It is interesting that visitors to our region display so little fear of long journeys in speeding automobiles, participation in hazardous adventure activities and involvement in other, potentially, life-threatening pastimes - while being so concerned about contracting malaria. Motor vehicle accidents are a much greater threat than malaria in this part of the world.
People tend to be more apprehensive of the 'unknown'. The malaria threat has often been mis-reported and exaggerated for the benefit of competitive tourism markets. There is a mass of reliable information in numerous publications, many of which are not readily available to visitors.
It may be of benefit to summarise the known facts to help develop a better understanding of malaria and mosquitoes. This article will be confined to the insect vectors and their life cycles. Malaria and associated 'diseases' will be dealt with in the next issue. The name 'mosquito' derives from the Spanish word meaning 'little fly'.
With only one pair of visible wings, all mosquitoes belong to the family Culicidae (mosquitoes) in the order Diptera (flies), which contains many other blood-sucking species. There are about 3200 species of mosquito recorded worldwide. They occur in most regions on earth. Around 113 species are known from southern Africa.
The food preferences of most species are plant juices. Only female mosquitoes suck blood. In certain species, even though both sexes are plant feeders, the female requires a 'blood meal' to enable her to develop and lay eggs. The mouthparts of males are designed differently to those of females and males are therefore unable to pierce skin and suck blood.
Males can normally be identified by their feathery antennae, with which they locate females and they generally have a louder 'whine' than females. Although the eggs of some species can survive in a desiccated condition, the eggs of all species of mosquito require water in which to hatch. The larvae all require water in which to pupate.
Very few species can breed in temperatures below 20°C and optimal conditions are between 25°C and 35°C. The species that suck blood and known to 'attack' humans are divided into two groups, each named after the principle genera in each group - the 'Culicines' and the 'Anophelines'. ONLY SOME FEMALE ANOPHELES ARE CAPABLE OF RANSMITTING THE MALARIA PARASITE.
There are some very large mosquitoes found seasonally in the tundra and Arctic regions. The biggest species found in this part of Africa is the 'Elephant Mosquito' (Toxorhynchetes brevipalpis). It has whitestriped legs and body and occurs along the northern and eastern areas down the coastal strip south to eastern Cape. Adult males and females are plant feeders and do not suck blood.
This species normally breeds in water held by certain plants and holes in tree trunks. It is mentioned as an example of a useful species. Their larvae have strong teeth and are known to prey on the larvae of other mosquitoes in the same water body - thus helping to control nuisance species.
It is suspected that within each of the genera - Aedes - Culex and Anopheles - there exists a complex of sibling species, able to mutate and adapt to the transmission of various viruses and parasites under changing conditions. The diseases mentioned here are those known to be related to a particular species under present climatic conditions.
All mosquitoes in this division can easily be identified by the way adults of both sexes sit parallel to the surface. Aedes species lay their eggs separately in water, while the Culex species lay their eggs attached together in small 'rafts'. The eggs hatch within a short period and the larvae of this group swim at an angle of 45 degrees to the water surface.
The larvae breath air through two 'spiracles' in the tail and shed their skin four times within a few days to two or three weeks, depending on the species. After the fourth moult they change to a comma-shaped pupa, which is mobile and floats under the surface. The pupal stage lasts only a few hours, in some cases - but up to several days in others - before the adult emerges and flies off after a short time.
Adults rarely travel more than a couple of kilometres from their breeding site and normally remain within 300 to 500 metres. Because of their breathing habits the larvae and pupae can be suffocated by oiling the water surface of breeding sites. This should not be done to natural pools as it will kill most other beneficial water life.
None of the species in this group transmit the malaria parasite. The small, black and white-striped 'bush mosquito' (Aedes aegyptii) is common in our area. Within South Africa it has a similar distribution to Toxorhynchites but extends further south to the western Cape. Its original home is believed to be West Africa but it has been spread by human commerce and is now found throughout the world's tropical and subtropical regions.
It has a decided preference for human blood and is commonly found in and around human dwellings. This is the nuisance species that bites during the day and early evening. They breed easily in water trapped by garden plants and domestic containers. They do not transmit malaria. Further north they are the principle carriers of yellow fever which is endemic to West Africa and the warmer parts of the Americas.
There is no yellow fever in South Africa but with the vector present there needs to be constant watch against the possibility of its spread - hence the need for inoculations to prevent people bringing in the virus from infected areas. Another species that occurs further north in Africa is Aedes caballus - a vector for the Rift Valley Fever which is a serious livestock disease.
This virus can cause blindness and even death in humans. Culex univittatus is the vector, in South Africa, for West Nile and Sindibs virus which affects humans and birds. Epidemics of the virus, after seasons of heavy rainfall, cause fever, rashes and joint and muscle pains in humans but are seldom fatal.
The common 'house mosquitoes' - Culex fatigans (sometimes called - C. quinquefasciata) and Culex pipiens generally breed in any water lying around domestic areas. They readily enter houses where the females will feed on humans or domestic animals. They DO NOT TRANSMIT MALARIA but are vectors for various other diseases, including 'filariasis', which will be discussed in the next article.
Some adult females of this group transmit Malaria. The adults of both sexes can be identified by the way they sit with their tails up at an angle of about 45 degrees to the surface. They generally breed in natural water bodies (rather than in domestic situations) and their eggs are laid singly. Their larvae and pupae develop in similar fashion to those of the Culex species but the larvae lie horizontally - parallel to the water surface.
These mosquitoes bite normally during the hours of darkness and locate their 'prey' through carbon dioxide emissions and body heat and odour. They have the ability to pierce thin clothing. Although a number of Anopheles species are found throughout Africa only a few of these are known to be carriers of malaria.
The rest are comparatively harmless, either because the malaria parasites cannot survive and multiply in the insects, or because they do not enter dwellings and attack human beings but prefer the blood of animals.
The Anopheles gambiae complex, which includes Anopheles arabiensis and Anopheles cinereus are all important malaria vector species. They generally breed in temporary pools exposed to sunlight where they multiply rapidly.
They spread widely after heavy rains and are largely responsible for epidemic outbreaks of malaria. They readily enter houses to feed on human beings after which they
habitually rest on internal walls, where they can be controlled by residual insecticides.
The Anopheles funestus complex are also important transmitters of malaria. They prefer to breed in slow flowing streams and permanent pools of water that are shaded by vegetation. They enter houses where they appear to prefer human blood. This complex is mainly responsible for the endemic malaria in regions where it is found.
Anopheles quadriannulatus of the A. gambiae complex feeds mainly on cattle and other animals and rarely enters human dwellings. It is therefore not an important vector of malaria. Some years ago a member of the Anopheles gambiae complex was carried accidentally from Africa to Brazil where it established and multiplied rapidly, causing one of the worst and most widespread outbreaks of malaria ever known in that country. This is an example of why aeroplanes on international flights are sprayed with insecticides.
On noticing a mosquito - If it sits horizontal to the surface it cannot carry malaria. If it sits at 45 degrees to the surface it is an Anopheles. If it has feathery antennae it is a male and can't bite you. If it is a female Anopheles it could carry malaria if there are infected people within that area.
By Dave Rushworth