Comparison of numerous delivery systems for the induction of cytotoxic T lymphocytes by immunization

A variety of vaccine delivery systems including peptides with various adjuvants, recombinant particles, live recombinant viruses and bacteria and plasmid DNA were tested for their ability to induce CD8+ cytotoxic T lymphocytes (CTL) against a well-defined epitope (amino acids 252-260) from the circumsporozoite (CS) protein of Plasmodium berghei. We compared routes of immunization that would be applicable for the administration of a malaria vaccine in humans. The majority of these vaccines did not induce high CTL responses in the spleens of immunized mice. However, both a yeast-derived Ty virus-like particle expressing the optimal nine-amino acid epitope SYIPSAEKI from the CS protein (CSP-VLP) and a lipid-tailed peptide of this same sequence induced high levels of the major histocompatibility complex (MHC) class I-restricted CTL with one and three subcutaneous immunizations, respectively. Moreover, these CTL were able to recognize naturally processed antigen expressed by a recombinant vaccinia virus. The levels of CTL induced by CSP-VLP could be augmented by co-immunization with certain cytokines. Target cells pulsed with CSP-VLP were recognized and lysed, showing that the particles were effectively processed and presented through MHC class I presentation pathway. The levels of CTL induced using CSP-VLP and lipopeptides are comparable to those observed after immunization with multiple doses of irradiated sporozoites.

The effect of atovaquone (566C80) on the maturation and viability of Plasmodium falciparum gametocytes in vitro

Atovaquone (566C80), a hydroxynaphthoquinone, was investigated for activity against Plasmodium falciparum gametocytes (NF54 strain) in vitro. After 96 h of continuous exposure to the drug at 1.4 x 10(-7) M (a concentration achievable in humans 14 d after administration of a therapeutic dose of 10 mg/kg) reductions of 75%, 54% and 20% in the number of gametocyte stages 2, 3 and 4, respectively, were achieved. A small increase (14%) in stage 5 gametocytes was seen. At the same concentration, atovaquone showed greater activity against the asexual stages of P. falciparum, reductions of 93%, 96% and 43% in the number of rings, schizonts and trophozoites, respectively, being achieved. These data are consistent with inhibition of maturation of trophozoites. The observed effect on maturation of gametocytes is similarly consistent with blockade of gametocyte recruitment from merozoites produced by the preceding schizogony, or to stasis of intraerythrocytic sexual development before the formation of stage 2 gametocytes.

Induction of anti-malarial transmission blocking immunity with a recombinant ookinete surface antigen of Plasmodium berghei produced in silkworm larvae using the baculovirus expression vector system

We have studied Pbs21, a major ookinete surface protein of Plasmodium berghei, for the development of a model transmission blocking immunogen. In the mouse, recombinant Pbs21 expressed in the Escherichia coli expression system (EcrPbs21) is not as effective in inducing transmission blocking antibodies as native Pbs21 (nPbs21), possibly because of differences in post-translational processing between EcrPbs21 and nPbs21. In an attempt to improve the efficacy of the recombinant molecule, we describe here the use of a baculovirus expression vector system in the silkworm Bombyx mori. Following an injection of recombinant baculovirus containing Pbs21 cDNA, B. mori larvae produced recombinant Pbs21 (BmrPbs21) with a molecular weight indistinguishable from nPbs21. Fifty micrograms of BmrPbs21 could be purified from the hemolymph of each infected larva using affinity chromatography. Immunization of Balb/c mice with BmrPbs21 induced high anti-BmrPbs21 and anti--ookinete antibodies but low anti-EcrPbs21 antibody. In contrast, EcrPbs21 induced high anti--EcrPbs21 antibody but low anti-BmrPbs21 and anti-ookinete antibodies. This suggests that most B-cell epitopes on nPbs21 are conformational and that many of the linear epitopes in EcrPbs21 are not normally exposed in nPbs21. Oocyst formation in Anopheles stephensi mosquitoes, which fed on mice immunized with purified BmrPbs21 and infected with P. berghei, was blocked by 85.5-97.1%. These results suggest that the baculovirus-silkworm system produces useful quantities of recombinant Pbs21 which in limited studies is structurally and immunogenically indistinguishable from the native molecule.

Inhibitory activity of the anti-malarial atovaquone (566C80) against ookinetes, oocysts, and sporozoites of Plasmodium berghei

Ookinete formation from mature Plasmodium berghei gametocytes in vitro was partially inhibited by 0.05-0.1 microM atovaquone and almost totally blocked at a concentration of 0.25 microM. Microgametocyte exflagellation was not affected by atovaquone at concentrations up to 300 microM. Ookinete formation was also inhibited in culture when addition of 0.20 microM atovaquone was delayed by 4 hr, by which time DNA replication was likely to have been completed. Inhibition of ookinete formation by atovaquone was not reversed by orotic acid. Plasmodium berghei-infected Anopheles stephensi mosquitoes were fed a second blood meal 4, 7, 14, and 20 days postinfection (p.i.) from mice that had been treated with atovaquone or control diluent 8 hr previously. Atovaquone blood feeds on day 4 reduced oocyst numbers on days 6-12, although sporozoite numbers in the thorax and abdomen on day 20 were not significantly reduced. Blood feeds on day 7 slowed oocyst growth, blood feeds on day 14 did not significantly reduce sporozoite numbers, and feeds to mosquitoes on day 20 p.i. had no effect on transmission to naive mice. Sporozoite invasion of human hepatoma cells was unaffected by the presence of atovaquone.

Expression of the Plasmodium berghei ookinete protein Pbs21 in a baculovirus-insect cell system produces an efficient transmission blocking immunogen

A surface protein of Plasmodium berghei ookinetes, Pbs21, was expressed in a baculovirus-insect cell system in cell culture and in Heliothis virescens larvae. Groups of BALB/c mice received two intraperitoneal inoculations of either i) Tris-buffer or homogenized H. virescens larvae infected with wild-type baculovirus; ii) enriched, homogenized ookinetes, or iii) homogenized H. virescens larvae expressing recombinant Pbs21 (rPbs21). All animals immunized with ookinetes or with rPbs21 had high titres of antibodies (IgG isotype) that bound to native Pbs21. The large majority of antibodies in immune sera of both groups recognized the antigen under non-reducing but not under reducing conditions. The predominant IgG-subclasses in mice immunized with ookinetes was IgG1 and in mice immunized with rPbs21, the subclasses were IgG1 and IgG2a. Immunization with rPbs21 reduced the infectivity of P.berghei to mosquitoes by 91% compared to a 99% reduction following immunization with ookinetes. This preliminary data indicate that rPbs21 expressed in this eukaryotic system induces a transmission-blocking immunity, which is more effective than that achieved using rPbs21 expressed in Escherichia coli (Matsuoka et al. 1994).

Specific and nonspecific responses to Plasmodium falciparum blood-stage parasites and observations on the gametocytemia in schoolchildren living in a malaria-endemic area of Mozambique

We have observed specific and nonspecific reactivities to the asexual states and gametocytes of Plasmodium falciparum and examined the effect of chloroquine and Fansidar (pyrimethamine/sulfadoxine) on the dynamics of gametocytemia. Schoolchildren peripheral blood films positive for P. falciparum gametocytes were identified in a malaria-endemic area of Mozambique. The children were randomly allocated into two groups to receive chloroquine or pyrimethamine/sulfadoxine, and were followed for 28 days after treatment. In patients harboring drug-sensitive parasites, asexual parasitemias were cleared by day 4, but gametocytes persisted for an additional 17 days. The prevalence of the asexual parasites was 67.6% in the chloroquine-treated group at day 0 and 61.1% at day 28, whereas in the pyrimethamine/sulfadoxine treated group, the initial parasite prevalence of 70.7% was reduced to 2.4% at day 28, suggesting a high prevalence of chloroquine-resistant parasites. On day 0, gametocyte prevalence was 59.5% in the chloroquine-treated group and in 68.3% in the pyrimethamine/sulfadoxine-treated group; these values were reduced to 5.6% and 2.4%, respectively, at day 28. Our results suggest strongly that there is no induction of gametocytogenesis by either course of chemotherapy.

In situ detection of Pbs21 mRNA during sexual development of Plasmodium berghei

The patterns of expression of ribosomal RNA and mRNA encoding the parasite surface antigen Pbs21 have been investigated during the sexual stages of development of the malaria parasite, Plasmodium berghei, using the technique of non-radioactive in situ RNA hybridisation. An RNA probe complementary to a region of the small subunit of P. berghei ribosomal RNA hybridised to parasites at all stages of development in a smear of blood taken from mice infected with P. berghei. Messenger RNA encoding Pbs21, in contrast, was detected only within parasites committed to sexual development within the vertebrate host and, furthermore, was shown to be expressed in a sex-specific manner, exclusively within female gametocytes. At later stages of sexual development, Pbs21 mRNA was detected at high levels in female gametes and ookinetes. We have previously shown that Pbs21 protein is first detectable only after the initiation of gametogenesis which occurs following transmission to the insect vector. These results suggest, therefore, that post-transcriptional mechanisms operate to regulate the translation of Pbs21 mRNA as it accumulates during female gametocytogenesis.

Characterization of the modes of action of anti-Pbs21 malaria transmission-blocking immunity: ookinete to oocyst differentiation in vivo

The impact of immune sera, and peripheral blood cells (PBC) from mice immunized with Plasmodium berghei ookinetes; and of purified immunoglobulin or Fab fragments from anti-Pbs21 monoclonal antibody 13.1, upon establishment of oocyst infections in the mosquito was studied. Infections were initiated either from gametocyte-infected mice, or membrane feeders which contained either gametocytes or mature ookinetes. PBC from ookinete-immunized mice presented with non-immune serum failed to show any transmission-blocking activity. Anti-ookinete serum, intact anti-Pbs21 monoclonal antibody 13.1 or its Fab fragments, all inhibited oocyst formation significantly. When gametocyte-infected mice or gametocytes in membrane feeds were used, inhibition did not directly correlate with antibody concentration. In membrane feeders that contained ookinetes and antibody, concentration-dependent inhibition usually occurred. The efficacy of purified 13.1 IgG was dependent upon the ookinete concentration. The ookinete plasmalemma and cytoplasm were significantly disturbed after 12 h in bloodmeals that contained antibody 13.1, but not in the isotype controls. These changes may have caused the observed failure of the ookinete to migrate as rapidly as the controls from the destructive environment of the bloodmeal.

Relationship between prevalence and intensity of Plasmodium falciparum infection in natural populations of Anopheles mosquitoes

Wild-caught Anopheles gambiae s. l. and An. funestus were dissected and their midguts were examined for the presence of Plasmodium falciparum oocyst infections. The mean intensity of infection and the prevalence of infected mosquitoes were determined for each sample, with one sample representing the mosquitoes caught in a single house at any given time. The patterns of infection were investigated using the relationships between prevalence, intensity, and variance within samples, and were found to be consistent with laboratory infections. The overall distribution of oocysts is characterized by a mixture of negative binomial distributions with means determined by the infectiousness of the human hosts, and a constant degree of aggregation (k = 0.0767) presumably determined by the development of oocysts within mosquitoes. The prevalence/intensity relationship was treated as a bivariate distribution to ascertain the effect of sample size on the accuracy of estimation, and to allow inference of intensity from prevalence. In mathematical models fitted to the collected data, sample size affected directly the minimum possible prevalence of infection, and the accuracy of both mean and prevalence estimations. Based on minimum possible positive prevalence rates, data from samples of less than 20-25 mosquitoes would provide unacceptable errors in prevalence estimations. However, natural oocyst rates are consistently higher than the minimum prevalence, and it is suggested that any interpretations from samples of less than approximately 40 mosquitoes must be treated with some caution. Such variation in natural samples means that prediction of intensity of infection from prevalence (or vice versa) is extremely inaccurate.

Sexual development of malaria parasites is inhibited in vitro by the neem extract azadirachtin, and its semi-synthetic analogues

We have shown that azadirachtin, a compound from the neem tree, Azadirachta indica, and selected semi-synthetic derivatives, block the development of the motile male malarial gamete in vitro. Changes in the hemiacetal group at position C11 in the molecule result in a loss of activity in this assay. The motility of fully formed male gametes, and other selected flagellated cells, is unaffected by azadirachtin in vitro. These findings raise the possibility of developing azadirachtin-based compounds as antimalarials with transmission-blocking potential, as well as permitting the further study of structure-activity relationships in these compounds.

Characterization of the effector mechanisms of a transmission-blocking antibody upon differentiation of Plasmodium berghei gametocytes into ookinetes in vitro

The transmission-blocking monoclonal antibody 13.1, which recognizes the ookinete surface antigen Pbs21 of Plasmodium berghei, and an IgG2a isotype control antibody 26.37 were purified by caprylic acid and ammonium sulphate precipitation. Fab fragments were prepared by papain digestion. IgG but not Fab from antibody 13.1 reduced ookinete formation by P. berghei in culture by as much as 94% at a concentration of 100 micrograms/ml. There was little difference in antibody efficacy in the range 6.25-400 micrograms/ml in this assay. The parasite was most sensitive to antibody activity in the first 6-9 h of culture, i.e. the gamete/zygote and early retort stages. Peripheral blood leucocytes (PBL) were essential to achieve maximal inhibition by mAb 13.1 (activity was abrogated totally if PBL were removed). Together the data suggest that one of the mechanisms of action of this antibody is antibody-mediated PBL killing. Phagocytosis of parasites was noted in these experiments in all cultures. We have not attempted in this study to distinguish between Fc-mediated opsonization, as opposed to antibody-dependent cellular cytotoxicity.

Effects of hormones and cysteine protease modulators on infection of HepG2 cells by Plasmodium berghei sporozoites in vitro determined by ELISA immunoassay

An enzyme-linked immunosorbent assay (ELISA) detecting a Plasmodium berghei liver-stage-specific protein Pbl-1 is described. The quantitative detection limits ranged from 0.01 to 0.05 microgram of parasite protein. Qualitatively the assay detected as little as 0.001 microgram Pbl-1 per well. Using the ELISA dexamethasone and insulin together was shown to promote higher parasite infections in HepG2 cells compared to unsupplemented medium. Anti-cowpea-protease cysteine inhibitor significantly increased hepatocyte invasion as compared to controls, whereas a significant decrease was recorded in the presence of the protease inhibitor E64. Partial involvement of cysteine proteases in HepG2 invasion by P. berghei sporozoites is therefore suggested.

Inhibitory action of the anti-malarial compound atovaquone (566C80) against Plasmodium berghei ANKA in the mosquito, Anopheles stephensi

The activity of atovaquone against Plasmodium berghei ANKA during sporogonic development has been examined. Anopheles stephensi mosquitoes were fed on gametocyte infected mice which had been treated 8 h previously with atovaquone or diluent alone. Mosquito midguts were examined for oocysts, and salivary gland infections were estimated using an ELISA for the circumsporozoite protein (CSP). The number of oocysts per midgut fell by at least 97% when mosquitoes were fed on mice dosed with 0.1-10 mg atovaquone/kg body weight. This was paralleled by a decrease in the prevalence of oocyst-infected mosquitoes from 70-90% in controls to 40% or 10% respectively. No oocysts were observed at a dose of 100 mg/kg. CSP ELISA results indicated that mosquitoes fed on atovaquone failed to produce sporozoites. Mosquitoes which fed on gametocytaemic, atovaquone-treated mice (0.1-100 mg/kg) did not transmit malaria to naive mice. These results demonstrate that atovaquone has a highly potent inhibitory activity against the mosquito stages of P. berghei.

Plasmodium berghei: serum-mediated inhibition of infectivity of infected mice to Anopheles stephensi mosquitoes

The transmission of Plasmodium berghei-infected mice to Anopheles stephensi mosquitoes showed a peak number of oocysts early in the infection prior to the peak of gametocytaemia. This was followed by a precipitous decline on Days 4 and 5 (see also Dearsley et al., Parasitology, 100, 359-368, 1990). By measuring percentage relative infectivity (using membrane feeds with viable gametocytes), we have shown that serum collected daily during the course of a blood-induced infection blocked infectivity from Day 6 postinfection onward. Although there was a correlation between anti-blood stage antibody levels and the loss of infectivity, a comparison of the infectivity pattern of P. berghei-infected BALB/c and SCID mice (the latter being incapable of antibody production) revealed the same pattern of inhibition in both mouse strains, suggesting that antibody alone is not responsible for this suppression. Sera taken from SCID mice late in the infection tested in vitro demonstrated a decline in infectivity similar to that observed in vivo, suggesting that a non-antibody serum factor(s) is responsible for the sustained decline in infectivity of P. berghei to A. stephensi mosquitoes.

Studies on the immunogenicity of a recombinant ookinete surface antigen Pbs21 from Plasmodium berghei expressed in Escherichia coli

Plasmodium berghei ookinete surface antigen (Pbs21), was produced as a fusion product with maltose binding protein (MBP) in Escherichia coli and used to induce transmission-blocking immunity in mice. Specificity of induced antibody was confirmed by Western blotting with native ookinete Pbs21, and by the indirect immunofluorescent antibody test on ookinete bloodfilms. Immunized mice were infected with P. berghei and transmission to Anopheles stephensi mosquitoes determined by both the intensity and prevalence of oocyst infections. Compared with a control group immunized with MBP alone the maximum blockade of oocyst intensity was 66% in the mice immunized with recombinant MBP-Pbs21. Over nine experiments blockade averaged only 33%. By comparison with native Pbs21 protein, which usually induces > or = 90% blockade, our data suggests the recombinant protein produced in this bacterial system is a less effective immunogen despite expressing epitopes recognized by known transmission-blocking monoclonal antibodies.

Attempted isolation of the gene encoding the 21 Kd Plasmodium berghei ookinete transmission blocking antigen from Plasmodium yoelli and Plasmodium vivax

The 21kD ookinete antigen of Plasmodium berghei (Pbs 21) has been shown to elicit an effective and long lasting transmission blocking immune response in mice. Having cloned and sequenced this antigen (Paton et al. 1993) the sequence was compared to the genes of the same family previously identified in P. falciparum, P. gallinaceum (Kaslow et al. 1989) and P. reichenowi (Lal et al. 1990). Four conserved areas were identified in this comparison, to which degenerate oligonucleotides were designed. PCR amplification and screening of genomic libraries was then carried out using these oligonucleotides. The P. yoelii gene was successfully cloned and a number of novel P. vivax genes identified but the P. vivax homologue of Pbs21 remains elusive.

The effect of transmission-blocking antibody ingested in primary and secondary bloodfeeds, upon the development of Plasmodium berghei in the mosquito vector

The effects of purified monoclonal immunoglobulins from control, or transmission-blocking anti-Pbs21 antibodies, upon the infection of Anopheles stephensi by ookinetes of Plasmodium berghei are compared. Anti-Pbs21 antibody reduced mean intensity and prevalence of infection by 94.7 and 58.7% respectively if added to the infectious bloodfeed at a concentration of 100 micrograms/ml. Fab fragments were of similar efficacy. No transmission enhancement was detected with declining antibody concentrations. Addition to subsequent (second) feeds reduced mean oocyst intensity but not prevalence. The reduction in blockade declined from 41% at day 2, to 4% at day 8. Second bloodfeeds, containing control globulin taken 4 or 6 days (but not 2 days) after infection, increased sporozoite burden in the salivary glands. At all times anti-Pbs21 reduced sporozoite number in the thorax compared to time-matched controls, but again highest gland intensities were obtained when the second bloodfeed was given on day 4. We conclude that second bloodfeeds containing transmission-blocking antibody simultaneously serve two opposing roles, (1) inhibition of parasite development and (2) the supply of nutrients which permit more sporozoites to be produced by each oocyst.

Kinetics of expression of two major Plasmodium berghei antigens in the mosquito vector, Anopheles stephensi

Expression of a 21 kDa determinant (Pbs21), first detected on the surface of ookinetes, and of the circumsporozoite protein (CSP) was studied by immunofluorescence and Western blots during the developmental cycle of Plasmodium berghei in the mosquito Anopheles stephensi. The expression of Pbs21 was predominantly localised on the ookinete surface one day after the infectious blood meal, and thereafter reactivity declined to a minimum on days 2 and 3, the time of onset of oocyst development. A gradual increase in fluorescence was observed on the oocysts from day 6 that was retained until day 17 post-infection. In contrast, sporozoites released from oocysts or salivary glands showed little or no antibody labelling with anti-Pbs21. Circumsporozoite protein was not detectable in any midgut preparations until 5-6 days after feeding, when reactivity was observed against immature oocysts. Expression then continued and increased throughout oocyst and sporozoite development. Western blots confirmed that Pbs21 was expressed minimally during the oocyst development but was not detectable in sporozoites. Co-localisation of anti-Pbs21 and anti-CSP monoclonal antibodies to the 50 kDa and 60 kDa bands in Western blots of sporozoite suggests immunological cross-reactivity between the CSP and the anti-21 kDa antibodies.

Factors regulating natural transmission of Plasmodium berghei to the mosquito vector, and the cloning of a transmission-blocking immunogen

Naturally occurring factors that regulate the infectivity of P. berghei infected rodent hosts to the mosquito vector in vivo have been compared in T.O., Balb/C and immunodeficient SCID mice. No detectable differences in infectivity were observed suggesting B and T cell mediated factors are not involved. Further studies investigated roles for macrophage colony stimulating factors, the cytokines IFN gamma and TNF alpha, of neutrophils, and of nitric oxide in the SCID mouse, but have failed to demonstrate an important role in vivo for any factor examined. Differences between these results and those obtained in vitro on the human and primate parasites must therefore be explained by biological differences between the parasite/host combinations, or by technical differences in experimental designs. Induced immunity to the ookinete surface antigen Pbs 21 of P. berghei can totally block the transmission of the parasite from the gametocyte infected host to the vector. We have cloned the gene encoding Pbs 21 and shown it bears striking structural similarities to Pfs 25, Pgs 25 and more particularly Pgs 28 in that it has a high cysteine content (9.5%), 4 EGF-like domains and hydrophobic amino-'signal'--and carboxyl-'anchor' sequences. The encoding gene is on chromosome 5 and is found also in P. chabaudi, P. yoelii and P. vinckei.

Structure and expression of a post-transcriptionally regulated malaria gene encoding a surface protein from the sexual stages of Plasmodium berghei

The sexual stage-specific protein Pbs21 of the rodent malaria parasite Plasmodium berghei, expressed on the surface of zygotes and ookinetes, has been shown to induce an effective and long-lasting transmission blocking immunity. The gene encoding Pbs21 was cloned by screening a cDNA library prepared from enriched zygotes and ookinetes using the monoclonal antibody 13.1.15, which is capable of blocking subsequent parasite sexual development in the mosquito vector. The Pbs21 gene encoded a protein of 213 amino acids which contained a putative amino-terminal signal sequence and a putative carboxy-terminal hydrophobic membrane anchor. The amino-acid sequence was characterised by a large number of cysteine residues which were organized into 4 epidermal growth factor-like domains. The spacing of the cysteine residues was highly conserved when compared to the 25-kDa ookinete proteins of Plasmodium falciparum (Pfs25), Plasmodium reichenowi (Prs25) and Plasmodium gallinaceum (Pgs25) which were approximately 45%, 45% and 40% homologous to Pbs21 respectively. The gene is located on chromosome 5 and cross-hybridizes to a similarly defined gene unit in the other rodent malaria species Plasmodium chabaudi, Plasmodium vinckei and Plasmodium yoelii. The gene is internally disposed and not in the subtelomeric region of chromosome 5. The gene is transcribed in a stage-specific manner giving rise to an abundant 1.5-kb transcript. This mRNA is synthesised in the precursor cells to female gametes (gametocytes) however the protein is observed only after activation of the gametes, suggesting that translation of the mRNA is controlled by a post-transcriptional process. The Pbs21 gene and the P. berghei parasite system provide an excellent vehicle for the study of stage-specific transcriptional and post-transcriptional control in malaria.

Heterogeneity in patterns of malarial oocyst infections in the mosquito vector

Oocyst prevalence and intensity have been recorded in 349 laboratory infections of Anopheles stephensi with Plasmodium berghei. Intensity and prevalence of infection are shown to be predictably related. The structure and heterogeneity in the infections has been analysed with the objective of describing the biological mechanisms by which the observed negative binomial oocyst distributions are generated. The analysis has revealed that the most likely processes lie within the population dynamic events of malaria within the mosquito, namely gametogenesis, fertilization and mortality. The distribution is similar in all Plasmodium-mosquito combinations examined so far, whether they are of laboratory (P. gallinaceum in Aedes aegypti) or field (P. vivax in An. albimanus and P. falciparum in An. gambiae s.l. and An. funestus) origin. Further we conclude that there is competition between parasites in the vector. Oocyst frequency distribution analysis shows that under natural conditions of transmission intensity, and even under the best laboratory conditions, significant numbers (> 10%) of fully susceptible mosquitoes will not be infected under conditions where the mean infection is as high as 250 oocysts. Failure to infect is not therefore an absolute indicator of refractoriness. In assessing transmission data it is shown that sample sizes should not be less than 50, and ideally 100 mosquitoes, if reliable data are to be obtained. In field it is suggested that difficulties in determining the low natural intensity of oocyst infections indicate that prevalence estimates are a useful and accessible parameter to measure.(ABSTRACT TRUNCATED AT 250 WORDS)