Falciparum Sporozoite Infection Research Articles

The role of Anopheles arabiensis and Anopheles coustani in indoor and outdoor malaria transmission in Taveta District, Kenya

BackgroundThe scaling up of malaria vector control efforts in Africa has resulted in changing the malaria vectorial systems across different ecological settings. In view of the ongoing trends in vector population dynamics, abundance, species composition and parasite infectiousness, there is a need to understand vector distribution and their contribution to malaria transmission to facilitate future planning of control strategies. We studied indoor and outdoor malaria transmission dynamics and vector population variability of Anopheles mosquitoes in Taveta district along the Kenyan Coast.MethodsAnopheles mosquitoes were collected indoors and outdoors in 4 ecologically different villages using CDC light traps (both indoor and outdoor) and aspiration method (day resting indoors; DRI) methods. Mosquitoes were examined for infection with P. falciparum sporozoites and blood feeding preferences using enzyme linked immunosorbent assay (ELISA). The An. gambiae and An. funestus complexes were identified by PCR technique to determine the sibling species composition.ResultsA total of 4,004 Anopheles mosquitoes were collected consisting of 34.9%% (n = 1,397) An. gambiae s.1., 28.1% (n = 1,124) An. funestus s.l., 33.5% (n = 1,340) An. coustani and 3.6% (n = 143) An. pharoensis. A total of 14,654 culicine mosquitoes were collected, mainly Cx. quinquefasciatus. Of the total Anopheles collected, 3,729 were tested for P. falciparum sporozoite infection. The sporozoite transmission was found to be occurring both indoors and outdoors. The overall sporozoite infectivity was 0.68% (n = 2,486) indoors and 1.29% (n = 1,243) outdoors. Indoor and outdoor sporozoite infectivity and the vectorial systems varied across the 4 ecological villages. Entomological inoculation rates for the 4 villages indicate that there was site-to-site variation. In the 4 villages, Mwarusa had the highest EIRs with An. arabiensis, An. funestus and An. coustani contributing to 23.91, 11.96 and 23.91 infectious bites per person per year ib/p/year respectively. In Kiwalwa and Njoro outdoor EIR was significantly higher than indoors.ConclusionsThis study shows that malaria transmission is occurring both indoors and outdoors. The main vectors are An. arabiensis, An. funestus and An. coustani indoors while An. coustani is playing a major role in outdoor transmission. Effective malaria control programmes, should therefore include tools that target both indoor and outdoor transmission.

Development of a quantitative flow cytometry-based assay to assess infection by Plasmodium falciparum sporozoites

The human malaria parasite Plasmodium falciparum causes the most deadly parasitic disease worldwide, necessitating the development of interventions that block infection. Yet, preclinical assays to measure inhibition of infection date from the 1980s and are based on microscopy. Here, we describe the development of a simple flow cytometric assay that can be used to quantitatively assess P. falciparum sporozoite infection in vitro in low and medium throughput. We demonstrate the utility of this assay for assessing both drug inhibition of infection and measuring efficacy of antibodies in blocking parasite infection. This methodology will aid in assessing functional antibody responses to vaccination and novel drugs that prevent mosquito-to-man transmission of malaria.

Species Composition and Role of Anopheles Mosquitoes in Malaria Transmission Along Badagry Axis of Lagos Lagoon, Lagos, Nigeria

Three communities along Badagry axis of the Lagos lagoon were sampled for indoor resting Anopheles mosquitoes in order to determine their species composition, relative abundance, density and contribution to malaria transmission in the coastal ecosystem. A total of 1938 adult female Anopheles mosquitoes collected from 2005 to 2007 constituted three species viz Anopheles gambiae, An melas and An. nili. The Polymerase Chain Reaction (PCR)–-based tests indicated that more than three-fourth of the An. gambiae s.l (75.8%) population belongs to An. gambiae s.s the remaining were An. melas. Further analysis showed that all the An. gambiae s.s was the M form. ELISA-based analyses indicated that An. gambiae s.s and An. melas were the main vectors of malaria in this area with an overall P. falciparum sporozoite infection rate of 4.8% and 6.5% respectively. Both species also maintained relatively high EIR indicating their prominent roles in malaria transmission in the study area. All the An. nili tested were negative for P. falciparum sporozoite infection. This study provides baseline information for planning vector control programme relevant to reduction of malaria transmission in the coastal areas of Nigeria.

The impact of a small-scale irrigation scheme on malaria transmission in Ziway area, Central Ethiopia

To assess the impact of a small-scale irrigation scheme in Ziway area, a semi-arid area in the Central Ethiopian Rift Valley, on malaria transmission. Parasitological, entomological and socio-economic studies were conducted in a village with and a village without irrigation. Blood smear samples were taken from individuals during the dry and wet seasons of 2005/2006. Socio-economic data were collected from household heads and key agricultural and health informants through interviews and questionnaires. Larval and adult mosquitoes were sampled during the dry and short wet seasons of 2006. Female anopheline mosquitoes were tested by enzyme-linked immunosorbent assay for blood meal sources and sporozoite infections. Malaria prevalence was higher in the irrigated village (19%, P < 0.05) than the non-irrigated village (16%). In the irrigated village, malaria prevalence was higher in the dry season than in the wet season while the reverse occurred in the non-irrigated village. Households with access to irrigation had larger farm land sizes and higher incomes, but also higher prevalence of malaria. Larval and adult abundance of the malaria vectors, Anopheles arabiensis and Anopheles pharoensis, was higher in the irrigated than in the non-irrigated village throughout the study period. Furthermore, the abundance of An. pharoensis was significantly higher than that of An. arabiensis during the dry irrigated period of the year. Canal leakage pools, irrigated fields and irrigation canals were the major breeding habitats of the two vector mosquitoes. Plasmodium falciparum sporozoite infection rates of 1.18% and 0.66% were determined for An. arabiensis and An. pharoensis in the irrigated village. Peak biting activities of the vectors occurred before 22:00 h, which is a source of concern that the effectiveness of ITNs may be compromised as the mosquitoes feed on blood before people go to bed. Irrigation schemes along the Ethiopian Rift Valley may intensify malaria by increasing the level of prevalence during the dry season. To reduce the intensity of malaria transmission in the small-scale irrigation schemes currently in operation in Ethiopia, year-round source reduction by using proper irrigation water management, coupled with health education, needs to be incorporated into the existing malaria control strategies.

Improved T cell responses to Plasmodium falciparum circumsporozoite protein in mice and monkeys induced by a novel formulation of RTS,S vaccine antigen

Protection against Plasmodium falciparum sporozoite infection can be achieved by vaccination with the recombinant circumsporozoite protein-based vaccine RTS,S formulated with the AS02A Adjuvant System. Since this protection is only partial and wanes over time, we have developed a new RTS,S-based vaccine adjuvanted with AS01B. RTS,S/AS01B-induced high specific antibody titers and increased the frequency of mouse CD4(+) and CD8(+) T cells expressing IFN-gamma, and of monkey CD4(+) T cells expressing IL-2 and/or IFN-gamma and/or TNF-alpha upon stimulation with vaccine antigens. Our data provides clear evidence that combining RTS,S antigen with a potent adjuvant induces strong humoral and cellular responses in vivo.

Expression of human CD81 differently affects host cell susceptibility to malaria sporozoites depending on the Plasmodium species

Plasmodium sporozoites can enter host cells by two distinct pathways, either through disruption of the plasma membrane followed by parasite transmigration through cells, or by formation of a parasitophorous vacuole (PV) where the parasite further differentiates into a replicative exo-erythrocytic form (EEF). We now provide evidence that following invasion without PV formation, transmigrating Plasmodium falciparum and Plasmodium yoelii sporozoites can partially develop into EEFs inside hepatocarcinoma cell nuclei. We also found that rodent P. yoelii sporozoites can infect both mouse and human hepatocytes, while human P. falciparum sporozoites infect human but not mouse hepatocytes. We have previously reported that the host tetraspanin CD81 is required for PV formation by P. falciparum and P. yoelii sporozoites. Here we show that expression of human CD81 in CD81-knockout mouse hepatocytes is sufficient to confer susceptibility to P. yoelii but not P. falciparum sporozoite infection, showing that the narrow P. falciparum host tropism does not rely on CD81 only. Also, expression of CD81 in a human hepatocarcinoma cell line is sufficient to promote the formation of a PV by P. yoelii but not P. falciparum sporozoites. These results highlight critical differences between P. yoelii and P. falciparum sporozoite infection, and suggest that in addition to CD81, other molecules are specifically required for PV formation during infection by the human malaria parasite.

Hepatocyte CD81 is required for Plasmodium falciparum and Plasmodium yoelii sporozoite infectivity.

Plasmodium sporozoites are transmitted through the bite of infected mosquitoes and first invade the liver of the mammalian host, as an obligatory step of the life cycle of the malaria parasite. Within hepatocytes, Plasmodium sporozoites reside in a membrane-bound vacuole, where they differentiate into exoerythrocytic forms and merozoites that subsequently infect erythrocytes and cause the malaria disease. Plasmodium sporozoite targeting to the liver is mediated by the specific binding of major sporozoite surface proteins, the circumsporozoite protein and the thrombospondin-related anonymous protein, to glycosaminoglycans on the hepatocyte surface. Still, the molecular mechanisms underlying sporozoite entry and differentiation within hepatocytes are largely unknown. Here we show that the tetraspanin CD81, a putative receptor for hepatitis C virus, is required on hepatocytes for human Plasmodium falciparum and rodent Plasmodium yoelii sporozoite infectivity. P. yoelii sporozoites fail to infect CD81-deficient mouse hepatocytes, in vivo and in vitro, and antibodies against mouse and human CD81 inhibit in vitro the hepatic development of P. yoelii and P. falciparum, respectively. We further demonstrate that the requirement for CD81 is linked to sporozoite entry into hepatocytes by formation of a parasitophorous vacuole, which is essential for parasite differentiation into exoerythrocytic forms.

Cutting edge: a new tool to evaluate human pre-erythrocytic malaria vaccines: rodent parasites bearing a hybrid Plasmodium falciparum circumsporozoite protein.

Malaria vaccines containing the Plasmodium falciparum Circumsporozoite protein repeat domain are undergoing human trials. There is no simple method to evaluate the effect of vaccine-induced responses on P. falciparum sporozoite infectivity. Unlike the rodent malaria Plasmodium berghei, P. falciparum sporozoites do not infect common laboratory animals and only develop in vitro in human hepatocyte cultures. We generated a recombinant P. berghei parasite bearing P. falciparum Circumsporozoite protein repeats. These hybrid sporozoites are fully infective in vivo and in vitro. Monoclonal and polyclonal Abs to P. falciparum repeats neutralize hybrid parasite infectivity, and mice immunized with a P. falciparum vaccine are protected against challenge with hybrid sporozoites.

Anopheline Ecology and Malaria Infection during the Irrigation Development of an Area of the Mahaweli Project, Sri Lanka

A study on adult anopheline ecology and malaria vector incrimination was carried out from 1986 to 1989 during irrigation development in an area of the Mahaweli Project in eastern Sri Lanka. Eleven potential vector species were collected resting indoors or by using human or bovid bait, and from light trap catches. Overall, Anopheles vagus (Donitz), An. annularis (van der Wulp), and An. subpictus (Grassi) were the most prevalent, and An. culicifacies (Giles) and An. barbirostris (van der Wulp) were the least prevalent species. The abundance of An. aconitus (Donitz), An. jamesii (Theobald), An. pallidus (Theobald), and An. subpictus increased after irrigation development, while An. annularis, An. barbirostris, An. culicifacies and An. varuna (Iyengar) decreased. Populations of An. nigerrimus (Giles), An. tessellatus (Theobald), and An. vagus did not change substantially. Seven species were infected with malaria parasites, as determined by enzyme-linked immunosorbent assays (ELISA) done on mosquito head-thorax triturates. The main species involved were An. annularis, at peak abundance during the 1986-1987 period of development leading to the onset of irrigation, and An. subpictus, during times of seasonal abundance in the post-irrigation period of 1988 to 1989. Although occurring at low abundance, An. culicifacies was involved in malaria transmission irregularly throughout the study period. While there was strong ELISA-based evidence implicating An. subpictus as a major post-irrigation vector, confirmation of its vector status must await dissection and ELISA-based evidence of P. vivax and P. falciparum sporozoite infection rates in salivary glands.