Anopheles gambiae complex mosquitoes are present throughout tropical Africa and its offshore islands. Recent work has shown that at least 6 cryptic or “sibling” species are involved. They comprise 2 salt-water species, A. melas and A. merus, 3 freshwater species—provisionally known as species A, B, and C, and a mineralwater species known as species D. Artificial inter-species crosses yield sterile hybrid males. Rarity of hybridization in nature proves the reproductive isolation and valid genetic barriers between these 6 species.

Morphological identification of most individuals of both saltwater and the mineralwater species is possible for larvae and adult females, using meristic features and other variable dimensions and ratios. Differential identification of the 3 freshwater species relies almost completely on cytotaxonomic methods.

Species A and B occur together in most areas, extending southwards to sub-tropical latitudes and eastward to Mauritius. Proportions of mixed A-B populations may depend directly or indirectly on relative humidity, with A favoured when nocturnal humidities approach saturation. Species B is often absent from areas of highest humidity, but thrives in relatively arid savannas and steppes. Species C and D have relict distributions. Both saltwater species are coastal: melas in West Africa and merus in East Africa and larger islands except Zanzibar; merus also spreads inland.

Apart from species C, which is always zoophilic, all members of the complex are proven or probable vectors of human malaria and filariasis, but with some wide contrasts in their levels of vectorial efficiency. Transmission of some arboviruses (Tataguine, O'nyong-nyong) is associated with species B, and perhaps with A also. Species B may transmit setariasis of cattle; melas and merus may also carry enzootic filariae.

Much of the confusing ecophenotypic plasticity of A. gambiae sensu lato is attributable to the differential biological characteristics of these 6 species with their dissimilar geographical distributions, behavioural contrasts and asynchronous population dynamics. Shifts in the species balance occur regularly between A and B and between freshwater and saltwater populations. Species C does not interchange so much with B under natural conditions, but may survive at high densities after control of A or B.

Additional versatility is caused by genetic polymorphism in some of the species, notably B. This species is the most widespread, and individual females tend to be either endophilic or exophilic, anthropophagic or zoophagic, early biters or late biters, and doubtless other alternatives, according to the arrangement of their floating chromosome inversions.

Control measures have to be considered separately for each of the sibling species. House spraying with residual insecticides against endophilic species A is possibly sufficient to break disease transmission (assuming favourable response of ancillary vectors) and even to eradicate A completely. Efficacy of this method against other species in the complex is reduced by their exophily, which can be enhanced by behaviouristic avoidance due to the extremely low threshold of irritability exhibited by gambiae s.l. adults in general. Genetic polymorphism of species B may lead to true behaviouristic resistance. Larvicidal control of species A and B is beneficial, but made operationally difficult by their tendency to utilize temporary breeding-sites. Effectiveness of DDT and cyclodiene insecticides is further limited by the development of physiological insecticide resistance by species A and B. Adequate control of both saltwater species, and probably the mineralwater species also, can be attained by antilarval measures.

Prospects of reduction of malaria and filariasis where melas, merus and species A and D are principal vectors may be much better than many people imagine. Widespread prevalence and inherent resilience of species B represents an insurmountable public health obstacle at present. Continued research may provide new control methods for integrated use against these mosquitoes. Concepts such as seeding breeding sites with pathogenic microsporidians or fungi, releasing sterile hybrid males or chemosterilized males, and other even more elaborate genetic control techniques, may be of special relevance to control of gambiae complex mosquitoes and diseases they transmit.

Anopheles gambiae complex and disease transmission in Africa

The call for malaria control, over the last century, marked a new epoch in the history of this disease. Many control strategies targeting either the Plasmodium parasite or the Anopheles vector were shown to be effective. Yet, the emergence of drug resistant parasites and insecticide resistant mosquito strains, along with numerous health, environmental, and ecological side effects of many chemical agents, highlighted the need to develop alternative tools that either complement or substitute conventional malaria control approaches. The use of biological means is considered a fundamental part of the recently launched malaria eradication program and has so far shown promising results, although this approach is still in its infancy. This review presents an overview of the most promising biological control tools for malaria eradication, namely fungi, bacteria, larvivorous fish, parasites, viruses and nematodes.

The biological control of the malaria vector.

The Microsporidia are a large diverse group of obligate, intracellular parasites. They are singlecelled eukaryotic microorganisms that have small genomes in the size range of prokaryotic cells and are now thought to be highly evolved fungi (Keeling and Fast 2002). They are exclusive parasites of other eukaryotes and possess a unique and highly specialized mechanism for invading host cells via infectious spores. Spores are the only stage that can exist outside a living host cell and they are the primary vehicles for horizontal transmission between hosts (peroral) and vertical transmission (transovarial) within the host. Spores are diagnostic, especially at the ultrastructural level (Fig. 1), and are distinguished by their small size (2–20 mm), thick walls (consisting of an endospore and exospore), and presence of a unique set of organelles that function as an extrusion apparatus. These include a tightly coiled polar filament (tube) that is attached to an anchoring disc at the anterior pole of the spore, a membranous polaroplast, and a posterior vacuole which collectively function to explosively inoculate the spore content or ‘‘sporoplasm’’ through the polar filament into a host cell to initiate infection. Microsporidia are ubiquitous in nature and exhibit a very broad host range within the animal kingdom. They have been described as parasites in all classes of vertebrates, including humans, and most invertebrates, but are particularly common to arthropods and fish. Microsporidia represent one of the largest and most diverse groups of parasitic organisms associated with mosquito populations in nature. They have been described from 14 different genera worldwide, and it is quite likely that all mosquitoes serve as hosts for one or more of these parasites. The group currently includes a heterogeneous assemblage of over 150 described species from 23 recognized genera, 15 of which are monotypic (only one species is known) (Table 1). Members of these genera exhibit extensive variation in their development and life cycles but generally fall into 2 broad categories. The first includes the monomorphic forms such as Anncaliia (formerly Nosema and then Brachiola) (Franzen et al. 2006) and Vavraia. These microsporidia have comparatively simple life cycles involving only one sporogonic sequence (Fig. 2). They develop asexually (merogony or schizogony) and produce a single spore type that is orally infectious to mosquito larvae. Vertical transmission may additionally occur via oral ingestion of spores on contaminated eggs (transovum), but there is no separate developmental sequence leading to ovarian infection in female hosts. These microsporidia have a very broad host range and are mildly pathogenic to mosquitoes, generally producing low larval mortality. Other genera from which only one developmental sequence has been described include Aedispora, Crepidulospora, Polydispyrenia, Senoma, Trichotosporea, and Tricornia. However, the complete life cycles and modes of transmission of members of these genera have yet to be resolved and they may be polymorphic. The second group includes the true polymorphic forms. These parasites are more common in nature and exhibit some of the most complex life cycles yet described for any microsporidia (Fig. 2). These include elements of asexual (schizogony, merogony, and sporogony) and sexual (karyogamy, gametogenesis, and plasmogamy) reproduction; the formation of multiple spore types in various stages of the host; host sexand tissue-dependent development; and separate developmental sequences leading to vertical (transovarial) and horizontal transmission. Many species, such as Edhazardia aedis, require two successive host generations to complete their life cycle, and at least 4 genera, Amblyospora, Duboscqia, Hyalinocysta and Parathelohania require obligatory development in an intermediate copepod host. These microsporidia generally exhibit higher levels of host specificity and although they do not cause any acute mortality or detectable morbidity in adult female hosts that go on to transmit infections transovarially, they have at least one phase of development that typically kills larval hosts during the last stadium. Mortality in larvae results from destruction of various host tissues and subsequent depletion of essential energy reserves necessary for pupation. The production of entomopathogenic toxins has never been documented. Copyright E 2007 by The American Mosquito Control Association, Inc.

https://bioone.org/journals/journal-of-the-american-mosquito-control-association/volume-23/issue-sp2/8756-971X(2007)23[3:MPOM]2.0.CO;2/MICROSPORIDIAN-PARASITES-OF-MOSQUITOES/10.2987/8756-971X(2007)23[3:MPOM]2.0.CO;2.pdf

Microsporidian parasites of mosquitoes.

Publisher Summary This chapter discusses a wide range of animal models available for laboratory studies requiring mosquito transmission of plasmodia. The malarial infections are normally initiated when an infected mosquito, in the act of feeding, injects sporozoites into a susceptible host. However, the majority of experimental studies start with infections initiated by the injection of infected blood. In Plasmodium vivax malaria in humans, characteristics of the infection such as length of the prepatent period, duration and severity of clinical attacks, tendency to relapse, and the latent period between relapses are determined by the number of sporozoites injected to initiate infection. The sporozoite-induced infections differ markedly from infections induced by the injection of infected blood. The total reliance on syringe-passed parasites gives misleading results and bears little relationship to mosquito-borne infections. Only mosquito-produced sporozoites can initiate an infection comparable to those experienced in nature. The sexual development of the parasite takes place in the mosquito gut, and studies on the genetics of the parasite require the establishment of a system of mosquito transmission to understand the mechanisms involved.

The Transmission by Mosquitoes of Plasmodia in the Laboratory

Developpement du parasite, de la gametogenese du gamete, de la formation du zigote, du developpement de l'ookinete, de l'oocyste, du sporocyste. Inhibition de la gametogenese, inhibition de la fertilisation, inhibition du developpement de l'ookinete et de sa fonction, modification de la physiologie

Richard H. Hulls

Papers

The adverse effects of a microsporidan on sporogony and infectivity of Plasmodium berghei