Biology, diseases and control of Culex mosquitoes

LISA O’HALLORAN: Hello. My name is Lisa O’Halloran, and I’m a research assistant at ARCTEC at the London School of Hygiene and Tropical Medicine. I will be taking you through the step on biology and control of Culex mosquitoes. During this step of the course, we will go over the basic biology of Culex mosquitoes and their traits as medically important vectors. We will then look at how these traits may be problematic or exploited for vector control. Like many insects, the taxonomy of Culex mosquitoes is intricate and widely debated. Culex mosquitoes belong to the subfamily Culicine, which also includes Aedes, Mansonia, Haemogogus, and Sabethes mosquitoes. Approximately 770 species of Culex are described and grouped into 26 subgenera.

Within the sub genus Culex linnaeus exists the medically important subcomplexes pipiens and vishnui. The pipiens subcomplex contains four closely-related species, Culex pipiens, Culex quinquefasciatus, Culex australicus, and Culex globocoxitus. Culex pipiens is further divided into two forms, Culex pipiens pipiens and Culex pipiens pallens, although there is a debate on whether these are two separate species. Culex pipiens pipiens also has another epidemiologically distinct form, Culex pipiens molestus. The vishnui subcomplex includes important factors such as Culex vishnui, Culex tritaeniorhynchus, and Culex pseudovishnui. Culx mosquitoes will lay eggs in highly polluted waters, although they will also lay in fresher, clearer, open water sources. Culex eggs are laid upright in distinct rafts of up to 300 eggs on the surface of the water.

Larvae will hatch from eggs within 24 to 48 hours of being laid. The first level instars, L1, are 1 to 1.5 millimetres in size. The morphology of Culex larvae allows us to differentiate them from Anophelene larvae, as they have a respiratory syphon, and therefore rest at an angle to the water’s surface. They also differ from Aedes and Mansonia in the size and shape of their syphon. Culex undergo four larval instar stages before developing into pupae. The pupal stage can last up to two days. Pupae do not feed, but breathe through respiratory trumpets, which can be seen in the photograph on the left. The first question when identifying an adult mosquito is whether it is Culicine or Anophelene.

Female Culicine mosquitoes, as seen on the right, have shorts palps, while male Culicine mosquitoes have long, unclubbed palps. Anophelene mosquitoes have palps as long as their proboscis, and male palps are clubbed at the ends. Anopheles also rest at a 45 degree angle to the surface, while Culicines rest parallel. Culex mosquitoes are typically a dull brown colour, and though some have markings on the legs, most do not display the dark and light wing patterns as seen in Anophelenes. Culex mosquitoes can also be recognised by their blunt abdomen. Culex females are small to medium in size, with the body coated in thin and broad scales. In some species, a pale ring around the proboscis can also be seen.

Culex mosquitoes have a cosmopolitan distribution. This map shows the range of the Culex pipiens subcomplex across the world. These mosquitoes vector diseases such as lymphatic filariasis, Japanese encephalitis, and West Nile virus. This map shows the distribution of a species within the Culex vishnui subcomplex, which is present across subtropical and tropical regions, notably India and Southeast Asian countries. This complex is important in the transmission of Japanese encephalitis. This map shows the distribution of a species within the Culex vishnui subcomplex, which is present across subtropical and tropical regions, notably India and Southeast Asian countries. This complex is important in the transmission of Japanese encephalitis.

Culex larval habitats are known to be diverse, and can include a variety of water sources, which may be found in both urban and rural settings. Urbanisation with a lack of sanitary infrastructure increases these larval habitats, such as pit latrines and open septic tanks. Agricultural runoff may provide breeding grounds for Culex, and so education about this is very important. Culex species which breed in the trains can be targeted with expanded polystyrene beads, which reduce surface water exposure and egg laying. Culex species living in anaerobic waters could not be targeted by biological control, as biocontrol species, such as fish, could not withstand these conditions. Feeding and resting behaviour is important to target vector control activities.

Mosquitoes of medical importance are often highly anthropophilic and present around indoor dwellings and human settlements. They may therefore bite and rest indoors, although some species are known to bite outdoors, as well. Some mosquitoes prefer to feed on animal hosts and are referred to as zoophilic. As well as being a biting nuisance, Culex mosquitoes also spread some serious diseases. West Nile virus naturally occurs between mosquitoes, namely Culex species and births. It can also affect humans, horses, and other mammals, although it is rarely fatal. Japanese encephalitis is closely related to West Nile virus and St. Louis encephalitis. It is transmitted to humans through the bites of Culex mosquitoes. The virus cycle is maintained between mosquitoes, vertebrates, mainly pigs, and wading birds.

Transmission is primarily in rural agricultural areas associated with irrigation flooding and rice fields. In some parts of Asia, these are situated near urban areas. In temperate climates around Asia transmission, of Japanese encephalitis is seasonal, peaking in the summer and autumn. In the tropics transmission can be all year round, peaking in the monsoon season. St. Louis encephalitis is also maintained in the peridomestic cycle by mosquitoes feeding on birds and spreading the virus. Humans and other mammals can become infected, but are dead end hosts. Lymphatic filariasis is a worm infection which is transmitted when microfilariae within infected mosquitoes break through mosquito mouth parts and bury into the host’s skin.

They then grow into adult worms, where they can migrate to and block the lymphatic system. Microfilariae can then circulate in the blood, where they can be picked up again during another blood meal. Culex mosquitoes are not the only vectors of Bancroftian LF. Aedes, Anopheles, and Mansonia are also vectors. Some Culex species also vector diseases in animals, such as Venezuelan equine encephalitis and dog heartworm. Diseases are transmitted from an infected host to a mosquito when the mosquito takes an infected blood meal. Pathogens from the blood meal replicate inside the mosquitoes, then migrate to the salivary glands to be inoculated into the next host’s blood when bitten, or they are mechanically transmitted on the mouth parts of the mosquito.

A mosquito ingests the microfilariae during a blood meal. After ingestion, the microfilariae lose their sheathes, and some of them work their way through the wall of the proventriculus and cardiac portion of the mosquito’s midgut and reach the thoracic muscles. There, the microfilariae develop into first stage larvae, and subsequently into third stage infective larvae. The third stage infective larvae migrate through the hemocele to the mosquito’s proboscis, and can infect another human when the mosquito takes a blood meal. Due to their tendency to bite at night, and often indoors, insecticide-treated nets are suitable means of control for Culex. Insecticides can sometimes be problematic to use against adults or for larvaciding.

This is because resistance has been reported in some species, Including Culex quinquefasciatis, towards DDT, pyretrhoids, malatheon, deltamethrin treated net materials, as well as Tanaffos. House screening is also a method used to control mosquito house entry. However, papers have found these methods to work better against Aenopheles, as opposed to Culex. While predacious fish can successfully reduce larval abundance, they cannot survive in highly polluted environments, where Culex species can thrive. Therefore, they are a good form of integrated vector control to use in conjunction with other methods. Expanding polystyrene beads are also used for Culex lacks control to reduce egg laying in open water sources. In this step, we have explored the biology and medical importance of Culex mosquitoes.

We have also looked at aspects of their biology and behaviour, which can be exploited for vector control. Here is a summary of the characteristics of Culex mosquitoes and how they inform choices of control method. Examples of these traits include the tendency of some species, especially within the Culex pipiens complex, to rest and bite indoors. This allows them to be targeted with indoor residual spraying of insecticides. The ability for Culex larvae to survive in very polluted waters allows us to use larval control methods like polystyrene beads to reduce their numbers. Methods like these are important, as Culex populations in areas of heavy insecticide use are becoming resistant to both larvacidal and adulticide insecticides.

What traits does your target species have, and how might these traits lend themselves to vector control methods.