Tropical Health Matters

This week started with articles that drew attention to the challenges of malaria in low transmission areas and with low density infections. Malaria Journal has provided several insightful articles toward this end.

Being an island has certainly helped Zanzibar make progress toward malaria elimination as witness the fact that malaria prevalence has remained below 1% for the past decade. Not only does Zanzibar still face threats of infection from the mainland, it may also experience an upsurge locally if residual transmission and the role of human behavior and community actions are not well understood. April Monroe et al. conducted in-depth interviews with community members and local leaders across six sites on Unguja, Zanzibar as well as semi-structured community observations of night-time activities and special events to learn more.

While there was high reported ITN use, there were also times when people were exposed t mosquitoes while being outdoors during biting times. This could be around the house, or at special night events like such as weddings, funerals, and religious ceremonies. Men spent more time outdoors than women. Clearly appropriate interventions and needed and should be promoted in culturally appropriate ways in order to further reduce and eventually eliminate transmission.

Angela Early and colleagues presented findings on a diagnostic process of deep sequencing for understanding the dynamics and complexity of Plasmodium infections, but stress that knowing the lower limit of detection is challenging. They present “a new amplicon analysis tool, the Parallel Amplicon Sequencing Error Correction (PASEC) pipeline, is used to evaluate the performance of amplicon sequencing on low-density Plasmodium DNA samples.”

The authors learned that, “four state-of-the-art tools resolved known haplotype mixtures with similar sensitivity and precision.” They also cautioned that, “Samples with very low parasitemia and very low read count have higher false positive rates and call for read count thresholds that are higher than current default recommendations.” Better understanding of the genetic mix of plasmodium infections as countries move toward low transmission and elimination is crucial for selecting appropriate interventions and evaluating their outcomes.

Hannah Edwards and co-researchers examined conditions for malaria transmission along the Thailand-Myanmar border in areas approaching malaria elimination. While prevalence may be less than 1%, residual transmission still occurs. Transmission occurs not only around residences but in the forests where people work. The researchers therefore looked at the behavior of both humans and insects. Overall, they found that, “Community members frequently stayed overnight at subsistence farm huts or in the forest. Entomological collections showed higher biting rates of primary vectors in forested farm hut sites and in a more forested village setting compared to a village with clustered housing and better infrastructure.”

While mosquitoes preferred to bite inside huts, their threat was magnified by those who did not use long lasting insecticide-treated nets (LLINs). While out in the farms and forests, people tended to wake early and increase their likelihood of being bitten. The authors discuss the challenges of dual residences in terms of LLIN ownership and even concerning the potential access to indoor residual spraying. The definition for universal net coverage needs to expand from one net per two people to include adequate nets wherever people are located.

The Amazonian area of Brazil is another area working toward malaria elimination, in particular, Plasmodium vivax. Felipe Leão Gomes Murta et al. also looked at the human side of the equation and identified misperceptions by both community members and health workers that could inhibit elimination efforts. They found, “many myths regarding malaria transmission and treatment that may hinder the sensitization of the population of this region in relation to the use of current control tools and elimination strategies, such as mass drug administration (MDA),” and LLINs.

Problematic perceptions included mention by both groups that the use of insecticide-treated nets, may cause skin irritations and allergies. Both community members and health professionals said malaria is “an impossible disease to eliminate because it is intrinsically associated with forest landscapes.” They concluded that such perceptions can be a barrier to control and elimination.

Efforts to eliminate malaria from low transmission settings are an essential to the overall global goals. These four articles tell us that close attention to and better understanding of humans, parasites and mosquitoes is still needed to achieve these goals.

On this final day of the American Society of Tropical Medicine and Hygiene’s 62nd Annual Conference, there is a featured symposium on Malaria: Biology and Pathogenesis – Human Responses to falciparum. Presentations are listed below with links to the abstracts online.

Demonstration of enhanced strain-specific Plasmodium falciparum multifunctional T cell cytokine expression among Malian children immunized with the FMP2.1/AS02A vaccine by Shawna F. Graves et al.

The Study suggests that AMA1 vaccination induced an AMA1-specific CD4+ response; however, recognition of the vaccine antigen is not dependent upon c1L alone. In summary, AMA1-specific T cell cytokine expression was notably increased in children vaccinated with an AMA1-based vaccine compared to rabies. The possible role of CD4+ TNF-?+IL-2+-expressing T cells in vaccine-induced strain-specific protection against clinical malaria requires further exploration.

Fatal Pediatric Cerebral Malaria is Associated with Intravascular Inflammation and Coagulation that is Exacerbated by HIV-1 Co-infection by Sarah Hochman and colleagues.

We hypothesize that the intravascular inflammation and coagulation seen in CM autopsies contribute to the pathogenesis of pediatric CM and that dysregulation of these processes in HIV infection contribute to CM mortality.

Immune responses of rhesus monkeys to a self-assembling protein nanoparticle (SAPN) vaccine displaying Plasmodium falciparum CSP B- and T-cell epitopes by David E. Lanar and co-researchers.

We have previously studied in mice the immune responses induced against Plasmodium falciparum circumsporozoite protein (PfCSP) epitopes using a self-assembling protein nanoparticle (SAPN) platform. In conclusion, a PfCSP-KMY-SAPN vaccine for malaria was safe and immunogenic in rhesus monkeys. Immune responses to the vaccine were greatly enhanced if the nanoparticle was formulated with the adjuvant GLA-SE.

Non-invasive Pulse Oximetry to Predict Mortality in African Children with Malaria by Andrea L. Conroy et al.

The mortality rate for children admitted with malaria was 3.1%. We evaluated whether non-invasive pulse oximetry would predict disease outcome in malaria and compared the findings to venous lactate, an established prognostic marker in malaria. These data suggest that pulse oximetry alongside assessment of venous lactate may be useful in the triage and treatment of children with severe malaria. Additional advantages in pulse oximetry are low operating costs and real-time patient monitoring.

Placental malaria induces excessive vasculogenesis by  Tara C. Bracken and colleagues.

Placental malaria (PM) results from sequestration of Plasmdium falciparum-infected erythrocytes and the resulting inflammatory responses in the maternal placental blood space. PM induces maternal anemia, preterm birth, low birth weight, or stillbirth, especially in primigravidae. The active/active chronic group had a significantly higher percentage of excessive vasculogenesis.

Retinal microvascular dysfunction in pediatric cerebral malaria is associated with death and neurological sequelae by Ian J. MacCormick and co-authors.

Our results suggest that central nervous system ischemia and leakage across blood-tissue barriers may be important contributors to the severity of pediatric Cerebral Malaria.

Bismarck Dinko of the School of Basic and Biomedical Sciences, University of Health and Allied Sciences, Ho, Ghana and his colleagues Teun Bousema and Colin Sutherland from the Department of Immunology and Infection, London School of Hygiene and Tropical Medicine, London, UK. Presented their research findings at the just concluded Multilateral Initiative for Malaria 6^th Pan African Malaria Conference in Durban. Bismark Dinko can be contacted as bismcck@gmail.com, bdinko@uhas.edu.gh for more information.

Malaria transmission-reducing interventions are key to malaria control and possibly elimination.¹ Therefore, the development of new tools targeting malaria transmission reduction would mean a major leap forward in malaria control efforts. However, little is known about the immune responses directed at circulating P. falciparum gametocytes in the human host, knowledge of which will be useful in developing transmission reducing interventions targeting gametocytes.

Studies in the Gambia showed P. falciparum gametocytes carry antigens (GSA) which were recognized by malaria patients’ antibodies. These anti-GSA antibodies were found to be associated with lower duration of gametocyte carriage in these patients^2,3.  Thus, we aimed to determine the presence of anti-GSA antibodies in an asymptomatic population and their relevance to gametocytaemia.

The study was conducted in Ahafo Ano South District, Ashanti Region, Ghana. 274 asymptomatic children aged 6-17yrs were screened by microscopy for malaria, 66% were asymptomatic parasite positive. 155 were treated with DHA-piperaquine upon second visit and enrolled. Enrolled children were followed-up for finger-prick blood donation weekly for 1 month.

Gametocytaemia were determined by Microscopy and QT-NASBA.Gametocytes were produced according to established protocols.³   Mature stage V gametocytes were magnet-purified and tested with plasma samples for antibody recognition by flow cytometry as described elsewhere³ and we present here a summary of the findings.

From a cohort of 113 children, all the children harboured plasma antibody responses that recognized GSA on a proportion of mature gametocyte-infected RBCs of 3D7 by flow cytometry. However, 56% of the children exhibited strong antibody responses to GSA (immune response above the median within the cohort per sampling time) by both the proportion of mature gametocytes bound to antibodies and the intensity of the antibody binding to GSA. Longitudinal data provided an additional 10% developing strong GSA responses during the 1 month follow-up.

There were some children with antibody responses fluctuating around the median immune response within the cohort. Children with GSA antibodies present at enrolment were less likely to develop new gametocytaemia at subsequent visits (odds ratio = 0.29, 95% CI 0.06 – 1.05; P = 0.034).

3D7 is a laboratory adapted parasite line, so a selection of positive plasma samples was tested against mature gametocyte preparations from HL1204, and strong plasma antibody binding was again shown. No binding to the surface of RBCs infected with immature gametocytes of HL1204 was detected.

In conclusion, a proportion of malaria-infected children carry antibodies that recognized cultured stage V P. falciparum gametocytes from 3D7 and clinical isolates. Strong plasma antibody responses may contribute to gametocytaemia control in vivo. Further work is currently being carried out to identify GSA in collaboration with colleagues at Johns Hopkins School of Public Health, Baltimore, USA.

References

1. Alonson et al., 2011. PLoS Med, 8, e1000406.
2. Sutherland, 2009, Mol Biochem Parasitol, 166, 93-8.
3. Saeed et al., 2008, PLoS One, 3, e2280.

Note that Bismarck Dinko was supported by a MIM travel award.

When the small copepod or cyclops swallows a guinea worm larva, seeing it as food, several things may happen as the larva develops – either someone swallows the cyclops when drinking the pond water, continuing the guinea worm cycle, or the larva grows so large that the cyclops is destroyed. Control measures have included putting temephos in ponds to kill the cyclops. All of these mean death for the cyclops. None of these alternatives bode well for the cyclops.

It is not surprising to learn, as reported in PLoS Biology that disease vectors or intermediate hosts themselves are not very ‘happy’ to get infected with disease organizms that are later passed to humans. In essence Anophleles gambiae mosquitoes have genes that encode ‘essential components of the mosquito immune defense against malaria parasites.’

Furthermore, these genes are not static. Rottschaefer and colleagues report that, “Our data indicate that functionally variable APL1 alleles are evolutionarily maintained to combat diverse pathogens, perhaps including but probably not restricted to Plasmodium species.”

Most malaria control measures to date that involve the vector are aimed at killing, repelling or sterilizing.  These range from the newest, a toxic sugar bait, to the widely used instecticide treated bednets. There is exploration into a human vaccine that would prevent the parasite from developing in the mosquito.

While mosquitoes are certainly a nuissance in their own right, they are not necessarily the main enemy in the fight against malaria. We should certainly continue using bednets and indoor spraying and in appropriate cases larviciding, as major gains have been made from these. It would be hard though to completely eliminate the vectors.  Therefore continued research is needed on vaccines and genetic modifications that make mosquitoes a hostile host to plasmodium species.