Anopheles

Larva

Anopheles is a genus of mosquito in the family Culicidae that inhabits practically the entire world including Europe, Africa, Asia, America and Oceania, with special intensity in the temperate, tropical and subtropical zones. There are 465 formally recognized species of Anopheles, of which 50 can transmit the four different species of parasites of the genus Plasmodium, which cause human malaria (since there are many others Plasmodia species that cause malaria in mice (P. bergei), birds (P. gallinaceum), apes (P. yoeli), etc.). The species Anopheles gambiae is one of the best known, because it transmits the most dangerous plasmodium, Plasmodium falciparum. In Europe one of the most important species is Anopheles atroparvus.

Life Cycle

Like all mosquitoes, Anopheles go through four stages: egg, larva, pupa, and adult. The first 3 stages take place in an aquatic environment, between 5 and 14 days, depending on the species and environmental factors such as temperature. It is in the adult stage, and only in the case of females, that the mosquito acts as a malaria vector. Adult females can live up to a month (somewhat more in captivity), and it is natural not to exceed 2 weeks of life.

Eggs

Adult females lay between 50 and 200 eggs. The eggs are deposited one by one on the water. They are characterized by having a kind of floats on both sides. Eggs do not withstand dryness. After 2-3 days they hatch and a larva comes out of each one. Hatching can take longer (up to 2-3 weeks) if the temperature is low.

Larvae

Mosquito larvae have a developed and prominent head from which a kind of whiskers are born that they use to feed; thorax and abdomen (without legs). Unlike many other mosquitoes, Anopheles larvae do not have a respiratory siphon, which is why they need to have their bodies parallel to the surface of the water. The larvae breathe through spiracles located on the eighth abdominal segment. Since they need to breathe regularly, they periodically rise to the surface. Due to this lack of respiratory siphon, it is enough to add a thin film of immiscible oil to the water to exterminate a larval population of the insect in that phase of its life. The larvae feed on algae, bacteria, and other microorganisms on the surface. Only occasionally do they descend to the bottom. To dive they use sudden or jerky movements, or use their mouth whiskers as propellers.

Pupae

The pupa presents a "comma" appearance, when viewed from the side. The head and thorax merge into a cephalothorax, and the abdomen curves under it. Periodically they ascend to the surface to breathe, thanks to the organs that they have in the cephalothorax. After a few days of metamorphosis, the dorsal part of the cephalothorax breaks and the adult mosquito emerges from it.

Adults

The time that elapses between the deposition of the egg and the appearance of an adult mosquito usually varies according to the species and is greatly influenced by the environmental temperature. Under natural conditions the process takes 10-14 days, but it can be accelerated and last only 5 days.

Like all mosquitoes, adults have a body divided into a head, thorax, and abdomen.

• The head has obviously specialized for feeding, and also for the catching of signals. In the head stands a pair of long and very segmented antennas. Thanks to the antennas, the mosquito can detect the smell of the hosts or the smell of a place in which to deposit the eggs. In addition to the antennas, the eyes are distinguished, the long and outstanding prognosis used to feed, and two sensory palps as long as the prognosis. The eyes of the insect can also "see" in the region of the near infrared, detecting by heat the victims more than by sight itself.

• The chest is specialized to move, as it is attached the three pairs of legs and the pair of wings.

• The abdomen contains specialized organs for digestion and for the development of eggs in the case of females. After feeding, the mosquito's abdomen increases considerably in size.

The Anopheles can be distinguished from other genera by their palps, or by the presence of scales on the wings. Although without a doubt, the best way to easily distinguish it is by observing its resting posture, since its abdomen is steeply arranged, as opposed to the abdomen parallel to the resting surface that is observed in other genera.

Adult mosquitoes usually mate within a few days of hatching from the pupa. In most species, males swarm together, usually around sunset, and females fly to swarms to mate with one or more males.

Males usually do not live more than a week, during which they feed on nectar and other sources of sugar. The females, in addition to sugar, need a source of protein to develop the eggs: blood. After a blood meal, the female rests for a few days digesting while the eggs develop with the extracted nutrients. In 2-3 days, the eggs are already developed enough to be deposited. After spawning, the cycle repeats itself until the death of the female, which occurs after 7-14 days (or more, in captivity).

Ecology and behavior

Understanding the biology of Anopheles can help to understand how malaria is transmitted and can therefore help in designing more effective control strategies. Some factors that affect the ability of the mosquito to transmit malaria are its innate susceptibility to Plasmodium, the availability of hosts to feed on, its longevity and factors that affect it, susceptibility to insecticides, existence of spawning grounds, etc.

Not all Anopheles are transmitters of the plasmodium that causes malaria, since it does not develop well (or at all) inside them in all species (or even in individuals of a species in the one that does develop). Strains of A. gambiae that encapsulate and kill the parasite once it has invaded their stomach wall; While waiting to find out the molecular basis of this process, there is speculation about the release of genetically modified mosquitoes that do not transmit malaria, with the intention of gradually replacing the wild strains that do.

It is known that in Africa the malaria transmitters Anopheles gambiae and Anopheles funestus have significant anthropophilia, that is, that humans are their main source of nutrition. Therefore, little can be remedied by controlling animal hosts, except in cases of proven bestiality by the vector.

Most Anopheles are crepuscular or nocturnal. Some feed inside homes (endophagy), while others prefer to feed outside (exophagy), and after ingesting blood, some mosquitoes prefer to rest inside (endophilia) and others outside (exophilia); however, this pattern of behavior changes depending on the geographic location, the characteristics of the home, and the microclimatic conditions. The use of impregnated insecticide nets arranged around beds as a physical barrier and screens that prevent mosquito access through windows are two proven methods that significantly reduce cases of bites. Anopheles of the nocturnal-endophageal type. Endophilic mosquitoes can best be controlled by spraying the interior with insecticide. On the other hand, the exophagic and exophilic are better controlled by destroying the breeding areas.

Although it is little known, in Europe there was malaria transmission until the middle of the 20th century and thus, for example, in Spain the eradication was not certified until 1964. In the Mediterranean countries, the most important species in the transmission of this disease were An. sacharovi, An. atroparvus and An. labranchiae. Control measures using insecticides, such as indoor spraying or protective netting, are the main way to fight mosquitoes and thus malaria. However, the indiscriminate and unjustified use of these substances has created in many situations a resistance to the insecticide (similar to the resistance to antibiotics of some pathogenic bacteria).

Some species of Anopheles can also serve as vectors for the heartworms Dirofilaria immitis (causing canine heartworm), Wuchereria bancrofti and Brugia malayi.

Medical significance

Malaria

As the only mosquitoes that transmit malaria, the genus Anopheles is of global medical importance. This genus can transmit Plasmodium falciparum, P. vivax, P. ovale and P. malariae. Recent studies have shown a fifth species P. knowlesi, known to infect macaque macaques (Macaca) can also infect humans.

Other parasites

Some species of Anopheles can transmit threadworms of Wuchereria bancrofti, Brugia malayi and Brugia timori which cause lymphatic filariasis in humans, being W. bancrofti the most prevalent and distributed.

B. timori is known to be found only in the islands of eastern Indonesia, affects one million people, and is only vectored by An. barbirostris. B. timori is characterized by being rural, since the hatcheries of An. barbirostris are found in rice-growing areas. B. malayi, instead it affects 12 million people and is transmitted by Anopheles spp. The breeding sites of this vector are related to freshwater bodies in rural areas. W. bancrofti affects more than 100 million people, Anopheles spp. is the main vector in Africa, New Guinea and parts of southern Asia. One of the reasons why it is so prevalent is that it can be transmitted not only by Anopheles spp., but also by Aedes and Culex.

Arboviruses

In addition to these parasites, Anopheles has been found to transmit around 20 viruses, one of which is the cause of O'nyong'nyong fever, associated with the an. gambiae and An. funestus.

Control methods

Control methods in Anopheles have focused on the different stages of the mosquito's life cycle to reduce its population. For the larval stage, the most common methods are the use of growth regulators. These may be pyriproxyfen and Bacilulus thuringiensis var. israelensis. Its application is direct in hatcheries. Larval predatory fish belonging to the Gambusia species have also been used.

Most control methods have been developed for adult mosquitoes. The use of residual insecticides inside houses and impregnated awnings with insecticides are the main methods used against Anopheles. The use of residual insecticides is wide and consists of applying on walls, ceilings and floors, DDT has been widely used with this method. Bed nets are physical barriers that prevent vector contact with humans, in addition to they are impregnated with insecticide. These barriers were able to reduce morbidity in children by up to 50% and their mortality by 20-30%.