Bridging Vectors of Dengue Fever: The Endless Cycle

Abstract

Within the past 10 years, there has been a resurgence of arboviral disease outbreaks within the sub-Saharan region of Africa due to the geographic expansion of both the mosquito vectors and their resistance to insecticides. The reasons for this resurgence are not well understood, migration of people, movement of disease vectors, and deforestation as a result of rapid and unplanned urbanization may lead to increased erosion of their natural habitats leading to contact with humans, and/or previously obligate sylvatic species might acclimatize to new urban environments and hosts, potentially with a greater role as vectors. And lack of effective control methods for Aedes mosquitoes. The possibility of arboviruses to adapt to new vectors rapidly occur, and this can have great significant consequences. Other Aedes species such as Aedes africanus and Ae. luteocephalus. play a vital role in the transmission of arboviruses in Africa because they are involved in sylvatic arbovirus transmission cycles and can also act as a bridge vector to humans. Bridge vectors may initiate a human outbreak, but large epidemics typically occur only when virus transmission involves urban populations of Ae. aegypti or Ae. albopictus, which has the ability to feed on both humans and other vertebrates.

1. Introduction

The Aedes mosquito is a significant carrier of arboviruses, including the Zika virus, dengue virus, chikungunya virus, and yellow fever virus [1]. Aedes aegypti originated in Africa, spread to other continents through trade and travel, and is now distributed worldwide. These vectors have accelerated the urban spread of these viruses in both tropical and temperate climates, Figure 1 [2]. Tropical urbanization and the extremely effective and anthropophilic Aedes aegypti’s colonization of their increasing habitat pose the biggest health danger from arboviral disease emergence [3].

Figure 1.

A world map showing risk transmission of Dengue fever.

Despite extensive attempts to help contain or eradicate their outbreaks, the majority of arboviral diseases continue to be more prevalent in Africa for a variety of reasons [4, 5]. Arboviral illnesses are not exempt from concerns about public health. Recent epidemics of arboviral infections in numerous countries have enhanced the significance of Aedes vectors in sub-Saharan Africa. Due to their associations with human arboviral infections like Zika, dengue, chikungunya, and Yellow fever, Aedes aegypti and Aedes albopictus have received significant attention [4, 6]. The vectors have been implicated in most epidemics within sub-Saharan Africa [7]. In the past five years, dengue epidemics have occurred in Burkina Faso in West Africa. Faso [8, 9] Cote d’Ivoire [10, 11], Senegal [12], yellow fever in Cote d’Ivoire [13], and Nigeria [14]. The recent confirmation of dengue cases has occurred in Ghana [15, 16, 17].

In the African continent, arboviral diseases have become a major public health threat [18]. The re-emergence of arboviruses such as the Dengue virus, and Chikungunya virus, is associated with urbanization, trade, and travel [3]. With 10–20 million cases reported annually in Africa. Furthermore, about 250,000–500,000 cases of Dengue hemorrhagic fever, 20,000 fatalities, and 264 disability-adjusted life years per million people each year have been documented [19]. No single intervention will be enough to control arboviral diseases, according to research and arboviral control experts, regardless of the effectiveness of future initiatives [20].

Currently, there are no effective vaccines or treatments for several important human-infecting arboviruses including the Dengue virus and Zika virus [21]. Therefore, the control of mosquito vectors is still the main tool to eradicate, or at least reduce, the incidence of arboviral diseases. This vector control relies heavily on the use of insecticides, the effectiveness of which may be impacted by resistance. The emergence of resistance of vectors to the four major classes of insecticides (i.e., organochlorides (OCs), pyrethroids (PYs), carbamates (CAs), and organophosphate (OPs) are highly widespread [4]. This has reached an extensive level geographically and across vector species [22, 23, 24].

Several newly emerging arthropod-borne viruses (arboviruses), including dengue, yellow fever, chikungunya, and zika viruses, are a result of sylvatic transmission cycles, in which bridging Aedes mosquitoes spread the viruses among non-human primates. A crucial, but poorly understood phase in the formation of arboviruses is the initial virus overflow from the sylvatic cycle to the human population. This review discusses bridging vectors of arboviral diseases from the standpoint of a One-Health control strategy.

2. Aedes as vectors of arboviruses

Aedes is a genus of mosquitoes originating from the tropical and subtropical regions [25]. However, these vectors are now distributed on all continents except for Antarctica. The visible black and white markings on their body and legs are distinctive of Aedes mosquitoes. These vectors are diurnal, with peak biting periods early in the morning and in the evening before dusk [26].

Some species of the Aedes genus are well-known for various arboviral diseases, but the most prominent species that transmit arboviruses leading to epidemics are Aedes aegypti and the highly invasive Aedes albopictus [6].

2.1 Aedes aegypti

The Ae. aegypti can be identified by white markings on its legs and a marking in the form of a lyre on the superior surface of its thorax, Figure 2. This mosquito originated in Africa [27].

Figure 2.

A pictorial morphological identification of Aedes aegypti and Ae. albopictus.

Only the female bites for blood, which is essential to induce egg laying and for maturing and nourishing her eggs. To find a host, these mosquitoes are attracted to chemical compounds (cues) emitted by mammals, including ammonia, body temperature (heat), carbon dioxide from sweat and breathing, lactic acid from certain bacteria, octanol from sweat, cholesterol, folic acid, skin lotions, and perfume [28]. Adults of the Ae. aegypti are highly domesticated mosquitoes and highly anthropophilic [29], and typically endophilic. Although Aedes aegypti mosquitoes most commonly feed at dusk and dawn, in shady areas, or when the weather is cloudy, they can bite and spread infection all year long and at any time of the day [28, 29]. The Aedes aegypti is more closely associated with human habitation The larvae develop preferentially in artificial containers [30, 31], including discarded car tires, toilet tanks, and water storage vessels often in urban settings. Although the lifespan of an adult Ae. aegypti is two to four weeks depending on environmental conditions [32], the eggs can be viable for over a year in a dry state, which allows the mosquito to re-emerge after hibernation or aestivation. The anthropophagic behavior of the Ae. aegypti is dependent on the expression of the odorant receptor AeegOr4 [33].

Ae. aegypti breeds in both sylvatic and domestic environments in artificial containers within or in proximity to human habitation whereas larvae of the sylvatic ecotype are bred in natural habitats such as rock pools, tree holes, plant axils, and fruit husks [31]. Larvae of the two Ae. aegypti ecotypes are exposed to different bacterial groups in their respective breeding sites, possibly resulting in variances in vectorial capacity [34]. Naturally, two morphological subspecies have been identified that generally inhabit these ecotypes: Ae. aegypti aegypti and Ae. aegypti formosus. Evidence however shows that, Ae. aegypti formosus is increasingly found in urban environments [31], and the indicative morphological characteristics i.e. presence/absence of white abdominal scaling patterns [35] often differentiate the variety. On the contrary, clear genetic boundaries are absent, probably as a result of widespread current or recent historical gene flow [36, 37].

2.2 Aedes albopictus

Aedes albopictus, denoted as the Asian tiger mosquito, the most invasive species of the Aedes genus, occurs even in temperate regions Figure 2. In recent times the distribution of Ae. albopictus from Asia to Africa, Europe, and the Americas through the used tire trade has heightened [38]. Aedes albopictus in contrast to Ae. aegypti is usually exophagic and bites humans and animals opportunistically [36], but it has also been shown to exhibit anthropophilic behavior similar to Ae. aegypti [2, 36]. Aedes albopictus thrives in a diverse range of breeding habitats than Ae. Aegypti. They also show comparable larval development behavior in artificial containers such as Ae. aegypti. This diversity of habitats of Ae. albopictus explains its abundance in rural as well as peri-urban areas and shady city parks, feeding readily on a diversity of mammalian and avian species [39].

2.3 Other Aedes species

The possibility of arboviruses adapting to new vectors rapidly occurs, and this can have great significant consequences [4]. Other Aedes species play a pivotal role in the transmission of arboviruses in Africa because they serve as a link between the sylvatic and human transmission cycles and/or are involved in sylvatic arbovirus transmission cycles. Aedes africanus is considered the main vector of yellow fever virus in Africa within the sylvatic environment [4] and can also act as a bridge vector to humans, together with Ae. luteocephalus, Ae. taylori, Ae. bromeliae, Ae. furcifer, Ae. metallicus, Ae. opok, Ae. vittatus, and species of the Ae. simpsoni complex [40]. Sylvatic dengue viruses in Africa are transmitted among non-human primates by Ae. furcifer and Ae. luteocephalus within the sylvatic habitat, and usually cross over to humans through biting by Ae. furcifer [40]. Bridge vectors may initiate a human outbreak, but large epidemics typically occur only when virus transmission involves urban populations of Ae. aegypti or Ae. albopictus, though there can be exclusions. The mainstream of these Aedes vector species are established in rural or forest areas, and so, are less likely to present a threat in the urban environments where Ae. aegypti populations thrive. Nevertheless, increasing erosion of their natural breeding habitats could lead to human-vector contact, and/or previously obligate sylvatic species might acclimatize to new urban environments and hosts, potentially with a greater role as vectors [3]. Many readily feed on animals both domestic and wild non-human primates, as well as humans, hence their potential importance as bridging vectors and zoonotic transmissions [41].

3. Dengue fever: a zoonotic disease

Many zoonotic diseases are caused by various contacts and frequently intricate cycles of transmission between people and animals, both vertebrates and invertebrates, as well as evolving social and environmental factors. Prior research has demonstrated that environmental, animal, and human factors all contribute to imported dengue cases and cyclical epidemics, which pose a threat to public health, Figure 3 [42].

Figure 3.

Transmission cycles of Dengue fever.

Continuous human land use in biome ecotopes for habitation, agriculture, or livestock increases the risk of spillover occurrences and the transmission of zoonotic diseases [43].

Public health institutions should ideally be structured around principles such as; integration, personnel empowerment (favoring prompt decision-making by health agents on the ground), community engagement by educating communities about best practices and bolstering control efforts, and flexibility to assign health agents in accordance with the current emerging or seasonal public health treat [44]. Tropical illness monitoring, however, is segmented and autonomous from one another in the majority of Sub-Saharan African countries.

Adopting a One Health approach enables the inclusion of more interconnected factors, such as the environment, land use, and management (such as the disposal of plastic containers, methods of water storage due to the availability of piped water, etc.), as well as social and climatic factors that affect disease transmission patterns. Due to their high occurrence rates, vector-borne diseases are of the utmost importance for public health. Among these, the dengue virus (DENV), which is spread by Aedes mosquitoes, causes disease with a high global morbidity and fatality rate.

4. Urbanization: a cause of arboviral disease spillage

According to [45], urban landscapes affect the spatial variability of mosquito abundance, community structure, mosquito-host interactions, and infection rates. Aedes mosquitoes are regarded as important vectors for public health due to their vector competence, proximity to, and ability to feed on human blood. Therefore, reducing the risk of human arbovirus infection requires a better knowledge of how urban settings affect mosquito numbers, blood-feeding behavior, and infection status in Aedes mosquitoes.

5. Insecticide resistance in Aedes mosquitoes

One of the effective approaches to swiftly interject the transmission of arboviruses is to employ safe and effective insecticides against mosquito vector populations that include both adults and larvae [20]. While insecticide-based interventions have effectively reduced Aedes mosquito populations for many years, resistance has recently emerged due to the reliance on a few active components approved for use in public health [46]. An insecticide’s effectiveness or level of control may be reduced due to insecticide resistance (IR), which is a shift in the mosquito population’s susceptibility to the substance. Insecticide resistance has emerged in Ae. aegypti in all four classes.

There is a dearth of information on pesticide resistance in Aedes mosquitoes worldwide, with the majority of the reports received from South-East Asia and Latin America [46]. It has been reported from every region where DDT has been tested that both Ae. aegypti and Ae. albopictus have developed widespread resistance to the insecticide [47]. There have been proven reports of Ae. aegypti from the regions of West, Central, and East Africa indicating patchy resistance to pyrethroids (mainly permethrin and deltamethrin) [46]. However, it should be highlighted that because Ae. aegypti discriminant doses are lower, pyrethroid doses are used for An. gambiae are frequently used to analyze pyrethroids in Aedes mosquitoes, which may lead to an underestimation of resistance,

In Yaoundé, Cameroon, recent bendiocarb testing on both Ae. albopictus and Ae. aegypti revealed resistance [48]. Fortunately, the first-line biological and chemical larvicides, Bti and temephos, have not recorded any resistance.

Bacillus thuringiensis israelensis’s (Bti) complex method of toxicity and the lack of any recent reports of resistance in Aedes field populations predict susceptibility. Temophos resistance is highly prevalent in Asia and Latin America [46]. However, due to Africa’s reportedly complete susceptibility, temephos is viewed as a potential option for water treatment. In contrast to Ae. aegypti, resistance in Ae. albopictus seems to be relatively low [4]. This could be because Ae. aegypti mosquitoes have had more prior exposure to indoor spray treatments and home insecticides than Ae. albopictus mosquitoes. Insecticide resistance will almost certainly eventually have a detrimental impact on our ability to control this vector shortly due to the expansion of Ae. albopictus populations into areas with high insecticide use; adulticides, or selection pressure from agriculture in its new breeding sites [47].

There are a variety of potential adaptations that allow a mosquito to endure dangerous levels of an insecticide; these adaptations are typically categorized based on their biochemical/physiological features as either mechanism of lower exposure i.e. increased excretion or reduced absorption and detoxification, or mechanisms of decreased reactivity to the insecticides (changes in the target site) [49]. Most of the time, the insecticide is either detoxified or sequestered before it reaches its target site due to variances in detoxifying enzymes or changes in the sensitivity of the insecticide target caused by mutations, which reduce the insecticide’s affinity for its target [50].

6. Aedes vector control strategies: a One-Health perspective

To gather epidemiological data for use in informing decisions and taking action, participatory rural evaluation methods are applied in entomological surveillance and disease monitoring. The epidemiological situation, spatiotemporal distribution, and risk of disease transmission are significantly improved as a result of this method [51]. Understanding host-pathogen-environment relationships, developing tools and technologies, modifying people’s behavior, and assessing the efficacy of interventions are all part of entomological surveillance and disease monitoring. Interest in multisectoral, socioeconomic, systems-based, collaborative (MSC) study techniques such as One Health is spurred by the need to adequately forecast, prevent, and respond to infectious diseases that emerge unexpectedly from human-animal-environmental systems. MSC research, which can be categorized as a form of “pragmatic research,” may be particularly helpful in documenting changes in complex human-animal-environmental systems, expediting the research-to-action process, and assessing the efficacy of interventions [52].

Using the frameworks of adaptive management and one-health, a plan will be created to identify, collect, and share linkages between important elements of regional complex systems of arboviral disease. Based on currently available scientific knowledge and input from stakeholders, significant causal relationships between social, economic, and environmental factors that are a determinant of arboviral disease could be identified at different levels, and assumptions that guide interventions may be offered. Implementing a One Health strategy thoughtfully and comprehensively can be difficult, especially in the face of a perceived crisis.

7. Conclusions

Vector control for Aedes mosquitoes is one of the main strategies against arboviral disease transmission, but it is mostly insecticide-based, which induces resistance in mosquitoes and also may target non-target species and cause damage to the environment. This resistance is probably due to the lack of regulation in use and the dosage of each case. Dengue fever control and prevention around the world should implement the One-Health approach. Furthermore, a global strategy and a global framework for Dengue fever control will be suitable for one health strategy which uses a multidisciplinary sector for this effort. One-Health approach will manage the strategy of the health workforce in multidisciplinary and other communities to provide health services and collaborate to control all factors involved in the transmission of DF, such as human health, animal health, environmental, socioeconomic, politics, and other sectors related. This review supports the need to generate mosquito control strategies using a One-Health approach for sustainable and effective vector control of the Dengue vector.

Acknowledgments

My sincere gratitude goes to God Almighty and my colleagues at the Department of Medical Microbiology, University of Ghana Medical School.

Conflict of interest

The author declares there is no conflict of interest