Although superseded by more recent and biologically realistic studies, Crofton's (1971b) model of host–parasite associations remains of interest as the simplest model which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.

Although three other Ixodes species were found, Ixodes ricinus L., when it occurs, is by far the most important tick parasite of wild and domestic animals on hill and moorland grazings in northern England. In the following, ‘tick’ or ‘ticks’ refer to I. ricinus.

The previous list of British mammal hosts, wild and domestic, to the tick (any stage) is largely confirmed, and increased approximately from twenty-three to twenty-nine species; and birds, similarly, from twenty-one to thirty-nine species. The preponderance of small mammals and birds among the twenty-four new-recorded host species confirms the impression that previous workers tended to confine their attentions to the larger animals. The host list is still incomplete, and the suggestion that the tick will parasitize any bird or mammal it may meet is strongly reinforced.

No host occurs below the reptile level among vertebrates; and among British reptiles, only the common lizard is a host—rarely. There appear to be no hosts among invertebrates.

All the farm and domestic mammals from horse down to cat are hosts to female ticks, but poultry have never been recorded—probably because they do not much frequent the tick-infested pastures.

Only the larger wild mammals and birds are hosts to female ticks. In northern England these include eight mammals and three birds. For Britain as a whole two other mammals are known, while a further four mammals and one bird are likely hosts.

With or without the inclusion of domestic animals, proportionately more mammal than bird species are hosts to female ticks. This is because there are more mammal than bird species large enough to be hosts.

All the recorded British hosts, except shrew and possibly mole, are hosts in some degree to nymphs, and all, without exception, are hosts to larvae.

A certain amount of self-deticking is practised by birds, and very probably by some of the smaller mammals. Birds never have ticks attached within beak-reach.

Appetite and growth were studied in primary schoolboys (6–10 years) infected with Ascaris lumbricoides (86%) and Trichuris trichiura (100%) who received a single dose of pyrantel pamoate (which has little or no effect on Trichuris trichiura ) or a placebo. Boys were examined, allocated at random by descending Ascaris egg count to pyrantel (PR, n = 36) or placebo (PL, n = 36) groups, treated, and re-examined 3 and 7 weeks later. The 2 groups did not differ significantly before treatment in helminth infections, appetite, or growth. Three and 7 weeks after treatment, the PR group exhibited significantly greater increases than did the PL group in weight (0·2 kg and 0·4 kg more, respectively) and percentage weight-for-age (0·6% and 1·7% points more, respectively). Appetite increased significantly in the PR group at 3 and 7 weeks (P < 0·0005 and P < 0·01, respectively) but not in the PL group. The prevalence and intensity of A. lumbricoides infection were greatly reduced in the PR group at 3 and 7 weeks (both P < 0·0001) but not in the PL group. We conclude that treatment with pyrantel pamoate may improve appetite and growth in schoolchildren in areas where A. lumbricoides infections and poor growth are highly prevalent.

Conventional mice and congenitally athymic, nude mice were infected with 20 metacercariae of the intestinal trematode Echinostoma revolutum. The sequential events in the pathological changes in the intestine were studied at different intervals post-infection. By day 11 onwards the conventional mice displayed dilatation of the region of the intestine which harboured the parasites. The mucosa in the dilated region showed marked crypt hyperplasia, villous atrophy and subepithelial fibrosis as the most conspicuous features which, together with a hypertrophy of the muscular layers, made the wall of the gut in the dilated region thicker than normal. The changes were thymus-independent as they were found to be as severe in the athymic, nude mice as in the conventional mice. The main histological features observed in the mice are discussed in relation to other conditions with similar changes, such as coeliac disease, nippostrongyliasis and trichinellosis. It is concluded that the present results support the view that there may be more than one effector mechanism of the change.

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.

Myxobolus cerebralis, the myxozoan parasite responsible for whirling disease in salmonid fishes, has a complex life-cycle involving an invertebrate host and 2 spore stages. Water flow rate is an environmental variable thought to affect the establishment and propagation of M. cerebralis; however, experimental data that separates flow effects from those of other variables are scarce. To compare how this parameter affected parasite infection dynamics and the invertebrate and vertebrate hosts, dead, infected fish were introduced into a naïve habitat with susceptible hosts under 2 experimental flow regimes: slow (0·02 cm/s) and fast (2·0 cm/s). Throughout the 1-year study, uninfected fry were held in both systems, the outflows were screened weekly for spores and the annelid populations were monitored. We found clear differences in prevalence of infection in the worms, prevalence and severity of infection in the fish, and host survival. Both flows provided environments in which M. cerebralis could complete its life-cycle; however, both the parasite and its invertebrate host proliferated to a greater extent in the slow flow environment over the 1-year study period. This finding is of significance for aquatic systems where the flow rate can be manipulated, and should be incorporated into risk analysis assessments.

The use of parasites as biological tags to discriminate among marine fish stocks has become a widely accepted method in fisheries management. Here, we first link this approach to its unstated ecological foundation, the decay in the similarity of the species composition of assemblages as a function of increasing distance between them, a phenomenon almost universal in nature. We explain how distance decay of similarity can influence the use of parasites as biological tags. Then, we perform a meta-analysis of 61 uses of parasites as tags of marine fish populations in multivariate discriminant analyses, obtained from 29 articles. Our main finding is that across all studies, the observed overall probability of correct classification of fish based on parasite data was about 71%. This corresponds to a two-fold improvement over the rate of correct classification expected by chance alone, and the average effect size (Zr = 0·463) computed from the original values was also indicative of a medium-to-large effect. However, none of the moderator variables included in the meta-analysis had a significant effect on the proportion of correct classification; these moderators included the total number of fish sampled, the number of parasite species used in the discriminant analysis, the number of localities from which fish were sampled, the minimum and maximum distance between any pair of sampling localities, etc. Therefore, there are no clear-cut situations in which the use of parasites as tags is more useful than others. Finally, we provide recommendations for the future usage of parasites as tags for stock discrimination, to ensure that future applications of the method achieve statistical rigour and a high discriminatory power.

1. The nomenclature and host specificity of Toxascaris leonina is reviewed and the conclusion reached that this parasite occurs in a wide range of feline and canine hosts. Notice is given of application to the International Commission on Zoological Nomenclature, for the suppression of Ascaris leptoptera Rudolphi 1809 and A. microptera Rudolphi, 1819, which appear to be senior synonyms of Toxascaris leonina. The synonyms and hosts of T. leonina are tabulated.

2. The distribution and growth of the larvae of T. leonina in experimentally infected mice, cats, and dogs is reported. A feline and a canine strain were used for these infections and infection of cats and dogs was produced both by feeding the eggs containing second-stage larvae and by feeding mouse tissues containing third-stage larvae.

The ‘Muguga cocktail’ live vaccine, delivered by an infection and treatment protocol, has been widely deployed in Eastern, Central and Southern Africa to protect cattle against East Coast fever, caused by Theileria parva. The vaccine contains 3 component stocks (Muguga, Serengeti-transformed and Kiambu 5). In a previous study, parasites from vaccinated and unvaccinated animals were genotyped with a panel of micro- and minisatellite markers (Oura et al.2004a) and it was shown that only the Kiambu 5 stock establishes a long-term carrier state but there was no evidence for the transmission of this stock. Also parasite genotypes different from the 3 component vaccine stocks were identified in vaccinated animals. We now report a follow-up study on the same farm, some 4 years after the initial vaccination, aimed at establishing the source of the novel parasite genotypes identified in vaccinated cattle, determining the longevity of the carrier state established by the Kiambu 5 vaccine stock and re-examining whether vaccine transmission can occur over a longer time-scale. To do this, samples were taken from vaccinated and unvaccinated cattle and the parasites were genotyped with a series of micro- and minisatellite markers. The data indicate that the vaccine stabilates contain at least 6 parasite genotypes, the Kiambu 5 stock can be detected in many but not all vaccinated cattle for up to 4 years and can be transmitted to unvaccinated cattle which share grazing and that some of the vaccinated animals become infected with local genotypes without causing overt disease.

Analysis of the seasonal occurrence of Argulus coregoni on Oncorhynchus masou and Salmo gairdneri indicates that the parasites overwinter as eggs and that there are 1 or 2 generations/year. Larvae which hatch from the overwintered eggs are abundant from May through July. Individuals hatched in May and June attain maturity and lay eggs in August. Larvae of the second generation are abundant in September, and they lay eggs in October–November which overwinter. Hatched larvae from overwintered eggs may mature in July and deposit overwintering eggs in September. The sex ratio of A. coregoni is about 1:1. A. coregoni shows an age-specific site preference on O. masou. Small parasites occur on almost all parts of the skin and fins, with slightly heavier concentrations on and around the pectoral and pelvic fins. Large parasites are found mainly on the skin around these fins.

Up to 200 laboratory reared Anopheles dirus mosquitoes were fed on each of 496 symptomatic Thai men who had patent, naturally acquired Plasmodium vivax gametocytaemia. Mean gametocyte densities were 455/mm3 (range: 0–3281), geometric mean oocyst number was 9 (0–142), mean frequency of infection was 43% (0–100%), and mean sporozoite number in salivary glands was 9525 (0–285 000). There was little relation between gametocyte density and either oocyst number or frequency of mosquito infection. There were, however, statistically strong positive correlations between oocyst numbers and frequency of infection, and between number of oocysts and number of salivary gland sporozoites. The data suggest that each oocyst contributed about 850 sporozoites to a gland infection.

Single receptors seen with the electron microscope to consist of a terminal cilium embedded in a nerve bulb have been found in the skin parasitic monogenean Gyrodactylus sp., adult and larval Entobdella soleae, Leptocotyle minor (adult) and in the endoparasitic juvenile of Amphibdella flaviolineata. Their distribution in Gyrodactylus sp. has been mapped using phase-contrast microscopy, and staining with the indoxyl acetate method for non-specific esterases revealed a nervous connexion between the lateral ventral nerve cord and the ‘tangoreceptor’. The general relationships of the nervous system in Gyrodactylus were investigated by staining with the thiocholine method for cholinesterase. The ‘sensory’ neurone contains vesicles, microtubules and mitochondria and the whole nerve bulb is sealed into the epidermis by means of septate desmosomes. The basal body of the cilium is not greatly modified and there is no obvious rootlet system. The terminal cilium of single receptors in Entobdella soleae has a 9 + 2 structure. A compound sense organ from the head of Entobdella soleae has also been described. The significance of the fibre arrangement in the terminal cilia and the possible roles of these sense organs have been discussed.

I should like to thank Dr D. L. Lee for training in electron microscope techniques and for his encouragement, Dr P. Tate for his continued interest and the staff of the Marine Biological Laboratory, Plymouth, especially Mr G. Best and Mr J. E. Green, for their assistance. This work was conducted during tenure of a Science Research Council Fellowship and the Tucker-Price Research Fellowship of Girton College.

Recent work has highlighted and enumerated the economic annual losses due to Neospora caninum abortions worldwide, which should provide strong motivation for the control of bovine neosporosis. However, with the recent withdrawal from sale of the only commercially available vaccine, control options for N. caninum have become more restricted. While researchers continue to work on developing alternative efficacious vaccines, what are the control options presently available for the cattle industries? At the practical level, recommendations for ‘Test-and-cull’, or ‘not breeding from seropositive dams’ stand diametrically opposed to alternative options put forward that suggest a primary producer is better advised to keep those cows in the herd that are already seropositive, i.e. assumed to be chronically infected, and indeed those that have already aborted once. Treatment with a coccidiostat has been recommended as the only economically viable option, yet no such treatment has gained official, regulatory approval. Dogs are central to the life cycle of N. caninum and have repeatedly been associated with infection and abortions in cattle by epidemiological studies. Knowledge and understanding of that pivotal role should be able to be put to use in control programmes. The present review canvasses the relevant literature for evidence for control options for N. caninum (some of them proven, many not) and assesses them in the light of the authors’ knowledge and experience with control of N. caninum.

Eggs of T. colubriformis and H. contortus were separated from faeces. Their response to desiccation was compared by subjecting them, at different stages of development, to a range of relative humidities. Many eggs of T. colubriformis at all stages of development survived desiccation at relative humidities down to 75%, whereas the only H. contortus eggs to survive were those exposed to the highest relative humidity and which had already undergone a substantial amount of development before desiccation.

Hosts are typically simultaneously co-infected by a variety of microparasites (e.g. viruses and bacteria) and macroparasites (e.g. parasitic helminths). However, the population dynamical consequences of such co-infections and the implications for the effectiveness of imposed control programmes have yet to be fully realised. Mathematical models may provide an important framework for exploring such issues and have proved invaluable in helping to understand the factors affecting the epidemiology of single parasitic infections. Here the first population dynamic model of microparasite-macroparasite co-infection is presented and used to explore how co-infection alters the predictions of the existing single-species models. It is shown that incorporating an additional parasite species into existing models can greatly stabilise them, due to the combined density-dependent impacts on the host population, but co-infection can also restrict the region of parameter space where each species could persist alone. Overall it is concluded that the dynamic feedback between host, microparasite and macroparasite means that it is difficult to appreciate the factors affecting parasite persistence and predict the effectiveness of control by just studying one component in isolation.

The parasite community of animals is generally influenced by host physiology, ecology, and phylogeny. Therefore, sympatric and phylogenetically related hosts with similar ecologies should have similar parasite communities. To test this hypothesis we surveyed the endoparasites of 5 closely related cheilinine fishes (Labridae) from the Great Barrier Reef. They were Cheilinus chlorourus, C. trilobatus, C. fasciatus, Epibulus insidiator and Oxycheilinus diagramma. We examined the relationship between parasitological variables (richness, abundance and diversity) and host characteristics (body weight, diet and phylogeny). The 5 fishes had 31 parasite species with 9–18 parasite species per fish species. Cestode larvae (mostly Tetraphyllidea) were the most abundant and prevalent parasites followed by nematodes and digeneans. Parasites, body size and diet of hosts differed between fish species. In general, body weight, diet and host phylogeny each explained some of the variation in richness and composition of parasites among the fishes. The 2 most closely related species, Cheilinus chlorourus and C. trilobatus, had broadly similar parasites but the other fish species differed significantly in all variables. However, there was no all-encompassing pattern. This may be because different lineages of parasites may react differently to ecological variables. We also argue that adult parasites may respond principally to host diet. In contrast, larval parasite composition may respond both to host diet and predator-prey interactions because this is the path by which many parasites complete their life-cycles. Finally, variation in parasite phylogeny and parasite life-cycles among hosts likely increase the complexity of the system making it difficult to find all-encompassing patterns between host characteristics and parasites, particularly when all the species in rich parasite communities are considered.

A morphological study has been carried out to determine the effect of the active sulphoxide metabolite of the benzimidazole anthelmintic, albendazole (ABZ-SO) on the adult liver fluke, Fasciola hepatica. Whole flukes were treated with ABZ-SO for 12 and 24 h at a concentration of 10 μg/ml. The changes in response to drug treatment were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and tubulin immunocytochemistry (ICC). No surface changes were apparent following 12 h ABZ-SO treatment, but localized blebbing was observed after 24 h, which became more extensive towards the posterior region of both surfaces. TEM of sections from the posterior midbody region revealed that ABZ-SO caused the accumulation of secretory bodies in the tegumental cells and in their cytoplasmic connections and, after 24 h, just above the basal plasma membrane. Localized blebbing of the apical membrane also occurred. The morphology of the Golgi complexes within the tegumental cells began to change after 12 h treatment with ABZ-SO and, by 24 h, few complexes were observed. A distinct increase in tubulin immunoreactivity occurred after 12 h treatment, but this decreased after 24 h. The results obtained are consistent with those expected for microtubule inhibition. They are discussed in relation to the action of established microtubule inhibitors, as well as the benzimidazole derivative, triclabendazole.

The pathogenicity and pathology of Ascaridia galli infections of poultry were studied using repeated doses of varying numbers of eggs. No significant effect due to infection was seen on p.c.v. and Hb. values of infected chickens. Haemo concentration occurred parallel with sporadic diarrhoea, which follows high levels of repeated dosing. This phenomenon complicated the interpretation of blood protein values. A/G ratios were, however, worked out and analysed. No significant differences were noticed between the groups and it was concluded that under the levels of inoculation employed during the study there were no significant effects of A. galli infections on blood protein values as well as p.c.v. and Hb.values of infected chickens.

Symptomatic developments, macroscopic lesions and pathological changes of intestinal tissues due to infection were followed up in detail. They were most severe in chickens on the least nutritive diet and sustaining the highest levels of repeated doses. Pathogenicity of larval and adult parasites was described. The presence of large numbers of adult parasites was associated with intestinal obstruction in host. This was seen mostly in chickens on low nutrition and sustaining either low or high levels of repeated inoculation, and very rarely also in chickens under high plane of nutrition but sustaining repeated inoculation of small numbers of eggs. Cellular infiltration as a result of infection was found to be extremely rare.It was present as fibroblasts and only in association with dead larvae.

The author is indebted to the Department of Technical Cooperation, under whose Fellowship the study was carried out; the Department of Zoology, Edinburgh University, for facilities provided; the Ministry of Agriculture, Fisheries and Food; Lasswode Poultry Research Laboratories, for the free supply of chickens used here; Dr J. A. Campbell of Edinburgh University for his keen interest, encouragement and advice throughout this work; and finally to G. Obiwelozor for reproducing the photographs from the original manuscript.

The individual and combined effects of cadmium (Cd) and 3,3′,4,4′,5-pentachlorobiphenyl (PCB 126) on the antibody response of fish against metazoan parasites were tested. Eels experimentally infected with the swim bladder nematode Anguillicola crassus were exposed to sublethal concentrations of Cd and PCB 126. Cd was added to the water resulting in an effective concentration of 21·7±12·8 μg/l (mean±S.D.). PCB 126 was applied orally at a dose of approximately 100 ng PCB 126 per g body weight. At the end of the experiment, 76 days post-infection (p.i.), eels were found to be infected with 2–3 worms. Immunoblot analyses revealed that the body wall of adult worms was the most suitable crude antigen, and was subsequently used for an ELISA to evaluate the immune response of A. anguilla under various conditions. A significant increase of Anguillicola-specific antibodies in the peripheral blood was first detected 61 days p.i., indicating that it was not the invasive larvae but the adult worms which elicit the antibody response. The presence of Cd in the concentrations applied did not appear to modulate the production of antibodies. In contrast, the exposure to PCB 126 resulted in a complete suppression of the antibody response. A similar effect was also found for the combined exposure of the infected eels to Cd and PCB 126. A suppressed immune response, as demonstrated here, may be the reason why hosts exposed to environmental pollution became often much more easily infected than unexposed conspecifics.

Mice inoculated with microfilariae of the filarial nematode Onchocerca lienalis clear their parasites over a period of 3–4 months and are highly resistant to re-infection. We have investigated the comparative roles of the eosinophil, macrophage and neutrophil in effecting this parasite clearance, employing agents specifically to perturb cell function in vivo. Using the anti-IL-5 monoclonal antibody TRFK-5, we show that eosinophils are of primary importance in effecting resistance to re-infection. Ablation of macrophages (with carbon) and neutrophils (with the monoclonal antibody NIMP-R14) had no effect on parasite clearance following re-infection. Neutralization of these 3 cell types during a primary infection showed that while the removal of both eosinophils and macrophages caused a small but significant delay in parasite clearance, the depletion of neutrophils had no effect. This report describes the first direct evidence for eosinophil-mediated killing of microfilariae in vivo, and is consistent with Th-2 cell responses previously described in this model.