Invasion of Aedes albopictus (Diptera: Culicidae) into central Africa: what consequences for emerging diseases?
Current distribution of Ae. albopictus
The global spread of Ae. albopictus is due mainly to human activities, such as increase in intercontinental trade, especially
in the past three decades 5]. In central Africa, Ae. albopictus was first described in 2000 in Cameroon 8], then in 2003 in Equatorial Guinea 15], in 2007 in Gabon 16], in 2009 in the Central African Republic (CAR) 17] and in 2011 in Republic of Congo 18] (Figure 1). In Cameroon, entomological investigation on a macro-geographical scale revealed
that Ae. albopictus is present only in the southern part of the country, which is characterized by an
equatorial climate, whereas the native species Ae. aegypti is present throughout the country 9],10]. A study in CAR showed that Ae. albopictus predominated over Ae. aegypti at all sites where both species were sympatric 4], and data on the spatial distribution of Ae. albopictus showed that this invasive species is widespread in southern sites, such as Mbaïki,
Batalimo, Mongoumba, Boda and Berberati, except in Bouar (located near Cameroon at
6°N latitude), where Ae. aegypti is found alone. High densities of Ae. albopictus were also reported in several cities in Gabon (Libreville, Lastouville in the south-east,
Franceville, Oyem and Cocobeach in the north-west) 19]-21]. These observations are consistent with the hypothesis that invasive species are
more likely to establish themselves in environments that are similar to their native
environment but can also evolve to adapt better to their new environment 22]. The absence of Ae. albopictus above 6°N in Africa suggests a climatic limitation for invasion of the species 9],10].
Figure 1. Chronology of invasion byAe. albopictusin central Africa. The black circle represent the continental African countries infested by Ae. albopictus.1st: Cameroon in 2000; 2ed: Equatorial Guinea in 2003; 3rd: Gabon in 2007; 4th: Central
African Republic in 2010; 5th: Republic of Congo in 2011.
Biology
i.) Breeding sites of Ae. albopictus
Aedes albopictus has strong ecological plasticity, which allows its rapid adaptation to a wide range
of habitats. Studies in central Africa show that its larval breeding sites are diverse,
ranging from natural sites (e.g. tree holes, snail shells, rock holes, cacao shells,
coconut shells and leaf axils) to artificial containers (e.g. water storage containers,
used tyres, tin cans, car wrecks, flower-pots) 4],8]-10] (Figure 2). Detailed characterisation of larval ecology in Cameroon and CAR showed that Ae. albopictus breeds mainly in used tyres, discarded tanks and flower-pots and prefers containers
with plant debris and/or surrounded by vegetation. The most productive containers
were used tyres, follow by discarded tanks 4],10].
ii.) Feeding hosts and daily dynamics of host-seeking activity
Aedes albopictus has long been considered mainly zoophilic and able to feed on most groups of cold-
and warm-blooded vertebrates, including reptiles, birds and amphibians 2],23],24]. Analysis of ingested blood in outdoor-resting females in Cameroon showed that Ae. albopictus preferentially fed on humans rather than on domestic animals (95% of blood meals
contained human blood) 25]. These results conflict with the assumption that Ae. albopictus is mainly zoophilic 2],23],26] and are consistent with observations made in regions outside Africa, such as Thailand
27], the USA 28], Italy 29] and La Réunion 30]. These results indicate that the authors chose sites where animals were available,
while Ae. albopictus prefers to feed on humans. The propensity of Ae. albopictus females to feed on humans in urban areas in Cameroon is a concern, as it suggests
a risk for human–human pathogen transmission. Moreover, observation of a few blood
meals in pigs and reptiles, and especially mixed animal–human meals, confirms that
this species could act as a bridge vector for zoonotic pathogens 25]. Mosquito collection with a double-net device in Cameroon demonstrated that Ae. albopictus females feed during daytime, from 05:00 to 19:00, with a peak from 15:00–19:00 25]. Although Ae. albopictus is sometimes observed indoors, it is generally considered exophilic and exophagic
in Africa and elsewhere 5],23],30].
Figure 2. Examples of larval breeding sites ofAe. albopictus.A. tree holes; B. leaf axil; C. used tyres; D. flower-pot saucer.
Interaction with indigenous species Ae. aegypti
Numerous studies on the spatial coexistence of Ae. aegypti and Ae. albopictus have been conducted outside Africa, where the two species are sympatric 31],32]. In North America 33] and Brazil 31], the two species have similar larval ecological niches and often share the same larval
habitat. Likewise, in Mayotte, Ae. albopictus co-exists with Ae. aegypti in 40% of larval habitats 34]. As suggested by Paupy et al. 5], however, the apparent coexistence of the two species could be a transient situation,
followed by a reduction 35]-37] or displacement 38],39] of the resident species; interspecific larval competition for resources is the most
likely reason for this process.
In central Africa, when Ae. albopictus was widespread, it was suspected to have played a major role in transmission of the
viruses of dengue and chikungunya. Most of the studies therefore focused on viral
detection or isolation, and few studies have been conducted on its interactions with
the resident species Ae. aegypti. Nevertheless, two studies conducted in Cameroon 10] and CAR 4] provide more detail (such as building density, type of container, vegetation around
the container and plant debris inside the container) on the spatial distribution and
interactions between the invasive species Ae. albopictus and the resident species Ae. aegypti. Data obtained showed that immature stages of both species colonized a variety of
artificial natural breeding sites and were often found together at the same larval
development sites. Ae. albopictus, however, colonizes preferentially containers containing plant debris or surrounded
by vegetation. Thus, although the two vectors are sympatric, significant differences
in their relative proportions and their spatial distribution are likely, due to environmental
factors (e.g. climate, vegetation and building density). In the detailed study in
Bangui (CAR), Ae. aegypti species represented the majority in the early rainy season, whereas Ae. albopictus was most abundant in the late rainy season. This is probably due to the better tolerance
of Ae. aegypti eggs to desiccation than those of Ae. albopictus, as suggested by Juliano et al. 40]. All the studies undertaken in the sympatric area in central Africa suggest that
Ae. albopictus tends to supplant the resident species Ae. aegypti4],10],20],41].
Population genetics and phylogeography
Since introduction of Ae. albopictus into central Africa, genetic studies have been conducted only in Cameroon 42],43] and CAR 4]. Analyses of Cameroonian samples with microsatellite markers showed moderate, statistically
significant overall genetic differentiation between samples. No obvious relation between
genetic and geographical distances was found, suggesting that the genetic structure
has been shaped by additional biotic or abiotic factors. Analysis of mtDNA sequences
revealed four haplotypes each for the COI and ND5 genes, with a dominant haplotype shared by all Cameroonian samples 42]. Phylogeographical analysis based on COI polymorphism indicated that Ae. albopictus populations in Cameroon are related to tropical rather than temperate or subtropical
outgroups 42]. Similar analysis of the CAR samples also showed little overall mtDNA diversity 4], which is consistent with the recent introduction of a few founder females or may
be related to ubiquitous Wolbachia infection in populations of this species, as suggested by Armbruster et al. 44]. Phylogeographical analysis based on COI polymorphism indicated that the Ae. albopictus haplotype in the CAR population segregated into two lineages (Figure 3), suggesting multiple sources 4]. The moderate genetic diversity observed among Cameroonian and CAR Ae. albopictus isolates is in keeping with recent introduction and spread in these countries.
Figure 3. Phylogeographical tree ofAe. albopictusfrom CAR and Cameroon based on COI. A. Subtropical or temperate region: GREE, Greece; REU, Reunion; HAWAI, Hawai; FRAN,
France; MADA, Madagascar; USA, United State of the America. B. Tropical region: CAR, Central African Republic; CAM, Cameroon; BRAZ, Brazil; VIET,
Vietnam; THAI, Thailande; INDIA, India; CAMB, Cambodge.
Impact on health
Invasive mosquito species are defined by their ability to colonize new territories
and can affect human health by concurrently harbouring novel pathogens, transmitting
native pathogens or transmitting novel pathogens introduced independently 39]. Changes in the epidemiology of arboviruses after the introduction of invasive species
have been seen throughout the world, including simultaneous introduction of Ae. aegypti and yellow fever virus in the Americas between the 16th and 17th centuries 38], the re-emergence of dengue in Asia after introduction of Ae. aegypti45] and the emergence of dengue in Hawaii after Ae. albopictus was established in 2001 46].
The introduction of Ae. albopictus and its subsequent rapid spread in numerous countries of central Africa is particularly
disturbing, as it is suspected to have played a major role in the transmission of
Chikungunya virus (CHIKV) in Cameroon in 2006 47] and was the main vector of CHIKV and dengue virus (DENV) in Gabon in 2007 20],21],41],48]. In Cameroon, additional Ae. albopictus populations were shown to be orally susceptible to DENV-2 and highly competent for
CHIKV 41]. In Republic of Congo more recently, Mombouli et al. 49] confirmed that Ae. albopictus together with the native species Ae. aegypti played a role in the dissemination and spread of CHIKV during the 2011 outbreak,
after 39Â years of absence. The role of Ae. albopictus in the transmission of DENV and CHIKV has been recognized since 2009 5], and, in 2013, Grard et al. 50] provided the first direct evidence of human Zika virus (ZIKV) infection in the Asian
tiger mosquito, Ae. albopictus, in Gabon. Phylogenetic analysis placed the Gabonese ZIKV at a basic position in
the African lineage, in agreement with previously obtained complete sequences of ZIKV
strains, indicating an African lineage and an Asian lineage 51]. Therefore, the emergence of ZIKV in Gabon was not due to an imported strain but
rather to the diversification and spread of an ancestral strain belonging to the African
lineage. These data from Libreville in 2007 are the first proof of human ZIKV infection
in an urban environment during concurrent CHIKV and DENV outbreaks and its first occurrence
in the invasive mosquito Ae. albopictus.
The introduction in central Africa of a new vector that is now known to be competent
for more than 20 arboviruses is a public health problem, because three arboviruses
(CHIKV, DENV and ZIKV) that are endemic in the region have re-emerged. Ae. albopictus can also transmit filarial nematodes, which are primarily parasites of dogs but can
also affect humans. Evidence of its transmission by Italian Ae. albopictus populations 52],53] has been linked to an increased prevalence of human dirofilariosis 54]. The emergence of a new strain of CHIKV in Gabon shows that Ae. albopictus can interfere with the indigenous virus-vector system and augment viral emergence.
This is a particular problem in areas such as central Africa where malaria is still
a public health problem because of the diversity of pathogens transmitted by mosquitoes.
In CAR, where a high infestation index of Ae. albopictus has been reported, there is thus an imminent risk for large outbreaks of arboviral
infections, such as dengue, chikungunya and zika, as observed elsewhere in the region.
It would be interesting to evaluate the vector competence of numerous arbovirus for
Ae. albopictus populations and Ae. aegypti in central Africa to assess the risk for emergence or re-emergence in the region.
Control of Ae. albopictus
In view of the occurrence in central Africa of large outbreaks of dengue and chikungunya,
the main diseases transmitted by Ae. albopictus, preventive measures are required in all countries of the region, because there is
no vaccine or specific treatment against these diseases. Surveillance of invasive
species is therefore essential to assess the risks for mosquito-borne diseases and
to prepare for a disease outbreak. The conventional strategies for controlling Ae. albopictus are based on reduction of breeding sites and using larvicides such as temephos and
Bti in natural and/or peridomestic breeding sites 55]. If treatment with larvicides fails and in emergency situations, space spraying with
pyrethroids or organophosphates can reduce the density of adult mosquitoes 55]. Alternative strategies consist of biological control (e.g., the use of larvivorous
organisms or bioinsecticides), reduction of human-to-vector contact with insect repellents
and insecticide-treated materials and genetic control (e.g., releasing factory-produced
sterile insects or genetically modified mosquitoes that are unable to transmit diseases
to humans). Unfortunately, few studies have shown effective, sustainable control of
the Aedes mosquitoes with these methods 5]. Meanwhile, biological control, using copepod in genus Mesocyclops has allowed eliminating immature stage of Ae. aegypti in water storage containers in Vietnam 56]. Recent data showed that all Ae. aegypti and Ae. albopictus samples collected in Cameroon and Libreville (Gabon) were susceptible to Bti and temephos, and both species were fully susceptible to deltamethrin, except in
Yaoundé, where the Ae. albopictus population had a mortality rate of about 80%, strongly suggesting resistance. WHO
bioassays on adult mosquitoes showed resistance to dichloro-diphenyl trichlorethane
(DDT) in one Ae. aegypti population in Gabon and two Ae. albopictus populations in Cameroon and suspected resistance to DDT in an Ae. albopictus sample from another site in Cameroon 57]. Vector surveillance and enhanced disease surveillance will enable early detection
of cases and prompt implementation of control measures.