Mosquito vectors of ape malarias: Another piece of the puzzle Alvaro Molina-Cruza and Carolina Barillas-Murya,1
It is remarkable that the two Plasmodium species that infect millions of humans around the world today, Plasmodium falciparum and Plasmodium vivax, both originated in Africa from single common ancestors that infected wild-living apes. Plasmodium species of the subgenus Laverania, which include P. falciparum, exhibit strong host specificity, and no host transfers between humans, gorillas, and chimpanzees have been documented in nature. In contrast, host transfers take place frequently in nature for parasites of the subgenus Plasmodium that includes P. vivax and Plasmodium malariae (1–4). P. falciparum appears to have originated as a result of a single transfer of Plasmodium praefalciparum from gorillas to humans (1). The ability of gorilla parasites to invade the human erythrocytes appears to be a major barrier for interspecies transfer that P. praefalciparum had to overcome (5). However, although P. falciparum does not infect apes in nature, species transfers to bonobos and chimpanzees have been documented in sanctuaries where apes come in close contact with infected humans. Furthermore, experimental infections of chimpanzees with P. falciparum have been achieved multiple times under laboratory conditions (6). This evidence indicates that there is no strong biological barrier for P. falciparum to transfer back from humans to certain ape species. In PNAS, Makanga et al. (7) investigate the potential role of anopheline mosquito vectors as a barrier for interspecies transfer of Plasmodium parasites in an extensive longitudinal survey. Sylvatic anopheline species were collected in two forested wildlife reserves in Gabon (Central Africa) for a period of 15 mo., and 18 different anopheline mosquito species were identified based on morphology and molecular taxonomy. Mosquito whole body and salivary gland were screened for the presence of Plasmodium parasites by PCR amplification and sequencing of a fragment of the cytochrome-b (Cyt-b) gene. This approach made it possible to carry out a phylogenetic analysis, including sequences of several known Plasmodium species that served as references. The sequences clustered with parasites that infect several different
Fig. 1. Role of anopheline mosquitoes in the transfer of ape malarias to humans. The diagram presents a possible two-step scenario for the transfer and establishment of ape malarias in humans. Sylvatic anopheline vectors (green) were transmitting malaria between apes; in the first step, one (or more) bridge vector(s) (black), would also transfer infective Plasmodium parasites to humans. These sylvatic bridge vector(s) may have maintained the initial transmission to humans. In a second step, human-adapted Plasmodium parasites adapted to domesticated mosquito vector species (blue, orange, red), such as An. gambiae, that share the same ecological niche with humans.
mammals, such as gorillas, chimpanzees, rodents, bats, and ungulates. This study revealed a high prevalence (up to 5%) of Plasmodium-infected mosquitoes in the African tropical forest. Three mosquito species Anopheles vinckei, Anopheles moucheti, and Anopheles marshallii, were infected with parasites that infect different apes, with An. vinckei having the highest prevalence of infection. The authors confirmed that the three mosquito species carried Plasmodium sporozoites in their salivary glands, thus providing direct evidence that they are malaria vectors. Transmission was higher during the rainy season, when mosquito populations thrive, and was also higher at midheight than on the ground or at canopy level, presumably because ape nests are located at midheight in the forest. They conclude that, because the three mosquito vectors of ape malarias are infected with parasites that infect both gorillas and chimpanzees, the species specificity of Laveranians in primates is not due to selective transmission by specific mosquito vectors.
a Mosquito Immunity and Vector Competence Section, Laboratory of Malaria and Vector Research, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Rockville, MD 20852 Author contributions: A.M.-C. and C.B.-M. wrote the paper. The authors declare no conflict of interest. See companion article on page 5329. 1 To whom correspondence should be addressed. Email: [email protected].
www.pnas.org/cgi/doi/10.1073/pnas.1604913113
PNAS | May 10, 2016 | vol. 113 | no. 19 | 5153–5154
COMMENTARY
COMMENTARY
This study represents a major advance to identify the mosquito vectors transmitting different Plasmodium species that infect a broad range of mammals living in forested areas of Central Africa, especially apes. The authors were able to document that the sylvatic mosquitoes infected with ape parasites also bite humans, when available, and are potential bridge vectors between apes and humans. The confirmation of An. moucheti as a potential bridge vector is particularly relevant because this anopheline is a major malaria vector of human malaria in some regions of Central Africa. The transfer of P. praefalciparum from gorillas to humans undoubtedly involved a human receiving a bite from an infected mosquito. Some anopheline mosquitoes like Anopheles gambiae are highly anthropophilic, preferring to feed on humans rather than on other animals, whereas others like Anopheles albimanus are more zoophilic, preferring to feed on animals over humans (8). However, host preference is not absolute: An. albimanus, for example, is a major vector of human malaria in the coastal areas of Mexico and Central America. One can envision that the dispersal of malaria to human populations was a two-step process, in which the first infected human, probably living in proximity to gorillas, was able to transmit the infection to other humans, most likely through the bite of the same vectors infecting gorillas (Fig. 1). The establishment of a human-to-human transmission cycle in the forest would have allowed the parasite to adapt to humans without having to adapt to a different mosquito vector(s). According to Makanga et al. (7), a vector like An. vinckei, An. marshallii, or An. moucheti could have carried out this step. Nevertheless, the broad dispersal of P. falciparum malaria throughout the African continent, and then globally, would also require a second step (Fig. 1). Malaria-infected humans would need to come in contact with mosquito vectors, such as An. gambiae, that breed in close proximity to human settlements, are well adapted to feed on humans, and are able to transmit the parasite. Little is known about the susceptibility of An. gambiae to different Laveranian species from apes, but there is evidence that this mosquito is not susceptible to infection by Plasmodium reichenowi (9, 10). Even if some parasites were able to adapt to humans living in a habitat shared by sylvatic mosquito vectors that infect apes, the lack of a domesticated vector would have hindered the broad spread to humans. Anopheles mosquito species are highly adapted to specific ecological niches and generally inhabit the geographic regions where they originally evolved. There appears to be a strong ecological barrier between mosquito vectors of apes and human
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
malarias, because some of the major vectors of human malaria were not found in this extensive mosquito collection in forested wildlife reserves by Makanga et al. (7). One should also consider that Plasmodium compatibility with a host may not be an all-or-nothing phenomenon, and two mosquito vectors or vertebrate hosts that become infected with a certain parasite may exhibit broad differences in susceptibility that favor transmission of the parasite to a given host. The lack of interspecies transfer of Laveranian parasites may be due to the combined effect of the susceptibility of the vertebrate host and its ability to generate infectious gametocytes, the susceptibility of the vector to infection and its ability to generate infectious sporozoites, the probability that infected mosquitoes of a given species come in contact with the vertebrate hosts and how much a mosquito likes to bite them. The additive effect of these factors over many reproductive cycles can exert a very strong selective force that minimizes the probability of interspecies transfers in nature. The plasticity of P. falciparum and P. vivax to adapt to different anophelines is evident from their global dispersion in a relatively short time following human migration out of Africa. Plasmodium has been able to adapt to more than 70 anophelines worldwide (11), some of them evolutionarily distant from the African vectors (12). However, adaptation to new vector(s) could have resulted in strong selection of the initial parasite population. For example, there is clear evidence that two mosquito vectors, Anopheles pseudopunctipennis and An. albimanus, are selecting the P. vivax populations circulating in the highlands and coastal areas in southern Mexico, respectively (13). Moreover, recent studies indicate that the immune system of some anopheline mosquito species can select the P. falciparum parasites circulating in a given geographic region (14). Whole-genome sequence analysis of two Laveranians that infect chimpanzees, P. reichenowi and Plasmodium gaboni, found that they are 10-fold more diverse than P. falciparum, in agreement with the proposed recent transfer to humans. It also revealed a Laverania-specific expansion of a multigene family involved in erythrocyte remodeling that was horizontally transferred into a recent ancestor of P. falciparum (15). The elegant study by Makanga et al. (7) has added one more piece to this complex puzzle, but many questions still remain. The future availability of the complete genome sequence of P. praefalciparum would provide key information regarding the genes involved in the adaptation of the parasite to humans and may also provide evidence of selection by mosquito vectors highly adapted to humans.
Liu W, et al. (2010) Origin of the human malaria parasite Plasmodium falciparum in gorillas. Nature 467(7314):420–425. Liu W, et al. (2014) African origin of the malaria parasite Plasmodium vivax. Nat Commun 5:3346. Boundenga L, et al. (2015) Diversity of malaria parasites in great apes in Gabon. Malar J 14:111. Prugnolle F, et al. (2013) Diversity, host switching and evolution of Plasmodium vivax infecting African great apes. Proc Natl Acad Sci USA 110(20):8123–8128. Wanaguru M, Liu W, Hahn BH, Rayner JC, Wright GJ (2013) RH5-Basigin interaction plays a major role in the host tropism of Plasmodium falciparum. Proc Natl Acad Sci USA 110(51):20735–20740. Su X, Hayton K, Wellems TE (2007) Genetic linkage and association analyses for trait mapping in Plasmodium falciparum. Nat Rev Genet 8(7):497–506. Makanga B, et al. (2016) Ape malaria transmission and potential for ape-to-human transfers in Africa. Proc Natl Acad Sci USA 113:5329–5334. Sinka ME, et al. (2010) The dominant Anopheles vectors of human malaria in the Americas: Occurrence data, distribution maps and bionomic precis. ´ Parasit Vectors 3:72. Bray RS (1957) Studies on malaria in chimpanzees. III. Gametogony of Plasmodium reichenowi. Ann Soc Belge Med Trop (1920) 37(2):169–174. Collins WE, Skinner JC, Pappaioanou M, Broderson JR, Mehaffey P (1986) The sporogonic cycle of Plasmodium reichenowi. J Parasitol 72(2):292–298. Sinka ME, et al. (2012) A global map of dominant malaria vectors. Parasit Vectors 5:69. Molina-Cruz A, Barillas-Mury C (2014) The remarkable journey of adaptation of the Plasmodium falciparum malaria parasite to New World anopheline mosquitoes. Mem Inst Oswaldo Cruz 109(5):662–667. Joy DA, et al. (2008) Local adaptation and vector-mediated population structure in Plasmodium vivax malaria. Mol Biol Evol 25(6):1245–1252. Molina-Cruz A, et al. (2015) Plasmodium evasion of mosquito immunity and global malaria transmission: The lock-and-key theory. Proc Natl Acad Sci USA 112(49): 15178–15183. Sundararaman SA, et al. (2016) Genomes of cryptic chimpanzee Plasmodium species reveal key evolutionary events leading to human malaria. Nat Commun 7:11078.
5154 | www.pnas.org/cgi/doi/10.1073/pnas.1604913113
Molina-Cruz and Barillas-Mury
It is remarkable that the two Plasmodium species that infect millions of humans around the world today, Plasmodium falciparum and Plasmodium vivax, both originated in Africa from single common ancestors that infected wild-living apes. Plasmodium species of the subgenus Laverania, which include P. falciparum, exhibit strong host specificity, and no host transfers between humans, gorillas, and chimpanzees have been documented in nature. In contrast, host transfers take place frequently in nature for parasites of the subgenus Plasmodium that includes P. vivax and Plasmodium malariae (1–4). P. falciparum appears to have originated as a result of a single transfer of Plasmodium praefalciparum from gorillas to humans (1). The ability of gorilla parasites to invade the human erythrocytes appears to be a major barrier for interspecies transfer that P. praefalciparum had to overcome (5). However, although P. falciparum does not infect apes in nature, species transfers to bonobos and chimpanzees have been documented in sanctuaries where apes come in close contact with infected humans. Furthermore, experimental infections of chimpanzees with P. falciparum have been achieved multiple times under laboratory conditions (6). This evidence indicates that there is no strong biological barrier for P. falciparum to transfer back from humans to certain ape species. In PNAS, Makanga et al. (7) investigate the potential role of anopheline mosquito vectors as a barrier for interspecies transfer of Plasmodium parasites in an extensive longitudinal survey. Sylvatic anopheline species were collected in two forested wildlife reserves in Gabon (Central Africa) for a period of 15 mo., and 18 different anopheline mosquito species were identified based on morphology and molecular taxonomy. Mosquito whole body and salivary gland were screened for the presence of Plasmodium parasites by PCR amplification and sequencing of a fragment of the cytochrome-b (Cyt-b) gene. This approach made it possible to carry out a phylogenetic analysis, including sequences of several known Plasmodium species that served as references. The sequences clustered with parasites that infect several different
Fig. 1. Role of anopheline mosquitoes in the transfer of ape malarias to humans. The diagram presents a possible two-step scenario for the transfer and establishment of ape malarias in humans. Sylvatic anopheline vectors (green) were transmitting malaria between apes; in the first step, one (or more) bridge vector(s) (black), would also transfer infective Plasmodium parasites to humans. These sylvatic bridge vector(s) may have maintained the initial transmission to humans. In a second step, human-adapted Plasmodium parasites adapted to domesticated mosquito vector species (blue, orange, red), such as An. gambiae, that share the same ecological niche with humans.
mammals, such as gorillas, chimpanzees, rodents, bats, and ungulates. This study revealed a high prevalence (up to 5%) of Plasmodium-infected mosquitoes in the African tropical forest. Three mosquito species Anopheles vinckei, Anopheles moucheti, and Anopheles marshallii, were infected with parasites that infect different apes, with An. vinckei having the highest prevalence of infection. The authors confirmed that the three mosquito species carried Plasmodium sporozoites in their salivary glands, thus providing direct evidence that they are malaria vectors. Transmission was higher during the rainy season, when mosquito populations thrive, and was also higher at midheight than on the ground or at canopy level, presumably because ape nests are located at midheight in the forest. They conclude that, because the three mosquito vectors of ape malarias are infected with parasites that infect both gorillas and chimpanzees, the species specificity of Laveranians in primates is not due to selective transmission by specific mosquito vectors.
a Mosquito Immunity and Vector Competence Section, Laboratory of Malaria and Vector Research, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Rockville, MD 20852 Author contributions: A.M.-C. and C.B.-M. wrote the paper. The authors declare no conflict of interest. See companion article on page 5329. 1 To whom correspondence should be addressed. Email: [email protected].
www.pnas.org/cgi/doi/10.1073/pnas.1604913113
PNAS | May 10, 2016 | vol. 113 | no. 19 | 5153–5154
COMMENTARY
COMMENTARY
This study represents a major advance to identify the mosquito vectors transmitting different Plasmodium species that infect a broad range of mammals living in forested areas of Central Africa, especially apes. The authors were able to document that the sylvatic mosquitoes infected with ape parasites also bite humans, when available, and are potential bridge vectors between apes and humans. The confirmation of An. moucheti as a potential bridge vector is particularly relevant because this anopheline is a major malaria vector of human malaria in some regions of Central Africa. The transfer of P. praefalciparum from gorillas to humans undoubtedly involved a human receiving a bite from an infected mosquito. Some anopheline mosquitoes like Anopheles gambiae are highly anthropophilic, preferring to feed on humans rather than on other animals, whereas others like Anopheles albimanus are more zoophilic, preferring to feed on animals over humans (8). However, host preference is not absolute: An. albimanus, for example, is a major vector of human malaria in the coastal areas of Mexico and Central America. One can envision that the dispersal of malaria to human populations was a two-step process, in which the first infected human, probably living in proximity to gorillas, was able to transmit the infection to other humans, most likely through the bite of the same vectors infecting gorillas (Fig. 1). The establishment of a human-to-human transmission cycle in the forest would have allowed the parasite to adapt to humans without having to adapt to a different mosquito vector(s). According to Makanga et al. (7), a vector like An. vinckei, An. marshallii, or An. moucheti could have carried out this step. Nevertheless, the broad dispersal of P. falciparum malaria throughout the African continent, and then globally, would also require a second step (Fig. 1). Malaria-infected humans would need to come in contact with mosquito vectors, such as An. gambiae, that breed in close proximity to human settlements, are well adapted to feed on humans, and are able to transmit the parasite. Little is known about the susceptibility of An. gambiae to different Laveranian species from apes, but there is evidence that this mosquito is not susceptible to infection by Plasmodium reichenowi (9, 10). Even if some parasites were able to adapt to humans living in a habitat shared by sylvatic mosquito vectors that infect apes, the lack of a domesticated vector would have hindered the broad spread to humans. Anopheles mosquito species are highly adapted to specific ecological niches and generally inhabit the geographic regions where they originally evolved. There appears to be a strong ecological barrier between mosquito vectors of apes and human
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
malarias, because some of the major vectors of human malaria were not found in this extensive mosquito collection in forested wildlife reserves by Makanga et al. (7). One should also consider that Plasmodium compatibility with a host may not be an all-or-nothing phenomenon, and two mosquito vectors or vertebrate hosts that become infected with a certain parasite may exhibit broad differences in susceptibility that favor transmission of the parasite to a given host. The lack of interspecies transfer of Laveranian parasites may be due to the combined effect of the susceptibility of the vertebrate host and its ability to generate infectious gametocytes, the susceptibility of the vector to infection and its ability to generate infectious sporozoites, the probability that infected mosquitoes of a given species come in contact with the vertebrate hosts and how much a mosquito likes to bite them. The additive effect of these factors over many reproductive cycles can exert a very strong selective force that minimizes the probability of interspecies transfers in nature. The plasticity of P. falciparum and P. vivax to adapt to different anophelines is evident from their global dispersion in a relatively short time following human migration out of Africa. Plasmodium has been able to adapt to more than 70 anophelines worldwide (11), some of them evolutionarily distant from the African vectors (12). However, adaptation to new vector(s) could have resulted in strong selection of the initial parasite population. For example, there is clear evidence that two mosquito vectors, Anopheles pseudopunctipennis and An. albimanus, are selecting the P. vivax populations circulating in the highlands and coastal areas in southern Mexico, respectively (13). Moreover, recent studies indicate that the immune system of some anopheline mosquito species can select the P. falciparum parasites circulating in a given geographic region (14). Whole-genome sequence analysis of two Laveranians that infect chimpanzees, P. reichenowi and Plasmodium gaboni, found that they are 10-fold more diverse than P. falciparum, in agreement with the proposed recent transfer to humans. It also revealed a Laverania-specific expansion of a multigene family involved in erythrocyte remodeling that was horizontally transferred into a recent ancestor of P. falciparum (15). The elegant study by Makanga et al. (7) has added one more piece to this complex puzzle, but many questions still remain. The future availability of the complete genome sequence of P. praefalciparum would provide key information regarding the genes involved in the adaptation of the parasite to humans and may also provide evidence of selection by mosquito vectors highly adapted to humans.
Liu W, et al. (2010) Origin of the human malaria parasite Plasmodium falciparum in gorillas. Nature 467(7314):420–425. Liu W, et al. (2014) African origin of the malaria parasite Plasmodium vivax. Nat Commun 5:3346. Boundenga L, et al. (2015) Diversity of malaria parasites in great apes in Gabon. Malar J 14:111. Prugnolle F, et al. (2013) Diversity, host switching and evolution of Plasmodium vivax infecting African great apes. Proc Natl Acad Sci USA 110(20):8123–8128. Wanaguru M, Liu W, Hahn BH, Rayner JC, Wright GJ (2013) RH5-Basigin interaction plays a major role in the host tropism of Plasmodium falciparum. Proc Natl Acad Sci USA 110(51):20735–20740. Su X, Hayton K, Wellems TE (2007) Genetic linkage and association analyses for trait mapping in Plasmodium falciparum. Nat Rev Genet 8(7):497–506. Makanga B, et al. (2016) Ape malaria transmission and potential for ape-to-human transfers in Africa. Proc Natl Acad Sci USA 113:5329–5334. Sinka ME, et al. (2010) The dominant Anopheles vectors of human malaria in the Americas: Occurrence data, distribution maps and bionomic precis. ´ Parasit Vectors 3:72. Bray RS (1957) Studies on malaria in chimpanzees. III. Gametogony of Plasmodium reichenowi. Ann Soc Belge Med Trop (1920) 37(2):169–174. Collins WE, Skinner JC, Pappaioanou M, Broderson JR, Mehaffey P (1986) The sporogonic cycle of Plasmodium reichenowi. J Parasitol 72(2):292–298. Sinka ME, et al. (2012) A global map of dominant malaria vectors. Parasit Vectors 5:69. Molina-Cruz A, Barillas-Mury C (2014) The remarkable journey of adaptation of the Plasmodium falciparum malaria parasite to New World anopheline mosquitoes. Mem Inst Oswaldo Cruz 109(5):662–667. Joy DA, et al. (2008) Local adaptation and vector-mediated population structure in Plasmodium vivax malaria. Mol Biol Evol 25(6):1245–1252. Molina-Cruz A, et al. (2015) Plasmodium evasion of mosquito immunity and global malaria transmission: The lock-and-key theory. Proc Natl Acad Sci USA 112(49): 15178–15183. Sundararaman SA, et al. (2016) Genomes of cryptic chimpanzee Plasmodium species reveal key evolutionary events leading to human malaria. Nat Commun 7:11078.
5154 | www.pnas.org/cgi/doi/10.1073/pnas.1604913113
Molina-Cruz and Barillas-Mury
