The effect of Toxoplasma gondii on rat behaviour was assessed in a colony of 36 free-ranging wild/laboratory hybrid rats allowed to compete freely for food and mates in a (100 m2) outdoor naturalistic enclosure. T. gondii infection had no effect on social status or mating success, both the product of costly and competitive activities. However, the propensity to explore novel stimuli in their environment was higher in infected than uninfected individuals. These results are consistent with the hypothesis that T. gondii only affects the behavioural traits which selectively benefit the parasite, rather than causing a general alteration of rat behaviour.

The technique of Random Amplified Polymorphic DNA (RAPD) PCR has been used to detect DNA polymorphisms among Toxoplasma gondii strains. Seven arbitrary oligonucleotides (10-mer) were used as primers to amplify total genomic DNAs and significant genetic heterogeneity was detected among 11 T gondii strains with different virulence for mice. The polymorphisms observed allowed relationship dendrograms of T. gondii strains to be constructed by PHYLIP and PAUP analyses. The genetic relationships of the T. gondii strains generated by 2 analyses using completely different assumptions were similar. Both analyses revealed 2 groups of T. gondii strains, one formed by the 6 virulent strains and the other formed by the 5 avirulent strains. This suggests that the genus Toxoplasma may actually contain 2 groups, correlated with their virulence, which have probably evolved independently following their initial separation. Significant polymorphisms were also detected between 2 different laboratory stocks of the T. gondii RH strain.

The effect of temperature on early Plasmodium falciparum development was examined in Anopheles stephensi. The rates of both ookinete development and bloodmeal digestion were lengthened as temperatures decreased from 27 to 21 °. However, low temperatures (21–27 °) did not significantly influence infection rates or densities of either ookinetes or oocysts. In contrast, high temperatures (30 and 32 °) significantly impacted parasite densities and infection rates by interfering with developmental processes occurring between parasite fertilization and ookinete formation, especially during zygote and early ookinete maturation. This study demonstrates clearly that temperature affects the sporogonic development of P. falciparum in anophelines by altering the kinetics of ookinete maturation. These studies not only confirm the ookinete as the key development stage affecting the probability of vector infectivity, they provide new insights into the epidemiology of P. falciparum infections.

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.

We have examined the similarities and differences in the organization of tubulin genes in New World Leishmania by restriction endonuclease digestion of genomic DNA and Southern blot analysis, using heterologous and homologous tubulin gene probes. As judged by the hybridization pattern and the restriction fragment length polymorphism (RFLP), there were large differences in both the restriction and hybridization patterns of the β-tubulin sequences between stocks of the mexicana and braziliensis complexes. There were similarities in the hybridization patterns of different speciesof the mexicana complex. In contrast, a high heterogeneity was found between species of the braziliensis complex which includes intraspecific variation. The results suggest that this polymorphism may be associated with random mutations. The same analysis gave evidence of large differences in the β-tubulin gene restriction pattern between New and Old World Leishmania. This variation in the β-tubulin gene region was sufficient to distinguish between New and Old World Leishmania groups and between stocks of the mexicana and braziliensis complexes.

Giardia that infect humans are known to be heterogeneous but they are assigned currently to a single species, Giardia intestinalis (syn. G. lamblia). The genetic differences that exist within G. intestinalis have not yet been assessed quantitatively and neither have they been compared in magnitude with those that exist between G. intestinalis and species that are morphologically similar (G. duodenalis) or morphologically distinct (e.g. G. muris). In this study, 60 Australian isolates of G. intestinalis were analysed electrophoretically at 27 enzyme loci and compared with G. muris and a feline isolate of G. duodenalis. Isolates of G. intestinalis were distinct genetically from both G. muris (approximately 80% fixed allelic differences) and the feline G. duodenalis isolate (approximately 75% fixed allelic differences). The G. intestinalis isolates were extremely heterogeneous but they fell into 2 major genetic assemblages, separated by fixed allelic differences at approximately 60% of loci examined. The magnitude of the genetic differences between the G. intestinalis assemblages approached the level that distinguished the G. duodenalis isolate from the morphologically distinct G. muris. This raises important questions about the evolutionary relationships of the assemblages with Homo sapiens, the possibility of ancient or contemporary transmission from animal hosts to humans and the biogeographical origins of the two clusters.

The study of parasite population dynamics has been one of the major developments in ecology over the last 15 years (Kennedy, 1975). The seminal articles of Crofton (1971) and Anderson & May (1978, 1979; May & Anderson, 1978, 1979) began this process by illustrating the potential role of parasites in regulating or destabilizing the dynamics of wildlife host populations. Since then, a variety of empirical and theoretical studies (reviewed by Grenfell & Dobson, 1995) have explored the role of parasites in natural populations. In parallel with these population dynamical developments, a growing interest in the evolutionary ecology of parasites has also led to a large literature, examining the evolutionary impact of parasites and the importance of host-parasite coevolution (Hamilton, 1982; May & Anderson, 1990; Lively & Apanius, 1995; Read et al. 1995; Herre, this volume).

In the blood of a Cercocebus albigena and of a C. galeritus agilis monkey, the infection with Plasmodium gonderi was found to follow its well-known chronic course; P. georgesi seemed to occur as a relapsing type of malaria parasite; P. petersi was found for only a few days and at a low level in C. albigena (end of an attack?). As shown by using polarized light, the pigment granules appeared mostly as fine dots in P. georgesi, short rods in P. gonderi and long needles in P. petersi. The three species can be distinguished by the morphological appearance of the nucleus of the young trophozoites, and also by the measurement of its surface area (Sa): small round nucleus (Sa= 0·81 ± 0·06 μm2) in P. gonderi, large 2-coloured nucleus (Sa= 1·43 ± 0·21 μm2) in P. petersi, and long crescent-shaped nucleus (Sa= 2·18±0·25 μm2) in P. georgesi. The first colour illustrations of the blood-stages of P. georgesi are presented. The dynamics of single and mixed blood infections in primate malaria parasites are discussed, with a proposal to classify them into 3 types.

To understand the relationship between mode of transmission and virulence, we investigated 2 microsporidian parasites in Daphnia magna laboratory populations. Pleistophora intestinalis is only transmitted horizontally, while Tuzetia sp. is transmitted vertically with high efficiency from mothers to parthenogenetic male and female offspring. We were not able to transmit Tuzetia horizontally in the laboratory. Tuzetia reduces host life-time reproductive success and host survival to a much greater extent than does P. intestinalis. Tuzetia-infected hosts were rapidly outcompeted by uninfected hosts. We suspect that Tuzetia infections may persist in natural populations by an as yet undiscovered horizontal transmission. It is possible that an alternate host species may be involved. We present a mathematical model to analyse the conditions for the persistence of a parasite with perfect vertical and an additional degree of horizontal transmission. We show that horizontal and vertical transmission contribute additively to a parasite's ability to invade and persist. Since the fitness contribution of horizontal transmission increases with population size, only very low transmission probabilities per host-to-host contact are necessary for the persistence of parasites occurring in large populations such as those commonly found for Daphnia. The detection of such low rates of horizontal transmission appears unlikely under laboratory conditions since the necessary number of host-to-host (or host-to-spore) contacts is not feasible. We review mechanisms that can maintain vertically transmitted parasites in nature.

Microsporidia are increasingly recognized as causing opportunistic infections in immunocompromised individuals. Encephalitozoon cuniculi is probably the most studied mammalian microsporidian that infects insects and mammals, including man. In this study, 8 E. cuniculi isolates were compared and were found to fall into 3 strains. Strain type I includes the rabbit type isolate, as well as isolates from an additional rabbit, a dwarf rabbit, and a mouse. Strain type II includes 2 murine isolates and strain type III includes 2 isolates obtained from domestic dogs. By SDS-PAGE, the 3 strains differ primarily in the molecular weight range of 54–59 kDa where strain type I displays an apparent broad singlet at 57 kDa, strain type II displays an apparent doublet at 54 and 58 kDa, and strain type III displays an apparent broad band at 59 kDa. Antigenic differences were detected in the molecular weight regions of 54–58 kDa as well as 28–40 kDa by Western blot immunodetection using murine antisera raised against E. cuniculi, Encephalitozoon hellem, and the Encephalitozoon-like Septata intestinalis. Polymerase chain reaction (PCR) products containing only small subunit rDNA sequences from the different E. cuniculi isolates formed homoduplexes whereas PCR products containing intergenic rRNA gene sequences formed heteroduplexes in mobility shift analyses. Fok I digestion of the PCR products containing the intergenic rRNA gene region resulted in unique restriction fragment length polymorphism patterns, and DNA sequencing demonstrated that in the intergenic spacer region, the sequence 5'-GTTT-3' was repeated 3 times in strain type I, twice in strain type II, and 4 times in strain type III. This study indicates that there exist at least 3 E. cuniculi strains which may become important in the epidemiology of human E. cuniculi infections. Furthermore, as additional E. cuniculi isolates are characterized, these strains will be named or reclassified once the criteria for taxonomy and phylogenetic tree construction for microsporidia become better defined.

Although parasites are reported to alter host responses to predators, little is known about the neurochemical mechanisms involved. Using an odour preference test, we examined the effects of an acute, subclinical infection with the naturally occurring, single host, enteric protozoan parasite, Eimeria vermiformis, on the responses of male laboratory mice, Mus musculus, to a predator. Uninfected mice avoided the odour of a predatory cat, spending a minimal amount of time in a Y-maze in the vicinity of the cat odour. In contrast, mice infected with E. vermiformis, spent a significantly greater amount of time in the proximity of the cat odour, showing a reduced avoidance of the cat odour and a reduction in predator-induced fear or anxiety. This was not related to augmented opioid activity and decreased pain sensitivity in the infected mice, as neither treatment with the exogenous opiate, morphine, nor restraint stress-induced augmentation of endogenous opioid activity, had any significant effects on the responses of uninfected mice to cat odour. The altered responses of the infected mice to the cat odour were reduced by peripheral administration of the gamma-aminobutyric A (GABAA) antagonists, bicuculline and picrotoxin, but were not significantly affected by either the benzodiazepine antagonist, Ro 15–1788, the opiate antagonist, naloxone, or the excitatory amino acid, N-methyl-D-aspartate (NMDA) antagonist, MK-801. These results indicate that infection with E. vermiformis in mice reduces the avoidance of predator odour through neurochemical systems associated with anxiety involving, at least in part, GABAA receptor mechanisms.

Infectious agents have considerable potential to regulate or constrain the population growth of vertebrate hosts in natural habitats. A broad theoretical framework provides many insights into how the biology of the parasite and the demography of the host interact to determine this impact. It may manifest itself as a steady influence over time via stable endemic infection or in a recurrent epidemic fashion, sometimes with unpredictable intervals between epidemics depending on the generation time of the pathogen (time from infection to recovery or host death), its ability to induce lasting immunity and the population growth rate of the host species. Building on these notions, the paper focuses on recent work on the population dynamics of genetically variable pathogen populations and examines the factors that determine the evolution of virulence and the maintenance of genetic diversity in both host and pathogen. Recent research extends conventional theoretical templates to include population genetic elements and the within-host dynamics of the parasite and its interaction with the vertebrate immune system.

Immune responses of 4 inbred lines of chickens, that differ in resistance to Eimeria maxima and E. tenella, were examined. Significant differences were found in in vitro proliferation of peripheral blood lymphocytes to E. maxima sporozoite antigen, the more resistant lines C and 72 having higher responses than the more susceptible line 151. These differences existed pre-infection and were enhanced following both primary and a second infection. The proportions of lymphocyte subsets in the peripheral blood following primary infection also differed between lines, with significantly higher percentages of CD8 + and TCR1 + lymphocytes circulating in the more resistant birds. In contrast, there were few differences between lines in either resistance or in in vitro proliferation of peripheral blood lymphocytes to E. tenella sporozoite antigen either pre-infection or following a primary infection. However, after a second infection when there were significant differences in resistance between lines, as measured by oocyst excretion, there were also significant differences in lymphoproliferation with the more resistant lines 151 and 62 having higher responses than the more susceptible line C. Thus for E. maxima there is a direct relationship between resistance to infection and lymphoproliferation in response to parasite antigen. This implies that differences in cellular immunity may account for differences in resistance between lines, and since these specific responses are enhanced by infection they may also reflect important immune mechanisms. For the rather less immunogenic E. tenella, the correlation between resistance and lymphoproliferation is not so clear. However, where there were significant differences between lines, i.e. after a second infection, the direct relationship between resistance and lymphoproliferation was upheld.

A gene, CRP136, from the intestinal protozoan parasite Giardia duodenalis, expressed at a high level in a metronidazole-resistant line, encodes a 136 kDa protein with 23 copies of a 40 amino acid repeat. The protein is cysteine-rich and has the typical membrane-spanning region and CXXC amino acid motifs of a family of Giardia cysteine-rich surface proteins (CRSPs). The repeat unit in CRP136, shares 57% homology with the gene encoding the precursor of the sarafotoxins, a group of snake toxins from the burrowing adder known to cause symptoms similar to those of humans acutely infected with Giardia. The sarafotoxins are low molecular weight sulphydryl cross-linked peptides which are proteolytically cleaved from a precursor polyprotein. CRP136 has homology over the entire length of the sarafotoxin precursor, and the repeats are of the same length. Thus CRP136 represents the first evidence for a potential Giardia toxin. The genomic copy number of CRP136 appears to be the same in both the parent and drug-resistant lines and expression of this gene, and at least one other, is associated with a conserved partial duplication, but not amplification, of one chromosome.

The genomic organization of gp63 genes in 4 and 7 isolates of Leishmania braziliensis and L. peruviana, respectively was studied by RFLP analysis with 3 restriction enzymes (Bgl I, Sal I and Apa I). Our results showed a marked polymorphism among isolates. Some characters were specific to L. braziliensis or to L. peruviana, and others specific to the respective biogeographical populations of L. peruviana. The average minimum copy number of gp63 genes was found to be higher in L. braziliensis (71) than in L. peruviana (46), suggesting that deletion of gp63 genes might be partially involved in the size decrease of the chromosome bearing gp63 genes, observed between those 2 species (from 700 to 610 kb). Our results may suggest the existence of at least 2 arrays of heterologous gp63 repeats, varying in relative copy number between L. braziliensis and L. peruviana, and among isolates of the latter species. Rearrangement of the gp63 genes was observed during long-term in vitro maintenance of a reference strain of L. braziliensis. These observations document the existence of a dynamic gp63 gene organization in Leishmania of the braziliensis complex.

Anopheles stephensi mosquitoes showed a reduction in fecundity over 3 successive gonotrophic cycles, after becoming infected with Plasmodium yoelii nigeriensis. This effect could be observed at high oocyst burdens (> 75) or at low oocyst burdens (mean of 4·36). Mean bloodmeal size of the infected mosquitoes was significantly reduced only when feeding upon a mouse with a high gametocytaemia and the conversion of the bloodmeal into eggs by the infected mosquitoes was disrupted. Patterns of infected mosquito mortality, over the 3 gonotrophic cycles, varied with severity of infection. Although in 1 case increased mortality and decreased bloodmeal size may have affected fecundity, this could not have accounted for all of the observed fecundity reduction. We propose that other, unknown parasite related factors, are involved.

The vector competences of 6 geographic strains of Aedes polynesiensis for Wuchereria bancrofti were studied using two types of experimental infections. Experimental infection of laboratory-bred mosquitoes fed on the carriers' forearms with different levels of microfilaraemia showed that microfilariae (mf) uptake was directly proportional to the carrier's mf density and, as mf densities decreased, concentration capacity of Ae. polynesiensis increased. It was also shown that infection has an important effect on mosquito mortality, and that the mortality rate differed among mosquito strains. In infections using artificial feeders, the mf uptake was closely regulated, thus showing differences in the vectorial efficiency of Ae. polynesiensis related to the geographic origin of the mosquito strain. The mosquitoes from the Society archipelago were more efficient intermediate hosts than geographically distant strains when infected with W. bancrofti from an island within the archipelago (Tahiti). Mosquito strains from the Society archipelago developed the highest proportion of infective-stage larvae and exhibited the lowest mortality rate when infected with sympatric Tahitian W. bancrofti.

Heritability of susceptibility to infection with Theileria parva was estimated from full sib families of Rhipicephalus appendiculatus ticks. Male and female ticks of 2 stocks were mated singly. Nineteen full sib families of the Muguga stock and 17 full sib families of the Kiambu stock were obtained. Nymphae of these families were fed on cattle infected with T. parva so that the ticks became replete on days 16 and 17 after infection when the blood was parasitaemic with intraerythrocytic piroplasms. The T. parva infections were assessed in the resultant adult ticks of each full sib group and the abundance of infection, the number of salivary gland acini infected/tick, was found to be the most useful parameter for analysis. Estimates of heritability of the susceptibility to infection with T. parva for the Kiambu and the Muguga tick stocks were 0·24 and 0·26 respectively. Using only the data from ticks which fed on day 16, the heritability estimates were 0·39 for the Kiambu stock and 0·59 for the Muguga stock. These results indicate that tick lines of high or low susceptibility for T. parva infection could be produced through selection.

Neospora caninum is an apicomplexan, protozoan parasite, which causes severe disease in dogs and cattle. It has previously been isolated only in the United States. A 5-week-old Boxer pup with a progressive hindlimb paresis was diagnosed as suffering from neosporosis on the basis of clinical signs and the presence of anti-Neospora antibodies in it, 2 litter-mates and its darn. Despite treatment with sulphonamides, the pup was euthanased 3 days later. The diagnosis of neosporosis was confirmed by immunohistochemical examination of muscle and CNS tissue sections from the pup. Parasites were isolated into Vero cell culture from the cerebrum, and confirmed as Neospora caninum by immunofluorescence with specific antibody, tachyzoite ultrastructure and 16S-like ribosomal RNA sequences. This isolate (designated NC-Liverpool) has been continuously passaged every 7–10 days. Its growth characteristics, ultrastructure and antigenic profile, as revealed by immunoblotting, have revealed no major differences from the American NC-1 isolate. Furthermore, no difference was seen when comparing the sequences of 16S-like ribosomal RNA and the ITS1 region of the two isolates.