Malaria mosquitoes can rightly be considered the world's most dangerous animal, causing 400,000 - 700,000 deaths per year worldwide.
Malaria is transmitted to humans via the female mosquitoes of the Malaria mosquito genus (Anopheles). Female mosquitoes suck blood in order to produce eggs, and these blood suckings are the link between humans, mosquitoes and malaria parasites.
Malaria parasites live inside malaria mosquitoes and are the cause of human malaria. For the parasites to develop inside the mosquito, the environment must have the right temperature and humidity. Unlike humans, mosquitoes are not particularly harmed by the presence of parasites in their bodies.
Malaria mosquitoes kill between 400,000 and 700,000 people annually - and before 2000, the number was much higher
Which mosquitoes can transmit malaria?
There are around 3500 mosquito species in the world, which are divided into 41 genera. Humans can only contract malaria by being bitten by female mosquitoes of the Malaria mosquito genus.
However, it should be noted that there are around 430 species of mosquitoes in the Malaria mosquito genus and only 41 of these are malaria hosts and can transmit the malaria parasite (which ultimately results in the disease malaria in humans).
Malaria mosquito hosts
Although most species of malaria mosquitoes can infect both animals and humans with malaria, there are some species that can only infect animals and some that only infect humans.
In Africa - the continent with the most extensive malaria problems in the world - the Anopheles gambiae and Anopheles funestus the two most widespread species of malaria mosquitoes, and both species have in common that they primarily infect humans. The two species are considered the most effective malaria hosts in the world.
Malaria's route of transmission
The malaria mosquito must suck blood from a malaria-infected host in order to transmit the malaria parasite to new hosts (and thereby infect them with malaria). Some mosquito species are better intermediate hosts than others - and in some, the malaria parasite cannot survive at all.
Species A. gambiae for example, has an immune response that encapsulates and kills the parasites in the mosquito's abdominal wall. Researchers are now investigating the genetics behind this mechanism in the hope of using it to fight malaria.
The challenges of the malaria parasite
Among the species that can transmit malaria, the following criteria must be met for transmission to take place:
- Immune systemAs mentioned above, some mosquito species have complex defense mechanisms against parasites. This limits the survivability of parasites when they are swallowed by a mosquito.
- Number of: For the malaria parasite to spread effectively, there must be a sufficient number of intermediate hosts (malaria mosquitoes) and hosts (humans and animals) in the local area.
- LifetimeOnce the malaria parasites are ingested by a mosquito, they need time to develop in the mosquito before they can be transmitted to humans. The duration of this period is 10 - 21 days and depends on the species of parasite and the temperature of the environment. Given that adult female mosquitoes are likely to survive only a few weeks in the wild, this doesn't leave the parasite much time to be transferred to a new host. In other words, the mosquitoes need to survive long enough to infect a new host with malaria.
- Capacity: Each individual mosquito must carry a sufficient number of parasites in their saliva to infect a new host.
- Contact with peopleMalaria mosquitoes need to live close to humans in order to infect new hosts.
The spread of malaria mosquitoes
Malaria mosquitoes are found all over the world - with the exception of the South Pole (Antarctica) - but not all places infect humans. This is due to two things:
- Different species live in different places (as mentioned, only around 8 - 10% of the malaria mosquito species can transmit malaria to humans)
- The malaria parasite is eradicated in many places (even if the malaria mosquitoes are present, the disease is eradicated)
In Denmark, for example, there are 3 species of malaria mosquitoes, but in practice they do not infect humans with malaria because the disease (i.e. the malaria parasite) has been eradicated since 1946.
The places where there is a risk of contracting malaria are generally divided into two groups:
- Epidemic areas (i.e. places where humans are relatively infected with malaria - in these areas the risk of infection is high)
- Former epidemic areas (i.e. places where malaria is considered eradicated, but where there is always a risk of reintroduction of malaria-carrying mosquitoes)
The map shows the distribution of the different species of malaria mosquitoes in the world (the map comes from cdc)
Life cycle
Like all mosquitoes, the life cycle of malaria mosquitoes is divided into 4 stages:
- Egg stage
- Larval stage
- Puppet stage
- Adult stage
The first 3 stages take place underwater and last 5-14 days depending on the species and temperature. The adult stage starts when the female mosquito becomes host to the malaria parasite.
The period from egg to adult varies greatly from species to species and is also highly dependent on temperature. Malaria mosquitoes can develop in as little as 5 days, but the development time is usually 10-14 days in tropical climates.
Egg stage
Adult female mosquitoes lay 50 - 200 eggs per laying. The eggs are approximately 0.5 x 0.2 mm. They are laid directly on water and are unique in that they have floating bodies on both sides. The eggs are not drought resistant. They usually hatch within 2-3 days, but in colder climates it can take up to 2-3 weeks.
The eggs can be laid in a variety of habitats, but most species prefer clean, uncontaminated water. Malaria mosquito eggs and larvae have been found in freshwater and saltwater marshes, mangrove swamps, rice fields, grassy ditches, the sides of streams and rivers and in puddles. Many of the species prefer habitats with vegetation, while others prefer habitats without vegetation.
Larval stage
Mosquito larvae have well-developed heads with "bristles" on the mouthparts that are used for feeding. The larvae are legless with a segmented abdomen.
The larvae of most mosquito species breathe underwater using a tube-like respiratory organ, which is the larva's only connection to the water surface. Malaria mosquito larvae, however, do not have this organ, but instead float on the surface of the water (so their bodies are parallel to the water surface). They breathe via the 8th segment of their abdomen and therefore often have to rise to the water's surface.
The larvae spend most of their time feeding on algae, bacteria and other microorganisms in the surface water. If disturbed, the larvae dive below the surface. The larvae can swim both by wiggling their bodies and by propulsion generated by their brush-like mouthparts.
The larval stage is divided into 4 stages, all of which end in a molt where the larva sheds its exoskeleton.
Puppet stage
The pupa is shaped like a comma when viewed from the side. The head and upper body are joined together in a cephalothorax (a fused joint), while the abdomen protrudes underneath.
Like the larvae, pupae often have to rise to the surface of the water to breathe. Breathing itself is done via a pair of trumpet-like respiratory organs located on the cephalothorax. After a few days in the pupal stage, the dorsal part of the cephalothorax splits and the adult mosquito emerges.
Adult stage
Like all mosquitoes, malaria mosquitoes' bodies are divided into 3 sections: head, torso and abdomen.
Malaria mosquitoes can be distinguished from other mosquitoes by their palps (sensory organs around their mouthparts), which are as long as their proboscis (which they use to suck blood). Another distinguishing feature is their wings, which have blocks of white and black scales. Then there's their resting position: malaria mosquitoes' abdomens face upwards (end up), whereas other mosquitoes lie roughly parallel to the surface they're sitting on.
Adult mosquitoes usually mate within a few days of pupal emergence. In most species, males form large swarms that females fly into to mate.
Males live for a week or so. They feed on nectar and other sugar sources. Females also consume sugar for energy but need blood to develop eggs. Once the female mosquito has consumed a full blood meal, she rests for a few days to digest the blood and develop the eggs. The duration of this process depends on the temperature, but in tropical climates it usually takes 2-3 days. Once the eggs are fully developed, the female lays them and then seeks a new host.
This process repeats until the female dies. Adult female mosquitoes can live for a month or more in captivity, but probably only live around 1-2 weeks in the wild (it's not possible to measure their lifespan in the wild). Their chances of survival depend on temperatures and humidity, but also their ability to suck blood without being killed by their host. In addition, adult malaria mosquitoes also have their predators, of course.
Fighting malaria
Fighting malaria and malaria mosquitoes has long been a high priority globally. This is especially true in Central and South Africa, where millions of people have lost their lives to the disease.
Malaria is an ancient disease, but it was possible to eradicate it in most of the world (Europe, North America, the Caribbean, parts of Asia and the southern part of Central America) through a series of eradication campaigns in the 1940s. However, this has never been successful in Africa as it is not an easy task.
It's worth noting that effective control doesn't necessarily mean eradicating all malaria mosquitoes (which is very difficult), but rather eradicating the malaria parasite - as is the case in Europe and North America, for example.
Avoid malaria mosquito bites
If you are traveling to a country or area where there is a risk of contracting malaria, you should consult a doctor who knows about travel medicine first. They will typically provide you with anti-malarial medication, but also advice on how to minimize your risk of contracting malaria and other local diseases. The doctor will also be able to tell you about the symptoms of malaria, treatment, diagnosis, etc,