suggests that it may also be a period of increased risk of infection to Plasmodium spp. Here we review the literature and directly compare and contrast the epidemiology, clinical pathogenesis and immunological features of P. vivax and P. falciparum in pregnancy, with a particular focus on studies performed in areas co-endemic for both species. Furthermore, we review the intriguing epidemiology literature of both P. falciparum and P. vivax postpartum and relate observations to the growing literature pertaining to malaria immunology in the postpartum period.

The protonephridium of platyhelminths including Schistosoma mansoni plays a pivotal role in their survival by excretion of metabolic wastes as well as xenobiotics, and can be revealed in the living adult parasite by certain fluorescent compounds which are concentrated in excretory tubules and collecting ducts. To determine the presence of the multidrug resistance-associated protein (MRP) as a possible transporter in protonephridial epithelium, adult schistosomes were exposed to a fluorescent Ca2+ indicator, fluo-3 acetyloxymethyl ester, which is a potential substrate of mammalian MRP. Specific fluorescence related to fluo-3/Ca2+ chelate delineated the whole length of the protonephridial system. Simultaneously, a fluorescent substance was accumulated in the posterior part of collecting ducts and the excretory bladder. Similarly, when other fluorogenic substrates for mammalian MRP such as monoclorobimane, fluorescein diacetate, and 5(6)-carboxyfluorescein diacetate were applied to adult schistosomes, these fluorescent markers were observed in the excretory tubules through to the excretory bladder. The excretory system of mechanically-transformed schistosomula was not labelled with any of these 4 fluorescent markers. These findings suggest that the protonephridial epithelium of adult schistosomes, but not schistosomula, might express the homologue of the mammalian MRP transporting organic anionic conjugates with glutathione, glucuronate or sulphate as well as unconjugated amphiphilic organic anions.

Dientamoeba fragilis is an inhabitant of the human bowel and is associated with gastrointestinal illness. Despite its discovery over a century ago, the details of Dientamoeba's life cycle are unclear and its mode of transmission is unknown. Several theories exist which attempt to explain how Dientamoeba may be transmitted. One theory suggests that animals are responsible for the transmission of Dientamoeba. However, reports of Dientamoeba in animals are sporadic and most are not supported by molecular evidence. Another theory suggests that Dientamoeba may be transmitted via the ova of a helminth. Given that the closest relative of Dientamoeba is transmitted via the ova of a helminth, this theory seems plausible. It has also been suggested that Dientamoeba could be transmitted directly between humans. This theory also seems plausible given that other relatives of Dientamoeba are transmitted in this way. Despite numerous investigations, Dientamoeba's mode of transmission remains unknown. This review discusses the strengths and weaknesses of theories relating to Dientamoeba's mode of transmission and, by doing so, indicates where gaps in current knowledge exist. Where information is lacking, suggestions are made as to how future research could improve our knowledge on the life cycle of Dientamoeba.

The host specificity and host sharing of avian haemoparasites (genera Haemoproteus and Plasmodium) is still poorly known, although they infect a large proportion of several studied bird populations. This study used molecular techniques to detect haemoparasites in marsh warblers and in other passerines that feed in reed beds, at 4 sites in Portugal. The host-specificity of the parasite lineages was analysed and compared with other cases described in the literature to assess whether apparent host specificity changes according to the studied system. Nine lineages of Haemoproteus and 15 of Plasmodium were found, of which only 10 Plasmodium were proven to have local transmission. Each lineage was confined to a distinct set of host species. The distribution of parasites in the host species was non-nested, meaning that specialist lineages did not always share hosts with generalists. The most prevalent lineages were those with a wider host range, indicating that the ability to infect more hosts will enhance a parasite's prevalence in its entire host range. We also found that in our areas, a specialist parasite (H. MW1) appears to have a more generalist character than described in the literature, suggesting that a parasite's apparent specialization can depend on the type of host species sampled.

Failure of snail reinfection by Schistosoma mansoni has been demonstrated in susceptible Biomphalaria glabrata infected with 1 miracidium and subsequently re-exposed to 1 or 5 homologous parasite larvae. The acquisition of ‘resistance’ to secondary parasite infection was time dependent since complete inhibition was observed at 2 weeks and longer following monomiracidial exposure. This phenomenon was still observed in snails challenged 8 weeks after primary infection. Histological observations revealed that sporocysts from the challenge infection were free of encapsulation, their development was stopped and they degenerated slowly in the absence of haemocytic reaction of the host. Under the hypothesis of an acquired homologous resistance mechanism, this strongly suggests that 1 or several unidentified humoral factors are responsible for the non-development of the sporocysts from the challenge infection. However, considering the time-dependent nature of the phenomenon, an intraspecific larval antagonism process between sporocysts resulting from the primary infection and those from the challenge infection may be involved.

Trichinella spiralis is a parasitic helminth that can infect almost all mammals, including humans. Trichinella spiralis infection elicits a typical type 2 immune responses, while suppresses type 1 immune responses, which is in favour of their parasitism. DNA vaccines have been shown to be capable of eliciting balanced CD4+ and CD8+ T cell responses as well as humoral immune responses in small-animal models, which will be advantage to induce protective immune response against helminth infection. In this study, serine protease (Ts-NBLsp) was encoded by a cDNA fragment of new-born T. spiralis larvae, and was inserted after CMV promoter to construct a DNA vaccine [pcDNA3·1(+)-Ts-NBLsp]. Ts-NBLsp expression was demonstrated by immunofluorescence. Sera samples were obtained from vaccinated mice, and they showed strong anti-Ts-NBLsp-specific IgG response. Mice immunized with the pcDNA3·1(+)-Ts-NBLsp DNA vaccine showed a 77·93% reduction in muscle larvae (ML) following challenge with T. spiralis ML. Our results demonstrate that the vaccination with pcDNA3·1(+)-Ts-NBLsp plasmid promoted the balance of type 1 and 2 immune responses and produced a significant protection against T. spiralis infection in mice.

The immune response of European sea bass after intracaelomic immunization with Sphaerospora dicentrarchi was studied. Fish were injected with S. dicentrarchi spores (DIC), with spores plus adjuvant (DIC+FCA), with adjuvant alone (FCA) or with PBS. Several parameters of the immune response were measured. Serum lysozyme increased significantly in DIC fish 1 week after immunization (p.i.) and it remained significantly higher in DIC+FCA fish 4 weeks p.i., and in DIC fish 8 weeks p.i. than in PBS-injected fish. The number of nitroblue tetrazolium-positive blood cells was significantly higher in DIC+FCA fish 1, 4 and 8 weeks p.i, but the highest values were detected 1 week p.i. The highest stimulation index was detected in phagocytes from DIC+FCA fish. The number of S. dicentrarchi antibody-secreting cells was significantly higher in DIC+FCA fish than in DIC fish. Serum from DIC and DIC+FCA fish, stained the polar capsules and the valves of S. dicentrarchi spores in immunohistochemistry. Serum antibodies could not be detected using immunoblot assay. All these results show that immunization with S. dicentrarchi resulted in the activation of the non-specific immune response, mainly 7 days p.i. A specific humoral response against the parasite was also demonstrated but it had a low magnitude.

Mouse peritoneal macrophage cells suspended in a TC 199 calf serum medium and cultured in Leighton tubes, were infected with Trypanosoma (Schizotrypanum) cruzi culture forms in order to study the development of this parasite in vitro. Three cycles of development were thought to occur: 1. Epimastigotes or trypomastigotes were taken up by macrophages and transformed into amastigotes. These multiplied by binary fission and ruptured the infected cell after 4‐5 days. Released amastigotes were taken up by uninfected macrophages and the cycle was repeated. 2. A small proportion of intracellular amastigotes developed into ‘ovoid forms’ which transformed progressively into promastigotes, epimastigotes and trypomastigotes. 3. Some amastigotes became ‘round forms’ from which sphaeromastigotes developed. These transformed directly into trypomastigotes without the formation of epimastigotes.

More than 20 species of the entomopathogenic nematode Steinernema have been described; to date, all reproduce exclusively by cross-fertilization of male and female individuals. Steinernema sp. strain T87 from Indonesia was found to consist largely of self-fertile hermaphrodites. Progeny were produced by morphological females both in insects (Galleria mellonella) and in hanging drops of insect haemolymph inoculated with a single infective juvenile. Sperm were present in the oviduct of unmated morphological females. Approximately 1% of infective juveniles developed into males, and males were also present in the second generation where they constituted 1–6% of the population. Under the same conditions the related species Steinernema longicaudum strain CB2B displayed typical steinernematid reproduction: cross-fertilization and a 1:1 sex ratio. It is argued that the development of hermaphroditism in Steinernema sp. T87 represents convergent evolution with Heterorhabditis, the other major genus of entomopathogenic nematode.

Cystoisospora belli is a coccidian parasite of humans, with a direct fecal–oral transmission cycle. It is globally distributed, but mainly found in tropical and subtropical areas. Many cases of C. belli infections have been reported in patients with HIV, and in patients undergoing immunosuppressive therapy for organ transplants or those treated for tumours worldwide. Unsporulated or partially sporulated oocysts of C. belli are excreted in feces. When sporulated oocysts in contaminated water or food are ingested, asexual and sexual stages of C. belli are confined to the epithelium of intestines, bile ducts and gallbladder. Monozoic tissue cysts are present in extra-intestinal organs (lamina propria of the small and large intestine, lymph nodes, spleen, and liver) of immunosuppressed humans. However, a paratenic host has not been demonstrated. Cystoisospora belli infections can be persistent, lasting for months, and relapses are common; the mechanism of relapse is unknown. Recently, the endogenous stages of C. belli were re-examined and attention was drawn to cases of misidentification of non-protozoal structures in the gallbladder of patients as C. belli. Here, we review all aspects of the biology of C. belli, including morphology, endogenous stages, prevalence, epidemiology, symptoms, diagnosis and control.

Populations of Schofield strain, random-bred mice were shown to have a bimodal variation in ability to bring about immune expulsion of the nematode Trichuris muris. This variation was genetically determined and independent of the size of infection experienced. The proportion of mice unable to achieve worm expulsion (non-responders) was relatively constant in various populations of the strain but was increased by selective breeding from mice of known status. Crosses made between non-responder and responder mice produced progeny that were almost all (92%) of responder phenotype, showing that the ability to achieve worm expulsion was inherited as a dominant characteristic. It is suggested that the genetic control involves a small number of genes; the possible immunological mechanisms by which control is mediated are briefly discussed.

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.

1. Oviposition by L. sericata has been induced on a moist clipped fleece by incorporating ammonium carbonate and indole, hence attraction is not completely dependent on some factor produced by the living animal.

2. Chemistry of attractants. A new method for determining small amounts of hydrogen sulphide is described. Larval excreta contains hydrogen sulphide but no indole or skatole. Following incubation the ammonia and sulphydryl contents of various natural attractants are markedly increased.

3. Field trials on sheep. 0·002% ethyl mercaptan in conjunction with 0·1% ammonium carbonate was able to induce oviposition by L. sericata. The mercaptan concentration could be reduced to 0·001%, providing 0·0001% hydrogen sulphide was present. 0·01% mercaptan and aqueous solutions of hydrogen sulphide alone are not attractive.

4. The results obtained support the hypothesis that attraction is the result of the breakdown of cystine, with the production of sulphur compounds, coupled with the presence of ammonium carbonate (or of some putrefactive material).

5. Chemical baits. During the period 17 August–21 September 1943, various combinations of ammonium carbonate, hydrogen sulphide and indole in conjunction with sheep wool proved to be attractants for females of L. caesar.

In 1917 Montgomery described a tick-borne gastro-enteritis of sheep and goats for which the name Nairobi sheep disease has passed into general use. In a comprehensive paper dealing with his work at Kabete he was able to show that the causal agent was a filtrable virus, transmitted by Rhipicephalus appendiculatus, the ordinary brown tick and common vector of East Coast fever in cattle. In spite of Montgomery's careful description of the lesions, of his record of failure to infect cattle, and of his successful filtration experiments, there has until recently been considerable confusion between this disease and heartwater, and opinions have been expressed that Montgomery was in all probability working with a strain of heartwater. During the course of certain investiga tions on heartwater the opportunity was taken to compare the two diseases and to confirm and extend Montgomery's observations.

Of the unicellular eukaryotes, formerly Protozoa, now considered to belong to five separate phyla, only the neogregarines and microsporidia are serious contenders for a role in biological control of invertebrate pests. Ciliates of the genus Lambornella, which penetrate their hosts via cuticular cysts, have potential in mosquito control but have not been investigated in depth.

‘Protozoa’ generally kill their hosts by overwhelming numbers, destroying the normal function of organs or depleting the host of essential reserves. Because they are slow-acting pathogens they cannot be used on their own when pests have already reached a high level of abundance nor can they be relied upon when the damage threshold of pests is low. Their principal use will be as the slow-acting component of a 2-pathogen or pathogen-plus-chemical formulation, which can be used when a degree of damage is tolerable.

A comparison is made between two microsporidia in lepidopteran hosts, Vairimorpha necatrix and Nosema pyrausta. The former causes high mortality in a wide range of hosts, when bacterial septicaemia ensues after disruption of the gut wall by the microsporidian invasion process. Some larvae may survive this period and live to damage crops, but none survives to adulthood. There is no transovarial transmission and the parasite is rarely found in natural populations. V. necatrix could be used as a microbial pesticide for short-term control. N. pyrausta is restricted to a single host, the European corn borer. It has low pathogenicity, causing some larval mortality especially under conditions of environmental stress. Most hosts survive to adults but show reduced longevity and fecundity. The parasite is transmitted transovarially and is highly prevalent in the field. It is not considered pathogenic enough to be used as a microbial pesticide but is an important factor in regulating natural populations. These examples illustrate the inverse relationship between pathogenicity and prevalence and show how cycles of host population abundance may be driven by pathogens of moderate to low pathogenicity.

Two kinds of transovarial transmission mechanisms are discussed. With the microsporidia of winter moth, vegetative stages and spores, even when abundant in egg yolk, do not gain access to larval tissues but are confined to the meconium in larvae just before eclosion. Larvae are not infected when they hatch but the spores are carried over in the eggs to the next period of larval feeding activity. In contrast, some genera of microsporidia in haematophagous diptera, e.g. Amblysopora in mosquitoes, actually infect the cells of developing larvae, which are already infected when they hatch.

The prospects for biological control with ‘Protozoa’ are reviewed for vectors of medical importance and for pests of pasture, field crops, forests and stored products. Particular attention is given to the use of microsporidia in combination with low concentrations of compatible chemical insecticides and with other pathogens (e.g. viruses).

Spores for field application can be produced in natural or experimental hosts by feeding or intrahaemocoelic inoculation. Yields vary according to the species of parasite and host. Examples are Nosema locustae in Melanoplus bivittatus yielding 3·9 × 109 spores/grasshopper enough to treat more than 1 hectare of rangeland, and Vairimorpha necatrix in Heliothis zea, yielding 1·67 × 109 spores per larva, with 2·5 × 1012 spores/hectare required for field application. In vitro culture is at present a laboratory tool only, with yields too low for economic returns.

Spores can be stored, according to species, dry or in distilled water with antibiotics at 4 °C. This gives good survival for months or years. In field applications feeding-bait formulations are more efficacious than sprays because they concentrate the spores for uptake by the target species and give the spores some protection from harmful ultraviolet radiation.

Pheromone lures have been used for the introduction of spores by males into pest infestations in stored grain. Males are lured to sites dusted with spores and return to the grain after removal of the lure, to contaminate females and larvae. The use of these lures, first as traps to monitor pest population increases, then to effect a controlled pest growth curve by introduction of pathogens, is an attractive innovation. Protozoa are considered safe for field application on the limited evidence available.