Since it is concerned with filling in certain details of a picture, long since sketched in outline, the new work in this paper permits only a somewhat disjointed summary.

From a study of Ixodes ricinus L. in nature, the main new facts emerging are:

(I) Unfed adults in a deep vegetation layer such as presented by rough hill or moorland pasture dominated by bents, heather or bracken. (i) Practically all inactive unfed ticks are in the underlying mat, and nearly all of these in the upper portion of the mat. This distribution does not change, summer or winter. (ii) When the tick is active its progression is practically confined to the vertical, i.e. between its niche in the mat and the vegetation tips immediately above. Random undirected movements due to certain circumstances may achieve a 0–8 in. (average 2 in.) change of position in the horizontal plane; but there is apparently no ruling urge to move in the horizontal plane. (iii) The unfed tick comes to the vegetation tips to await a host for only a limited time during the 3-month activity season. Commonly this amounts to about five periods of 4–5 days each if no host is forthcoming. Between the periods of activity it returns to the upper mat. If it stayed constantly at the tips its survival time would be much shorter than normal. (iv) From nymphs engorged in spring, adults emerge in autumn but remain inactive for a considerable period. About half of them may perish within the first 100 days after emergence. Survivors become active during the following March–June. Effective life ends when the tick, if still unfed, finally stops coming to the vegetation tips, the only position from which a host can normally be achieved. After emergence, the effective life of the unfed adult deprived of a host is less than one year; and the actual life is not much more than one year at best. Thus adults not finding a host in spring will be ineffective, i.e. die without reproducing before the next activity season.

The development and survival of the eggs and larvae of T. colubriformis was studied by spreading the daily faecal output of an infected sheep for 1 or 2 weeks on grass plots 6 ft square. The observations were carried out for 3 years and broadly similar results were obtained in each year. Eggs placed on the ground from November to February failed to develop. A large proportion of the eggs placed on the ground in March disintegrate but those which develop do so slowly. Infective larvae are not produced for 8–12 weeks and they are relatively short lived. As conditions become more favourable development becomes more rapid, taking 8 weeks in April 6 weeks in May and only 1 week in June, July and August. A larger proportion of the eggs develop into infective larvae and their time of survival increases. On most plots larvae did not die out until the following April. The prolonged period of snow during the winter of 1962–63 tended to preserve larvae rather than kill them. The relevance of these results to the control of trichostrongylosis is discussed and it is concluded that it would be impossible under British conditions to devise a system of rotational grazing which would satisfactorily control trichostrongylosis and at the same time ensure economic utilization of the herbage.

We are grateful to Mr C. R. Phillips and Mr T. Doy for technical assistance during the course of this investigation.

The long-term, in vivo effects of a single, subcurative dose (200 mg/kg body weight of mouse) of praziquantel on the structure of adult Schistosoma mansoni and on the process and speed of tegumental repair are described. In both male and female worms praziquantel caused often extensive damage to the tegument, in the form of surface blebbings, swellings and lesions, and vacuolization and disruption of the subtegumental tissues. Repair of the drug-induced tegumental damage occurred slowly with partial and, more rarely, complete repair only being seen after 65 days post-treatment (p.t.), although signs of damage were still observed, particularly in male worms, at 100 days p.t. In contrast, repair of damage to the subtegumental/parenchymal tissues including the tegumental perikarya occurred relatively quickly, with the majority of worms examined appearing normal by 8–12 days p.t. The possible role(s) of the host immune response in relation to the speed of tegumental repair in vivo is discussed.

In the hybrid zone of the two mouse subspecies Mus musculus musculus and Mus musculus domesticus, mice with hybrid genotypes harbour, on the average, more helminth parasites (cestodes and nematodes) than mice of the two parental taxa. In order to determine the roles played by genetic parameters in this phenomenon, mice with recombined and parental genotypes were experimentally infected with the intestinal pinworm Aspiculuris tetraptera, a natural parasite of the house mouse. The results showed that the high susceptibility of the hybrid zone mice is genetically determined. In addition, this study shows the occurrence of variability among resistant parental populations.

Introduction. Several reports indicate high prevalences of both onchocerciasis and epilepsy in some regions of Africa. This raises the question of whether these diseases are associated. We therefore investigated people with epilepsy and/or onchocerciasis living in an area in Tanzania endemic for Onchocerca volvulus (O. volvulus). Methods. We collected clinical information, skin snips, and blood from 300 individuals, and cerebrospinal fluid (CSF) from 197. Participants were allocated to 4 groups consisting of people with epilepsy and onchocerciasis (n=135), those with either epilepsy (n=61) or onchocerciasis only (n=35), and healthy individuals (n=69). Samples were evaluated for microfilaria, IgG4 antibodies against O. volvulus, O. volvulus antibody index (CSF/serum), and CSF routine parameters. Polymerase chain reaction (PCR) was performed on skin snips and CSF. Results. No difference was found in microfilarial density between participants with and without epilepsy (P=0·498). The antibody index was raised in 2 participants. CSF PCR was negative in all samples tested. Discussion. Our results do not give evidence of a relationship between O. volvulus and epilepsy. Despite the fact that 2 participants had raised antibody index, the existence of cerebral onchocerciasis caused by migration of microfilariae into the CSF appears unlikely. However, to date unexplored reactions to the infestation with O. volvulus causing epilepsy cannot be excluded.

Intergenic region typing by restriction analysis of the ribosomal internal transcribed spacer (ITS) and mini-exon provide diagnostic markers for some Leishmania. Here, we evaluate restriction analysis of these targets for genotyping and phylogenetic analysis within the Leishmania donovani complex (agents of visceral leishmaniasis). Each method was useful for genotyping of both L. donovani complex strains and Old World Leishmania species. The targets produced less robust groups than gp63 intergenic regions, but support the need for re-evaluation of the taxonomy of the L. donovani complex.

Plasmodium merozoites are covered by a complex coat of surface proteins. Several of the Merozoite Surface Proteins (MSPs) that make up this coat have been proposed as vaccine candidates although some of the MSPs are known to be highly polymorphic. We present here the first survey and analysis of the polymorphism in the recently characterized P. vivax surface protein PvMSP-3α. Full length or partial sequences were obtained for the Pvmsp-3α gene from isolates originating in Central and South America, Asia and the Pacific. The Pvmsp-3α sequence is remarkably diverse, but this extensive diversity is largely restricted to certain domains of the encoded protein. An acidic C-terminal domain and a smaller hydrophilic N-terminus are relatively conserved, while a central domain containing coiled-coil heptad repeats is highly polymorphic and in some isolates of P. vivax is partially deleted. Unlike other MSPs, there is no evidence of allelic families of PvMSP-3α gene sequences, and no evidence that certain patterns of polymorphism group within isolates of similar geographical origin. The distribution and nature of polymorphism suggest that there are functional restrictions on mutations in this gene, and have implications for inclusion of PvMSP-3α as a candidate in a P. vivax vaccine.

A potential control strategy for nematode infection in sheep is the implementation of a breeding programme to select for genes associated with resistance. The Texel breed is more resistant to gastrointestinal nematode infection than the Suffolk breed, based on faecal egg count, and this difference should enable the identification of some of the genes responsible for resistance. The objective of this study was to determine if variation at the ovine MHC-DRB1 locus was associated with variation in faecal egg count in Suffolk and Texel sheep. Ovar-DRB1 alleles and faecal egg count were determined for Texel (n=105) and Suffolk (n=71) lambs. Eight Ovar-DRB1 alleles, including 1 previously unknown allele, were identified in the Texel breed by sequence-base-typing. Seven Ovar-DRB1 alleles were identified in the Suffolk breed. Two Ovar-DRB1 alleles were common to both breeds, but were among the least frequent in the Suffolk population. In the Suffolk breed 1 Ovar-DRB1 allele was associated with a decrease in faecal egg count and 2 alleles with an increase in faecal egg count. This locus accounted for 14% of the natural variation in faecal egg count in Suffolks. There was no evidence for an association between Ovar-DRB1 alleles and faecal egg count in the Texel breed and the Ovar-DRB1 locus accounted for only 3% of the phenotypic variation in faecal egg count. These results suggest that the Ovar-DRB1 gene plays an important role in resistance to nematode infection in the Suffolk breed. The difference in faecal egg counts between these breeds may be attributable in part to the different allele profile at the Ovar-DRB1 locus.

Infection by the ciliate protozoan Ichthyophthirius multifiliis Fouquet, 1876 causes significant economic losses in freshwater aquaculture worldwide. Following the ban on the use of malachite green for treating food fish, there has been extensive research aimed at identifying suitable replacements. In this paper we critically assess drug and non-drug interventions, which have been tested for use or have been employed against this parasite and evaluate possibilities for their application in farm systems. Current treatments include the administration of formaldehyde, sodium chloride (salt), copper sulphate and potassium permanganate. However, purportedly more environmentally friendly drugs such as humic acid, potassium ferrate (VI), bronopol and the peracetic acid-based products have recently been tested and represent promising alternatives. Further investigation, is required to optimize the treatments and to establish precise protocols in order to minimize the quantity of drug employed whilst ensuring the most efficacious performance. At the same time, there needs to be a greater emphasis placed on the non-drug aspects of management strategies, including the use of non-chemical interventions focusing on the removal of free-swimming stages and tomocysts of I. multifiliis from farm culture systems. Use of such strategies provides the hope of more environmentally friendly alternatives for the control of I. multifiliis infections.

Gene flow maintains the genetic integrity of species over large spatial scales, and dispersal maintains gene flow among separate populations. However, body size is a strong correlate of dispersal ability, with small-bodied organisms being poor dispersers. For parasites, small size may be compensated by using their hosts for indirect dispersal. In trematodes, some species use only aquatic hosts to complete their life cycle, whereas others use birds or mammals as final hosts, allowing dispersal among separate aquatic habitats. We performed the first test of the universality of the type of life cycle as a driver of parasite dispersal, using a meta-analysis of 16 studies of population genetic structure in 16 trematode species. After accounting for the geographic scale of a study, the number of populations sampled, and the genetic marker used, we found the type of life cycle to be the best predictor of genetic structure (Fst): trematode species bound to complete their life cycle within water showed significantly more pronounced genetic structuring than those leaving water through a bird or mammal host. This finding highlights the dependence of parasites on host traits for their dispersal, suggesting that genetic differentiation of parasites reflects the mobility of their hosts.

Two species of microsporidia, Enterocytozoon bieneusi and Septata intestinalis have been reported as intestinal parasites of AIDS patients. In attempts to establish E. bieneusi in vitro, spores were concentrated from stool samples from 4 AIDS patients with biopsy-proven E. bieneusi infections. After sterilization of the concentrate in antibiotic solution, the spores were added to monolayers of RK13 cells grown on the membranes of Transwells. Cultures were established from 7 stool samples from the 4 patients but in every case the species established was S. intestinalis not E. bieneusi. On retrospective examination of the stools, a very small number of spores of a size comparable to that of S. intestinalis was found but this species was not detected in biopsies. Typical septate vacuoles containing Type I tubules were observed in vitro but in contrast to the original description, meronts were intravacuolar and sporogony was mainly disporoblastic. The cultivation system, used for the first time for microsporidia, revealed the presence of unsuspected S. intestinalis infections and indicates that this species may be much more common than hitherto suspected. S. intestinalis has not previously been cultured.

The persistence of Toxoplasma gondii tissue cysts in organs of cats (definitive host) and rodents (intermediate hosts) was studied. Nine cats, 12 rats, and 12 mice were fed T. gondii oocysts and their organs were digested in pepsin and then bioassayed for bradyzoites in mice. Of 9 cats killed 37 or 51 days after feeding 102 (2 cats), 103 (3 cats) or 104 (4 cats) oocysts of the VEG strain, tissue cysts were found in each cat; in the tongue of 9, in the heart of 5, in the brain of 4, and in the eyes of 1 cat. The dose had no effect on the distribution of tissue cysts in cats. Twelve rats were each fed 105 oocysts of the VEG strain of T. gondii and killed 21, 29, 64 or 237 days later. At each time-period, 11 tissues of 3 rats were pooled and bioassayed in mice. Tissue cysts were found in the brain, skeletal muscle, heart and kidneys of rats at each killing time; in the lungs, intestines, and mesenteric lymph nodes in 3 of 4 instances; in the tongue, liver, and eyes in 2 instances and in the spleen in 1 instance. Also, using the same procedures and sampling the same 11 tissues as used for rats, tissue cysts were seen in all organs except in the tongue and liver of 3 mice killed on day 82 after feeding the VEG strain. In 9 mice (3 with each strain) fed oocysts of the ME-49, GT-1, or P89 T. gondii strain and killed 62–130 days later, tissue cysts were found consistently only in the brain. Thus, in rats and mice, most tissue cysts were found in the brain and rarely in the tongue. This was in marked contrast to the distribution of tissue cysts in cats.

Infection with Schistosoma mansoni was studied in 5 troops of olive baboons (Papio cynocephalus anubis) in Gombe Stream National Park, Tanzania. Three troops were infected with S. mansoni. An aggregated distribution of parasites was observed among hosts. Troop membership was found to be the most significant factor influencing parasite prevalence. Age and reproductive status had no significant effect, but there was a trend for males to acquire higher levels of infection. However, age–prevalence curves showed a high infection in young baboons declining in the older baboons. Behavioural components of exposure – as measured in water-contact pattern – may be related to parasite burden. A ‘peak shift’ between infection in different age-classes in the different troops was observed: troops with higher schistosome prevalences displayed an earlier peak in prevalence of infection. The baboon troop with the most contact with people showed highest prevalence of infection possibly due to longer exposure to the parasite than the other troops and/or higher host density.

Toxoplasma gondii is a zoonotic pathogen defined by three main clonal lineages (types I, II, III), of which type II is most common in Europe. Very few data exist on the prevalence and genotypes of T. gondii in the UK. Wildlife can act as sentinel species for T. gondii genotypes present in the environment, which may subsequently be transmitted to livestock and humans. DNA was extracted from tissue samples of wild British carnivores, including 99 ferrets, 83 red foxes, 70 polecats, 65 mink, 64 badgers and 9 stoats. Parasite DNA was detected using a nested ITS1 PCR specific for T. gondii, PCR positive samples were subsequently genotyped using five PCR–RFLP markers. Toxoplasma gondii DNA was detected within all these mammal species and prevalence varied from 6·0 to 44·4% depending on the host. PCR–RFLP genotyping identified type II as the predominant lineage, but type III and type I alleles were also identified. No atypical or mixed genotypes were identified within these animals. This study demonstrates the presence of alleles for all three clonal lineages with potential for transmission to cats and livestock. This is the first DNA-based study of T. gondii prevalence and genotypes across a broad range of wild British carnivores.

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

Apicomplexans, including species of Eimeria, pose a real threat to the health and wellbeing of animals and humans. Eimeria parasites do not infect humans but cause an important economic impact on livestock, in particular on the poultry industry. Despite its high prevalence and financial costs, little is known about the cell biology of these ‘cosmopolitan’ parasites found all over the world. In this review, we discuss different aspects of the life cycle and stages of Eimeria species, focusing on cellular structures and organelles typical of the coccidian family as well as genus-specific features, complementing some ‘unknowns’ with what is described in the closely related coccidian Toxoplasma gondii.

To determine whether prolonged in vitro passage would result in attenuation of virulence in vivo, Neospora caninum tachyzoites were passaged for different lengths of time in vitro and compared for their ability to cause disease in mice. Groups of Balb/c mice were inoculated intraperitoneally with 5×106 or 1×107 of low-passage or high-passage N. caninum tachyzoites. The mice were monitored for changes in their demeanour and body weight, and were culled when severe clinical symptoms of murine neosporosis were observed. Mice inoculated with the high-passage parasites survived longer (P<0·05), and showed fewer clinical symptoms of murine neosporosis, compared to the mice receiving the low-passage parasites. The parasite was detected in the brains of inoculated mice using immunohistochemistry and ITS1 PCR. Tissue cysts containing parasites were seen in mice inoculated with both low-passage and high-passage parasites. When the in vitro growth rates of the parasites were compared, the high-passage parasites initially multiplied more rapidly (P<0·001) than the low-passage parasites, suggesting that the high-passage parasites had become more adapted to tissue culture. These results would suggest that it is possible to attenuate the virulence of N. caninum tachyzoites in mice through prolonged in vitro passage.

The discovery of Trichinella zimbabwensis in farm crocodiles of Zimbabwe has opened up a new frontier in the epidemiology of the Trichinella genus. The objective of the present study was to investigate the infectivity of encapsulated species (T. spiralis, T. nativa, T. britovi, T. murrelli and T. nelsoni) and non-encapsulated species (T. pseudospiralis, T. papuae and T. zimbabwensis) in caimans (Caiman crocodilus), varans (Varanus exanthematicus), pythons (Python molurus bivittatus) and turtles (Pelomedusa subrufa) raised at their natural temperature range (26–32 °C). Mice and chickens were used as controls. At 6 days post-infection (p.i.), adult worms were detected in the small intestine of reptiles infected with T. papuae and T. zimbabwensis, of chickens infected with T. pseudospiralis and of mice infected with all encapsulated and non-encapsulated species. At 60 days p.i., T. papuae and T. zimbabwensis adult worms were collected from the intestine of varans and caimans and larvae from muscles of the four reptile species, T. pseudospiralis larvae from muscles of chickens, and larvae of all Trichinella species from mouse muscles. The highest reproductive capacity index of both T. papuae and T. zimbabwensis was observed in varans. The results show that T. papuae and T. zimbabwensis are able to complete their entire life-cycle in both poikilothermic and homoiothermic animals.

The mechanism of action of praziquantel (PZQ), the drug of choice against schistosomiasis, is still unclear. Since exposure of schistosomes to the drug is associated with calcium influx and muscular contraction, calcium channels have been suggested as the target, although direct combination of PZQ with their subunits was never demonstrated. We report a hitherto unknown effect of PZQ, namely the inhibition of nucleoside uptake, as observed in living worms using radio-isotope labelled adenosine and uridine. This effect is clearly seen in schistosomes but is absent in mammalian cells in culture. Moreover it is a specific pharmacological effect seen exclusively with the active levo-R(−)stereo isomer of the drug, and is shared by at least one benzodiazepine having antischistosomal activity. This novel effect acquires significance given that schistosomes cannot synthesize purine nucleosides de novo. A possible relationship between this novel effect and the known action of PZQ on calcium channels is discussed, since adenosine is known to bind to specific receptors and to behave as an indirect antagonist of calcium release in mammalian cells. If calcium channels were correlated with adenosine receptors also in schistosomes, as they are in mammals, this would support the hypothesis that PZQ-induced calcium influx may be correlated to adenosine receptor blockade.

The method of attachment of the cattle tick Boophilus microplus (Canestrini) has been investigated on cattle of different breeds and susceptibility to infestation.

Despite their differing sizes, the larval, nymphal and adult mouthparts penetrate to a similar depth towards the base of the Malpighian layer. Penetration may occur within 5 min of the arrival of the tick on the host.

Attachment is accomplished by the secretion of a cement substance in which the mouthparts are embedded and which adheres firmly to the hosts skin.

The cement consists of two main components: a cortex of carbohydrate-containing protein, stabilized by quinone tanning and disulphide linkages, and an internum which is lipoprotein in nature.

The start of the secretion of cement and the initial cutting of the host tissues by the chelicerae occur simultaneously. Final rapid engorgement of each stage is preceded by a secondary secretion of cement into a fluid-filled cavity which forms directly beneath the mouthparts and provides additional support.

It is concluded that attachment is an entirely mechanical process by an inert cement which does not provoke the development of host-parasite factors inimical to the tick.

It is with much pleasure that we acknowledge generous financial support by the Australian Cattle and Beef Research Committee which has made possible this project.