Mosquitoes

Morphology and Systematics

The mosquito vectors all belong to the subfamily Culicinae and nearly all belong to 3 genera namely Culex (Cx.), Aedes (Ae.) and Ochlerotatus (Oc.). In the case of Aedes, 2 different subgenera Aedimorphus (Adm.) and Neomelaniconion (Neo.) are important while all the important  Culex belong to the subgenus Culex (Cux.). This module on mosquitoes describes the main features of mosquito morphology and shows how these are used to identify the 3 genera. Identification of individual species is a specialist's job but some of the vector species have striking features that make them readily identifiable – these will be described.  It is also shown how the morphology of the male genitalia and the larva is essential for distinguishing members of the important Cx. pipiens complex.

Biology

All mosquitoes have “complete metamorphosis” with a life cycle of egg – larva - pupa - adult, but the biology of Culex differs from that of Aedes and Ochlerotatus, particularly in the egg stage.

Culex species are mainly bird feeders biting at night and feed on mammals to a lesser extent.  In contrast Aedes and Ochlerotatus are avid feeders on livestock and humans that bite mainly in the daytime. Culex deposits a raft of eggs on water while the other genera deposit their eggs singly on a moist substrate such as mud at the margin of a flooded area. Provided that the humidity is adequate in this microhabitat these eggs can survive for long periods until the next rains. Hence, Culex must bridge the winter or dry season as either quiescent or hibernating larvae and/or adults while the other 2 genera can survive as quiescent or diapausing eggs buried just below the soil surface. Shortly after emerging from the pupae the adults mate and the female seeks a blood-meal. Both sexes feed on floral nectar and plant sap, the male relying entirely on these for his sustenance.

Distribution

Rift Valley fever is confined to Africa and Saudi Arabia, while WSL is confined to Africa except for an unconfirmed occurrence in Thailand. Until recently WN virus was known only in Africa, Israel, India, Pakistan and France but has now spread to further parts of Europe and widely through the USA. Of the equine encephalitis viruses, EEE and WEE are endemic to the USA while VEE occurs in the tropical and subtropical Americas. Mosquito vectors of these viruses have been identified in all these areas and will be listed in this module.

Importance

Arboviruses by definition multiply in both vertebrates and certain arthropods including mosquitoes as considered here. These mosquito vectors are therefore most important as they both play a role in maintenance of the viruses and also transmit them to livestock and humans to cause illness and sometimes mortality. The transmission cycles for each of the viruses are described in this module. Vectors of RVF virus are species of Aedes (Aedimorphus), Ochlerotatus (Ochlerotatus) and Culex (Culex). The interepidemic period usually lasts for years and vertical (transovarial) transmission may be one mechanism for virus survival through this period. Wesselsbron virus has a very similar ecology to RVF except Culex species are not involved as vectors. West Nile virus is transmitted between wild birds by Culex (Culex) species in an endemic cycle and from this cycle infection is transmitted to horses and humans sometimes by additional link vectors. The equine encephalitis viruses (EEE and WEE) are similar to WN in that they also have endemic cycles between birds and mosquitoes but further link vectors transmit these viruses to horses and humans. VEE virus has a complex ecology involving rodents and a number of mosquito species and there are also endemic and epidemic subtypes of this virus.

Control

There are a variety of ways stabled horses can be protected against mosquito bites which will be described.  Alternatively, stables and barns can be sprayed with carefully chosen insecticides as will be recommended.  As to RVF and WSL outbreaks on farms in Africa, immunization of stock should be the first line of defence. Mosquito densities can then be reduced, if necessary, by using an appropriate larvicide such as the insect growth regulator Methoprene.

Collection methods

The reasons for collecting insects will be listed but they are mainly directed at determining which species are vectors in a particular locality. To find out which species are present in an area,it is necessary to employ a variety of collection methods, including larval collections, as will be described. The main methods for collecting adults depend on either animal bait (humans or sheep) or CO2 bait. Also the “gravid trap” that collects gravid Culex mosquitoes as they come to oviposit will be explained. Sources for obtaining traps will be mentioned as will ways of killing mosquitoes and preserving specimens.