**5.** *Wolbachia***: a paratransgenic agent**

*Wolbachia* is an obligate intracellular gram-negative bacterium belonging to the family Rickettsiales; it is known to be part of the microbiota of insects, isopods, nematodes, and mites (**Figures 1** and **2**). As an obligate parasite, they infect the cytoplasmic vacuoles of their host cell, including gonads. *Wolbachia* can be vertically transmitted or maternally inherited and are therefore considered as potential targets for paratransgenic systems [48, 49]. Many mosquito species (especially those

#### **Figure 1.** *Electron micrograph of* Wolbachia *within an insect cell [46].*

#### **Figure 2.**

*Distribution of* Wolbachia *(in green) in somatic tissues of various hosts as detected by PCR and fluorescent cytology [47].*

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

of epidemiological importance) are known to be susceptible to *Wolbachia* infection; however, the prevalence of this bacterium is notably high in wild *Ae*. *albopictus* and *Cx*. *pipiens* population. Different phylogenetic strains of *Wolbachia* induce distinct extended phenotypes in the mosquito they infect; the effect induced by this bacterium in their host can be cytoplasmic compatibility, incompatibility or compatibility in only one direction [50]. The persistence of *Wolbachia* population through the generation of mosquitoes is known due to the bacterium's ability to induce a severe selective pressure that rapidly drives its transovarial transmission [51, 52].

Basic approaches to using *Wolbachia* for paratransgenic control of vectors of infectious diseases include:


The ability of *Wolbachia* to induce transovarian transmission of itself is considered a major boost in paratransgenic systems. This means once the bacterium has been introduced into the host, they can persist for several generations in the insect; hence, once introduced, there is no need for subsequent re-introduction [53, 54]. Interestingly, the effect induced by *Wolbachia* is species-dependent [55]. For example, infected *Aedes aegypti* with different strains of *Wolbachia* resulted in three outcomes: shortened lifespan [54]; reduced susceptibility to dengue or chikungunya virus and *Plasmodium* infection [18]; and, depending on the infecting strain, cytoplasmic incompatibility was observed, with apparent high horizontal transmission and no high fitness cost [54]. The foregoing underscores the importance of capacity development in the areas of research and laboratorybased surveillance systems in ensuring the successful introduction, establishment, and maintenance of *Wolbachia* populations wherever paratransgenesis is used as a biocontrol method as part of an integrated vector control strategy.

## **6. Wolbachia in Africa**

The presence of *Wolbachia* in wild *Anopheles gambiae* mosquitoes was first demonstrated by Baldini et al. [15] in Burkina Faso. Hughes et al. [56] demonstrated that a stable maternally transmissible *Wolbachia* population can be achieved in *An. gambiae* and *An. stephensi* by suppressing other members of the insect microbiota with the use of antibiotics. Furthermore, Shaw et al. [5] demonstrated the ability of the *wAnga* strain to stably infect reproductive tissues (ovaries), and certainly somatic tissues where the *Plasmodium* development occurs, with the potential to effectively compete for resources or upregulate the immune response to kill the malaria parasite. Similar results were reported in Mali with a new anopheline *Wolbachia* strain (*wAnga*-Mali) [17]. Moreover, reports have shown that there are native *Wolbachia* infections in 16 out of 25 wild African *Anopheles* species, including both vectors and non-vectors of malaria [16, 57]. These reports and more recent reports [58] confirm that natural *Wolbachia* infection in anopheline mosquitoes is more common than expected and underscores the need for further studies in the diversity of anopheline *Wolbachia* strains towards identifying suitable strains that may serve to impede the development of *Plasmodium* parasites in mosquitoes and other *Wolbachia* strains associated with non-malaria vectors that are responsible for other infectious agents of health importance.

### **7. Conclusions and recommendations**

The fact that more researchers in Africa in recent years are looking and finding *Wolbachia* [14, 15, 17, 58] in African mosquito populations is a welcomed change, unlike previously when there was no activity in this area of research in Africa. However, none of these strains are yet to be found to confer Cytoplasmic Incompatibility (CI), a condition needed to spread rapidly in natural populations and as such disrupt disease transmission. In laboratory experiments, environmental factors such as temperature and availability of food have been shown to affect the expression of CI. For example, rearing males at temperatures higher than 25°C and low levels of nutrition was found to lead to increases in cytoplasmic incompatibility [59], although the environmental factors were found to be mediated by bacterial density. On the other hand, it may be expedient to consider developing a genetically modified *Wolbachia* to induce CI or to select *Wolbachia* strains that can spread efficiently in natural mosquito populations.

Five strategic areas of development have been identified as critical to the establishment of impactful IVM programs in Africa; enhanced advocacy, intra, and intercollaboration, integrated approach, capacity building, particularly human resource development [60]. Apart from these strategic areas, basing decisions increasingly on local evidence, and community involvement and empowerment to ensure sustainability have also been identified as key components of successful IVM programs in Africa [61]. There are wide variations to the extent of adoption and promotion of these prerequisites to successful IVM among the African countries with the consequent variations in success rates. While some countries are still grappling with the consolidation of strategic and operational frameworks, others have advanced to the point of adopting IVM as a national policy, and have implemented its key elements in different measures of success [61].

Using IVM strategies, progress has been achieved with increased intervention coverage, reduced risk of transmission, and reduced VBD burden, particularly for malaria, in some African countries, including, Namibia [62], Swaziland [63], Botswana [64], Zambia and Zimbabwe [65]. These successes however may not be entirely attributed to vector control alone but also to effective case management, community mobilization, and sensitization, including changing climatic and environmental factors. These kinds of successes can be replicated in Africa if the best practices are adopted by more countries in Africa.

Developing the required technical capacity and infrastructure for entomological surveillance is another area of focus that needs to be developed in Africa, particularly, sub-Saharan Africa. This has been identified as a major challenge for most African countries [62]. Although it may take some time to develop this capacity, reports show that in countries where targeted training of entomological technicians have been conducted, such as Burundi, Eritrea, Guinea, and Zambia, the corresponding reduction in the malaria burden by up to 99% was achieved in some cases [60].

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

Moreover, since vector control of mosquito-borne diseases, must rely on insecticides as its backbone, particularly via long-lasting insecticidal nets (LLIN) and indoor residual spraying (IRS), the development of insecticide resistance has been identified as a potentially limiting factor in IVM programs [66]. On the other hand, combination innovative approaches including genetically modified or transinfected mosquitoes (Wolbachia-based), durable wall linings, mosquito traps such as eave tubes and entomopathogenic bacteria traps, odor-baited traps, attractive toxic sugar baits, spatial repellents, and entomopathogenic fungus-impregnated targets are expected to be effective when used in support of the application of insecticides "backbone" [62].

In conclusion, a great potential for IVM has been demonstrated in various regions of Africa, particularly in the area of malaria vector control [67, 68]. However, deploying IVM strategies for effective vector control in Africa will require sustained funding, removal of governmental bureaucracy, strategic planning and human resource development, and synergy among stakeholders, including community-based groups and their collaboration with nongovernmental organizations, international and national research institutes, and various government ministries.
