**1. Introduction**

Of the three foremost medically important mosquito genera of public health significance, namely, *Anopheles*, *Aedes,* and *Culex*, the *Anopheles* mosquito is most important in the transmission of human malaria while members of the genera *Culex* and *Aedes* are more important in the transmission of arboviruses [1]. Since it is impractical to eliminate mosquitoes, reducing the number of competent vectors is a logical target for controlling malaria and arboviral vector-borne diseases. For some mosquito-borne arboviruses such as West Nile, chikungunya, dengue, Zika, and so on that lack licensed vaccines or viable therapeutics, in addition to the problems posed by the ever-plastic plasmodium parasite that continues to exhibit resistance to even the most potent combined therapeutic agents, this may actually be the only option left [2].

The present chapter is focused on the potential of using a proven biological vector control method, specifically the release of mosquitoes infected with endosymbionts that help to limit the reproductive capability of mosquitoes to reduce the population of the disease vectors in Africa. Many insect species are infected by intracellular bacteria, and these are known to sometimes exert deleterious effects on the host insects. *Wolbachia* is perhaps the best-known example of intracellular bacteria that can drastically reduce the reproductive capability of several insect species, particularly disease-bearing mosquitoes. *Wolbachia* is an alpha proteobacterium first described in *Culex pipens* by *Wolbachia* and for this reason, was named *Wolbachia pipientis* [3]. Similarly, *Wolbachia* has been isolated from *Drosophila*, *Aedes albopictus*, and other insect species; in fact, reports have shown that these bacteria only infect invertebrate hosts and are naturally found in more than 50% of all arthropod species and in several nematodes [4].

Today, *Wolbachia* is still relevant in biological control programs due to its potential as a safe vector for spreading cytoplasmic incompatibility and other means of reproductive isolation among disease-bearing vectors, such as induction of parthenogenesis, feminization, and male-killing [5, 6]. In recent times, there are notable examples of successful establishment of *Wolbachia* in mosquito populations aimed at suppressing mosquito-borne diseases [7–10]. Remarkably, the Australian *Wolbachia* project tagged "eliminate dengue" (www.eliminatedengue.com) has shown that *Wolbachia* bacteria can prevent Dengue virus (DENV) transmission in mosquitoes without high fitness costs. Moreover, a virulent *Wolbachia* strain in Drosophila melanogaster fruit flies (named *w*MelPop) is known to lower the lifespan of its host significantly. It has been shown to shorten the lifespan of mosquitoes [11].

In addition, a closely related avirulent *w*Mel strain was found to protect their native hosts, Drosophila fruit flies, against infection by pathogenic RNA viruses [12, 13]. Recent reports indicate that such strains that provide similar or better characteristics deployable in preventing the capacity of viruses to replicate in the vector or the ability to incapacitate the vector (such as *w*MelPop and *w*Mel strains) exist in Africa. An example is a report by the insect vector research group at the African Centre of Excellence for the Genomics of Infectious Diseases (ACEGID) laboratory recently reported finding *Wolbachia* in Ede (Osun State), which is the first report from Nigeria [14].

*Wolbachia* has been reported from countries in West Africa and even from *Anopheles* species initially thought not to be naturally infected by *Wolbachia*. African countries from which natural mosquito infections by *Wolbachia* have been reported include Burkina Faso [15]; Ghana, the Democratic Republic of the Congo (DRC) [5, 16], and Mali [17]. Since success rates of *Wolbachia* infections have been attributed to the relatedness of the donor and recipient hosts [16], the present chapter focuses on the great potential in developing indigenous strains of *Wolbachia* that might be used in artificial infections that can reduce the capacity of wild mosquito populations to reproduce and transmit human pathogens in Nigeria and possibly elsewhere in Africa. Moreover, the artificial infection of mosquitoes may produce inhibitory effects on arboviruses and *Plasmodium* parasites as observed in Australia and elsewhere in Asia [18, 19].

#### **2. The microbiome of mosquitoes**

As a result of their interactions with biotic and abiotic factors in their ecosystem, mosquitoes internalize diverse consortia of microbes, which have been shown to have a significant effect on this insect's physiology. Microbes belonging to diverse life

### *The Potential for Wolbachia-Based Mosquito Biocontrol Strategies in Africa DOI: http://dx.doi.org/10.5772/intechopen.104099*

forms (bacteria, protists, viruses, and yeasts) have been identified and characterized as established or occasional members of the mosquito microbiome. Some members of this symbiotic microbiota can either be beneficial (e.g. dietary supplementation, enhancement of digestive mechanisms, tolerance of environmental perturbations, protection from parasites and pathogens, and maintenance and/or enhancement of host immune system homeostasis) or detrimental (reducing the fitness or life span of


*Source: WHO [19]; Available from: www.who.int/news-room/fact-sheets/detail/malaria [Accessed on: 12 December, 2021].*

their host); while other members of this community are of medical significance to the host on which the insect feeds on [20–28].

The microbes that constitute the microflora of the mosquito are the causal organisms of infectious diseases of global public health importance. Consequently, the process of diseases vectoring by a mosquito may not be viewed as a deliberate act but rather an accidental act that happens during a normal blood meal, necessary for reproduction. Interestingly, the selective feeding pattern seen in mosquitoes creates a possibility of having infectious agents from an "unusual host" introduced into a completely susceptible new host. This is the basis for most emerging infectious diseases that are of zoonotic origin; mosquito, once infected, remains infectious for life [29]. According to the World Health Organization, the infectious diseases of public health importance that are vectored by mosquitoes include dengue, yellow fever, chikungunya, zika virus, japanese encephalitis, west nile virus, malaria, and lymphatic filariasis [19]. A list of these diseases, the global disease burden, and their mosquito vectors are presented in **Table 1**.

#### **3. Vector control as a means of disease control**

In the early twentieth century, vector control emerged as one of the main methods of disease control. During this era, environmental management of breeding sites, including larviciding, was employed in the reduction of mosquito vectors. Around the 1950s, insecticides (most especially DDT) were introduced and used extensively. Interestingly, by the 1970s most mosquitoes had developed resistance to these insecticides, and on discovering the environmental hazard these chemical agents place on the ecosystem, its continuous use was frowned upon [30]. This new development led to the re-evaluation of vector control programs. In 1982, WHO recommended an integrated vector control (IVC) program based on the Axtell principle of integrated pest management [30]. The Axtell principle is founded upon the combination of biological control methods such as the introduction of exotic natural enemies, larvivorous fish, microbial agents with source reduction methods such as intermittent irrigation, water level management, landfilling, channeling, and draining in combination with the use of chemicals, including insect growth regulators, adulticide, and larvicides integrated with the use of personal protection methods, such as bed nets and repellents, concurrently with health education in the various communities at the schools, on television and mass media. Of all the mosquito control components highlighted in the IVC strategy, only biological control has not been implemented successfully in Africa, although some baseline data necessary for implementation are recently being generated. Most of the problems preventing the incorporation of biological control methods in IVC strategies in Africa are due to limited capacity, as the implementation of biocontrol methods requires a high level of technical capability. Moreover, since other control measures like chemical control have inherent limitations of environmental toxicity and the emergence of resistant strains of the vector, IVC programs in Africa have not been so successful, largely due to the lack of mastery of the biological control component.

### **4. Biocontrol in IVC programs**

Biological control methods employ the use of natural enemies like fish, insects, protozoa, fungi, bacteria, and viruses to reduce the population of mosquitoes or

#### *The Potential for Wolbachia-Based Mosquito Biocontrol Strategies in Africa DOI: http://dx.doi.org/10.5772/intechopen.104099*

reduce their vectorial competence. The two most widely employed mosquito biological vector control methods include larvicides and larvivorous fish. The use of smallsized fishes that feed on mosquito larvae has the advantages of being cost-effective, environmentally safe, and long-term effective control measures against different varieties of mosquito species. On the other hand, this has some limitations such as it requires a large number, takes about 2 months (not suitable for quick intervention), less effective in waters with floating garbage or vegetation. Sometimes birds and in some African communities, humans prey on the fishes as some of the larvivorous species are delicacies in these African communities. Examples of larvivorous fish include *Gambusia spp* and *Poecilia spp* (Guppy) [31]. On the other hand, the use of bio-larvicides involves the use of bacteria for the control of mosquito larvae. *Bacillus sphaericus* and *Bacillus thuringiensis* H 14 are the two most widely used bio-larvicide usually available as granules and wettable powder, which contain lyophilized bacteria, spores, and toxic crystals. The mechanism of biolarvicide control employed by *B*. *thuringiensis* H 14 and *B*. *sphaericus* involves the production of endotoxins (Cry4A, Cry4B, and Cry11A) which result in gut paralysis and leakage of gut contents into the body cavity, which finally results in death due to osmotic shock. Toxins of *B*. *sphaericus* have been shown to be more effective in polluted water (polluted water is characteristic of *Culex* breeding sites). They are environmentally safe and do not pose any threat to humans and their livestock but are expensive [31–33].

The third mosquito biological vector control method is paratransgenesis involving the use of native bacteria flora in disease vectors to express effector molecules capable of interfering with pathogen transmission. Paratransgenesis begins by the screening of internal microbiota of the vector to isolate symbiotic bacteria that are genetically modified to express effector molecules, after which they are again reintroduced into the vector that is now introduced into the wild where they produce the desired effect [34–36]. Understanding bacteria diversity in mosquitoes is the bull's eye in paratransgenic control of mosquitoes, and this requires a detailed knowledge base of the biology of the local mosquitoes and their microflora. To be effective, the bacterial population in the local mosquito populations are screened in order to identify bacteria that are consistent and persistent in all generations and across a variety of mosquito species. For this reason, a bacterium is considered suitable as a paratransgenesis agent when it has an effector molecule that produces the desired effect; an exocytotic mechanism to discharge the effector molecule on its cell surface; and ability to survive long enough to produce the expected amount of effector molecules in the mosquito [37–39].

Gaio et al. [40] investigated the contribution of midgut bacteria to blood digestion and egg production in *Ae. aegypti*. Findings from this study showed that eradication of gut bacteria resulted in a slower growth rate and decline in fecundity. The researchers concluded that alteration of gut flora should be further investigated as a new approach for preventing the transmission of pathogens and controlling mosquito populations.

Paratransgenic management of infectious disease and their insect vector is considered to have advantages of increased bacteria number after ingestion of blood (by the vector), which will invariably cause an increase in the secretion of effector molecules by the genetically modified bacteria. The expected outcomes of paratransgenesis include a reduction in mosquito's vectorial competence; obstruction of pathogen transmission; loss of fecundity in mosquito (non-viable eggs and alteration of embryogenesis); and eventual death of the mosquito [41–45].
