Theileria sergenti Shintoku stock consists of 2 parasite populations bearing 2 atlelic forms of p33/32, an immunodominant piroplasm surface protein. Parasite population changes during parasite passages among cattle and tick vectors, and during persistent infection in individual calves were analysed by using allele-specific polymerase chain reaction (PCR). The parasite DNAs were prepared from piroplasms from calves which had been infected with Shintoku stock by inoculation of sporozoite stabilates or parasitized erythrocytes, and from sporozoite stabilates which had been prepared from Shintoku stock-infected ticks. Changes in a dominant parasite population were demonstrated during transmission from calves to vector ticks and from infected ticks to calves. Parasite population changes were also apparent during persistent infection in cattle over several months, and this change is thought to occur under host immune pressure. The results of this study indicate that expression of diverse forms of p33/32 may play a role in parasite persistence within mammalian hosts and its transmission from tick vector.

We compared parasite communities in fish taken from a polluted lake (L. Vatia) and two control lakes before (1986) and after (1995) nine years of markedly reduced chemical and nutrient loading from a pulpmill in central Finland. Discriminant analyses of the 1995 data, using a function based on the 1986 data, showed that the parasite communities in the fish from the two control lakes had changed relatively little, whereas those from L. Vatia had converged on those from the mesotrophic control lake, indicating substantial recovery from the effects of pollution. Only a few species of parasites provided evidence for recovery. These were anodontid glochidia, which had increased markedly in perch, Rhipidocotyle fennica in roach and R. campanula in both fish species. This suggests that the recovery of the polluted lake involved increased populations of anadontid clams in shallow waters. On the other side decrease of Dermocystidium percae on perch fins and Ichthyophthirius multifiliis on roach indicate increased immune responses in the fish, reflecting better water quality. Other parts of the system have apparently not yet recovered.

In a previous paper (Crofton, 1954) it was shown that the rise in egg counts occurring in ewe flocks in spring was related to the time of lambing. It was further shown that the rise in individuals occurred about 6 weeks after parturition and was normally of short duration. While the mean time of increased egg count for each individual was about 2 weeks, the increased egg count in the flock was spread over a period equivalent to the length of the lambing period. It is clear that the apparent rise in the flock as a whole will be greatest when all the flock lambs within a short period. With greater scatter of times of parturition the flock rise will be smaller but will extend over a greater period. This association of rise in egg count with time of parturition is obviously one of considerable selective advantage, in so far as an increase in infective stages will occur at a time when the susceptible lambs have a reduced milk intake and an increased grass intake. The synchronization of availability of infective stages and the occurrence of susceptible individuals is further emphasized by the relation of the period of increase to the time of birth of the lambs, the duration of the period of increase in the flock being directly proportional to the duration of the lambing period. This type of ‘escapement’ control is seen in so many other host-parasite relationships that it makes considerable appeal on general grounds. Other workers, Morgan, Parnell & Rayski (1951), have, however, related the rise in egg counts to climatological factors, but climatic control would not give the close synchronization described above.

Epidemiological modelling can be a useful tool for the evaluation of parasite control strategies. An age-structured epidemiological model of intestinal helminth dynamics is developed. This model includes the explicit representation of changing worm distributions between hosts as a result of treatment, and estimates the morbidity due to heavy infections. The model is used to evaluate the effectiveness of different programmes of age-targeted community chemotherapy in reducing the amount of morbidity due to helminth infection. The magnitude of age-related heterogeneities is found to be very important in determining the results of age-targeted treatment programmes. The model was verified using field data from control programmes for Ascaris lumbricoides and Trichuris trichiura, and was found to provide accurate predictions of prevalence and mean intensities of infection during and following different control regimes.

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.

Chagas' disease, caused by the protozoan Trypanosoma cruzi, represents a serious health problem in Latin America, and the available chemotherapy, which is based on 2 nitro-derivatives, is not satisfactory. In folk medicine, natural products including naphthoquinones have been employed for the treatment of different parasitic diseases. In the pursuit of alternative drugs for Chagas' disease, we investigated the mechanism of action of the triazolic naphthoquinone (TN; 2,2-dimethyl-3-(4-phenyl-1H-1,2,3-triazol-1-yl)-2,3-dihydronaphtho[1,2-b]furan-4,5-dione), which is the most active compound against T. cruzi trypomastigotes among a series of naphthofuranquinones. TN was active against the 3 parasite forms producing a dose-dependent inhibitory effect. In epimastigotes, TN induced reservosome disruption, flagellar blebbing, Golgi disorganization, the presence of cytosolic concentric membrane structures and abnormal multiflagellar parasites. The treatment also led to the appearance of well-developed endoplasmic reticulum profiles surrounding organelles that associated with an increase in monodansylcadaverine labelling, suggesting autophagy as part of the TN mechanism of action. Interestingly, no ultrastructural damage was detected in the mitochondria of naphthoquinone-treated epimastigotes. Flow cytometric analysis demonstrated an impairment of mitosis, an increase in ROS production and the maintenance of mitochondrial membrane potential. TN could be a good starting point in the investigation of a chemotherapeutic approach for the treatment of Chagas' disease.

Many marine fisheries in South American Atlantic coasts (SAAC) are threatened by overfishing and under serious risk of collapsing. The SAAC comprises a diversity of environments, possesses a complex oceanography and harbours a vast biodiversity that provide an enormous potential for using parasites as biological tags for fish stock delineation, a prerequisite for the implementation of control and management plans. Here, their use in the SAAC is reviewed. Main evidence is derived from northern Argentine waters, where fish parasite assemblages are dominated by larval helminth species that share a low specificity, long persistence and trophic transmission, parasitizing almost indiscriminately all available fish species. The advantages and constraints of such a combination of characteristics are analysed and recommendations are given for future research. Shifting the focus from fish/parasite populations to communities allows expanding the concept of biological tags from local to regional scales, providing essential information to delineate ecosystem boundaries for host communities. This new concept arose as a powerful tool to help the implementation of ecosystem-based approaches to fisheries management, the new paradigm for fisheries science. Holistic approaches, including parasites as biological tags for stock delineation will render valuable information to help insure fisheries and marine ecosystems against further depletion and collapse.

The Plasmodium falciparum erythrocyte membrane protein 1 antigens that are inserted onto the surface of P. falciparum infected erythrocytes play a key role both in the pathology of severe malaria and as targets of naturally acquired immunity. They might be considered unlikely vaccine targets because they are extremely diverse. However, several lines of evidence suggest that underneath this molecular diversity there are a restricted set of epitopes which may act as effective targets for a vaccine against severe malaria. Here we review some of the recent developments in this area of research, focusing on work that has assessed the potential of these molecules as possible vaccine targets.

The lack of an adequate model for Leishmania (Viannia) braziliensis infection is a limiting factor for studying American tegumentary leishmaniasis (ATL). The golden hamster (Mesocricetus auratus) is a promising model because besides being highly susceptible to dermotropic Leishmania infection, the lesions are very similar to cutaneous leishmaniasis (CL) in humans. However, different Leishmania isolates or species and/or protocols have resulted in different outcomes, whereas no study has evaluated the reproducibility of L. braziliensis infection in this model. The natural history of L. braziliensis infection in 34 hamsters was evaluated by using a single parasite isolate in 8 independent experiments under similar experimental conditions. Clinical, histological and immunological analyses were performed. The hamsters presented skin ulcers similar to those observed in ATL. The intra-experiment lesion increment tended to show an intermediary variance. Histological analysis of infected skins showed granulomatous reaction, scarce amastigotes, and Schaumann's bodies. Blood lymphocytes proliferated in response to leishmanial antigens. The severity of the infection was positively correlated to spleen weight, and the titres of anti-Leishmania IgG antibodies. Our findings indicate that the hamster is an appropriate model for immunopathogenesis studies of CL caused by L. braziliensis, supporting its use in clinical, vaccine and chemotherapy experimental protocols.

Lowland populations of the Plasmodium berghei group are compared with strains from the highlands of Katanga, Republic of Zaire, and it is concluded that the former warrant separate specific status. It is proposed that Plasmodium berghei yoelii of the Central African Republic be raised to a species, P. yoelii, and the lowland subspecies from the CAR, Brazzaville and Nigeria be moved to this species as P. y. yoelii, P. y. killicki and P. y. nigeriensis. P. berghei from Katanga would then revert to a monotypic species.

Differences between P. berghei and P. yoelii are in distribution, hosts, optimum temperatures of sporogony, sizes of mature oocysts and of sporozoites, rates of growth and minimum maturation times of tissue schizonts in the liver of the white rat, the forms of six enzymes and the DNA.

Differences between three subspecies of P. yoelii are in distribution, the sizes of mature oocysts and of sporozoites, sizes of tissue schizonts in the liver of the white rat, the effect of tissue forms on the nuclei of infected parenchymal cells and the electrophoretic forms of the enzyme GDH.

Habronema microstoma and Habronema muscae (Spirurida: Habronematidae) are parasitic nematodes which infect the stomach and/or skin of equids. The accurate diagnosis of gastric habronemosis is central to studying its epidemiology, but data on its distribution and prevalence are lacking, mainly due to the limitations of clinical and coprological diagnosis in live horses. To overcome this constraint, a two-step, semi-nested PCR-based assay was validated (utilizing genetic markers in the nuclear ribosomal DNA) for the specific amplification of H. microstoma or H. muscae DNA from the faeces from horses (n=46) whose gastrointestinal parasite status had been determined at autopsy and whose faeces were examined previously using a conventional parasitological approach. Of these horses examined at autopsy, some harboured adults of either H. microstoma (n=19) or H. muscae (n=4), and others (n=7) harboured both species. Most of them were also infected with other parasites, including strongylid nematodes (subfamilies Cyathostominae and Strongylinae), bots and/or cestodes; there was no evidence of metazoan parasites in 2 horses. Larvated spirurid eggs were detected in the faeces of 1 of the 30 horses (3·3%) shown to be infected with Habronema at autopsy. For this set of 46 samples, the PCR assay achieved a diagnostic specificity of 100% and a sensitivity of ~97% (being able to specifically detect as little as ~0·02 fg of Habronema DNA). The specificity of the assay was also tested using a panel of control DNA samples representing horse, the gastric spirurid Draschia megastoma and 26 other species of parasites from the alimentary tract of the horse. H. microstoma, H. muscae and D. megastoma could be readily differentiated from one another based on the sizes of their specific amplicons in the PCR. The results of this study showed that the performance of the PCR for the diagnosis of gastric habronemosis was similar to that of autopsy but substantially better than the traditional coprological examination procedure used. The ability to specifically diagnose gastric habronemosis in equids should have important implications for investigating the epidemiology and ecology of H. microstoma and H. muscae.

Schistocephalus solidus is a small avian pseudophyllidean cestode which, on account of its small size and the ease with which its plerocercoid may be cultured to maturity in vitro, has been used within recent years for the study of cestode physiology.

Biomphalaria glabrata is a major intermediate host for the helminth parasite Schistosoma mansoni. Beginning in the mid-20th century, studies were carried out with this snail species to identify the immunological and genetic components that might be involved in controlling schistosome development. A number of genetically well-defined snail stocks were derived as a direct result of these studies and have since played major roles in helping investigators to identify important cellular and humoral components in the snail/schistosome relationship. This review will explore the historical development of these stocks and describe some of the major advances in several areas of medical malacology that have been made possible by their use.

Marteilia refringens is a paramyxean parasite which infects the flat oyster Ostrea edulis and mussels (Mytilus galloprovincialis), where it has been attributed to a separate species, Marteilia maurini, by several authors. Doubts persist though as to the existence or not of two species of Marteilia in Europe. We have devised a molecular method for the diagnosis of M. refringens based on 358 bp nested-PCR of the rDNA intergene spacer (rDNA IGS) which is capable of detecting 0·5 fg of M. refringens DNA. Molecular characterization of this spacer indicates that the Marteilia parasites which infect oysters and mussels are two different strains of the same species.

Gyrodactylus salaris infects numerous salmonid species, ranging from the fully susceptible (Norwegian strains of Salmo salar), through species which, though initially susceptible, eventually eliminate their infections (Salvelinus alpinus and S. fontinalis) to entirely resistant (Salmo trutta) species. Here we describe experiments in which Salvelinus alpinus, S. fontinalis and Salmo trutta, implanted with hydrocortisone acetate to simulate stress-induced immunosuppression, were challenged with G. salaris. With previously uninfected Salvelinus fontinalis, G. salaris infections on fish treated with hydrocortisone acetate grew larger, and for longer, than on sham-treated controls. A similar result was obtained with S. trutta. Patterns of infection on Arctic charr, Salvelinus alpinus, were more complex, because individual fish varied from susceptible to highly resistant. Fish were therefore initially infected with G. salaris, and the most highly resistant group of individuals identified and disinfected. After 6 months recovery from this primary infection, hydrocortisone acetate was administered to half the fish, and all were challenged with G. salaris. Parasite populations on the hydrocortisone-treated individuals were consistently larger than those on the sham-treated controls, exceeding 30 parasites per fish after 5 weeks, in comparison with less than 10 parasites per fish on controls. These results indicate that hydrocortisone administration can lead to enhanced gyrodactylid populations on a range of salmonids. This suggests that the response to G. salaris is mediated by the immune system, and that the spectrum of responses observed in different species are, at least in part, due to the same mechanism. At a practical level, stress-induced immunosuppression during handling and transport of cultured salmonids may prove an important factor in the dissemination of G. salaris between watersheds.

Since the in vitro production of trypomastigote stages of Trypanosoma cruzi in cell culture is frequently limited by (1) host cell overgrowth and (2) by the unequal redistribution of parasites after cell division resulting in asynchronous release of trypomastigotes, a culture system was devised in which host cell mitosis was inhibited by irradiation prior to parasite infection. L-6 rat myoblast cells when exposed to 3000 rad. of gamma radiation lost their ability to divide but remained susceptible to infection with, and capable of supporting the intracellular growth of, T. cruzi. Using this approach it proved possible to have virtually 100% of cells infected and achieve much better synchronization of trypomastigote release than with conventional culture systems. Additionally, the total number of parasites provided approached 1 × 109 trypomastigotes/ 150 cm2 flask, a significant increase over other culture systems. Preliminary studies with Plasmodium fallax and Eimeria tenella indicate that irradiated host cells may be utilized to advantage for the cultivation of other intracellular protozoa.

1. Two species of Allantonematidae were found parasitizing five of twelve species of Drosophila collected at Rothamsted Experimental Station, Harpenden, Herts, England.

2. H. aoronymphium sp.nov. was found in 20·8 % of D. phalerata and 4·3 % of D. kuntzei examined. Parasitylenchus diplogenus sp.nov. was found in about 5 % of D. subobscura, D. obscura, and D. silvestris examined.

3. The type species and taxonomy of the genus Parasitylenchus are discussed.

4. The life history of H. aoronymphium is typical of most allantonematids except that the female is fertilized when she is a fourth-stage larva. P. diplogenus has two sexual generations per cycle.

5. Both nematodes have a precocious development of the male and a retarded development of the female genital system. Both destroy host gonads.

6. The specificity of the nematodes is not a result of dissimilar ovipositing habits of the hosts, but shows a similarity to the taxonomic assignment of the hosts to subgenera.

Although superseded by more recent and biologically realistic studies, Crofton's (1971b) model of host–parasite associations remains of interest as the simplest model which captures the essentials. Even if its simplifying assumptions are all accepted, Crofton's model has two defects: the first is that its general conclusions are drawn from numerical simulations for a very restricted range of parameter values; the second is that the probability for a parasite transmission stage to succeed in establishing itself in a host is not constrained to be less than unity, as biologically it must be. The present paper remedies these two defects, by giving analytical results valid for all values of the parameters, and by demanding that the parasite transmission factor indeed saturates to unity. Some of Crofton's conclusions remain intact, others are significantly altered.

(1) Crofton (1971b) has presented a mathematical model which aims to exhibit some of the essential dynamical properties of host–parasite associations. The extreme biological simplicity of this model (e.g. hosts and parasites have the same generation time) makes it applicable to few real systems, and later models (Anderson & May, 1977; May & Anderson, 1977) have added many more general biological features in an effort to makecontact with empirical data. Nevertheless, Crofton's model retains pedagogical value as the basic model.

(2) Even within its own framework ofsimple assumptions, Crofton's model has two defects. The first is thatthe general conclusions about its dynamical behaviour are drawn from numerical stimulations for a re stricted, and not necessarily representative, range of parametervalues. The second is that the factor describingthe input of parasite transmission stages into the next generation of hostsdoes not saturate to unity, as its biological definition implies it must. Thepresent paper gives an analytical account of the dynamical behaviour of Crofton's model, valid for all values of the relevant biological parameters, and with a parasite trans mission factor that does saturate to unity. The ensuing conclusions are in several respects significantly differentfrom Crofton's

(3) The intrinsic growth rates of the host and parasite populations are defined as λ and A; the negative binomial parameter k measures the overdispersion of parasites among hosts (small k corresponds to high overdispersion); and L characterizes thelethal level of parasites per host.Then unless λ1+1/k λ A exp (– 1[L) no equilibrium state is possible, andthe host population undergoes Malthusiangrowth that the parasites cannot check. This inequality tends to be satisfied if k is not too small, λ not too large, and A significantly larger than λ: see Figs 1, 2, and 4.This aspect of the model derives from the saturation of the parasite transmission factor, and is omitted fro Crofton's discussion.

(4) When an equilibrium does exist, the following observations can be made. The equilibrium host population H* is given by eq. (15): it de creases with increasing A; increases with increasing λ; is roughly inde pendent of L; and increases with increasing parasite overdispersion for small k (k < In λ);, while being roughly independent of k for larger k. Theequilibrium number of parasites per host m* is given by eq. (9): it is independent of A; increases roughly linearly with L; increases with increasing overdispersion or λ for small k (k < in λ); and increases slowly with λ, and is roughly independent of k, for larger k. The totalpopulation of parasites at equilibrium is given by P* = H*m*.

(5) The stability of the equilibrium, i.e. its ability to recover from disturbance, depends mainly on λ and on k, as illustrated in Fig. 4. Except for values of λ and k perilously close to the boundary where no equili brium is possible, the disturbed host and parasite populations will return to their equilibrium values by undergoing damped oscillations. The damping will tend to be weak if k is large, or if λ is small.

(6) These conclusions accord with those derived from more detailed and realistic host–parasite models.

(7) The general process, whereby thehost–parasite association can be stabilized by overdispersion of parasites, is dynamically similar to that whereby prey–predator or host–parasitoid associationscan be stabilized by differential aggregation of predators or by explicit refuges for the prey.

Previous observations, which showed that treatment of mice with hydrocortisone acetate around the time of their infection with Schistosoma mansoni reduced the mature worm burden, have been confirmed. The number of eggs produced by the surviving worms during early patency was also significantly reduced. Cyclophosphamide and another steroid immunosuppressant, betamethasone, also caused a reduction in fecundity of S. mansoni when given at the time of infection, as did T-cell deprivation of the mice by adult thymectomy and injection of anti-thymocyte serum 1 month before infection. There was no effect of these three treatments on worm numbers. In contrast, injection of mice with anti-thymocyte serum at the time of infection marginally increased the size of the mature worm burden. The deleterious effects of hydrocortisone acetate on S. mansoni worm numbers and fecundity were only apparent if the steroid was given within 1 week of infection. Indomethacin, a compound which inhibits tissue inflammatory reactions by inhibiting prostaglandin synthesis, and which therefore mimics one of the actions of corticosteroids, had no effect on S. mansoni worm maturation and fecundity.

1. Large numbers of minnows (Phoxinus phoxinus (L.)) from Fron Goch Pool, near Aberystwyth, have been found to be heavily infested with Diplostomulum pelmatoides Dubois ( = D. phoxini (Faust)), the parasites occurring in very large numbers in the brain, spinal cord and eyes.

2. The excretory system of the worm has been worked out in detail. There are fifty-two pairs of flame cells.

3. Infested heads, brains and eyes have been fed to the domestic duck and the adult stage of the worm Diplostomum pelmatoides Dubois obtained.

4. Some morphological details of the adult worm are described.

5. The natural bird host of the parasite in this country is not yet known, nor the stages in the molluscan host.