Transmission of the malaria parasite Plasmodium is influenced by many different host, vector and parasite factors. Here we conducted a field study at Mbita, an area of endemic malaria in Western Kenya, to test whether parasite transmission to mosquitoes is influenced by the severity of malaria infection in its human host at the time when gametocytes, the transmission forms, are present in the peripheral blood. We examined the infectivity of 81 Plasmodium falciparum gametocyte carriers to mosquitoes. Of these, 21 were patients with fever and other malaria-related symptoms, and 60 were recruited among apparently healthy volunteers. Laboratory-reared Anopheles gambiae s.s. (local strain) were experimentally infected with blood from these gametocyte carriers by membrane-feeding. The severity of the clinical symptoms was greater in febrile patients. These symptomatic patients had higher asexual parasitaemia and lower gametocyte densities (P=0·05) than healthy volunteers. Ookinete development occurred in only 6 out of the 21 symptomatic patients, of which only 33·3% successfully yielded oocysts. The oocyst prevalence was only 0·6% in the 546 mosquitoes that were fed on blood from this symptomatic group, with mean oocyst intensity of 0·2 (range 0–2) oocysts per mosquito. In contrast, a higher proportion (76·7%) of healthy gametocyte carriers yielded ookinetes, generating an oocyst rate of 12% in the 1332 mosquitoes that fed on them (mean intensity of 6·3, range: 1–105 oocysts per mosquito). Statistical analysis indicated that the increased infectivity of asymptomatic gametocyte carriers was not simply due to their greater gametocyte abundance, but also to the higher level of infectivity of their gametocytes, possibly due to lower parasite mortality within mosquitoes fed on blood from healthy hosts. These results suggest that blood factors and/or conditions correlated with illness reduce P. falciparum gametocyte infectivity.

The usage of alternative synonymous codons in the completely sequenced, extremely A+T-rich parasite Plasmodium falciparum was studied. Confirming previous studies obtained with less than 3% of the total genes recently described, we found that A- and U-ending triplets predominate but translational selection increases the frequency of a subset of codons in highly expressed genes. However, some new results come from the analysis of the complete sequence. First, there is more variation in GC3 than previously described; second, the effect of natural selection acting at the level of translation has been analysed with real expression data at 4 different stages and third, we found that highly expressed proteins increment the frequency of energetically less expensive amino acids. The implications of these results are discussed.

Giardia duodenalis isolates recovered from humans and dogs living in the same locality in a remote tea-growing community of northeast India were characterized at 3 different loci; the SSU-rDNA, elongation factor 1-alpha (ef1-α) and triose phosphate isomerase (tpi) gene. Phylogenetic analysis of the SSU-rDNA and ef1-α genes provided poor genetic resolution of the isolates within various assemblages, stressing the importance of using multiple loci when inferring genotypes to Giardia. Analysis of the tpi gene provided better genetic resolution and placed canine Giardia isolates within the genetic groupings of human isolates (Assemblages A and B). Further evidence for zoonotic transmission was supported by epidemiological data showing a highly significant association between the prevalence of Giardia in humans and presence of a Giardia-positive dog in the same household (odds ratio 3·01, 95% CI, 1·11, 8·39, P=0·0000).

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.

Amylopectin is used for carbohydrate storage in different life-stages of a number of apicomplexan parasites. We have performed an ultrastructural analysis of amylopectin granules from the oocyst residual body and sporozoites of Cryptosporidium parvum. Amylopectin granules were studied in situ and after isolation from ‘French’ press disrupted parasites, by conventional transmission electron microscopy (TEM) of sectioned oocysts and various negative staining and cryoelectron microscopy techniques. Within the membrane-enclosed oocyst residuum large amylopectin granules (0·1–0·3 μm) can be found besides a characteristic large lipid body and a crystalline protein inclusion. Smaller granules were detected in sectioned sporozoites. Negative staining of isolated amylopectin granules revealed some ultrastructural features not readily visible in sectioned material. The large amylopectin granules had a smooth surface with a ‘ball of string’-like inner structure. Granules isolated from sporozoites were more irregularly shaped and showed a rod-like particulate composition. With the exception of α-amylase, which led to some degree of damage of the surface of the particles, treatment of amylopectin granules with other glycohydrolases had little effect on the overall structure. However, granules adhered to one another. Only when the granules were boiled did the ‘ball of string’ structure gradually dissolve.

We characterized 14 trypanosome isolates from sylvatic mammals (9 from primates, 1 from sloth, 2 from anteaters and 2 from opossum) plus 2 human isolates of Brazilian Amazon. These isolates were proven to be Trypanosoma rangeli by detection of metacyclic trypomastigotes in the salivary glands of triatomines and by a specific PCR assay. Polymorphism determined by randomly amplified polymorphic DNA (RAPD) revealed that most (12) of the Brazilian T. rangeli isolates from the Amazon differed from those of other geographical regions, thus constituting a new group of T. rangeli. Four Brazilian isolates clustered together with a previously described group (A) that was described as being composed of being isolates from Colombia and Venezuela. Isolates from Panama and El Salvador form another group. The isolate from Southern Brazil did not cluster to any of the above-mentioned groups. This is the first study that assesses the genetic relationship of a large number of isolates from wild mammals, especially from non-human primates. A randomly-amplified DNA fragment (Tra625) exclusive to T. rangeli was used to develop a PCR assay able to detect all T. rangeli groups.

Mongolian jirds, Meriones unguiculatus, are susceptible to infection with Trypanosoma grosi, which naturally parasitizes Apodemus spp. The present study investigated T cell dependence of elimination of T. grosi from the bloodstream of jirds by in vivo T cell depletion using a monoclonal antibody (HUSM-M.g.15). In T cell-depleted jirds, elimination of T. grosi, particularly the dividing forms, from the bloodstream was significantly delayed, occurring at around week 3 p.i. The kinetics of serum levels of IgM and IgG specific to trypanosomes in T cell-depleted and control immunocompetent jirds were different; peak levels of IgM were noted on days 6–8 p.i. around the time of peak parasitaemia (day 6 p.i.) in immunocompetent jirds, whereas the serum levels began to increase abruptly after day 10 p.i., peaking at around day 18 p.i. in T cell-depleted jirds. Similarly, serum IgG increased after day 6 p.i. in immunocompetent jirds, in contrast to after day 12 p.i. in T cell-depleted jirds, and the level increased steadily even after disappearance of parasitaemia. Our findings indicate that T cells play a major role at least in the ‘first crisis’ during elimination of dividing T. grosi from the bloodstream.

The diversity and abundance of parasites vary widely among populations of the same host species. These infection parameters are, to some extent, determined by characteristics of the host population or of its habitat. Recent studies have supported predictions derived from epidemiological models regarding the influence of host population density: parasite abundance and parasite species richness are expected to increase with increasing host population density, at least for directly transmitted parasites. Here, we test this prediction using a natural system in which populations of the crucian carp, Carassius carassius (L.), occur alone, with no other fish species, in a series of 9 isolated ponds in Finland. The ectoparasite communities in these fish populations consist of only 4 species of monogeneans (Dactylogyrus formosus, D. wegeneri, D. intermedius and Gyrodactylus carassii); the total and relative abundance of these 4 species varies among ponds, with one or two of the species missing from certain ponds. Across ponds, only one factor, total fish population size, explained a significant portion of the variance in both the mean number of monogenean species per fish and the mean total abundance of monogenean individual per fish. In contrast, fish population density did not influence either monogenean abundance or species richness, and neither did any of the other variables investigated (mean fish length per pond, number of fish examined per pond, distance to the nearest lake, and several water quality measures). In our system, proximity among fish individuals (i.e. host population density) may not be relevant to the proliferation of monogeneans; instead, the overall availability of host individuals in the host population appeared to be the main constraint limiting parasite population growth.

A statistical method that analyses correlation structures in families to delineate the contribution of genetic, household and environmental factors on clustering of infection, has been applied to data collected in an area endemic for brugian filariasis in South Sulawesi, Indonesia. Infection was assessed both by microfilaraemia and by anti-filarial IgG4. The results confirmed earlier findings that genetic factors play an important role in clustering of infection. When clustering of infection was analysed in children (<10 years of age) and adults (>20 years of age) separately, it was found that the genetic factors influence clustering of infection in children more profoundly than environmental or household effects. In contrast, genetic factors could not fully explain the clustering of infection seen in adults, which seemed to be mainly determined by household and environmental effects. The data have implications for genotyping studies in brugian filariasis; they indicate that it may be important to concentrate on the younger age groups where individual environmental effects have not yet overruled the genetic influences on gain/loss of infection.

We found that the expression of mitochondrial apoptosis related genes (Bcl-2 associated protein X, BAX; apoptotic protease activating factor 1, Apaf-1; Caspase 9 and serine/threonine protein kinase, PKB) is elevated in Trichinella spiralis-infected muscles during encapsulation. Micro-dissection of the capsule and subsequent reverse transcription polymerase chain reaction (RT-PCR) confirmed that the expressions of these genes are restricted to the nurse cell. Immunocytochemistry revealed that pro-apoptosis factor (BAX, Apaf-1 and Caspase 9) are predominantly expressed in the basophilic cytoplasm (infected muscle cell origin) and anti-apoptosis factor (PKB) in the eosinophilic cytoplasm (satellite cell origin) of the nurse cell. Electron microscopy revealed that the pre-existing mitochondria in the muscle cells became swollen and disappeared immediately after newborn larva invasion, but new mitochondria of smaller size appeared in the cytoplasm. Nuclear fragmentation and condensation were observed in basophilic cytoplasm which is known to die. Together, the results suggest that the infected muscle cells transform but die through the process of apoptosis which is triggered by factors from the newly formed mitochondria. The anti-apoptosis factor may help the eosinophilic cytoplasm with its survival to ensure nurse cell function.

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.

Experimental intraperitoneal Taenia crassiceps cysticercosis in mice exhibits distinct genetical, immunological and endocrinological features possibly resulting from the complex interactive network of their physiological systems. Very notable is the tendency of parasites to grow faster in hosts of the female sex. It is also remarkable in the feminization process that the infection induces in chronically infected male mice, characterized by their estrogenization, deandrogenization and loss of sexual and aggressive patterns of behaviour. The proto-oncogene c-fos is a sex steroid-regulated transcription factor gene, expressed basally and upon stimulation by many organisms. In the CNS of rodents, c-fos is found expressed in association to sexual stimulation and to various immunological and stressful events. Hence, we suspected that changes in c-fos expression in the brain could be involved in the feminization of the infected male mice. Indeed, it was found that c-fos expression increased at different times during infection in the hypothalamus, hippocampus, less so in the preoptic area and cortex, and not in several other organs. The significant and distinctive regional changes of c-fos in the CNS of infected mice indicate that the brain of the host senses intraperitoneal cysticercosis and may also announce its active participation in the regulation of the host–parasite relationship. Possibly, the host's CNS activity is involved in the network that regulates the estrogenization and deandrogenization observed in the chronically infected male mice, as well as in the behavioural and immunological peculiarities observed in this parasitic infection.