The mathematical model of the in-host dynamics for malaria parasites of Anderson, R. M., May, R. M. & Gupta, S. (1989), Parasitology99 (Suppl.) S59–S79, and subsequently used by other authors, contains an error. This leads to very substantial over-estimates in parasite growth rates. A corrected form of the model is presented in this paper.

In the accompanying manuscript Saul (1998) points out that a model of within-host malaria population dynamics (Anderson, May & Gupta, 1989) can exhibit unrealistically large growth rates. He suggests that this error can be avoided by replacing the parameter r, which is the number of merozoites produced by each parasite at schizogony, with the value ln (r)+1. This substitution does not, however, address the true underlying problem with the model, namely that whilst in reality there is small variation in the distribution of Plasmodium spp. life-spans, the use of the constant rate α assumes an exponential, and hence much more variable, distribution. This can allow the population to increase, over the time period of the average life-span (48 h in the case of Plasmodium falciparum), by factors considerably larger than r. Saul identifies this assumption as unrealistic in terms of the biology of malaria, but it is not remedied in the proposed model.

Within the structure of the original model there are two ways of addressing the growth rate problem. Firstly, the ‘growth constant’ r can be replaced by the value ln (r)+1, so that if all parasites reinvade, the model increases by a factor r over 1 generation. As pointed out by Saul, this is an artificial device since it means that each parasite produces a reduced number of merozoites. Alternatively, a parasite can produce the observed number of merozoites, r, many of which do not reinvade. This is the situation in the original paper and Gravenor, McLean & Kwiatkowski (1995). In these papers the model does grow at a reasonable rate because the parameter β is estimated directly from observed growth rates. These points, however, are only an aside. Both the original and Saul's modified model can grow at the same rate and they both have the same distributional assumptions concerning parasite life-span.

Stochastic processes play a vital role in the early stages of the evolution of drug-resistant malaria. We present a simple and flexible method for investigating these processes and understanding how they affect the emergence of drug-resistant malaria. Qualitatively different predictions can be made depending on the biological and epidemiological factors which prevail in the field. Intense intra-host competition between co-infecting clones, low numbers of genes required to encode resistance, and high drug usage all encourage the emergence of drug resistance. Drug-resistant forms present at the time drug application starts are less likely to survive than those which arise subsequently; survival of the former largely depends on how rapidly malaria population size stabilizes after drug application. In particular, whether resistance is more likely to emerge in areas of high or low transmission depends on malaria intra-host dynamics, the level of drug usage, the population regulation of malaria, and the number of genes required to encode resistance. These factors are discussed in relation to the practical implementation of drug control programmes.

The BALB/c strain of mice is comparatively more resistant to sporozoite infections of Plasmodium berghei than the C57BL6 strain. Infection with live sporozoites results in the formation of small hepatic forms in the BALB/c liver that persist for as long as 6 days. Upon infection with small numbers of sporozoites, some of the parasites are destroyed in the liver whereas the rest persist as blocked forms. When larger numbers of sporozoites are injected the same process occurs but, in addition, a fraction of the liver-stage parasites complete full development and give rise to blood forms. Although blocked liver forms persist until day 6 post-infection they actually develop to only 24 h of maturity. The nature of these persistent forms is similar to those obtained from irradiated sporozoite immunization. There is a stronger cell proliferation to liver-stage antigens by spleen lymphocytes of irradiated sporozoite-immunized BALB/c mice in comparison to that of immunized C57BL6 mice suggesting that a stronger priming to liver-stage antigens, probably due to the presence of blocked hepatic forms in the liver for a longer period of time (as compared to C57BL6), occurs in the BALB/c mice. This could be a reason for the long-lasting protective memory observed in BALB/c mice.

Plasmodium falciparum merozoites have an array of 2–3 subpellicular microtubules, designated f-MAST. We have previously shown that colchicine inhibits merozoite invasion of erythrocytes, indicating a microtubular involvement in this process. Colchicine inhibition of invasion was reduced by the Taxol®-stabilization of merozoite microtubules prior to colchicine exposure. Immunofluorescence assays showed that the number and length of f-MASTs were reduced in colchicine-treated merozoites, confirming that microtubules were the target of colchicine inhibition. The dinitroaniline drugs, trifluralin and pendimethalin, were shown by immunofluorescence to depolymerize the f-MAST and both drugs were inhibitory in invasion assays. These results demonstrate that the integrity of the f-MAST is important for successful invasion. Fluorescence imaging demonstrated the alignment of mitochondria to f-MAST, suggesting that mitochondrial transport might be perturbed in merozoites with disorganized f-MAST. Depolymerizing mt in late-stage schizonts did not affect the allocation of mitochondria to merozoites.

DAMP (3-(2,4-dinitroanilino)-3′amino-N-methyldipropylamine), which differentially accumulates in acidic compartments, was used to identify such compartments in Toxoplasma gondii tachyzoites at the electron microscope level. In both free tachyzoites and dividing intracellular parasites the only sites of DAMP accumulation were mature and forming rhoptries. No labelling of other secretory organelles (micronemes and dense granules), the ER, Golgi or any other membrane-bounded organelles or anything resembling a lysosomal system was observed. Labelling of the forming rhoptries was higher and more homogenous than in mature rhoptries in which labelling was confined to the expanded ends of each organelle. The acid pH-dependent accumulation of DAMP in the forming and mature rhoptries was blocked by ammonium chloride and monensin, reagents known to abolish intracellular pH gradients. Estimates of rhoptry pH, based on the level of DAMP accumulation, show that the intralumenal pH of forming rhoptries is more acidic (pH 5·5–3·5) than the mature rhoptries (pH 7·0–5·0).

A gene (vsp417-3A-II) encoding a new member of the variant-specific surface protein (VSP) family of Giardia is described. Vsp417-3A-II is detected exclusively in isolates belonging to the genetic Assemblage A/Group II sublineage of G. intestinalis, and it is closely related to a previously characterized VSP gene (tsp11, herein renamed vsp417-2A-I) found in Group I isolates of G. intestinalis. Analysis of DNA amplified from the genomes of Group II isolates using consensus primers also revealed products corresponding to the vsp417-2 locus (vsp417-2A-II allele), indicating that vsp417-2A-II and vsp417-3A-II represent separate loci in these organisms. Southern hybridizations revealed a single genomic copy of vsp417-3A-II in Group II isolates, but efforts to detect this locus in Group I organisms were unsuccessful. Together with the vsp417-1 (tsa417) locus that was first identified in the genetic Group I isolate WB and which has been detected since in all tested genetic Group I isolates (as vsp417-1A-I) and in genetic Group II isolates (as vsp417-1A-II), vsp417-3A-II represents a third member of a stable subset of tsa417-like VSP genes in genetic Group II G. intestinalis. The encoded polypeptide, vsp417-3A-II (667 residues, predicted Mr 69120) possesses a high cysteine content (12·1 mol%), 28 copies of the metal-binding -CXXC- motif, and a highly conserved C-terminal hydrophobic domain – similar to all other characterized Giardia VSP. A revised nomenclature for Giardia VSP genes is proposed, prompted by the need to describe the homologues found in the various major genotypes of the species complex, G. intestinalis.

We describe a laboratory model using Eimeria falciformis and E. pragensis to investigate some of the interactions in double-species infections of Eimeria. Mice were given trickle infections by oral inoculation of 100 sporulated oocysts of one species at 3 or 4 day intervals throughout the experiments and, once immunity had developed, as indicated by cessation of oocyst production, the animals were challenged with a single inoculation of the other species. A trickle infection of E. falciformis gave a significant enhancement of oocyst output from E. pragensis infection as compared with animals that had not received E. falciformis. Histological examination of the mice infected with E. pragensis showed significantly more asexual parasites and a significantly higher female/male ratio in the mice that had received trickle E. falciformis infections than in those that had not. There was no evidence to suggest that extra asexual generations were occurring in these mice. In the converse experimental protocol, trickle E. pragensis infections had no significant effect on a single E. falciformis infection. We discuss the possible mechanisms for the interactions and also how these interactions may influence multiple species infections in animals in their natural habitats.

The circumstances which permit the establishment of Leishmania infections in sandflies were investigated by altering the growth conditions for L. donovani parasites in the unsuitable vector Phlebotomus papatasi. Only 5·0% of the sandflies harboured a few parasites 3 days after feeding on promastigotes in defibrinated blood. Heparinized blood or the addition of trypsin inhibitor to the meals allowed persistence of infections (day 6) in 9·9% and 25·8% of the flies respectively. Meals of erythrocytes, saline and amastigotes produced 44·4% fly infection on day 6, while similar promastigote-initiated infections remained in 70·3% of the flies. Proteolytic activities in the guts of sandflies fed on the above meals without parasites, were the highest after defibrinated bloodmeals. Erythrocytes with saline decreased the maximal alkaline protease level from 20·8 U to 13·5 U/fly; that of trypsin from 3·9 U to 1·8 U/fly and that of the aminopeptidase from 5·5 U to 3·9 U/fly. After meals of heparinized blood, the maximal alkaline protease activity (12·0 U/fly) was also much lower than after defibrinated blood-feeding. The different diets which resulted in comparatively low enzymatic activities, including blood with trypsin inhibitor, also promoted the survival of infections. This implies that the proteolytic activity in the sandfly gut modulates the vector susceptibility.

We review the theoretical framework for exploring the impact of individual and spatial heterogeneities in patterns of exposure and contamination and on the basic reproduction number, R0, for human schistosomes. Analysis of water contact data for 5 communities in Zimbabwe and Mali suggests that the impact is substantial, increasing R0 by factors of up to 6·5, mostly due to highly overdispersed distributions of contact rates among individuals. Several practical conclusions emerge: concentration of contacts at a single site should be avoided; the impact of control targeted at certain sites cannot be predicted without knowledge of how individuals' contacts are distributed among sites; control programmes targeted at individuals or sites contributing most to transmission can be very efficient but, conversely, will be ineffective if any of these individuals or sites are missed.

We compared a Trypanosoma cruzi clone unable to infect or induce pathology in mice (CL-14), with virulent T. cruzi (Y and CL strains) in terms of cruzipain expression, subcellular distribution and functional activity. Our results showed that (1) intracellular Y amastigotes expressed R1 (carboxy-terminal) and R2 (catalytic) domains concentrated in cytoplasmic vesicles, while CL-14 presented R1 labelling on membrane clusters and R2 in intracellular compartments, (2) CL-14-trypomastigotes presented R1 and R2 staining preferentially on flagellar and cellular membranes, similar to CL, but different from Y strain intracellular labelling pattern, (3) flow-cytometry revealed higher expression of R1 by CL-14-trypomastigotes than virulent strains, but R2 staining similar to CL-trypomastigotes, (4) CL-14-trypomastigotes presented normal cruzipain activity in gelatin gels, but different banding patterns were found in CL-14 versus CL and Y strains. Our data rule out failure in cruzipain expression, activity or subcellular distribution as an explanation for CL-14 biological behaviour, but suggest the expression of a different isoform. These results also cast doubt on the putative role of cruzipain as a target of immunopathological responses, since high levels of functional cruzipain are expressed by a non-pathogenic T. cruzi.

The survival characteristics of Discocotyle sagittata larvae are both age- and temperature-dependent. In laboratory studies at constant temperatures of 6, 10, 13, 18 and 22°C, oncomiracidia had a maximum life-span of 96 h at 6°C, declining with increasing temperature to 26 h at 22°C. Larval swimming activity was also age dependent, and continued for a large proportion of survival time. The relationship between time at which larvae stop swimming as a proportion of total survival time was statistically significant between temperatures. Proportionally, oncomiracidia swam for longer periods at lower temperatures. As oncomiracidia age, they become progressively less active, spending less time in the water column. In contrast to observations recorded in the literature, D. sagittata larvae are infective soon after emergence (within the minimum of 2 h studied), and feed on blood from the gill capillaries within 2 h post-exposure. The temperature-dependent characteristics of the larval stage are likely to have important implications for seasonal changes in parasite transmission.

Egg production by the freshwater gill monogenean Discocotyle sagittata infecting rainbow trout (Oncorhynchus mykiss) was monitored at 5, 13 and 18°C; eggs were incubated at 6, 10, 13, 15, 16, 18, 22 and 26°C and the hatching times determined. Mean per capita output at 13°C ranged from 0·8 eggs/worm/day (e/w/d) to 18·3 e/w/d for 25 hosts (burdens 1–17 worms/host). Average egg production rates for worms in single burdens varied from 3 e/d to 14·6 e/d at 13°C, mean 7·6 e/d (±0·63) (n=8). Egg production was temperature dependent, increasing from a mean of 1·5 e/w/d at 5°C to 12·0 e/w/d at 18°C. Developmental rate was also temperature dependent: larvae began hatching after 84 days at 6°C, 28 days at 13°C, and 20 days at 18°C. Eggs laid within the same 24 h period hatched over 3–7 days at 13–18°C; this range was extended at decreasing temperatures. Egg viability was generally high: between 13 and 18°C a mean of 92% of larvae successfully emerged. However, viability declined at the highest and lowest temperatures (23% hatching at 6°C, 55% at 8°C and 47% at 26°C). The implications for seasonal parasite transmission are discussed.

Infrapopulation dynamics of the nematode Rhabdias cf. hylae within naturally-infected Bufo marinus in north Queensland, Australia, were detailed. Over 80% of 580 toads were infected with Rh. cf. hylae with a mean intensity of 16·1. Distribution of Rh. cf. hylae within the toad population was aggregated, with an increase in the variance-to-mean ratio with increasing toad size. Intensity of infection and length of nematode were both correlated with length of toad in the smaller size classes. Length of nematode was not related to intensity of infection at any time. Mean intensity of infection rose significantly in small toads following initial infection after metamorphosis. Over the same period, average length of nematode did not increase implying constant re-infection of the toads. Larger toads were not reinfected to the same extent, and the number of uninfected toads in the larger size class increased which indicated a natural loss of infections. Changes in parameters of Rh. cf. hylae infection within B. marinus were attributed to seasonal rainfall and its subsequent effect on the behaviour of the toad.