Babesia microti (Apicomplexa: Piroplasmida) has historically been considered a common parasite of Holarctic rodents. However, human babesiosis due to this species has generally been limited to the northeastern seaboard of the United States and Minnesota and Wisconsin. The absence of reports of B. microti babesiosis from sites where the agent is enzootic, such as in western Europe, remains unexplained. Previous work focusing on the 18S rDNA demonstrates little sequence diversity among samples from allopatric host populations across a wide geographical area. It may be that genetic diversity is underestimated due to sample size or the gene analysed. Accordingly, we collected blood or spleen samples from American or Eurasian animals with parasites that were morphologically consistent with B. microti, amplified the 18S rDNA and beta-tubulin gene, and conducted phylogenetic analysis. Surprisingly, what was considered to be ‘B. microti’ by microscopy appears to be a diverse species complex. We identify 3 distinct clades within this complex, including parasites from non-rodent hosts. Rodent parasites comprise 2 clades, one representing zoonotic isolates, and the other apparently maintained in microtine rodents, and therefore their morphological detection within animals from a site does not necessarily imply a risk to public health.

This review considers the current status of urinary schistosomiasis, caused by infection with Schistosoma haematobium, and argues that greater research effort and focus are needed to improve understanding of this neglected tropical disease (NTD). The inappropriateness of relying solely on data concerning the much more extensively studied intestinal form of schistosomiasis caused by S. mansoni is highlighted. The current lack of genome and transcriptome information for S. haematobium is directly hindering further targeted research and must be quickly rectified. Recent molecular phylogenies caution the expectation of similarities between schistosome species and highlight the close relationships of species within the S. haematobium group. Treatment, current and prospective drugs and vaccines, together with diagnosis are considered, highlighting the differences associated with urinary schistosomiasis. This infection has a significant and specific impact on the urino-genital system and has a strong association with bladder cancer, leading to severe and chronic morbidity. There is a clear need for new clinical initiatives in this area to better quantify the disease burden. Furthermore, emerging associations with HIV and other pathogens need to be closely monitored. Research is urgently needed to improve current knowledge in order to develop the next generation of control tools.

The geographical distance between conspecific host populations is no doubt a key determinant of the likelihood that exchanges of parasite species occur between these populations. This variable must therefore be taken into account in studies that compare parasite species richness or similarity among host populations. This paper presents a multivariate approach, based on the permutation of matrices, that allows all pairwise geographical distances between host populations to be included as independent variables. The method is illustrated with 3 separate data sets on parasite communities of conspecific fish from different lakes. In 2 of 3 cases, geographical distances among lakes had a significant influence on the similarity of their parasite communities. The effect of geographical distance on species richness of parasite communities also proved important in 2 of the 3 case studies. These examples demonstrate the pervasive influence of distances among host populations on their parasite communities, and the need to properly control for them in statistical analyses.

In merozoites of Plasmodium knowlesi, rhoptries have a dense substructure of fine (2·5 nm diameter) granules and short rods. These are not altered by lipid extraction, and stain with ethanolic phosphotungstate indicating a proteinaceous composition. Various types of fixation also show multilamellar whorls with a periodicity of 5–7 rim in the tips of rhoptries or extruded at the merozoite apex. In merozoites fixed during invasions of red cells, membrane continuity typically occurs between the rim of the rhoptry canal and the red cell membrane, but where this contact has apparently been lost, extensive membranous whorls and blebs are often found at the apex of the parasite. Similar structures occur at the spices of merozoites within late-stage schizonts. It is suggested that the same mechanism which generates these lamellae forms the parasitophorous vacuole by inserting membranous elements formed by the parasite into the red cell membrane, so causing its invagination. A similar mechanism may be responsible for the release of merozoites from the late-stage schizont.

1. Activity in Ixodes ricinus was studied by laying down newly emerged ticks in natural clumps of vegetation from which all hosts (with the possible exception of ‘mice’) were excluded. After a period of quiescence the ticks climbed to the ‘active’ position at the vegetation tips. Activity was assessed either by simple observation (in adults which were marked individually) or by ‘brushing’ the vegetation with the hands (nymphs and larvae). 52 % of the adults and 44 % of the nymphs were recovered.

2. The seasonal activity behaviour was followed in three series of ticks set out during May, July and October 1945. The results were used in interpreting the time relations of seasonal activity in ‘wild’ populations. The timing of the activity cycle appears to be largely determined by the availability of unfed ticks rather than by the prevailing meteorological conditions.

3. The diurnal pattern of behaviour in active ticks was closely examined during a 24 hr. period. Many remained at the tips without moving. The others began or ended a phase of activity by night or by day; but among these, the proportion ending a phase at night was significantly greater. This is the main reason for the previously reported diurnal fluctuation of activity in a tick population.

4. Two hundred and seven adult ticks spent, on an average, a total of 9 days at the vegetation tips (individual limits 1 and 54 days). This activity was spread over an ‘active period’ (interval between first and last appearances) averaging 30 days. Very active ticks rarely remained continuously at the tips. The mean number of visits to the tips was 4 (limits 1 and 19), each lasting for an average of 2·5 days. Where long spells of activity alternate with short periods of quiescence, the behaviour is probably regulated by the humidity reaction, the quiescent phase providing an opportunity for restoring the depleted water balance.

5. Newly moulted adult ticks possess reserves of fat adequate for many months of quiescence. During activity these reserves are exhausted in a few weeks or even days. Duration of survival mainly depended on whether the onset of activity was immediate or delayed. Some adults remained quiescent for 1 year before becoming active for the first time.

6. Several types of orientations were observed in the field. In attaining the position at the tips favourable for encountering a host, the gravity response (upward-turning near the tip) is of major importance. The tick avoids wind and direct sunlight by sheltering behind its supporting stem. In sensing the approach of a ‘host’ (the observer's finger) the perception of eddies of warm air is particularly significant. The response (questing, then orientation) is elicited much less readily if the stimulus is applied from the leeward.

Cerebral malaria in A/J and CBA/H mice infected with Plasmodium berghei ANKA is accompanied by mononuclear cell infiltration, haemorrhage and cerebral endothelial cell damage. This damage is presumably one of the causes of the breakdown of the blood—brain barrier which was detected by measuring the movement of the dye Evans blue and radioisotope labelled albumin and erythrocytes. The density of brain tissue, measured by a Percoll gradient technique, was significantly reduced in mice exhibiting cerebral symptoms, suggesting the occurrence of cerebral oedema.

Isolates of the protozoan parasite Cryptosporidium parvum have been differentiated into 2 genotypes: genotype ‘H’, which is associated only with human infections, and genotype ‘C’, which is associated with both human and animal infections. To date, the analysis of polymorphisms of genes and of the small subunit ribosomal DNA have revealed no heterogeneity within the 2 genotypes. In the present study, a locus containing simple sequence repeats (microsatellites) was PCR amplified and sequenced from 94 C. parvum isolates, which were collected from humans (immunocompetent and immunocompromized individuals, outbreak and single cases) and from several animal hosts in 3 continents. The analysis revealed that genotype ‘H’ can be further differentiated into 2 subgenotypes, and genotype ‘C’ can be further differentiated into 4 subgenotypes. The 6 subgenotypes differ in terms of expansions/contractions of the microsatellite repeats and by point mutations. Some subgenotypes showed a wide geographical distribution, whereas others were restricted to specific regions. Therefore, microsatellites are informative markers for more defined studies on the epidemiology, the transmission routes, and the population structure of this parasite.

Sealworms or codworms, larvae of ascaridoid nematodes belonging to the Pseudoterranova decipiens species complex, infect the flesh of numerous species of marine and euryhaline fish, and have proven a chronic and costly cosmetic problem for seafood processors. Moreover, the parasite may cause abdominal discomfort in humans when consumed in raw, undercooked or lightly marinated fish. In this review, the phylogeny, life cycle and distributions of sealworms are discussed along with biotic and abiotic factors which may influence distributions of these parasites in their intermediate and final hosts. Also considered here are efforts to control the problem through commercial fishing practices, fish processing technology, and the reduction of infection parameters in marine fish populations by biological means. Ironically, concern over sealworm problem has subsided in some fisheries in recent years, not as a result of falling infection parameters in fish stocks or innovations in processing technology, but as a consequence of declines in abundance and size of groundfish.

Wild brown rats (Rattus norvegiens) from 11 rural UK farmsteads were found to carry 13 zoonotic and 10 non-zoonotic parasitic species, many of which (e.g. Cryptosporidium, Pasteurella, Listeria, Yersinia, Coxiella and Hantavirus) have rarely or never been previously investigated for wild rats. The study suggests that wild brown rats, serving as vectors of disease, represent a serious risk to the health of humans and domestic animals in the UK.

Host-parasitoid interactions are abundant in nature and offer great scope for the study of coevolution. A particularly fertile area is the interaction between internal feeding parasitoids and their hosts. Hosts have evolved a variety of means of combating parasitoids, in particular cellular encapsulation, while parasitoids have evolved a wide range of countermeasures. Studies of the evolution of host resistance and parasitoid virulence are reviewed, with an emphasis on work involving Drosophila and its parasitoids. Genetic variation in both traits has been demonstrated using isofemale line and artificial selection techniques. Recent studies have investigated the fitness costs of maintaining the ability to resist parasitoids, the comparative fitness of flies that have successfully defended themselves against parasitoids, and the degree to which resistance and virulence act against one or more species of host or parasitoid. A number of studies have examined geographical patterns, and sought to look for local adaptation; or have compared the traits across a range of species. Finally, the physiological and genetic basis of change in resistance and virulence is being investigated. While concentrating on Drosophila, the limited amount of work on different systems is reviewed, and other possible areas of coevolution in host-parasitoid interactions are briefly discussed.

The problems caused by tick and tick-borne diseases for livestock particularly cattle on the African continent are described and discussed. The control of ticks and tick-borne diseases must receive high priority in Africa with regard to both research and control application because of their widespread distribution in areas of high livestock potential and productivity. The conventional methods of tick and tick-borne disease control are discussed and are found to be inadequate in the conditions prevailing in Africa. Methods of integrated control are suggested and discussed in light of recent development in control methods and those still under development. Any one of these methods may not be adequate to control the problem on its own but when several of the methods are combined an economic and robust integrated control is likely to result. Encouragement is given to attempt this approach in Africa to solve what must be the largest animal health problem of livestock remaining in the world.

More than 200 species of avian Haemosporidia (genera Plasmodium, Haemoproteus, and Leucocytozoon) have been described based primarily on morphological characters seen in blood smears. Recent molecular studies, however, suggest that such methods may mask a substantial cryptic diversity of avian haemosporidians. We surveyed the haemosporidians of birds sampled at 1 site in Israel. Parasites were identified to species based on morphology, and a segment of the parasite's cytochrome b gene was sequenced. We compared 3 species concepts: morphological, genetic, and phylogenetic. Fifteen morphological species were present. Morphological species that occurred once within our dataset were associated with a unique gene sequence, displayed large genetic divergence from other morphological species, and were not contained within clades of morphological species that occurred more than once. With only 1 exception, morphological species that were identified from multiple bird hosts presented identical sequences for all infections, or differed by few synonymous substitutions, and were monophyletic for all phylogenetic analyses. Only the morphological species Haemoproteus belopolskyi did not follow this trend, falling instead into at least 2 genetically distant clades. Thus, except for H. belopolskyi, parasites identified to species by morphology were supported by both the genetic and phylogenetic species concepts.

Unlike other members of the genus, Echinococcus granulosus is known to exhibit considerable levels of variation in biology, physiology and molecular genetics. Indeed, some of the taxa regarded as ‘genotypes’ within E. granulosus might be sufficiently distinct as to merit specific status. Here, complete mitochondrial genomes are presented of 2 genotypes of E. granulosus (G1–sheep-dog strain: G4–horse-dog strain) and of another taeniid cestode, Taenia crassiceps. These genomes are characterized and compared with those of Echinococcus multilocularis and Hymenolepis diminuta. Genomes of all the species are very similar in structure, length and base-composition. Pairwise comparisons of concatenated protein-coding genes indicate that the G1 and G4 genotypes of E. granulosus are almost as distant from each other as each is from a distinct species, E. multilocularis. Sequences for the variable genes atp6 and nad3 were obtained from additional genotypes of E. granulosus, from E. vogeli and E. oligarthrus. Again, pairwise comparisons showed the distinctiveness of the G1 and G4 genotypes. Phylogenetic analyses of concatenated atp6, nad1 (partial) and cox1 (partial) genes from E. multilocularis, E. vogeli, E. oligarthrus, 5 genotypes of E. granulosus, and using T. crassiceps as an outgroup, yielded the same results. We conclude that the sheep-dog and horse-dog strains of E. granulosus should be regarded as distinct at the specific level.

Biomphalaria glabrata selected for genetic differences in susceptibility to infection with a Puerto Rican strain of Schistosoma mansoni were exposed to miracidia of a strain of S. mansoni from St Lucia. The St Lucian strain was less infective than the Puerto Rican. Results suggested that in snails susceptible to the Puerto Rican S. mansoni, differences in susceptibility to the St Lucian strain were determined by a single gene, with insusceptibility dominant.

Rates at which Trypanosoma brucei rhodesiense trypanosomes switch from expression of one variable antigen type (VAT) to that of another have been determined in cloned populations that have been recently tsetse-fly transmitted. Switching rates have been determined between several, specific pairs of VATs in each population. High rates of switching were observed in 2 cloned trypanosome lines, each derived from a separate cyclical transmission of the same parental stock and each expressing a different major VAT. Five estimates of the switching rate between one particular pair of VATs were consistently high (approximately 1 × 103 switches/cell/generation). These high switching rates were similar both in bloodstream populations of mice and in populations confined to subcutaneously implanted growth chambers in mice, thus indicating that the interaction of the bloodstream population with other trypanosome populations in the lymphatics or extravascular sites in systemic infections did not influence the estimates of the rate of switching. Fourteen estimates were made of VAT-specific switching rates in bloodstream infections involving 8 combinations from among 6 VATs. Switching rate estimates were VAT-specific and showed considerable variation between different combinations of VATs — from 1.9 × 10−6 to 6.9 × 10−3 switches/cel/generation. The rates of switching to different metacyclic-VATs were, however, very similar. Summation of between 3 and 5 VAT-specific switching rate values in each of 4 experiments conducted in bloodstream infections has provided minimum estimates of the overall rate of antigenic variation: 2.0−9.3 × 10−3 switches/cell/generation. These values are between 20 and 66000-fold higher than previously published estimates. It is likely that at least 1 in every 100 trypanosomes switches its VAT expression every generation.

The influence of adult female body size of Anopheles gambiae s.l. on development of midgut and salivary gland infections by the parasite Plasmodium falciparum was investigated in a field study carried out in Tanzania. The proportion of mosquitoes infected during a blood meal was independent of size. However, the number of oocysts harboured by infected mosquitoes increased with size of the mosquito. The proportion of mosquitoes with sporozoites, and thus potentially infective to humans, was highest in intermediate-sized mosquitoes, whereas the largest and smallest mosquitoes were less likely to have sporozoites. This pattern is interpreted as a combination of high survival rate of large, uninfected mosquitoes and of low survival rate of mosquitoes infected with many oocysts.

Using both correlational and experimental evidence, the relationship between parasite load and host activity was assessed in brown rats, Rattus norvegicus. Two hypotheses were tested – (1) that parasites with indirect life-cycles, involving transmission between a prey and its predator, will alter the activity of the intermediate host so as to increase its susceptibility to predation by the definitive host and (2) that activity levels in parasitized rats would be increased rather than decreased. Four groups of rats (n = 140) were examined. One group (n = 50) were wild brown rats trapped from 3 UK farmsteads, with naturally occurring parasites. The others were purpose-bred wild/laboratory hybrid rats with experimentally induced parasitic infections of either (n = 15) adult-acquired or (n = 15) congenitally-acquired Toxoplasma gondii (an indirect life-cycle parasite), or (n = 15) Syphacia muris (a direct life-cycle parasite). Uninfected hybrid rats (n = 45), matched for sex, age and weight, served as controls. Rats were housed individually in outdoor cages, and their activities were recorded on video-tapes for 6 non-consecutive 10 h nights. Exercise wheels were also available for the hybrid rats. Out of 6 parasite species detected in the wild rats, T. gondii was the only one which required predation by a definitive host to complete its life-cycle, and was also the only parasite to be associated with higher activity levels in infected than uninfected rats. Hybrid rats infected with T. gondii were also more active than those uninfected, whereas there were no differences in activity levels between S. muris infected and uninfected rats. This study shows that the indirect life-cycle parasite T. gondii can influence the activity of its intermediate host the rat. I suggest that this may facilitate its transmission to the cat definitive host.

A key aim of anthelmintic resistance research is to identify molecular markers that could form the basis of sensitive and accurate diagnostic tests. These would provide powerful tools to study the origin and spread of anthelmintic resistance in the field and to monitor strategies aimed at preventing and managing resistance. Molecular markers could also form the basis of routine diagnostic tests for use in surveillance and clinical veterinary practice. Much of the research conducted to date has focused on the investigation of possible associations of particular candidate genes with the resistance phenotype. In the future, as full parasite genome sequences become available, there will be an opportunity to apply genome-wide approaches to identify the genetic loci that underlie anthelmintic resistance. Both the interpretation of candidate gene studies and the application of genome-wide approaches require a good understanding of the genetics and population biology of the relevant parasites as well as knowledge of how resistance mutations arise and are selected in populations. Unfortunately, much of this information is lacking for parasitic nematodes. This review deals with a number of aspects of genetics and population biology that are pertinent to these issues. We discuss the possible origins of resistance mutations and the likely effects of subsequent selection on the genetic variation at the resistance-conferring locus. We also review some of the experimental approaches that have been used to test associations between candidate genes and anthelmintic resistance phenotypes and highlight implications for future genome-wide studies.

Anthelmintic resistance is a major problem for the control livestock parasites and a potential threat to the sustainability of community-wide treatment programmes being used to control human parasites in the developing world. Anthelmintic resistance is essentially a complex quantitative trait in which multiple mutations contribute to the resistance phenotype in an additive manner. Consequently, a combination of forward genetic and genomic approaches are needed to identify the causal mutations and quantify their contribution to the resistance phenotype. Therefore, there is a need to develop genetic and genomic approaches for key parasite species identified as relevant models. Haemonchus contortus, a gastro-intestinal parasite of sheep, has shown a remarkable propensity to develop resistance to all the drugs used in its control. Partly because of this, and partly because of its experimental amenability, research on this parasite has contributed more than any other to our understanding of anthelmintic resistance. H. contortus offers a variety of advantages as an experimental system including the ability to undertake genetic crosses; a prerequisite for genetic mapping. This review will discuss the current progress on developing H. contortus as a model system in which to study anthelmintic resistance.

A Neospora sp. was isolated from the brains of two aborted bovine foetuses and grown continuously in vitro in bovine cell cultures. A comparison of the antigenic reactivity of in vitro cultivated tachyzoites with polyclonal antisera to Neospora caninum, Hammondia hammondi or Toxoplasma gondii revealed that the bovine protozoal isolates were similar to N. caninum and antigenically distinct from T. gondii. Tachyzoites of both bovine isolates had similar ultrastructural features, including an apical polar ring, conoid, electron-dense rhoptries and micronemes. The orientation of the micronemes, presence of micropores and a large number of electron-dense granules in the posterior portion of the bovine isolate tachyzoites differed from previous descriptions of N. caninum in vivo. Tachyzoites of the bovine isolates were ultrastructurally more similar to in vitro cultivated N. caninum tachyzoites than to tachyzoites of T. gondii or H. hammondi. The antigenic and ultrastructural similarities between N. caninum and the protozoal parasites isolated from aborted bovine foetuses in this study support the proposition that these parasites belong to the genus Neospora.