Mosquitoes and Malaria Control – A Complex Problem for Entomologists to Unravel

Abstract

The control of malaria transmitting mosquitoes hinges on accurate species identification. This enables assessments of insecticide susceptibilities and important behavioural characteristics (such as feeding and resting behaviours) by species, leading to the design of coherent insecticide-based control strategies that can be enhanced by additional methodologies for malaria elimination. Malaria is a mosquito-borne parasitic disease that affects many vertebrates including humans. Prior to the 20th century the human malarias (Plasmodium falciparum, P. vivax, P. malariae, P. ovale and P. knowlesi) occurred in tropical and temperate regions but their distribution has since reduced to the tropical belt with by far the highest incidence in sub-Saharan Africa. Global incidence for 2017 was estimated by the WHO at 219 million cases corresponding to 435 000 deaths. It is also estimated that investment in malaria control and elimination amounted to $3.1 billion in 2017. The control (and elimination) of malaria largely hinges on the suppression of mosquito vectors, accurate diagnosis and case detection, and case management using appropriate antimalarial drug regimens. Controlling malaria vector mosquitoes (and of course other mosquito-borne diseases) means being able to identify that which needs to be controlled. This is not unlike the maxim of knowing one's enemy, and disease vector control is often phrased in militaristic terms. The arsenal of tools in the war against malaria vectors includes insecticides, bed nets, repellents, larvicides, endectocides, toxic baits and even modified genes. This call to arms against the transmitters of a deadly disease presupposes that the enemy can be identified, which, unfortunately, is not as easy as it sounds. Identifying malaria vectors to species has posed a significant challenge ever since Ronald Ross and Giovanni Grassi implicated dappled-winged Anopheles mosquitoes in malaria transmission. They could not have known the Pandora's Box they had opened, because several Anopheles species are cryptic. Many hide in cryptic species complexes and groups that confound straightforward morphological methods of identifying them. A species complex is a group of morphologically identical species that are very closely related, but nevertheless vary significantly in their feeding and resting behaviours, and mate assortatively (i.e. they recognise and tend only to mate with conspecific partners) enough that hybridisations between them are rare. Many member species of these complexes are sufficiently diverged that cross-mating between them yields infertile or non-viable offspring, but not in all cases. A species group is a looser assortment of related species whose morphological features match to a point where they are very nearly identical, often requiring specimens from more than one life stage to identify them. They also mate assortatively, and hybrids are rarer or simply never occur. The problem for malaria control is that several vector species, including many primary vectors, are members of cryptic complexes or groups. These invariably contain vector and non-vector species, requiring a complex and laborious system to unravel them and ascribe unambiguous genetic methods for their identification. Added to this complexity is the possibility that any Anopheles. species that takes human blood is a potential vector of the human malarias, with the added caveat that not all populations within a species are vectors. Some member species, and even populations within a species, feed either exclusively on humans (anthropophagy) and are potentially high transmission intensity vectors, or exclusively on livestock animals (zoophagy) making them non-vectors, or take blood from a range of sources including humans, becoming potential vectors of low to medium transmission intensity. An added layer of complexity is genetic heterogeneity between populations within a species. It can be argued that this complexity is not necessarily a problem for malaria control. After all, the aim of suppressing or even eliminating vector populations is the interruption of transmission, regardless of what species they are. But mosquito adaptability dictates otherwise. This is because the primary method of malaria vector control is deployment of specially formulated insecticides against adult mosquitoes, either by indoor residual spraying (IRS) or the treatment of bed nets. Mosquito adaptability has enabled a powerful response to these interventions, with resistance to insecticides becoming so widespread that fully insecticide susceptible malaria vector populations are now quite rare.