Is that a real oocyst? Insectary establishment and identification of Plasmodium falciparum oocysts in midguts of Anopheles mosquitoes fed on infected human blood in Tororo, Uganda

Quantification of sporozoite expelling by Anopheles mosquitoes infected with laboratory and naturally circulating P. falciparum gametocytes

It is currently unknown whether all Plasmodium falciparum-infected mosquitoes are equally infectious. We assessed sporogonic development using cultured gametocytes in the Netherlands and naturally circulating strains in Burkina Faso. We quantified the number of sporozoites expelled into artificial skin in relation to intact oocysts, ruptured oocysts, and residual salivary gland sporozoites. In laboratory conditions, higher total sporozoite burden was associated with shorter duration of sporogony (p<0.001). Overall, 53% (116/216) of infected Anopheles stephensi mosquitoes expelled sporozoites into artificial skin with a median of 136 expelled sporozoites (interquartile range [IQR], 34-501). There was a strong positive correlation between ruptured oocyst number and salivary gland sporozoite load (ρ = 0.8; p<0.0001) and a weaker positive correlation between salivary gland sporozoite load and number of sporozoites expelled (ρ = 0.35; p=0.0002). In Burkina Faso, Anopheles coluzzii mosquitoes were infected by natural gametocyte carriers. Among salivary gland sporozoite positive mosquitoes, 89% (33/37) expelled sporozoites with a median of 1035 expelled sporozoites (IQR, 171-2969). Again, we observed a strong correlation between ruptured oocyst number and salivary gland sporozoite load (ρ = 0.9; p<0.0001) and a positive correlation between salivary gland sporozoite load and the number of sporozoites expelled (ρ = 0.7; p<0.0001). Several mosquitoes expelled multiple parasite clones during probing. Whilst sporozoite expelling was regularly observed from mosquitoes with low infection burdens, our findings indicate that mosquito infection burden is positively associated with the number of expelled sporozoites. Future work is required to determine the direct implications of these findings for transmission potential.

Susceptibility of Anopheles gambiae to Natural Plasmodium falciparum Infection: A Comparison between the Well-Established Anopheles gambiae s.s Line and a Newly Established Ugandan Anopheles gambiae s.s. Line

Much of our understanding of malaria transmission comes from mosquito feeding assays using Anopheles mosquitoes from colonies that are well adapted to membrane feeding. This raises the question whether results from colony mosquitoes lead to overestimates of outcomes in wild Anopheles mosquitoes. We successfully established an Anopheles colony using progeny of wild Anopheles gambiae s.s. mosquitoes (Busia mosquitoes) and directly compared their susceptibility to infection with Plasmodium falciparum with the widely used An. gambiae s.s. mosquitoes (Kisumu mosquitoes) using gametocyte-infected Ugandan donor blood. The proportion of infectious feeds did not differ between Busia (71.8%, 23/32) and Kisumu (68.8%, 22/32, P = 1.00) mosquitoes. When correcting for random effects of donor blood, we observed a 23% higher proportion of infected Busia mosquitoes than infected Kisumu mosquitoes (RR, 1.23; 95% CI, 1.10-1.38, P < 0.001). This study suggests that feeding assays with Kisumu mosquitoes do not overestimate outcomes in wild An. gambiae s.s. mosquitoes, the mosquito species most relevant to malaria transmission in Uganda.

Malaria rapid diagnostic tests reliably detect asymptomatic Plasmodium falciparum infections in school-aged children that are infectious to mosquitoes

BACKGROUND

Asymptomatic malaria infections (Plasmodium falciparum) are common in school-aged children and represent a disease transmission reservoir as they are potentially infectious to mosquitoes. To detect and treat such infections, convenient, rapid and reliable diagnostic tools are needed. In this study, malaria rapid diagnostic tests (mRDT), light microscopy (LM) and quantitative polymerase chain reaction (qPCR) were used to evaluate their performance detecting asymptomatic malaria infections that are infectious to mosquitoes.

METHODS

One hundred seventy asymptomatic school-aged children (6-14 years old) from the Bagamoyo district in Tanzania were screened for Plasmodium spp. infections using mRDT (SD BIOLINE), LM and qPCR. In addition, gametocytes were detected using reverse transcription quantitative polymerase chain reaction (RT-qPCR) for all qPCR-positive children. Venous blood from all P. falciparum positive children was fed to female Anopheles gambiae sensu stricto mosquitoes via direct membrane feeding assays (DMFAs) after serum replacement. Mosquitoes were dissected for oocyst infections on day 8 post-infection.

RESULTS

The P. falciparum prevalence in study participants was 31.7% by qPCR, 18.2% by mRDT and 9.4% by LM. Approximately one-third (31.2%) of asymptomatic malaria infections were infectious to mosquitoes in DMFAs. In total, 297 infected mosquitoes were recorded after dissections, from which 94.9% (282/297) were derived from infections detected by mRDT and 5.1% (15/297) from subpatent mRDT infections.

CONCLUSION

The mRDT can be used reliably to detect children carrying gametocyte densities sufficient to infect high numbers of mosquitoes. Subpatent mRDT infections contributed marginally to the pool of oocyts-infected mosquitoes.

The Truman Show for Human Helminthic Parasites: A Review of Recent Advances in In Vitro Cultivation Platforms

Throughout history, parasites and parasitic diseases have been humankind's constant companions, as evidenced by the findings of tapeworm eggs in ancient, mummified remains. Helminths are responsible for causing severe, long-term, and debilitating infectious diseases worldwide, especially affecting economically challenged nations due to prevailing deficits in access to sanitation, proper hygiene practices, and healthcare infrastructure. Socio-ecological drivers, such as poverty, migration, and climate change, continue to contribute to parasites and their disease vectors being spread beyond known endemic zones. The study of parasitic diseases has had a fair amount of success leading to the development of new chemotherapeutic agents and the implementation of parasite eradication programs. However, further progress in this direction has been hampered by the challenges of culturing some of these parasites in in vitro systems for efficient availability, basic life cycle, infection studies, and effectiveness of novel treatment strategies. The complexity of the existing models varies widely, depending on the parasite and its life cycle, ranging from basic culture methods to advanced 3D systems. This review aims to highlight the research conducted so far in culturing and maintaining parasites in an in vitro setting, thereby contributing to a better understanding of pathogenicity and generating new insights into their lifecycles in the hopes of leading to effective treatments and prevention strategies. This work is the first comprehensive outline of existing in vitro models for highly transmissible helminth diseases causing severe morbidity and mortality in humans globally.

The Truman Show for protozoan parasites: A review of in vitro cultivation platforms

Protozoan parasites are responsible for severe disease and suffering in humans worldwide. Apart from disease transmission via insect vectors and contaminated soil, food, or water, transmission may occur congenitally or by way of blood transfusion and organ transplantation. Several recent outbreaks associated with fresh produce and potable water emphasize the need for vigilance and monitoring of protozoan parasites that cause severe disease in humans globally. Apart from the tropical parasite Plasmodium spp., other protozoa causing debilitating and fatal diseases such as Trypanosoma spp. and Naegleria fowleri need to be studied in more detail. Climate change and socioeconomic issues such as migration continue to be major drivers for the spread of these neglected tropical diseases beyond endemic zones. Due to the complex life cycles of protozoa involving multiple hosts, vectors, and stringent growth conditions, studying these parasites has been challenging. While in vivo models may provide insights into host–parasite interaction, the ethical aspects of laboratory animal use and the challenge of ready availability of parasite life stages underline the need for in vitro models as valid alternatives for culturing and maintaining protozoan parasites. To our knowledge, this review is the first of its kind to highlight available in vitro models for protozoa causing highly infectious diseases. In recent years, several research efforts using new technologies such as 3D organoid and spheroid systems for protozoan parasites have been introduced that provide valuable tools to advance complex culturing models and offer new opportunities toward the advancement of parasite in vitro studies. In vitro models aid scientists and healthcare providers in gaining insights into parasite infection biology, ultimately enabling the use of novel strategies for preventing and treating these diseases.

In light of the far-reaching social and economic repercussions of communicable, zoonotic parasitic diseases on human health, it is imperative to continue to strive toward developing in vitro models for in-depth scrutiny and understanding of pathogenicity, as well as for innovations toward combating these infections. This review, to our knowledge, is the first to offer a qualitative summary of the existing models for culturing protozoan parasites with major relevance to human health in vitro. The present work aims to provide a reference guide on the current state of in vitro culture of these protozoan parasites and offers a foundation to facilitate exchange of expertise among researchers, clinicians, and healthcare workers. This comprehensive review would aid in enabling discussions on new intervention approaches to fill in the knowledge gaps in the field of parasitic diseases affecting the global population.

Detection of Plasmodium falciparum in laboratory-reared and naturally infected wild mosquitoes using near-infrared spectroscopy

There is an urgent need for high throughput, affordable methods of detecting pathogens inside insect vectors to facilitate surveillance. Near-infrared spectroscopy (NIRS) has shown promise to detect arbovirus and malaria in the laboratory but has not been evaluated in field conditions. Here we investigate the ability of NIRS to identify Plasmodium falciparum in Anopheles coluzzii mosquitoes. NIRS models trained on laboratory-reared mosquitoes infected with wild malaria parasites can detect the parasite in comparable mosquitoes with moderate accuracy though fails to detect oocysts or sporozoites in naturally infected field caught mosquitoes. Models trained on field mosquitoes were unable to predict the infection status of other field mosquitoes. Restricting analyses to mosquitoes of uninfectious and highly-infectious status did improve predictions suggesting sensitivity and specificity may be better in mosquitoes with higher numbers of parasites. Detection of infection appears restricted to homogenous groups of mosquitoes diminishing NIRS utility for detecting malaria within mosquitoes.

Maintaining Plasmodium falciparum gametocyte infectivity during blood collection and transport for mosquito feeding assays in the field

BACKGROUND

Mosquito feeding assays using venous blood are commonly used for evaluating the transmission potential of malaria infected individuals. To improve the accuracy of these assays, care must be taken to prevent premature activation or inactivation of gametocytes before they are fed to mosquitoes. This can be challenging in the field where infected individuals and insectary facilities are sometimes very far apart. In this study, a simple, reliable, field applicable method is presented for storage and transport of gametocyte infected blood using a thermos flask.

METHODS

The optimal storage conditions for maintaining the transmissibility of gametocytes were determined initially using cultured Plasmodium falciparum gametocytes in standard membrane feeding assays (SMFAs). The impact of both the internal thermos water temperature (35.5 to 37.8 °C), and the external environmental temperature (room temperature to 42 °C) during long-term (4 h) storage, and the impact of short-term (15 min) temperature changes (room temp to 40 °C) during membrane feeding assays was assessed. The optimal conditions were then evaluated in direct membrane feeding assays (DMFAs) in Burkina Faso and The Gambia where blood from naturally-infected gametocyte carriers was offered to mosquitoes immediately and after storage in thermos flasks.

RESULTS

Using cultured gametocytes in SMFAs it was determined that an internal thermos water temperature of 35.5 °C and storage of the thermos flask between RT (~ 21.3 °C) and 32 °C was optimal for maintaining transmissibility of gametocytes for 4 h. Short-term storage of the gametocyte infected blood for 15 min at temperatures up to 40 °C (range: RT, 30 °C, 38 °C and 40 °C) did not negatively affect gametocyte infectivity. Using samples from natural gametocyte carriers (47 from Burkina Faso and 16 from The Gambia), the prevalence of infected mosquitoes and the intensity of oocyst infection was maintained when gametocyte infected blood was stored in a thermos flask in water at 35.5 °C for up to 4 h.

CONCLUSIONS

This study determines the optimal long-term (4 h) storage temperature for gametocyte infected blood and the external environment temperature range within which gametocyte infectivity is unaffected. This will improve the accuracy, reproducibility, and utility of DMFAs in the field, and permit reliable comparative assessments of malaria transmission epidemiology in different settings.

A Plasmodium vivax experimental human infection model for evaluating efficacy of interventions

BACKGROUND

Interventions that interrupt Plasmodium vivax transmission or eliminate dormant P. vivax liver-stage parasites will be essential for malaria elimination. Development of these interventions has been hindered by the lack of P. vivax in vitro culture and could be accelerated by a safe and reproducible clinical model in malaria-naive individuals.

METHODS

Healthy, malaria-naive adults were enrolled in 2 studies to assess the safety, infectivity, and transmissibility of a new P. vivax isolate. Participants (Study 1, n = 2; Study 2, n = 24) were inoculated with P. vivax–infected red blood cells to initiate infection, and were treated with artemether-lumefantrine (Study 1) or chloroquine (Study 2). Primary endpoints were safety and infectivity of the new isolate. In Study 2, transmission to mosquitoes was also evaluated using mosquito feeding assays, and sporozoite viability was assessed using in vitro cultured hepatocytes.

RESULTS

Parasitemia and gametocytemia developed in all participants and was cleared by antimalarial treatment. Adverse events were mostly mild or moderate and none were serious. Sixty-nine percent of participants (11/16) were infectious to Anopheles mosquitoes at peak gametocytemia. Mosquito infection rates reached 97% following membrane feeding with gametocyte-enriched blood, and sporozoites developed into liver-stage schizonts in culture.

CONCLUSION

We have demonstrated the safe, reproducible, and efficient transmission of P. vivax gametocytes from humans to mosquitoes, and have established an experimental model that will accelerate the development of interventions targeting multiple stages of the P. vivax life cycle.

TRIAL REGISTRATION

ACTRN12614000930684 and ACTRN12616000174482.

FUNDING

(Australian) National Health and Medical Research Council Program Grant 1132975 (Study 1). Bill and Melinda Gates Foundation (OPP1111147) (Study 2).

Genetic changes of Plasmodium vivax tempers host tissue-specific responses in Anopheles stephensi

Recently, we showed how an early restriction of gut flora proliferation by Plasmodium vivax favors immune-suppression and Plasmodium survival in the gut lumen (Sharma et al., 2020). Here, we asked post gut invasion how P. vivax interacts with individual tissues such as the midgut, hemocyte, and salivary glands, and manages its survival in the mosquito host. Our data from tissue-specific comparative RNA-Seq analysis and extensive temporal/spatial expression profiling of selected mosquito transcripts in the uninfected and P. vivax infected mosquito’s tissues indicated that (i) a transient suppression of gut metabolic machinery by early oocysts; (ii) enriched expression of nutritional responsive proteins and immune proteins against late oocysts, together may ensure optimal parasite development and gut homeostasis restoration; (iii) pre-immune activation of hemocyte by early gut-oocysts infection via REL induction (p ​< ​0.003); and altered expression of hemocyte-encoded immune proteins may cause rapid removal of free circulating sporozoites from hemolymph; (iv) while a strong suppression of salivary metabolic activities, and elevated expression of salivary specific secretory, as well as immune proteins together, may favor the long-term storage and survival of invaded sporozoites. Finally, our RNA-Seq-based discovery of 4449 transcripts of Plasmodium vivax origin, and their developmental stage-specific expression modulation in the corresponding infected mosquito tissues, predicts a possible mechanism of mosquito responses evasion by P. vivax. Conclusively, our system-wide RNA-Seq analysis provides the first genetic evidence of direct mosquito-Plasmodium interaction and establishes a functional correlation.

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System-wide RNASeq analysis discloses direct mosquito-Plasmodium vivax ​interaction and establishes a functional correlation.

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Discovery of 4449 transcripts specific to ​P. vivax ​developmental stages sheds light on possible immune evasion mechanisms.

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Upregulation of nutritional related transcripts ensures optimal late oocyst development and midgut homeostasis.

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Hemocyte activation by early oocysts ensure removal of free circulatory sporozoites by proliferating immune transcripts.

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Heightened levels of salivary immune transcripts show that an active local immune response restricts salivary invaded sporozoite

A microsporidian impairs Plasmodium falciparum transmission in Anopheles arabiensis mosquitoes

A possible malaria control approach involves the dissemination in mosquitoes of inherited symbiotic microbes to block Plasmodium transmission. However, in the Anopheles gambiae complex, the primary African vectors of malaria, there are limited reports of inherited symbionts that impair transmission. We show that a vertically transmitted microsporidian symbiont (Microsporidia MB) in the An. gambiae complex can impair Plasmodium transmission. Microsporidia MB is present at moderate prevalence in geographically dispersed populations of An. arabiensis in Kenya, localized to the mosquito midgut and ovaries, and is not associated with significant reductions in adult host fecundity or survival. Field-collected Microsporidia MB infected An. arabiensis tested negative for P. falciparum gametocytes and, on experimental infection with P. falciparum, sporozoites aren't detected in Microsporidia MB infected mosquitoes. As a microbe that impairs Plasmodium transmission that is non-virulent and vertically transmitted, Microsporidia MB could be investigated as a strategy to limit malaria transmission.

Comparison of infectivity of Plasmodium vivax to wild-caught and laboratory-adapted (colonized) Anopheles arabiensis mosquitoes in Ethiopia

BACKGROUND

Mosquito-feeding assays that assess transmission of Plasmodium from man-to-mosquito typically use laboratory mosquito colonies. The microbiome and genetic background of local mosquitoes may be different and influence Plasmodium transmission efficiency. In order to interpret transmission studies to the local epidemiology, it is therefore crucial to understand the relationship between infectivity in laboratory-adapted and local mosquitoes.

METHODS

We assessed infectivity of Plasmodium vivax-infected patients from Adama, Ethiopia, using laboratory-adapted (colony) and wild-caught (wild) mosquitoes raised from larval collections in paired feeding experiments. Feeding assays used 4-6 day-old female Anopheles arabiensis mosquitoes after starvation for 12 h (colony) and 18 h (wild). Oocyst development was assessed microscopically 7 days post-feeding. Wild mosquitoes were identified morphologically and confirmed by genotyping. Asexual parasites and gametocytes were quantified in donor blood by microscopy.

RESULTS

In 36 paired experiments (25 P. vivax infections and 11 co-infections with P. falciparum), feeding efficiency was higher in colony (median: 62.5%; interquartile range, IQR: 47.0-79.0%) compared to wild mosquitoes (median: 27.8%; IQR: 17.0-38.0%; Z = 5.02; P < 0.001). Plasmodium vivax from infectious individuals (51.6%, 16/31) infected a median of 55.0% (IQR: 6.7-85.7%; range: 5.5-96.7%; n = 14) of the colony and 52.7% (IQR: 20.0-80.0%; range: 3.2-95.0%; n = 14) of the wild mosquitoes. A strong association (ρ(16) = 0.819; P < 0.001) was observed between the proportion of infected wild and colony mosquitoes. A positive association was detected between microscopically detected gametocytes and the proportion of infected colony (ρ(31) = 0.452; P = 0.011) and wild (ρ(31) = 0.386; P = 0.032) mosquitoes.

CONCLUSIONS

Infectivity assessments with colony and wild mosquitoes yielded similar infection results. This finding supports the use of colony mosquitoes for assessments of the infectious reservoir for malaria in this setting whilst acknowledging the importance of mosquito factors influencing sporogonic development of Plasmodium parasites.