Bringing Malaria to the United States

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QUESTION

I was recently diagnosed with Malaria still waiting to find out what strand, but either got it in Uganda or Rwanda. I have recently came back to the states and have been bit by mosquito. I have already started to take some medication for it, but I am concerned about spreading the virus to others in the states including my family. Is it possible that if I still have the infection to spread it to others or in the states? If so what should I do.

ANSWER

There actually used to be malaria in the United States, particularly in the southern areas, but concerted mosquito control efforts as well as public health initiatives eradicated it in the 1950s. I don’t think you need to worry too much about transmitting malaria—once you are on treatment, the number of parasites in your blood drops dramatically and it becomes more difficult to transmit the parasite to a mosquito. Also, there are lots of types of mosquitoes in the US, and only those of the genus Anopheles can transmit malaria. As such, if you are concerned about spreading malaria, you should take precautions to protect yourself from mosquitoes especially at night and at dusk and dawn, as this is when Anopheles mosquitoes are most active. The type of mosquitoes which bite during the day usually belong to the genus Aedes, and cannot transmit malaria. During these high risk times of day, you should take care to wear long-sleeved clothing, and also wear insect repellent, preferably containing DEET.

Malaria Mosquito Classification

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QUESTION

What is the classification of mosquito that cause malaria?

ANSWER

The mosquitoes that cause malaria in humans (and indeed also in all other mammals) all belong to the genus Anopheles. They belong to the family Culicidae, which also includes other disease vectors such as Culex and Aedes, which transmit other diseases such as dengue virus, lymphatic filariasis, West Nile virus and Japanese encephalitis, among many others. The Culicidae are part of the Diptera, or the “true flies” which possess a pair of wings and a pair of halteres. The Diptera are part of the class Insecta, which is found within the phylum Arthropoda, in the Kingdom Animalia.

Anopheles Mosquito

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QUESTION

What does Anopheles looks like?

ANSWER

Mosquitoes of the genus Anopheles are responsible for all transmission of malaria in mammals, including to humans. They are night-feeding mosquitoes, usually biting between dusk and dawn, though they may also be active during the day in heavily shaded environments.

Like all mosquitoes, Anopheles are usually found either as freshwater larvae, which prefer stagnant, still water, or flying adults, of which only the females feed on blood (the males exclusively feed on nectar).  Anopheles larvae lie parallel to the surface of the water where they live (in contrast to Aedes and Culex larvae which hang at an angle), whereas the adults rest with their bodies at a 45 degree angle upwards (again in contrast to Aedes and Culex adults, which rest parallel to their resting surface).

Anopheles Mosquito

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QUESTION

Why is it only the female anopheles mosquito alone can cause malaria but not the male anopheles mosquito or any other mosquitoes?

ANSWER

Malaria is actually caused by a single-celled parasite called Plasmodium—it is transmitted via the bite of a female mosquito, of the genus Anopheles, as she takes a blood meal from a human (or other mammal) host. Male mosquitoes do not feed on blood (they only feed on nectar), whereas females need the nutrients from blood in order to produce their eggs; as such, only female Anopheles transmit mosquito.

Why only Anopheles are able to transmit malaria to humans is interesting—birds and reptiles also can get Plasmodium (though different species than those that infect humans and other mammals), and these kinds of malaria can also be transmitted by other kinds of mosquitoes, such as Aedes and Culex. Other closely related blood parasites can even be transmitted by other flying insects, such as sand flies and black flies. However, it is true that only Anopheles can transmit human malaria.

Sterilizing Mosquitoes to Fight Malaria

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QUESTION

Is it possible to breed mosquitoes in the laboratory and then sterilize them and release them into the environment in order to reduce their rate of reproducing.

ANSWER

That is a very good question, and indeed efforts to genetically modify mosquitoes in order to control the various diseases they transmit are underway in many laboratories across the world.

For almost 15 years, scientists have had the ability to modify mosquitoes so that they are sterile. The aim, as you rightly describe, is then to release these sterile mosquitoes into the wild in order to reduce numbers. If the gene that causes sterility can be passed to future offspring, without any reduction in survival of the insect, then the eventual result will be a total population extinction.

To date, many of the major mosquito disease vector species have been successfully genetically modified, though there are many fewer instances of field testing of these modified insects. For example, in 2000/2001, a World Health Organisation-led project in India created sterile mosquitoes of one species of each of the three main disease vector genera: Culex, Aedes and Anopheles, the latter of which acts as vectors for malaria. However, the project did not, in the end, release any of the modified Anopheles vectors into the wild.

While many scientists applaud the benefits of this approach (such as being very species-specific and being more environmentally friendly than spraying), there are also causes for caution. For example, there are concerns that the loss of mosquitoes in the food chain will have a negative impact on animals that rely on them for food. Similarly, if mosquitoes vanish from an ecosystem, their “niche” may be filled by another organism that is equally or even more dangerous and destructive, such as a crop pest or another disease vector. There is also a worry that changing mosquitoes may have unexpected and dangerous effects on the disease itself, for example forcing it to evolve into a more severe disease or changing its epidemiological patterns in ways we cannot predict in advance.

Finally, not all scientists are convinced that the approach will work in the first place—the sterile mosquitoes will have to survive equally well or better than normal mosquitoes in order to establish in the population, and must be equally or more successful at reproducing. As such, while a lot of money is being poured into GM mosquitoes, it is still the center of vigorous debate.

Perhaps the best indication of this controversy came last year, when Oxitec, a British company, released sterile Aedes aegypti mosquitoes on the Cayman Islands. These mosquitoes are the vectors of dengue fever, and so all eyes are on this study to see whether indeed sterile mosquitoes can survive in a population, and if they do, what other effects they will have longer term on the population size of mosquitoes and the rest of the ecosystem. You can read more information about that here: Oxitec: GM Mosquito Factory.

Mosquito Types

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QUESTION

How many types of mosquito are there?

ANSWER

There are over 3,500 species of mosquito! However, most of these do not transmit any diseases to humans. Mosquitoes are usually divided into two sub-families, the Anophelinae and the Culicinae. The latter group consists of about 40 genera, including Culex and Aedes, which contain some species that transmit diseases to humans (such as yellow fever, dengue fever and West Nile). The former contains the genus Anopheles, which are the mosquitoes that transmit malaria. There are about 460 described species of Anopheles mosquito, of which about 100 can transmit malaria, though the vast bulk of transmission is usually limited to about 30 species.

Malaria and Ross River Fever

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QUESTION

Is malaria similar to Ross river Fever which is common in Australia? If you have Ross River and get bitten by a mosquito can the mosquito pass it on to some one else.

ANSWER

Ross River fever is similar to malaria in that both are transmitted by mosquitoes. However, Ross River fever is caused by a virus, whereas malaria is caused by a protozoan (single-celled) parasite. Also, the type of mosquitoes they use are different. Malaria can only be transmitted by mosquitoes of the genus Anopheles, whereas the Ross River fever virus is usually transmitted by Aedes or Culex mosquitoes.

Another difference is that Ross River virus also infects lots of other mammals, with kangaroos and wallabies key reservoir species in the wild, whereas the species of malaria which infect humans are more or less limited to us (though there have been cases of human malaria infecting closely related animals, such as gorillas and chimpanzees).

As for your other question, as far as I know there is no reason why a mosquito infected with Ross River virus couldn’t bite multiple humans or other mammals, and thus transmit the disease to several new hosts.

Evolution of Malaria

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QUESTION

how did malaria evolve?

ANSWER

Malaria in humans probably evolved independently several times, and both times likely due to a cross-over event from a closely related primate malaria. For example, Plasmodium vivax is evolutionarily closely related to several species of malaria found in macaque monkeys in south-east Asia, and so a cross-over of one of those species into human, with subsequent adaptation and speciation, is one hypothesis as to the origin of P. vivax. Conversely, some people argue that P. vivax emerged in Africa, due to the high prevalence of certain genetic traits in African populations (such as the Duffy negative antigen), which protect against P. vivax.

In contrast, P. falciparum is agreed to have emerged in sub-Saharan Africa, and likely in the Congo basin, though the exact source of its origin has been under recent scientific dispute. Until 2010, it was thought that P. falciparum had crossed over to humans from chimpanzees, as chimps are known to be infected with P. reichenowi, a species very closely related to P. falciparum. However, a paper was published in 2010 which had sampled Plasmodium parasites of gorillas and revealed new species of Plasmodium which are even more closely related to P. falciparum, suggesting that the cross-over occurred from gorillas to humans.

As you can see, humans are not the only primates to get malaria; many species of monkey and ape are also susceptible to Plasmodium species, and even lemurs have their own suite of Plasmodium parasites. Among the mammals, rodents also can get malaria, and bats are infected with Hepatocystis, a malaria-like parasite which also infects hippos, primates and rodents. However, no other species of mammal appears to be susceptible to Plasmodium/Hepatocystis, and the reasons for this are not entirely clear.

Plasmodium probably crossed over to mammals from birds or lizards, both of which are infected with a vast number of species of Plasmodium. It is unclear in which of these groups Plasmodium first emerged, though it likely evolved originally from another type of blood-borne parasite called Leucocytozoon, which infects birds and uses blackflies (genus Simulium) as vectors.

A sister group to Plasmodium, called Haemoproteus, also evolved from Leucocytozoon but utilises a variety of different vectors, including mosquitoes, biting midges (Culicoides), louse flies (Hippoboscidae) and tabanids (Tabanidae). Plasmodium, by contrast, exclusively uses mosquitoes as its vectors (apart from one species of lizard Plasmodium, P. mexicanum, which uses sandflies), but while mammalian Plasmodium is only transmitted by Anopheles mosquitoes, bird and lizard Plasmodium can be transmitted by Culex, Aedes, Culiseta, Anopheles, Mansonia and Psorophora. As such, understanding the patterns of vector and host switches within Plasmodium and related taxa can actually provide interesting insights into the genus’ evolutionary history.

What are mosquito larvae?

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QUESTION

What are mosquito larvae?

ANSWER

Larvae are one of the life stages of mosquitoes; they are baby mosquitoes, if you will. Adult mosquitoes lay eggs as a “raft” on the surface of a body of fresh water—they prefer still and stagnant pools. These eggs then hatch into the mosquito larvae, which live in the freshwater pool until they form a pupa, just under the surface. These pupae then hatch into adults again, completing the life cycle.

Mosquito larvae are omnivorous, eating algae and small organisms also living in the water. Despite living immersed in water, they require oxygen to breathe, which they inhale using two different methods: Aedes and Culex mosquitoes (the vectors of a number of diseases, including West Nile disease, dengue fever, yellow fever, encephalitis and filarisasis) have a specialised breathing organ, a bit like a snorkel, called a siphon, which they use to suck in air, whereas Anopheles mosquitoes (the main vectors of malaria) lack this organ and so have to lie next to the surface to take in air. The larvae moult four times while they live in water; after the fourth time, they are ready to pupate and become adults. The entire larval stage of a mosquito’s life usually take between one and two weeks, depending on the ambient temperature.