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Article

Nationwide Inventory of Mosquitoes and the Distribution of Invasive Aedes (Stegomyia) albopictus (Skuse, 1894) on the Islands of Sao Tome and Principe in Central Africa

by
Tsai-Ying Yen
1,2,
Chien-Fu Cheng
3,
Lien-Fen Tseng
3,
Ronalg Mendes Costa d’ Assunção Carvalho
4 and
Kun-Hsien Tsai
1,3,5,*
1
Institute of Environmental and Occupational Health Sciences, College of Public Health, National Taiwan University, Taipei 100025, Taiwan
2
Center for Diagnostics and Vaccine Development, Centers for Disease Control, Ministry of Health and Welfare, Taipei 115201, Taiwan
3
Taiwan Anti-Malaria Advisory Mission, Sao Tome, Sao Tome and Principe
4
Taiwanese Medical Mission, Sao Tome, Sao Tome and Principe
5
Global Health Program, College of Public Health, National Taiwan University, Taipei 100025, Taiwan
*
Author to whom correspondence should be addressed.
Insects 2024, 15(8), 560; https://doi.org/10.3390/insects15080560
Submission received: 29 June 2024 / Revised: 16 July 2024 / Accepted: 20 July 2024 / Published: 23 July 2024
(This article belongs to the Section Medical and Livestock Entomology)
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Abstract

:

Simple Summary

Simple Summary: Mosquito surveys conducted in the Democratic Republic of Sao Tome and Principe during 2000 to 2016 as a part of anti-malaria programs found the presence of four species of mosquitoes in the nation including Culex (Culex) poicilipes, Mansonia (Coquillettidia) annetti, Uranotaenia (Uranotaenia) alboabdominalis, and Uranotaenia (Uranotaenia) fraseri for the first time. Aedes albopictus (Skuse, 1894) was identified in 2015, and the follow-up survey confirmed that Aedes albopictus has become widespread across the nation. The larvae were predominant in artificial water-holding containers, with a positive rate up to 45.6% in used tires in Príncipe, while the native species, Aedes aegypti, preferred natural breeding sources. Phylogenetic analysis based on mitochondrial DNA revealed the introduced populations belonged to a clade involved in the worldwide spread of the species. Aedes albopictus is a public health threat due to its vectorial capacity for various arboviruses. Continuous vector surveillance and implication of interventions, such as source reduction to remove used tires, environmental management, and use of larvicides, were suggested.

Abstract

Aedes albopictus (Skuse, 1894), a mosquito originating in Asia, has been introduced to Africa since the 2000s. The mosquito is not only a nuisance but is capable of transmitting various arboviruses. The current study summarized our entomological surveys in the Democratic Republic of Sao Tome and Principe during 2000 to 2016. Adult mosquitoes were collected by sweep nets, human landing catches, and Centers for Disease Control (CDC) light traps, and the immatures were collected from water-filled habitats at 15 sentinel sites and reared to adulthood. Species identification was performed based on morphologic characteristics. Fragments of the cytochrome C oxidase subunit I (COI) and the Wolbachia surface protein (wsp) genes were amplified for mosquitoes collected in Principe. New records of four mosquito species were reported. Aedes albopictus was identified in 2015. The larvae were found distributed over the nation and were predominately in artificial water-holding containers (488/2698, 18.1%). The highest positive rate was observed in used tires in Príncipe (114/250, 45.6%). Mitochondrial DNA analysis revealed low genetic diversity among the invasive populations, but all tested specimens were superinfected by Wolbachia. The ability of Ae. albopictus to adapt to new environments and its involvement in disease transmission make the surveillance and control of this species particularly important.

1. Introduction

The emergence of the COVD-19 pandemic has sounded an alarm to the world about the threats of infectious diseases. Although Africa appeared to be less affected, vector-borne infectious diseases, such as malaria, still claim more than 550,000 lives each year [1]. Meanwhile, arboviral infections are becoming evident as public health concerns. Dengue virus (DENV) has been linked to at least 35 of 54 African countries [2,3]. Chikungunya outbreaks were reported in Gabon, Central Africa [4,5]. Zika virus (ZIKV), first identified in Uganda and dispersed across Asia, the Pacific, and Latin America, was shown to circulate in sub-Saharan Africa [6,7]. The worldwide spread of dengue, chikungunya, and Zika viruses is primarily attributed to Aedes (Stegomyia) aegypti (Linnaeus, 1762), but Aedes (Stegomyia) albopictus (Skuse, 1894) is a vector of increasing importance as well. Also called the Asian tiger mosquito, Ae. albopictus originates in Asia but has colonized on every continent except Antarctica with the aid of international trade and travel in the last three decades [8,9]. The first record of Ae. albopictus in Africa was in Cape Town, South Africa in 1989, but the invasive population was successfully controlled [10,11]. A few years later, Ae. albopictus was found in Nigeria, Cameroon, Equatorial Guinea, and Gabon [12]. The presence of Ae. albopictus on Sao Tome Island was not documented until 2017, while the mosquito was identified at four locations on the island [13]. Aedes albopictus was then recorded on Principe Island in limited sampling sites, with limited information about the ecological features and types of breeding sources [14,15].
The Democratic Republic of Sao Tome and Principe (DRSTP) is an island nation in the Gulf of Guinea (Figure 1). The climate is tropical with an average annual temperature of approximately 27 °C. The rainy season lasts from October to May. Lying on the equator, the DRSTP forms part of the Central African tropical forest ecosystem, which is rich in mosquito fauna [14,16]. However, most ecological and genetic research have focused on Anopheles spp. because the nation used to suffer greatly from malaria [17]. An early survey has documented five species of Aedes mosquitoes on the islands including Ae. aegypti, Aedes (Stegomyia) africanus, Aedes (Neomelaniconion) circumluteolus, Aedes (Aedimorphus) gandarai, and Aedes (Aedimorphus) nigricephalus [16]. Aedes (Catageomyia) tarsalis was identified in 2016, besides Ae. albopictus [14].
To date, prevention of vector-borne diseases, such as dengue, chikungunya, and Zika fever still relies on sustainable, locally adapted, and adequate vector control, as specific antiviral treatment or vaccines for these viral diseases are still unavailable. In fact, taking yellow fever as an example, the disease continued to impose a significant burden on Africa and Latin America despite the development of an effective vaccine [18]. To maximize the efficiency of vector control programs, researchers must first establish a firm knowledge of the vectors’ ecology. Accordingly, nationwide entomological surveys were successively conducted in the DRSTP. The surveillance of mosquitoes would provide fundamental information for risk assessments, control strategies, and effect evaluation.
This report summarized mosquito surveys conducted from 2000 to 2016 in the DRSTP. The distribution of Ae. albopictus in the nation was illustrated, and the ecological characteristics of breeding sources as well as the genetic features of the invasive populations were delineated.

2. Materials and Methods

2.1. Ethics Statements

All mosquito specimens analyzed in the present study were collected as part of the vector surveillance and malaria control programs. Approval for this study, including mosquito collection, was granted by the Ministry of Health of the DRSTP (OF° N° 20/P° CNE/2016) and the Institutional Research Ethics Committee (NTUH-REC 201110023RD).

2.2. Mosquito Sampling

Entomological surveys have been conducted periodically as a part of the malaria risk assessments since 2000 in the DRSTP. Adult mosquitoes were collected using human landing catches, double-net devices, and CDC light traps nocturnally at two to four sites in each district. Local health workers participating in the actions were provided with rapid tests and pre-exposure prophylaxis for malaria [17]. The collection was performed indoors and outdoors in villages, around villages as well as in the nearby edges of forested areas. Nationwide surveys were performed in 2000, 2006, and 2015, while mosquito collections were carried out at designated sentinel sites mainly located in Agua Grande District in 2004, 2005, 2010, 2011, and 2012. Captured mosquitoes were identified morphologically [19,20]. Then, the specimens were preserved in tubes with silica gel desiccants and sent for further laboratory analyses.
An investigation specifically targeting Aedes mosquitoes was carried out at 15 study sites across the nation during January to February and July to September in 2015 and 2016 as a follow-up investigation performed to assess the risks of arboviral diseases after the serological findings suggested DENV exposure in the DRSTP [21] (Figure 1). Agua Grande District represents the smallest district with a dense population, while Caue and Lemba districts are relatively remote and mountainous. Sampling was performed primarily around human-domesticated environments. Adult mosquitoes were collected by sweep nets during daytime. Larvae and pupae were obtained by searching for water-filled habitats. One hundred fifty to two hundred artificial water-holding containers were visited at each study site (averaged 180 containers per site), and the types as well as the numbers of containers with Aedes larvae or pupae were recorded. To confirm the colonization of Ae. albopictus, ovitraps were placed to attract gravid females to lay eggs at fixed sites in Campo de Milho, Agua Grande, and eggs on the strips were examined weekly during the study period. Aquatic-stage mosquitoes and eggs were brought back to the insectary and reared to adulthood. Species of mosquitoes were identified according to morphologic characteristics [20,22]. Newly emerged mosquitoes with wholly morphological features were pinned as vouchers and preserved in sample boxes with mothballs in the laboratory. Specimens for molecular analysis were stored in tubes containing silica gel desiccants.

2.3. Molecular Analysis

Genomic DNA was extracted from whole mosquito bodies using a DNA extraction kit (Geneaid Biotech Ltd., Taipei, Taiwan) according to the manufacturer’s instructions. The universal LCO and HCO barcoding primers were used for molecular analysis [23]. The PCR mixtures consisted of 2 μL of template DNA, 1 μL of each primer at 10 μM, and 12.5 μL of HotStarTaq Master Mix (Qiagen, Hilden, Germany), and a final volume of 25 μL was obtained with ddH2O. DNA amplification was performed using a T3000 Thermocycler (Biometra GmbH, Jena, Germany). The following reaction condition was used: an initial incubation of 95 °C for 15 min; 35 cycles of denaturation at 94 °C for 30 s, annealing at 47 °C for 30 s, and extension at 72 °C for 40 s; followed by a final extension at 72 °C for 10 min and a 4 °C hold. The PCR products were visualized on 2% agarose gels stained with EZ-Vision® DNA Dye (Amresco Inc., Solon, OH, USA) and sent for bidirectional sequencing at a commercial laboratory (Mission Biotech, Taipei, Taiwan). Completed sequences were aligned using DNASTAR Lasergene and the Clustal W algorithm implemented in BioEdit v7.2.5 software [24]. The sequences generated in the study were directly compared with those publicly available in GenBank using BLAST (http://blast.ncbi.nlm.nih.gov/, accessed on 26 October 2016). Partial mitochondrial cytochrome c oxidase subunit I (COI) sequences from 47 isolates of Ae. albopictus were aligned. The resulting alignment included 658 bases and was analyzed in MEGA11 using the Neighbor-Joining method [25].

2.4. PCR for Wolbachia-Specific Genes

Taking advantage of its cytoplasmic incompatibility and pathogen-blocking properties, the maternally inherited endosymbiont Wolbachia is being explored as a cost-effective tool for controlling arboviral transmission in many tropical countries [26,27,28]. To investigate the potential for future applications of Wolbachia in the DRSTP, Wolbachia infection in Ae. albopictus was screened with the wsp primer pair: 81F: 5′-TGGTCCAATAAGTGATGAAGAAAC-3′ and 691R: 5′-AAAAATTAAACGCTACTCCA-3′. Additional primers were used for differentiation of the infected groups, i.e., 136F/691R for group A (136F: 5′-TGAAATTTTACCTCTTT-TC-3′) and 81F/522R for group B (522R: 5′-ACCAGCTTTTGCTTGATA-3′) [29]. PCR products of individual mosquitoes were sent for sequencing (Mission Biotech, Taipei, Taiwan). The resulting sequences were compared with those in GenBank.

3. Results

3.1. Identification of Ae. albopictus during Mosquito Surveys

A total of 36 mosquito species were identified during the routine surveys conducted since 2000, of which four species, Culex (Culex) poicilipes, Mansonia (Coquillettidia) annetti, Uranotaenia (Uranotaenia) alboabdominalis, and Uranotaenia (Uranotaenia) fraseri, were newly recorded in the nation. Combined with previous records [14,16,30,31,32,33,34], a list of 44 mosquito species belonging to 11 genera has been documented in the DRSTP (Table 1).
An investigation into Aedes mosquitoes with additional specimen collection during daytime was carried out in 2015, and a total of 5530 adult mosquitoes were collected. The majority belonged to Culex mosquitoes (3547/5530, 64.1%) which included Culex (Culex) quinquefasciatus (3215/5530, 58.1%), Culex (Culiciomyia) cinerellus (228/5530, 4.1%), Culex (Culex) thalassius (37/5530, 0.7%), Culex (Culex) decens (32/5530, 0.6%), Culex (Culex) antennatus (22/5530, 0.4%), Culex (Eumelanomyia) inconspicuosus (6/5530, 0.1%), Culex (Eumelanomyia) micolo (4/5530, 0.1%), Culex (Culex) invidiosus (2/5530, 0.04%), and Culex (Culiciomyia) cambournaci (1/5530, 0.02%). Anopheles mosquitoes were the second most abundant species (1809/5530, 32.7%) including Anopheles (Cellia) coluzzii (1797/5530, 32.5%) and Anopheles coustani (12/5530, 0.2%) (Table 2). One hundred sixty-one Aedes mosquitoes (161/5530, 2.9%) were found in the collection, and to our surprise, Ae. albopictus was identified at four sites on Sao Tome Island (119/5530, 2.2%).

3.2. Distribution of Ae. albopictus and its Breeding Sources

Immatures of Ae. albopictus were discovered at all nine sentinel sites in six districts on Sao Tome Island as well as all six sites on Principe Island (Figure 1). Five species of Aedes larvae were observed in artificial water-holding containers in 2016, with Ae. albopictus being the dominant one (Table 3). Of 2698 containers with freshwater, 488 (18.1%) were positive for Ae. albopictus larvae or pupae. Ponte Graca in Agua Grande District had the highest positive rate for Ae. albopictus larvae (64/180, 35.6%) on Sao Tome Island, and the airport in Principe (Aeroporto) had the greatest overall container index for Ae. albopictus (114/250, 45.6%). The common types of breeding sources included used tires (n = 253) (Figure 2), household water-storage containers (n = 103), and waste cans (n = 65), but the larvae could also be found in natural sources, such as leaf axils and tree holes. The larvae of Ae. albopictus and Ae. aegypti co-occurred in some habitats (n = 7). However, Ae. aegypti preferred natural breeding sources to artificial containers in the DRSTP. Breeding populations of Ae. albopictus confirmed by eggs laid in ovitraps were found at two of six sites persistently.

3.3. COI-Based Molecular Analysis

At least three specimens at each sampling site on Príncipe Island were subjected to molecular analysis. In total, 27 specimens of Ae. albopictus collected from six locations were analyzed using the COI gene. A very low level of polymorphism was observed among the sequences, with only one single nucleotide polymorphism (SNP) at position 2142 (G; guanosine) in the females collected at Porto Real (Figure 3). The sequence was in line with the ST3 (JF309318) identified earlier on Sao Tome Island and identical to the reference sequences from Brazil (KX383924), China (JQ235749), Democratic Republic of Congo (MT345356), India (KJ410335), Portugal (MK995332), the USA (Los Angeles, CA) (KC690940), or Vietnam (HQ398900) in GenBank [13]. Other specimens had A (adenosine) at position 2142, which was in agreement with the ST1 (JF309317) [13], and matched perfectly with the sequences from China (KU738429), Democratic Republic of Congo (MT345384), Greece (MN005056), Malaysia (MF148282), Singapore (MW321942), Thailand (KX383928), the USA (Los Angeles, CA, USA) (KC690951), or Vietnam (MZ230338).

3.4. Wolbachia Infection in Ae. albopictus

All 27 specimens of Ae. albopictus, including both males (n = 3) and females (n = 24), were superinfected by group A (wAlbA) and group B (wAlbB) Wolbachia. The specimens generated identical sequences of wAlbA (508 bp), showing 100% similarity to those from China (KU738337), India (MF805776), Mexico (KX118691), Sri Lanka (MH777434), or Thailand (KY817484). PCR targeting partial wsp of wAlbB also yielded identical sequences (400 bp) for all the specimens, which matched with sequences from China (CP041924), India (JX629463), Panama (MH392336), Singapore (MT645169), Thailand (KY817494), or the USA (St. Augustine, FL) (CP041923).

4. Discussion

Mosquito surveys have been routinely conducted in the DRSTP as a countermeasure of malaria control since 2000. Two novel species, Coquillettidia saotomensis and Culiseta (Theomyia) wui, were identified in 2008, and the new record of Mimomyia (Etoreptiomyia) mediolineatafive was demonstrated in 2009 [32,33,34]. Culex poicilipes, Mn. annetti, Ur. alboabdominalis, and Ur. fraseri were first recorded in the nation in the study. In contrast, although Aedes africanus, Anopheles (Cellia) funestus, Anopheles (Anopheles) paludis, Anopheles (Cellia) pharoensis, Culex (Culex) pipiens, Culex (Culex) rima, Culiseta (Theomyia) fraseri, and Mansonia (Mansonioides) africana were documented in early reports, the mosquitoes were not encountered in recent investigations [14,16,30,31]. The results may be affected by sampling sites, sampling time, sampling methods, seasons as well as ecological changes of the environment. Insecticide has been extensively applied for the control of malaria in the DRSTP, for example, intra-domiciliary DDT spraying campaign was implemented in 1980 to 1982, and after that, three rounds of nationwide indoor residual spraying (IRS) using pyrethroid was applied during 2004 to 2006 [35,36]. From 2007 to 2013, larvicide of Bacillus thuringiensis israelensis (Bti) was used for outdoor larval control [17]. Long-term and intensive use of insecticide has been shown to have a great impact on biodiversity. Since 1986, only An. coluzzii and An. coustani were found on the islands [37]. The influence on non-target mosquitoes was supported by the evidence of resistance to DDT in the native populations of Ae. aegypti [38].
In our study, inspired by the findings of antibodies against DENV in pregnant women in the nation, diurnal collections of Aedes mosquitoes were included in the entomological surveys since 2015 [21]. The serological evidence suggested DENV exposure and implied the existence of a transmission cycle. Thus, an investigation of the abundance of Aedes mosquitoes would help to assess the risk of dengue. To our surprise, Ae. albopictus was detected in the mosquito collections. Since its appearance in South Africa, Ae. albopictus has been found in Nigeria (1991), Cameroon (2000), Equatorial Guinea (2003), Gabon (2007), the Central African Republic (2009), and the Republic of Congo (2011) in the Afrotropical Region [10,39,40,41,42,43,44]. The current study identified Ae. albopictus during an entomological survey in 2015, but the invasion could probably be dated back as early as 2013 [15]. Our findings illustrated the nationwide distribution of the invasive populations of Ae. albopictus, including the densely populated plains of Agua Grande, the mountainous area of Caue, and even the autonomous region of Principe Island, which was inhabited by only 5% of the total population. Ponte Graca is a hamlet right next to the capital, Sao Tome, and it was found to have the highest positive rate of Ae. albopictus larvae (64/180, 35.6%) on Sao Tome Island. The positive containers were mainly water-storage tanks. However, in other less populated areas, used tires provided ideal habitats for Ae. albopictus, e.g., the positive rate was over 40% in Aeroporto in Principe. The difference in breeding source choices in urban and rural areas was consistent with the later observation by Kamgang, et al. (2024) [38]. Larvae of Ae. albopictus were also found in phytotelms. The diversity of breeding sources indeed reflected the ecological plasticity and highly invasive nature of the species.
Larvae of Ae. albopictus were found to coexist with Ae. aegypti in some habitats in the study. Larvae coexisting in the same habitats were brought back to the insectary and reared to adulthood successfully without significant inhibitory effects between species. Nevertheless, several studies have shown that Ae. albopictus dominated in the interspecific larval competition. The populations of native Ae. aegypti declined rapidly after the invasion of Ae. albopictus, and the later species became particularly prevalent in urban settings as well as in remote forested areas [45,46]. In our findings, larvae of Ae. albopictus predominated in artificial water-holding containers (488/2698, 18.1%), while other Aedes mosquitoes may prefer natural breeding sources. Interestingly, Ae. aegypti was reported to comprise 55.5% of the specimens of Aedes mosquitoes collected in the dry season during the entomological survey in 2022 [38]. Whether there was a rebound of Ae. aegypti population after regaining balance between species needed to be clarified. Two subspecies of Ae. aegypti with distinct morphological and ecological features exist in sub-Saharan Africa. The sylvatic Ae. aegypti formosus was predominant in the DRSTP in our surveys, but the subspecies has been found active sympatrically with the domestic Ae. aegypti aegypti in urban environments in Africa as well [47]. Previous studies further indicated that climate change and urbanization may drive Ae. aegypti formosus to utilize artificial water-holding containers for breeding in prolonged dry seasons and increase human biting [48]. Thus, the rise of Ae. aegypti could also be attributed to changes in the balance of subspecies or mosquito behaviors. In addition, details of interactions between Ae. albopictus and Ae. aegypti in the DRSTP remained to be investigated. For instance, parasitism of Ae. albopictus by Ascogregarina taiwanensis has been demonstrated to affect its competitive ability against other Aedes larvae [49]. There is currently a lack of research elucidating the prevalence of Ascogregarina spp. in Ae. albopictus and the native populations of Ae. aegypti.
A very low polymorphism in the partial COI gene was detected in Ae. albopictus collected from Principe Island. Similar results, called the founder effect, have been shown by other studies in areas newly colonized by Ae. albopictus [50,51]. The sequences obtained in the study, although with a different reverse primer, were consistent with the ST1 (JF309317) and ST3 (JF309318) identified in Sao Tome, while Rader, et al. (2024) described three haplotypes, ST1, Principe 1, and Principe 2, in Principe [13,15]. The two haplotypes in current study both belonged to one of the haplogroups known to associate with the recent global spread (A1b in Figure 3). The haplogroup covered the mitochondrial DNA lineages in Thailand, Malaysia, the Papua New Guinea mainland, Cameroon, and Brazil as well as at lower frequencies in California and Texas, indicating that the Ae. albopictus populations in the DRSTP were likely to originate from tropical regions [52,53,54]. The analysis of mitochondrial DNA could provide new insights into how the vectors spread and help to identify the paths of importation.
Geographic expansion of a vector has been shown to play a crucial role in the spread of disease [55]. Aedes albopictus is a capable vector for a variety of Flavivirus, Alphavirus, and Bunyavirus. The mosquito was experimentally susceptible to 36 arboviruses, and the transmission was confirmed for 14 viruses [56]. The potential threat of the mosquito was even accentuated when a mutation in the E glycoprotein (A226V) of chikungunya virus detected during the epidemic in La Reunion was demonstrated to confer improved transmission by Ae. albopictus [57]. Although considered the second important vector, in some cases, Ae. albopictus was the sole vector in the regions. Taking Europe as an example, the occurrence of chikungunya and dengue epidemics in Italy and France have just proved the public health concerns about the increased risk of arboviral diseases resulting from the invasion of Ae. albopictus [58,59]. In Africa, the sylvatic Ae. aegypti was found to be less susceptible to all four DENV serotypes, whereas studies in Gabon have demonstrated that the introduced Ae. albopictus exhibited relatively high vector competence for ZIKV, which was further supported by the discovery of infected Ae. albopictus in the field [60,61,62,63,64]. In addition, unlike the domestic Ae. aegypti agypti, the sylvatic Ae. aegypti formosus was rather zoophilic, making the introduced Ae. albopictus the more anthropophilic mosquito by comparison [65]. As a result, the invasive populations were held responsible for the concurrent dengue and chikungunya outbreaks in Gabon [66,67]. One limitation of the current study was the lack of information about arboviruses detected in collected Aedes mosquitoes. In 2022, the first dengue outbreak in the DRSTP was reported [3]. Whether the invasive Ae. albopictus played a role in the event and its vector competence remains to be investigated.
Wolbachia spp. are intracellular symbiotic bacteria that commonly infect invertebrates. Recent studies have shown that certain strains of the endosymbiont Wolbachia were able to lower the vectorial capacity of mosquitoes and, therefore, provided new strategies for bio-control of mosquito-borne diseases, either by directly inhibiting the infection of pathogens or by shortening mosquito lifespans [27,28]. Aedes albopictus has been reported naturally harboring two Wolbachia groups, wAlbA and wAlbB, with different prevalence [68]. The current study showed both males and females of the invasive populations of Ae. albopictus were superinfected by wAlbA and wAlbB. The sequences of partial wsp were identical to those from Southeast Asia. Artificial introduction of a third Wolbachia strain has been demonstrated feasible, and the resulting populations were potentially capable of blocking dengue and chikungunya transmission [69,70]. The archipelago of DRSTP is a relatively controlled environment which might serve as a promising site for pioneer field trials of releasing transfected mosquitoes. Extensive investigation into the occurrence of Wolbachia in vector mosquitoes in nature is essential before applying any test releases [71].
Both Ae. aegypti and Ae. albopictus are daytime biters; as a result, they would be less affected by the application of bed nets or insecticide-treated nets, a useful measure for malaria prevention. The control strategy therefore relies on source reduction to remove larvae-inhabiting containers. However, water storage is a common practice for the residents in response to a lack of piped-water supplies. Proper management of storage containers, such as using lids that are fully sealed onto the containers or applying plastic net covers to tanks, has been shown to successfully decrease larval presence [72,73]. Separately, Bti has been administered as an effective larvicide for malaria control in the DRSTP. Although Anopheles and Aedes mosquitoes share less similarity in breeding sources, studies have provided promising evidence for the control of Aedes larvae by Bti [74]. Considering the potential risk of arbovirus transmission by the invasive mosquito, interventions are suggested to be implemented against Ae. albopictus. Continual vector and disease surveillance would provide prompt epidemic information and is essential for adopting efficient control strategies.

5. Conclusions

Our study has provided an update of the mosquito inventory in the DRSTP. The widespread status of Ae. albopictus across the nation was reported with the typology of breeding sources. Sequence analysis based on the COI gene revealed a very low polymorphism among the invasive populations, implying the recent introduction. The potential impact of the invasion of Ae. albopictus on disease transmission should be taken into consideration for the implementation of vector control strategies.

Author Contributions

Conceptualization, T.-Y.Y. and K.-H.T.; methodology, C.-F.C., L.-F.T., R.M.C.d.A.C. and K.-H.T.; software, T.-Y.Y.; validation, T.-Y.Y. and K.-H.T.; formal analysis, T.-Y.Y.; investigation, T.-Y.Y., C.-F.C., L.-F.T., R.M.C.d.A.C. and K.-H.T.; resources, K.-H.T.; data curation, T.-Y.Y. and K.-H.T.; writing—original draft preparation, T.-Y.Y.; writing—review and editing, T.-Y.Y. and K.-H.T.; visualization, T.-Y.Y. and K.-H.T.; supervision, K.-H.T.; project administration, K.-H.T.; funding acquisition, K.-H.T. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by the Taiwan Ministry of Science and Technology (MOST 103-2314-B-002-036-MY2) and MOHW-NTU Infectious Diseases Research and Education Center.

Data Availability Statement

All data supporting reported results can be found in context.

Acknowledgments

We thank the Taiwan Anti-Malaria Advisory Team for their enthusiastic participation and the Taiwan Ministry of Foreign Affairs for their full support. We are grateful to the Centro Nacional de Endemias, Ministério da Saúde de São Tomé e Príncipe for their resources. This article is in memory of the project leader, Jih-Ching Lien, and Arlindo Vicente de Assunção Carvalho from local health ministry. Jih-Ching Lien (1927–2022) was our leading entomologist and played a crucial role in the identification of mosquito specimens. Arlindo Vicente de Assunção Carvalho (1961–2024) not only provided professional suggestions for the study but assisted in the coordination with the local departments. The study would never have been accomplished without the efforts from every team member.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Sampling sites for Aedes albopictus in the Democratic Republic of Sao Tome and Principe (DRSTP). The continental African countries infested by Ae. albopictus are marked (green). 1. Ponte Graca; 2. Campo de Milho; 3. Boa Morte; 4. Santana; 5. Trindae; 6. Guadulupe; 7. Neves; 8. Sao Joao dos Angolares; 9. Porto Alegre; 10. Santo Cristo; 11. Porto Real; 12. Aeroporto; 13. Ponta do Sol; 14. Picao; 15. Sao Joaquim.
Figure 1. Sampling sites for Aedes albopictus in the Democratic Republic of Sao Tome and Principe (DRSTP). The continental African countries infested by Ae. albopictus are marked (green). 1. Ponte Graca; 2. Campo de Milho; 3. Boa Morte; 4. Santana; 5. Trindae; 6. Guadulupe; 7. Neves; 8. Sao Joao dos Angolares; 9. Porto Alegre; 10. Santo Cristo; 11. Porto Real; 12. Aeroporto; 13. Ponta do Sol; 14. Picao; 15. Sao Joaquim.
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Figure 2. Used tires were the most common breeding source of Ae. albopictus in the Democratic Republic of Sao Tome and Principe. The photo shows piles of used tires near the airport on Principe Island.
Figure 2. Used tires were the most common breeding source of Ae. albopictus in the Democratic Republic of Sao Tome and Principe. The photo shows piles of used tires near the airport on Principe Island.
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Figure 3. Phylogenetic analysis based on partial mitochondrial cytochrome c oxidase subunit I (COI) sequences of Aedes albopictus. The analysis was conducted in MEGA11 using the Neighbor-Joining method [25]. The evolutionary distances were computed using the Kimura two-parameter method in the bootstrap test (1000 replicates). A total of 658 positions were applied.
Figure 3. Phylogenetic analysis based on partial mitochondrial cytochrome c oxidase subunit I (COI) sequences of Aedes albopictus. The analysis was conducted in MEGA11 using the Neighbor-Joining method [25]. The evolutionary distances were computed using the Kimura two-parameter method in the bootstrap test (1000 replicates). A total of 658 positions were applied.
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Table 1. Mosquito surveys conducted in the Democratic Republic of Sao Tome and Principe, 2000–2016.
Table 1. Mosquito surveys conducted in the Democratic Republic of Sao Tome and Principe, 2000–2016.
GenusMosquito SpeciesYear/Period
2000 1200420052006 12010201120122015 12016
AedesAe. (Stegomyia) aegypti+++++++++
Ae. (Stegomyia) albopictus−−−−−−−++
Ae. (Stegomyia) africanus−−−−−−−−−
Ae. (Neomelaniconion) circumluteolus++++−+−+−
Ae. (Aedimorphus) gandarai−+++−−−−−
Ae. (Aedimorphus) nigricephalus++++++++−
AnophelesAn. (Anopheles) coustani−+++−++++
An. (Cellia) funestus−−−−−−−−−
An. (Cellia) coluzzii+++++++++
An. (Anopheles) paludis−−−−−−−−−
An. (Cellia) pharoensis−−−−−−−−−
CulexCx. (Culex) annulioris−−−+−−−−−
Cx. (Culex) antennatus++++−++++
Cx. (Culiciomyia) cambournaci++++−−−+−
Cx. (Culiciomyia) cinerellus+++++++++
Cx. (Culex) decens++++++++−
Cx. (Eumelanomyia) inconspicuosus++++−+−++
Cx. (Culex) invidiosus−+++−+−++
Cx. (Culiciomyia) macfiei−−−+−−−−−
Cx. (Eumelanomyia) micolo−−−+−−++−
Cx. (Culiciomyia) nebulosus−+−+−−−−−
Cx. (Culex) pipiens−−−−−−−−−
Cx. (Culex) poicilipes 3−−+−−−−−−
Cx. (Culex) quinquefasciatus+++++++++
Cx. (Culex) rima−−−−−−−−−
Cx. (Culex) tamsi+−−+−−−−−
Cx. (Culex) thalassius−+++−+++−
Cx. (Lutzia) tigripes++++−+ + +−
CoquillettidiaCq. saotomensis 2−−+−−−−−−
CulisetaCs. (Theomyia) fraseri−−−−−−−−−
Cs. (Theomyia) wui 2+−−−−−−−−
EretmapoditesEr. chrysogaster−+++−−−−−
MansoniaMa. (Mansonioides) africana−−−−−−−−−
Ma. (Coquillettidia) annetti 3−−−+−−−−−
MimomyiaMi. (Etoreptiomyia) mediolineata 3+++++++ +−
PolyleptiomyiaPo. gandarai−−−−−−−++
ToxorhynchitesTx. (Toxorhynchites) brevipalpis conradti−−−+−−−−−
Tx. (Toxorhynchites) capelai−+−−−−−−−
UranotaeniaUr. (Uranotaenia) alboabdominalis 3+−−−−−−−−
Ur. (Uranotaenia) balfouri+++++++++
Ur. (Pseudoficalbia) capelai−++−−−−−−
Ur. (Uranotaenia) fraseri 3+−−+−−−−−
Ur. (Pseudoficalbia) micromelas−+++−−−++
Ur. (Pseudoficalbia) principiensis−+−−−−−−−
Total species 16232127815132012
1 Surveys were conducted extensively in 2000, 2006, and 2015; 2 new species identified during our surveys; 3 species newly recorded during our surveys.
Table 2. Collection of adult mosquitoes in the Democratic Republic of Sao Tome and Principe, 2015–2016.
Table 2. Collection of adult mosquitoes in the Democratic Republic of Sao Tome and Principe, 2015–2016.
Mosquitoes2015 2016
AG 1CT 2MZ 3LO 4LE 5CU 6PR 7TotalAG 1CT 2MZ 3LO 4LE 5CU 6PR 7Total
Aedes
Ae. (Stegomyia) albopictus56834601501191400000014
Ae. (Stegomyia) aegypti22052010301300000013
Ae. (Aedimorphus) nigricephalus100000001000000000
Ae. (Neomelaniconion) circumluteolus 2000000200000000
Anopheles
An. (Cellia) coluzzii1001148241941692035817975487031861421835151575
An. coustani120000001270000007
Culex
Cx. antennatus220000002210000001
Cx. (Culiciomyia) cambournaci1000000100000000
Cx. (Culiciomyia) cinerellus1702301022302283211722036
Cx. decens320000003200000000
Cx. (Eumelanomyia) inconspicuosus6000000610000001
Cx. invidiosus2000000230000003
Cx. (Eumelanomyia) micolo4000000400000000
Cx. quinquefasciatus3029605326623032152300020025
Cx. thalassius370000003700000000
Lutzia
Lutzia (Metalutzia) tigripes2000000200000000
Mimomyia
Mi. (Etorleptiomyia) mediolineata1000000100000000
Polyleptiomyia
Po. gandarai6000000610000001
Uranotaenia
Ur. balfouri3000000330000003
Ur. (Pseudoficalbia) micromelas1000000120000002
Total4419239682442372655855306199132931461855151681
1 Agua Grande; 2 Cantagalo; 3 MeZochi; 4 Lobata; 5 Lemba; 6 Caue; 7 Principe.
Table 3. Surveys of Aedes immatures on Sao Tome and Principe islands, 2015–2016.
Table 3. Surveys of Aedes immatures on Sao Tome and Principe islands, 2015–2016.
RegionDistrictLocalityType of Artificial ContainerContainers with Larvae/Containers with Water (%)
2015 2016
Aal 1Aae 2Ani 3Aal 1Aae 2Ani 3Aga 4Aci 5
Sao Tome Agua GrandePonte GracaWaste cans, water-storage tanks20/156 (12.8)3/156 (1.9)1/156 (0.6)64/180 (35.6) 12/180 (6.7)3/180 (1.7)6/180 (3.3)0/180 (0)
Compo de MilhoDiscarded plastic containers, ovitraps placed at sentinel sites16/150 (10.7)1/150 (0.7)3/150 (2)45/179 (25.1)21/179 (11.7)0/179 (0)0/179 (0)0/179 (0)
Boa MorteUsed tires, water-storage tanks20/158 (12.7)13/158 (8.2)0/158 (0)24/151 (15.9)14/151 (9.3)0/151 (0)0/151 (0)3/151 (2.0)
CantagaloSantanaUsed tires, discarded tools, water-storage tanks, washbasinsndndnd22/186 (11.8)0/186 (0)0/186 (0)0/186 (0)0/186 (0)
MeZoxiTrindadeWaste cansndndnd42/181 (23.2) 0/181 (0)0/181 (0)2/181 (1.1)1/181 (0)
LobataGuadalupeUsed tires7/178 (3.9)0/178 (0)0/178 (0)23/186 (12.4)0/186 (0)0/186 (0)0/186 (0)0/186 (0)
LembaNevesUsed tiresndndnd45/185 (24.3)0/185 (0)0/185 (0)0/185 (0)0/185 (0)
CaueSao Joao dos AngolaresUsed tiresndndnd26/197 (13.2) 2/197 (1.0)0/197(0)0/197 (0)0/197 (0)
Porto AlegreUsed tiresndndnd6/118 (5.1)0/118 (0)0/118 (0)1/118 (0.8)0/118 (0)
Total 64/642 (10.0)17/642 (2.6)4/642 (0.6)297/1563 (19.0)49/1563 (3.1)3/1563 (0.2)9/1563 (0.6)3/1563 (0.2)
Principe PríncipeSanto CristoWater-storage tanksndndnd13/176 (7.4)0/176 (0)0/176 (0)0/176 (0)0/176 (0)
Porto RealUsed tiresndndnd25/191 (13.1)0/191 (0)0/191 (0)0/191 (0)0/191 (0)
AeroportoUsed tires, leakage of water towerndndnd114/250 (45.6)0/250 (0)0/250 (0)0/250 (0)0/250 (0)
Ponta do SolWaste cans, water-storage tanksndndnd7/146 (4.8)0/146 (0)0/146 (0)0/146 (0)0/146 (0)
PicaoWater-storage tanksndndnd20/203 (9.9)0/203 (0)0/203 (0)0/203 (0)0/203 (0)
Sao JoaquimBucketsndndnd12/169 (7.1)0/169 (0)0/169 (0)0/169 (0)0/169 (0)
Total ndndnd191/1135 (16.8)0/1135 (0)0/1135 (0)0/1135 (0)0/1135 (0)
Total 64/642 (10.0)17/642 (2.6)4/642 (0.6)488/2698 (18.1)49/2698 (1.8)3/2698 (0.1)9/2698 (3.3)3/2698 (0.1)
1 Aedes albopictus; 2 Aedes aegypti; 3 Aedes nigricephalus; 4 Aedes gandarai; 5 Aedes circumluteolus. nd: not determined.
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Yen, T.-Y.; Cheng, C.-F.; Tseng, L.-F.; Carvalho, R.M.C.d.A.; Tsai, K.-H. Nationwide Inventory of Mosquitoes and the Distribution of Invasive Aedes (Stegomyia) albopictus (Skuse, 1894) on the Islands of Sao Tome and Principe in Central Africa. Insects 2024, 15, 560. https://doi.org/10.3390/insects15080560

AMA Style

Yen T-Y, Cheng C-F, Tseng L-F, Carvalho RMCdA, Tsai K-H. Nationwide Inventory of Mosquitoes and the Distribution of Invasive Aedes (Stegomyia) albopictus (Skuse, 1894) on the Islands of Sao Tome and Principe in Central Africa. Insects. 2024; 15(8):560. https://doi.org/10.3390/insects15080560

Chicago/Turabian Style

Yen, Tsai-Ying, Chien-Fu Cheng, Lien-Fen Tseng, Ronalg Mendes Costa d’ Assunção Carvalho, and Kun-Hsien Tsai. 2024. "Nationwide Inventory of Mosquitoes and the Distribution of Invasive Aedes (Stegomyia) albopictus (Skuse, 1894) on the Islands of Sao Tome and Principe in Central Africa" Insects 15, no. 8: 560. https://doi.org/10.3390/insects15080560

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