A mathematical model of the life-cycle of Echinococcus granulosus in dogs and sheep in New Zealand is constructed and used to discuss previously published experimental and survey data. The model is then used to describe the dynamics of transmission of the parasite, and the means by which it may be destabilized. It is found that under the conditions that prevailed in New Zealand during the late 1950s, at the time of surveys of this parasite, the dog–sheep life-cycle was not regulated by any effective density-dependent constraint. In contrast there was evidence for an effective acquisition of immunity to reinfection by cattle. The long time to maturity of the cyst in sheep, together with the practice of feeding aged sheep to dogs, provides a time delay in the intermediate host. By comparison, the time to maturity of the adult stage in dogs is short, but it is of sufficient magnitude to be a key factor in the destabilization of the system by a regular dog-dosing programme. The model used to describe the life-cycle is a linear integrodifferential equation of the Volterra type. Such equations are intrinsically unstable in that a small perturbation in parameters can drive a previous equilibrium solution to zero. At the time of the surveys, the value of the basic reproductive rate, R0 was close to 1, and it has since been reduced below 1 by control measures.

The course of the migration of infective larvae of Brugiapahangi within the mosquito, Aedes aegypti, is described. The presence of blood or eggs in the abdomens of mosquitoes at a time when the infective larvae are beginning their migration out of the thorax leads to a significant increase in the proportion of larvae accumulating in the abdomen. The evidence supports the hypothesis that infective larvae may accumulate in the labium as a direct consequence of the insect's internal anatomy i.e. infective larvae on moving into a confined space tend to remain there. In addition, the rate at which infective larvae migrate into the labium is shown to be density dependent.

A glycoprotein cross-reactive among Schistosoma mansoni, Trichinella spiralis and Fasciola hepatica was identified and characterized by use of monoclonal antibodies prepared against S. mansoni glycoproteins. Four monoclonal antibodies recognized the same antigen which was one of the major S. mansoni glycoproteins precipitated by sera of hosts infected with either S. mansoni or T. spiralis. This antigen was expressed in S. mansoni cercariae, adult male and female worms, and eggs, and in S. haematobium but not in S. japonicum. Radio-immunoprecipitation and partial proteolytic digest mapping showed that the monoclonal antibodies each recognized a unique epitope. These epitopes were heat labile, sensitive to chaotropic agents, but resistant to reduction and alkylation or digestion with glycosidases, indicating that the recognition sites were amino acids and not carbohydrates. Epitopes recognized by the four monoclonal antibodies were expressed in F. hepatica, whereas only two were expressed in T. spiralis. Analysis by immunofluorescence microscopy showed that the antigen was expressed in S. mansoni in the parenchymal tissue and on the surface of the dorsal tubercles; in mature F. hepatica in the parenchymal tissue, vitelline glands and eggs; in immature F. hepatica only in the parenchymal tissue and in larval T. spiralis in the hypodermis.

The human malaria parasite, Plasmodium falciparum, is currently being actively studied by molecular biologists. It is hoped that the use of recombinant DNA techniques in this area will give new insights into the biology of the organism and, at the same time, provide new approaches to diagnosis and vaccine development.

Our own studies employ the blood stages of the parasite and cover three main areas: enzymes of importance in parasite metabolism; antigens of potential use in a subunit vaccine; and repetitive DNA as a probe able to distinguish genetically different isolates of P. falciparum and as a species-specific diagnostic tool in human and mosquito infections.

Human leishmaniasis is a world-wide public health problem with more than 400000 new reported cases/year (Anon, 1984) in the tropics and subtropics. Were (1985) has estimated that Leishmaniases affect about 20 million people in the Third World. In hospitals where treatment is available, initially all positive cases are treated equally; it is the prognosis and follow-up that varies according to the causative organism. A wide variety of species or subspecies of Leishmania may be present in a small verrucose lesion developing after an infected sandfly bite or appear in lesions as the disease progresses. Depending on which Leishmania is present a clinial decision must be made whether to treat with drugs of varying toxicity or not.

Protoscoleces of Echinococcus granulosus absorb the amino acids cycloleucine and α-aminoisobutyric acid (AIB) by a combination of mediated uptake and diffusion. After correcting for the latter, values for Kt and Vmax of 0·124 mM and 0·947 nmoles/mg protein/2 min for cycloleucine were calculated; corresponding values for AIB were 0·039 mM and 0·139 nmoles/mg protein/2 min. Both amino acids were accumulated against a concentration gradient and a comparison of Kt and Kt values determined in mutual inhibition experiments suggested that both cycloleucine and AIB share a common uptake locus (loci). Cycloleucine uptake was pH-dependent and could be inhibited by a variety of other amino acids. Neither D- nor L-proline inhibited cycloleucine absorption but D-methionine, D-alanine, D-leucine, D-valine and D-serine were much more effective inhibitors than their L-counterparts.

A variety of morphological forms developed during monophasic culture of ruptured brood capsules of equine Echinococcus granulosus. Attached to some protoscoleces were small posterior bladders, with a surface ultrastructure of truncated microtriches, and these never developed a laminated layer. In contrast, small free vesicles of similar size and ultrastructure, but devoid of an attached protoscolex, produced a layer by 20–30 days. The surface ultrastructure of both these forms is similar to that of the protoscolex attachment stalk and the brood capsule lining, and posterior bladders and small free vesicles are considered to be derived from portions of the stalk±part of the brood capsule lining retained at protoscolex release. Also present were large vesicles interpreted from their size and surface anatomy as ruptured, everted brood capsules. When devoid of attached protosoleces these formed cysts each with a laminated layer, whereas similar vesicles with several attached protoscoleces did not develop a layer, or only very occasionally after 70 or more days. In contrast, a few large vesicles with a single attached protoscolex produced a laminated layer. The possibility that protoscolex attachment to stalk remnants or brood capsules inhibits the cystic development of these structures is discussed.

We present an analysis of the survival and migration rates of the infective (L3) stages of Oslertagia ostertagi and Cooperia oncophora. Although the majority of laboratory studies show that the survival of the L3 stage depends upon temperature, moisture and the age of the larvae, a simple mathematical model of larval demography, in which their mortality and migration rates are held constant, provides as good agreement between observed and predicted larval counts as models in which these rate processes are made explicit functions of larval age and microclimate. Maximum-likelihood estimates of larval mortality rates in the faeces and on the herbage are 0·0284/day and 0·00887/day respectively. The average migration rate from faeces to herbage under temperate Northern European conditions is estimated as 0·00884/day. Finally, we discuss the probable scale of L3 larval losses due to desiccation and lavage (active or passsive migration into the soil).

The reaction of Balb/c mice to primary and secondary subcutaneous infection with chinoeoccu.s granulosus protoscoleces (PSC) is described. From 3 to 14 days following primary exposure to PSC, draining lymph nodes increase in weight and there is expansion of T and B lymphocyte populations, enhancement of in vitro lymphocyte transformational responses and production of PSC-specific 1gM and IgE antibodies. Despite the persistence of viable PSC in host tissues, lymphocyte responses decline to pre-infection values over the period 3–8 weeks post-infection. Secondary exposure to PSC immediately induces lympho proliferation, enhancement of transformational responses, production of IgE antibody and encapsulation of PSC by inflammatory cells. Although specific antibody levels remain high until at least 8 weeks after challenge infection, lymphocyte activity begins to decline after 4 days and is profoundly suppressed by 10 days. Parasite viability appears to be significantly reduced in secondary, as opposed to primary, infection and is associated with the accumulation of large numbers of eosinophils, mast cells and macrophages in infected tissues.

Acanthocephaloides geneticus sp.nov. is described from Arnoglossus laterna (Bothidae) from the Mediterranean littoral (Sète-France). The new species most closely resembles Acanthocephaloides propinquus (Dujardin, 1845) Meyer, 1932, whose type host is Gobius niger (Gobiidae). A genetic comparison of the parasites collected from these two Teleostean fishes has confirmed the value of morphological differences which seemed too slight to raise the populations to species status.

The tegumental surface of Schistosoma mansoni sporocysts is the site of both nutritive and immunological interactions with haemolymph cells and plasma of Biomphalaria glabrata, the schistosome intermediate host. Within minutes of being placed in host plasma, sporocysts acquire plasma antigens, and within 3 h host plasma antigens are present on the surface at near steady state. Though a wide variety of peptides is acquired, there is selection. Furthermore, some differences occur in the peptides acquired from the plasma of susceptible and resistant strains of snail. Acquired antigens are rapidly processed, and are predominantly undetectable in tegumental extracts after a few hours. In contrast, rabbit antibody on sporocysts remains in situ for at least 48 h, so under some conditions there is stable expression of certain tegumental antigenic determinants.These data, obtained using antibodies to snail plasma antigens and to sporocyst tegumental antigens, are discussed in the light of current ideas on the cellular and molecular basis of susceptibility and resistance in this host#parasite system.

Some effects of Hymenolepis diminuta on the fecundity of Tribolium confusum are described. Host fecundity is observed to be reduced exponentially with increasing parasite burden/host, although there are differences in the ability of individual hosts to respond to parasitism. Of particular interest is the finding that host fecundity is greatly reduced in young beetles on or by day 14 post-infection (p.i.). This age-related reduction in host fecundity is discussed in relation to the population dynamics of this hostr-parasite relationship.

Testes in the male early acanthella of Moniliformis dubius are cellular. The testes in the middle and late acanthellae and cystacanth stage also contain individual cells but these are embedded in a compact supporting syncytium which also forms the boundary of the testis. The testes are embedded in the cytoplasmic wall of the ligament sac in the early acanthella but after this stage they become exposed in the cavity of the sac when the surfaces of the testes become covered by a surface coat. Certain cells adjacent to testes primordia in the early acanthella appeared to have formed rudiments of vasa efferentia in the middle acanthella.

Interactions between tapeworm species in a single host offer intriguing opportunities for immunological studies that attempt to identify the mechanism(s) underlying protection against cestode infections. Mice that are immunized against Hymenolepis citelli infections were shown to be refractory to subsequent H. diminuta challenge infections. The reciprocity of the response was also demonstrated, although the protection recorded for H. diminuta when mice are sensitized with H. citelli is weaker than that observed when mice are primed with H. diminuta against H. citelli challenge. H. citelli was also shown to be expelled simultaneously during the rejection phase of H. diminuta in concurrent infections, indicating the susceptibility of the former tapeworm to the rejection mechanism initiated by the latter. H. microstoma immunized mice were shown to be strongly protected against heterologous H. citelli challenge. However, mice primed against H. citelli were not as strongly protected against H. microstoma challenge infections: a statistically significant protection was obtained only after a 12-cysticercoid H. citelli primary infection, although a 6-cyst infection did stunt the growth of H. microstoma challenge worms. It is presently suggested that the cross-protective responses observed in the study between H. citelli, H. diminuta and H. microstoma may have emanated from a specific immunological cross-reactivity due to the sharing of similar immunogens.

The role of serum in the adherence of roach (Rutilus rutilus) leucocytes to the plerocercoid of Ligula intestinalis has been investigated in vitro. Roach plerocercoids, either untreated, cultured or killed (fixation in 0·5 % buffered glutaraldehyde or heat at 50°C for 45 mm), were exposed to serum and 4×106 leucocytes obtained from the pronephros of non-infected roach using a discontinuous Percoll gradient. At normal roach serum (NRoS) dilutions of 1:5000 to 1:100, leucocyte adherence was observed on all living parasites but was negligible in killed parasites or in complement-depleted NRoS assays. The number of adherent cells increased with increasing NRoS concentration. Leucocytes bound more avidly in the presence of heat-inactivated immune serum (hi IRoS); at a serum dilution of 1:5000 the percentage of the parasite surface covered by leucocytes was approximately 3 times greater in hi IRoS than in NRoS. Ultrastructural observations on parasites revealed that plerocercoids exposed to cells and hi IRoS had, in some areas, abnormal inclusions in their tegument and lacked a microthrix border and distal cytoplasm. In addition, the efflux of radioactivity from [14C] cycloleucine-labelled worms was greater in parasites incubated in the presence of hi IRos.

Two simple methods of measuring the levels of inequality in reproductive success of different individuals in parasite populations are presented. These techniques are then applied to a number of sets of data for cestodes and acanthocephalans. The analysis suggests that both population density and host nutrition are important in determining the observed degree of inequality in reproductive success and body size. Cestodes, with a more flexible growth form, are shown to exhibit higher levels of inequality than acanthocephalans. The discussion outlines the evolutionary importance of considering variation in the reproductive success of different individuals.

Gametogenesis in Dirofilaria immitis has been studied principally by means of the aceto-orcein chromosomal squash technique, but with additional ultrastructural observations. A terminal germinative zone, in which a continuous and rapid division of germ cells occurs, has been identified in the gonoduct of both male and female worms. Approximately 20% of cells within these germinative zones were in arrested mitotic division following the incubation in vitro of excised gonads in 0·01% colchicine for 4 h. All primary spermatocytes within a 1–2 cm length of the testis proximal to the germinative zone were at the prophase of the 1st meiotic division. In the corresponding region of the ovary, the primary oocytes were similarly at the prophase of the 1st meiotic division in 75% of female worms examined but in the remaining 25% all primary oocytes possessed markedly less condensed, probably interphase nuclei. A possible hormonal control of the cyclical development of primary oocytes, but not primary spermatocytes in D. immitis is suggested. In most of the remaining length of the gonoducts beyond this region of cells at meiotic prophase, the chromatin material of both primary spermatocytes and oocytes is decondensed. Recondensation of chromosomes in the spermatocytes is observed just prior to entry into the seminal vesicle, where meiosis I is completed and meiosis II takes place. In the primary oocyte, completion of meiosis only occurs after fertilization within the seminal receptacle by an entire male gamete. Following the 2 meiotic divisions in the oocyte and subsequent extrusion of the 2 polar bodies, the haploid chromosome complement of the female unites with that of the male, re-establishing the diploid number of the zygote (2n = 10). Male chromosomes within the oocyte remain visible throughout late oogenesis and fusion occurs without the formation of pronuclei.

Interpretation of anthelmintic activity using in vitro screens has, until now, relied on the detection of drug-induced effects on nematode development, viability and motility. A novel biochemical parameter dependent upon the spectrophotometric assay of acetyicholinesterase (AChE), an enzyme secreted in large quantities by certain trichostrongylid nematodes, has been developed to replace these often subjective indices of activity. Using Nippostrongylus brasiliensis, a worm frequently employed for primary screening, the secretion of this enzyme in the presence or absence of a large number of drugs in vitro was determined. During a 4-day incubation period in a complex undefined medium without serum, AChE was secreted by normal 4th larval and immature adult stages of the worm in a linear fashion. All modern broad-spectrum veterinary anthelmintics, regardless of their mode of action, dramatically reduced the amount of enzyme secreted. Correlation between the biochemical and observa tional parameters was excellent and the selectivity of the assay when based solely on enzyme secretion was not lost. Other advantages were that the time required for the activity of certain slow-acting compounds to be detected was reduced from 7 to 4 days and that close microscopical examination of the worms was not necessary.