An update on the mosquito fauna and mosquito-borne diseases distribution in Cameroon

The expansion of mosquito-borne diseases such as dengue, yellow fever, and chikungunya in the past 15 years has ignited the need for active surveillance of common and neglected mosquito-borne infectious diseases. The surveillance should be designed to detect diseases and to provide relevant field-based data for developing and implementing effective control measures to prevent outbreaks before significant public health consequences can occur. Mosquitoes are important vectors of human and animal pathogens, and knowledge on their biodiversity and distribution in the Afrotropical region is needed for the development of evidence-based vector control strategies. Following a comprehensive literature search, an inventory of the diversity and distribution of mosquitoes as well as the different mosquito-borne diseases found in Cameroon was made. A total of 290 publications/reports and the mosquito catalogue website were consulted for the review. To date, about 307 species, four subspecies and one putative new species of Culicidae, comprising 60 species and one putative new species of Anopheles, 67 species and two subspecies of Culex, 77 species and one subspecies of Aedes, 31 species and one subspecies of Eretmapodites, two Mansonia, eight Coquillettidia, and 62 species with unknown medical and veterinary importance (Toxorhynchites, Uranotaenia, Mimomyia, Malaya, Hodgesia, Ficalbia, Orthopodomyia, Aedeomyia, and Culiseta and Lutzia) have been collected in Cameroon. Multiple mosquito species implicated in the transmission of pathogens within Anopheles, Culex, Aedes, Eretmapodites, Mansonia, and Coquillettidia have been reported in Cameroon. Furthermore, the presence of 26 human and zoonotic arboviral diseases, one helminthic disease, and two protozoal diseases has been reported. Information on the bionomics, taxonomy, and distribution of mosquito species will be useful for the development of integrated vector management programmes for the surveillance and elimination of mosquito-borne diseases in Cameroon.

According to the World Health Organization (WHO), mosquito-borne diseases account for about 17% of the total burden of all infectious diseases [1]. Mosquitoes are blood-sucking dipterans belonging to the Culicidae family, with 3583 valid species currently described worldwide [2]. In this publication, we used the composite Aedes valid species list, where species are classified within 41 genera and 44 subgenera [2]. The genus Aedes has the highest number of species, with 33 species of uncertain subgeneric status and 900 classified into 72 subgenera. The Culex genus is the second largest, with 763 species described within 26 subgenera, whereas the genus Anopheles comprises seven subgenera with about 460 species [3][4][5].
Cameroon has a diverse mosquito fauna with over 300 mosquito species thus far recorded in the country. The bionomics and distribution of Anopheline species have been extensively studied in Cameroon, and in many parts of the country, malaria transmission dynamics are complex and stable because of multiple vector species occurring in the same locality [7, 16, 22-25, 27, 32, 34-48]. The distribution and bionomics of almost all the remaining mosquito genera are still not well documented across Cameroon. Moreover, the implication of these mosquito species in diseases or pathogen transmission is not well documented. The present review provides a synopsis of current information on the bionomics, distribution, and role in disease transmission of the mosquito fauna in Cameroon.

Retrieval of information
We followed a similar methodology as Azari-Hamidian et al. [49] for searching the literature. Briefly, published reference documents on medical and veterinary entomology were reviewed to collect information on mosquitoes, diseases, and pathogens [50][51][52][53]. An online search of scientific papers on mosquito-borne diseases using different search terms was undertaken using online bibliographic databases such as PubMed, Google, and Google Scholar. A combination of the following search terms were used to select publications on mosquito-borne diseases or mosquito species: "mosquito borne pathogens", "mosquito borne diseases", "mosquito borne viruses", "mosquito borne bacteria", "mosquito borne rickettsia", "mosquito fauna'' , mosquito genera including "Aedes", "Culex", "Mansonia", "Anopheles", "arbovirus", "malaria", "filariasis", "avian malaria", "trypanosomiasis", "vectorborne diseases", and "Cameroon". In addition to the online bibliographic databases searched, data were also extracted from reports and theses.

Presentation of Cameroon
Cameroon (1°40-13°05N, 8°30-16°10E) is in Central Africa, along the Guinea Gulf and covers a surface area of 475,000 km 2 with a coastal border of about 400 km along the Atlantic Ocean. The country has a population of about 25 million inhabitants [54]. The mean density of the population is 49.5 person/km 2 . There is an increase in migration of the population from rural to urban settings, and it is estimated that about 2% of the population moves from rural to urban settings yearly. Neighbouring countries include Nigeria to the west, Chad to the north and east, Central African Republic to the east, and Democratic Republic of the Congo, Gabon, and Equatorial Guinea to the south [54,55].
The country is divided into 10 administrative units called regions and has highly heterogeneous landscapes, with mountain peaks approaching 4000 m (Mount Cameroon), plains, and plateaus. Ecological domains with particular associated climatic conditions result in a high variety of ecological settings across the country [7,56]. Annual rainfall varies from 400 mm/year in the Sahelian zone to 10,000 mm/year at the foot of Mount Cameroon. The average temperature varies between 18 °C and 28 °C [57].

Mosquito-borne diseases and pathogens circulating in Cameroon
A high variety of mosquito-borne pathogens responsible for diseases in humans and animals have been reported to circulate in Cameroon. These include parasites, filarial worms, and arboviruses (Table 1).

Malaria
Malaria is a parasitic disease that affects both humans and animals, including birds and great apes (gorillas and chimpanzees). Human malaria is endemic across the country with differences in its prevalence between regions [58]. Cameroon is among the 11 countries most affected by malaria in the world. In 2018, about 2,133,523 malaria cases and 3263 associated deaths were reported in health facilities in Cameroon [58]. Currently, it is estimated that 28% of the population suffers yearly from malaria attacks, and the prevalence of the disease increased by 4% in 2019 as compared to 2018 [58]. The East, Adamaoua, and Central regions exhibit the highest morbidity (about 162, 142, and 120 cases for 1000 inhabitants, respectively), while the overall mean of morbidity in the country is 102 cases for 1000 inhabitants [58]. Four Plasmodium (Haemosporidia, Plasmodiidae) species infecting humans have been reported in the country, namely P. falciparum, P. vivax, P. ovale, and P. malariae [7, [59][60][61][62][63]. Six Plasmodium (Laverania) spp. infecting primates have also been reported, including P. reichnowii, P. gaboni, and P. billcollinsi in chimpanzees and P. adleri, P. blacklocki, and P. praefalciparum in gorillas [64][65][66]. Anopheline species such as An. moucheti are considered to be a possible bridge vector between humans and apes [67]. Plasmodium falciparum-like parasites infecting great apes in southern Cameroon were not found to represent a recurrent source for human malaria [68].
Avian malaria caused by several Plasmodium and Haemoproteus (Haemosporida, Haemoproteidae) species occur in Cameroon [15]. Avian malaria affects a wide range of birds globally and is responsible for high lethality in bird populations in areas where previous exposure to the parasites was limited or absent [69]. Recent studies in the South-West region (Nguti) of Cameroon reported a high prevalence of avian malaria parasites in all bird families (16.1% for Plasmodium infections and 11.6% for Haemoproteus infections) [15]. Unlike human Plasmodium, whose vectors are found only among anophelines, species within the Culex and Aedes are considered as the primary vectors of avian Plasmodium species. Other species within the genera Culiseta (Diptera, Culicidae), Anopheles, Mansonia, Aedeomyia, Uranotaenia, and Coquillettidia have been implicated in the transmission of avian Plasmodium [29,[70][71][72][73]. Studies conducted so far in Cameroon have identified Culex spp., Coquillettidia spp., and Mansonia spp. as vectors of avian malaria [29,74] (Table 1). Studies on avian malaria in Cameroon are still at the exploratory phase, and there is still much more to investigate to unveil the complexities of transmission, distribution, and epidemiology of avian malaria.

Filariasis
Lymphatic filariasis (LF) is a neglected tropical disease that is targeted for elimination by 2030 [75]. It is caused by W. bancrofti, Brugia malayi, and B. timori (Spirurida, Onchocercidae) and is transmitted by Culex, Mansonia, and Anopheles mosquitoes [76]. In West Africa, the dominant vectors are Anopheles gambiae sensu lato and An. funestus, whereas in East Africa, it is transmitted by Cx. quinquefasciatus [75]. LF causes substantial morbidity and disabilities which can lead to social exclusion. The disease is on the decline in Cameroon due to the intensification of mass drug administration (MDA) campaigns across the country using the drugs albendazole and ivermectin [75]. A recent study conducted in 31 health districts of four endemic regions of the south of the country (central, east, south, and littoral) failed to find any cases of W. bancrofti infection [77]. Due to cross-reaction of the LF diagnostic test (filariasis test strip, FTS) with Loa loa (Spirurida, Filaridae), a microfilaria which is endemic in the southern part of the country, false positives are increasingly reported, and it is obvious that appropriate diagnostic tools avoiding false positive detections are needed to guide disease elimination efforts in the country [77,78]. With the intensification of travel and population migrations between East and West Africa and within regions, it is possible that cases could be imported from other endemic settings which will make W. bancrofti filariasis elimination in Cameroon more challenging.

Anopheles species
The genus Anopheles is the most important, with several species playing major roles in the transmission of malaria and lymphatic filariasis to humans. This genus is composed of seven subgenera with about 460 species  Table S1). Their distribution varies between ecological zones (see Additional file 1: Figure S1). and lymphatic filariasis [75], but studies conducted so far in Cameroon specifically have not incriminated anopheline in the transmission of these diseases. According to WHO, indoor residual spraying (IRS) and long-lasting insecticidal nets (LLINs) are the cornerstone in the fight against malaria vectors in most endemic countries. In Cameroon, LLINs are the only tools used for malaria vector control by the National Malaria Control Program [58,121]. Despite successes gained so far in the fight against malaria vectors, vector control faces diverse challenges due to multiple vector species, changes in feeding and biting behaviour of anopheline, and the rapid expansion of insecticide resistance in the main malaria vectors [22,102,113,122]. The rapid expansion of insecticide resistance in Cameroon seems to be driven by the increase use of insecticides in both public health and agriculture [123]. A recent review on insecticide resistance evolution in Cameroon [22] indicated high resistance to almost all insecticide classes (pyrethroids, carbamates, organochlorines, and organophosphates) driven by both target site-and metabolicbased mechanisms [7,22,113,[124][125][126]. In addition to insecticide resistance, behavioural changes could also affect the efficacy of control interventions, but these have so far received limited attention [102].

Culex species
Culex is a genus comprising more than 700 species worldwide belonging to 26 subgenera [127]. So far, 67 species and two subspecies of Culex have been collected in Cameroon [15,74,84,104,109,[128][129][130][131] (Additional file 5: Table S2). It is likely that the number of species known from the Cameroon fauna is underestimated due to difficulties associated with their identification, especially with species that differ only in male genitalia structure within the Culiciomyia and Eumelanomyia subgenera. Culex quinquefasciatus, Cx. antennatus, and Cx. duttoni appear to be the most common in urban settings [104,105,128,130], but the general distribution of species of this genus varies with collection sites or region of the country (Additional file 2: Figure S2).
Immature stages of Culex are found in different types of habitats. In forests, Culex larvae are typically found in rock pools, tree-rot holes, river ditches, forest pools, leaf axils, crab holes, and epiphyte plant leaf axils [53,104,132]. In urban settings, Culex larvae are found in a variety of habitats including catch basins, storm drains, temporary vernal habitats between houses, septic tanks and open sewage systems rich of organic matters, roadside ditches, and in artificial containers such as rainwater barrels, tires, and bottles [130,133]. The large adaptive capacity of some Culex mosquito species has facilitated their spread across the world. Some Culex, especially the more opportunistic blood-feeding species that feed on both humans and animals, are a nuisance and serve as arboviral bridge vectors in urban settings [134,135].
Few studies have investigated the impact of control interventions on Culex mosquitoes in Cameroon [143,144]. Some indoor biting Culex found in sympatry with Anopheles, particularly in urban settings, could be affected by LLIN and IRS control measures. However, one study indicated a low impact of these measures on Culex mosquito abundance [130]. As a result of intense selection pressure induced by insecticides used in agriculture and public health, Culex have likely become increasingly resistant to most chemicals used for vector control, such as permethrin, deltamethrin, DDT, and bendiocarb [130]. A larval control trial using Bacillus sphaericus (Bacillales, Bacillaceae), now named Lysinibacillus sphaericus, failed to control Cx. quinquefasciatus in the city of Maroua, Cameroon [144]. Larvivorous fishes such as Poecilia reticulata (Cyprinodontiformes, Poeciliidae) are frequently used in lakes and permanent water collection in urban settings to control Cx. quinquefasciatus [145].
Because little attention is generally paid to Culex in Africa, data on species diversity, bionomics, distribution, and the vectorial role of species within this genus remain incomplete in Cameroon. With the increased adaptive capacity and rapid expansion of insecticide resistance in some species, integrated strategies to control both Culex and Anopheles should be promoted.

Aedes species
The genus has the highest number of species worldwide with up to 1256 species [5,146]. A total of about 77 species and one subspecies have been reported in Cameroon (Additional file 5: Table S3). There is still an uncertainty on the true number of Aedes species distributed in Cameroon [147]. Four species are by far the most mentioned or studied in the country. These include Ae. aegypti, Ae. albopictus, Ae. africanus, and Aedes simpsoni [56,148]. These species are widely distributed across the country (Additional file 3: Figure S3). Aedes albopictus was reported for the first time in Cameroon in 2001 [149] and is now abundant in urban settings, whereas Ae. aegypti predominates in suburban and rural areas. Since the first report of Ae. albopictus, it has extended its distribution range to almost all the country except in the north and far-north regions (Fig. 2). Aedes africanus and Ae. simpsoni are abundant in rural and forest settings (see Additional file 3: Figure S3) [15,41,74,84,103,104,109,130].
In rural settings, Ae. aegypti, Ae. albopictus, and Ae. simpsoni larvae are mainly found in containers used to store water, but also in tree holes, cocoa pods, snail shells, and fallen large leaves holding water on the ground. In urban areas, larvae of these species are most often found in artificial containers such as tires, flower pots, broken bottles, plastic cups, tanks, cinder blocks, and abandoned containers [56,[149][150][151]. The eggs of Aedes are resistant to desiccation for several months [152], allowing them to survive the dry season [150], and the females usually bite during the day in shaded areas, with peak activity in the morning and late in the afternoon [149].
Multiple arboviruses have been isolated from different Aedes mosquito species in Cameroon, highlighting their role as epidemiologically important (Table 1 and Additional file 5: Table S3). Aedes aegypti and Ae. albopictus are the most well-known vectors of arboviruses such as dengue, chikungunya, yellow fever, zika, and West Nile viruses [153][154][155][156][157]. Cases of dengue, chikungunya, and yellow fever viruses have been frequently reported in Cameroon [158,159] with cases of yellow fever mostly documented in rural settings and dengue and chikungunya more prevalent in urban settings. The current expansion of Ae. albopictus in Cameroon [148] could result in increased risk of arbovirus disease transmission across the country, which deserves further investigations.
Knowledge of the bionomics of almost all Aedes species, except a little known for the ones mentioned above, is severely lacking in Cameroon. Changes in the environment due to deforestation, urbanisation, and agricultural practices will likely affect dynamics of "Aedes-borne" arboviral diseases which may put more people at risk of contracting diseases such as Rift Valley fever.
There is no specific intervention targeting Aedes populations in Cameroon, and the existing risk of arboviral infection outbreaks due to Aedes is unknown. The current national vector control strategies (LLINs) target only the indoor biting species and species that feed predominantly at night, and most Aedes species bite outdoors and during the day and at sunset. Different studies suggest increased tolerance of Aedes mosquito populations to DDT, permethrin, deltamethrin, and propoxur [150,151]. Insecticide resistance, which affects both Ae. aegypti and Ae. albopictus, is still emerging and limited to urban settings. Mechanisms involved in Aedes species resistance in Cameroon have not been fully investigated [150,151]. Controlling mosquito larvae with either B. sphaericus and/or B. thuringiensis (Bacillales, Bacillaceae) could be useful for Aedes control, although challenging, because of the diversity and numbers of small habitats in which immatures develop.

Other mosquito genera of medical and veterinary importance Eretmapodites species
The genus Eretmapodites consists of 48 species, three of which includes nominotypical and one subspecies [2]. All Eretmapodites species are restricted to the African continent [160]. Most Eretmapodites species occur in Cameroon; in fact, 31 species and one subspecies are recorded in this country (Additional file 5: Table S4).
Eretmapodites species found so far in Cameroon were collected in the West region (Dschang), Central region (Yaoundé), South region (Mvina, Dja et lobo), and South-West region (Nguti in Talangaye forest) [15,31,74,83,84,103,104,111] (see Additional file 4: Figure  S4). There could be more than the 32 species present in Cameroon because the methods used to capture mosquitoes in general mosquito surveys and traps do not capture Eretmapodites very well. Furthermore, most identifications of Eretmapodites are limited to the generic level because of the similarities in species morphologically, and most species can only be reliably identified based on male genitalia, requiring dissection and careful mounting on slides for examination [160,161].
Most Eretmapodites species occur in pristine thickly forested areas, with a few adapted to small riverine and mountainous wooded enclaves in savanna regions. Very little is known about the bionomics of Eretmapodites, but they seem to be quite opportunistic blood feeders with many species, including the Er. chrysogaster group and Er. parvipluma, reported to feed on humans during the day and crepuscular periods [160].
The few Eretmapodites species that have adapted to more open rural environments near human dwellings, such as Er. chrysogaster, are typically known as "container breeders". For example, immatures of the Er. chrysogaster group, which are usually abundant in large fallen leaves that hold water in forest floors, can be found in artificial containers such as discarded tins and pots, but only when these are in shade and contain water strongly tinged by decaying vegetable matter. In pristine forests, it is believed that Eretmapodites mainly select habitats such as epiphyte plant leaf axils, fallen fruit husks and leaves, and empty snail shells as oviposition sites [162]. Larvae of some species are facultative predators and feed on culicine larvae (Ae. simpsoni, Ur. ornata musarum, and Cx. nebulosus) which are also common in plant axils [163].
Due to their low epidemiological importance and local nuisance, there are no official control measures against these potential vectors in Cameroon. However, because they bite during the day and outdoors, the use of repellents could be effective against these mosquitoes, especially in areas where they can be a nuisance such as in banana plantations.
Some Coquillettidia species have been implicated in the transmission of protozoans and viruses to humans and animals [8,29,74,172]. In Cameroon, Cq. pseudoconopas, Cq. aurites, and Cq. metallica have been implicated in the transmission of avian malaria [29]. Coquillettidia species such as Cq. perturbans, a species found in the USA, was found to be a competent vector of West Nile virus in the laboratory [173]. Moreover, Cq. maculipennis and Cq. versicolor are potential vectors of chikungunya virus and Rift Valley fever virus, respectively [174].
Due to the minimal nuisance and low known disease importance, there have so far not been any interventions targeting African species of Coquillettidia. However, Coquillettidia mosquitoes can be controlled by removal of hydrophytic vegetation because of their reliance on submerged vegetation for gaseous exchange. In the USA, where these mosquitoes are important vectors of disease for humans and animals, control strategies were first based on the aerial spraying of insecticides (Arosurf ® MSF and fenitrothion) on flying adults or ponds [175,176] and traps [177,178]. Studies conducted so far have shown that adult management remains poorly effective or not easily applicable. Moreover, most conventional methods for controlling the immature aquatic stages of other mosquitoes are ineffective against this group because they do not rise to the water surface until eclosion and often occur in densely vegetated water bodies, which minimises effective exposure to larvicides [179,180]. Acceptable control has been obtained using an insect growth regulator such as methoprene to control Cq. perturbans in place of B. thuringiensis and temephos (organophosphate) [181]. Another control method for this group is the removal of vegetation in rivers, but this can lead to ecological disturbance or imbalance.

Mansonia species
In Cameroon, Mansonia africana and Ma. uniformis are quite prolific in areas near water supporting hydrophytic vegetation [74,84,111,130]. Both species are distributed countrywide and are mainly found in areas with slowflowing rivers and marshlands favourable for immature stage development (Additional file 4: Figure S4 and Additional file 5: Table S6). Mansonia africana and Ma. uniformis have been reported in the country since 1937 [111] (Additional file 5: Table S6). Mansonia uniformis is sometimes considered the most abundant species in Cameroon and in Central Africa [122]. Mansonia mosquitoes are active mostly at dusk and dawn but sometimes bite at night indoors and mostly outdoors [122,135,182] and occasionally are found resting indoors [122]. Mansonia feed on both humans and animals such as cattle, fowl, goats, and dogs [183,184]. Mansonia larvae are found in rivers supporting thick vegetation along the edges, and in swamps to which they attach by piercing their siphons to breathe from submerged plant stems (especially Pistia) [185,186].
Mansonia species are involved in the transmission of arboviruses [174] and lymphatic filariasis caused by W. bancrofti [187][188][189], B. malayi, and B. timori [190]. Several arboviruses including Banzi, Bunyamwera, chikungunya, Rift Valley fever, Sindbis, Spondweni, and Wesselsbron viruses have been reported to be transmitted by Mansonia species [174]. Studies conducted so far in Cameroon were unable to link the transmission of W. bancrofti to Mansonia species. Mansonia uniformis has been implicated in the transmission of avian malaria in forested areas of Cameroon [74] (Additional file 5: Table S6).
Occasional blood-feeding at night and indoor resting habits of both Mansonia species means they can be controlled by IRS and LLINs. Additional control tools such as repellents, coils, and screens on windows could be used for controlling this mosquito species. There are still no data on the susceptibility of this species to insecticides in Cameroon.

Mosquito species of unknown or minimal medical and veterinary importance
A list of 62 mosquito species belonging to other genera is provided in Additional file 5: Table S7. Toxorhynchites and Malaya are non-hematophagous and therefore of no consequence as disease vectors. Toxorhynchites larvae are predaceous, and they have been proposed as a biological means of control for major container disease vector species such as Ae. aegypti, although field attempts to do so have not been successful [191].
Mimomyia and Uranotaenia species are mostly herpetophilic and seldom feed on the blood of mammals, but are likely responsible for transmission of anuran intracellular apicomplexans and microfilariae, as found in South Africa [192][193][194].
Due to a lack of isolation and attempts to do so, the genera Hodgesia, Ficalbia, Orthopodomyia, Aedeomyia, Culiseta, and Lutzia have thus not been implicated as disease vectors in Cameroon and Africa more generally, and their disease relationships are unknown [195].

Conclusions
The present review provides information on the bionomics, distribution, and epidemiological role of mosquito species present in Cameroon. Even though multiple entomological surveys have been conducted in the country, only a few have investigated the diversity and distribution of non-malaria mosquito vectors, probably because of their perceived low medical and epidemiological importance at the national level. The review reveals a great diversity of the Cameroonian mosquito fauna, with over 300 species recorded. Among these, many species are vectors of human and animal pathogens. Cameroon has not yet developed a national strategy for integrated control of vector-borne diseases. There is still a huge shortage in well-trained entomologists able to identify non-Anopheles spp. In addition to training activities on basic entomology, courses on new molecular techniques are also needed to improve the capacity of technicians to properly identify and process all mosquito samples. The past decade has seen the development of new molecular tools and easy-to-use light technology such as matrixassisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS) which can significantly improve mosquito species identification [196][197][198][199], blood meal source detection [199], and pathogen detection [200]. Developing capacities in both new and classical techniques is paramount in driving efforts toward vector-borne disease elimination. Although increasing efforts have been made during the last decade for the elimination of diseases such as lymphatic filariasis and human African trypanosomiasis, most diseases are still prevalent in Cameroon and need additional efforts to achieve elimination targets. Involving communities in the removal of unnecessary water-holding containers and trash, and the use of improved irrigation practices could help ensure sustainability and the success of control interventions.
Additional file 1: Figure S1. Leaflet map of main Anopheles species found in Cameroon.
Additional file 2: Figure S2. Leaflet map of main Culex species found in Cameroon.
Additional file 3: Figure S3. Leaflet map of main Aedes found in Cameroon.
Additional file 4: Figure S4. Leaflet map of other mosquito groups found in Cameroon. Table S1. Anopheles species composition, pathogens transmitted and control interventions in Cameroon. Table S2. Culex species composition, pathogens transmitted and control interventions in Cameroon. Table S3. Aedes species composition, pathogens transmitted and control interventions in Cameroon. Table S4. Eretmapodites species composition, pathogens transmitted and control interventions in Cameroon. Table S5. Coquillettidia species composition in Cameroon, pathogens transmitted and control interventions. Table S6. Mansonia species composition, pathogens transmitted and control interventions in Cameroon. Table S7. Mosquito species of unknown or minimal medical and veterinary importance: species composition, pathogens transmitted and control interventions in Cameroon.