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Epidemiology

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What Is Epidemiology? | Factors That Determine The Occurrence of Malaria | Climate | Anopheles Mosquitoes | Humans | Parasites | Areas Where Malaria Is No Longer Endemic

Factors That Determine The Occurrence of Malaria

Factors that determine the occurrence of malaria are those that influence the three components of the malaria life cycle:

• Anopheles mosquitoes must be present, which are in contact with humans, and in which the parasites can complete the "invertebrate host" half of their life cycle
• Humans must be present, who are in contact with Anopheles mosquitoes, and in whom the parasites can complete the "vertebrate host" half of their life cycle
• Malaria parasites must be present.

Notes:

• In rare cases malaria parasites can be transmitted from one person to another without requiring passage through a mosquito (from mother to child in "congenital malaria", or through transfusion, organ transplantation or shared needles)
• Some species of monkeys and apes can be experimentally infected with some of the four "human" malaria parasite species, and in nature some apes are infected with parasites similar to those infecting humans. But for all practical purposes the role of an animal reservoir is negligible.

Climate

Climate can influence all three components of the life cycle. It is thus a key determinant in the geographic distribution and the seasonality of malaria.

Rainfall can create collections of water ("breeding sites") where Anopheles eggs are deposited, and larvae and pupae develop into adulthood, a process that takes approximately 9-12 days in tropical areas. Such breeding sites may dry up prematurely in the absence of further rainfall, or conversely they can be flushed and destroyed by excessive rains.

Once adult mosquitoes have emerged, the ambient temperature, humidity, and rains will determine their chances of survival. To transmit malaria successfully, female Anopheles must survive long enough after they have become infected (through a blood meal on an infected human) to allow the parasites they now harbor to complete their growth cycle ("extrinsic" cycle). That cycle takes 9-21 days at 25°C or 77°F. Warmer ambient temperatures shorten the duration of the extrinsic cycle, thus increasing the chances of transmission. Conversely, below a minimum ambient temperature (15°C or 59°F for Plasmodium vivax, 20°C or 68°F for P. falciparum), the extrinsic cycle cannot be completed and malaria cannot be transmitted. This explains in part why malaria transmission is greater in warmer areas of the globe (tropical and semitropical areas and lower altitudes), particularly for P. falciparum. It has been speculated that current trends of global warming may increase the geographic range of malaria and may be responsible for malaria epidemics.

Climate also determines human behaviors that may increase contact with Anopheles mosquitoes between dusk and dawn, when the Anopheles are most active. Hot weather may encourage people to sleep outdoors or discourage them from using bed nets. During harvest seasons, agricultural workers might sleep in the fields or nearby locales, without protection against mosquito bites.

Anopheles Mosquitoes

What types (species) of Anopheles are present in an area at a given time will influence the intensity of malaria transmission. Not all Anopheles are equally good "vectors" for transmitting malaria from one person to another. Some species are biologically unable to carry human malaria parasites, while others are readily infected and produce large numbers of sporozoites (the parasite stage that is infective to humans).

The anthropophilic Anopheles gambiae is an extremely effective vector and is one of the reasons why malaria is so prevalent in Africa.

Different Anopheles species may differ in selected behavior traits, with important consequences on their abilities as malaria vectors. In some species the females prefer to get their blood meals from humans ("anthropophilic") while in others they prefer animals ("zoophilic"). Some species prefer to bite indoors ("endophagic"), and others prefer outdoor biting ("exophagic"). All other factors being equal, the anthropophilic, endophagic species will have more frequent contacts with humans and thus will be more effective malaria vectors.

Some species prefer to rest inside the dwellings where they have just obtained their blood meals ("endophilic") while others prefer to rest outdoors ("exophilic"). The endophilic species will be more likely to acquire lethal doses of insecticides sprayed on the walls of the dwellings (a malaria control measure called "indoor residual spraying") while this will not be the case for the exophilic species.

An important biologic factor is insecticide resistance. If the mosquitoes are resistant to the insecticide(s) used locally for spraying or for treating bed nets, these measures will be ineffective in curtailing transmission.

See also: Anopheles Mosquitoes

Humans

Biologic characteristics (inborn and acquired) and behavioral traits can influence an individual's malaria risk and, on a larger scale, the intensity of transmission in a population.

more: Human Factors And Epidemiology of Malaria

Parasites

Plasmodium falciparum predominates in Africa south of the Sahara, one reason why malaria is so severe in that area.

Characteristics of the malaria parasite can influence the occurrence of malaria and its impact on human populations:

• Areas where P. falciparum predominates (such as Africa south of the Sahara) will suffer more disease and death than areas where other species predominate
• P. vivax and P. ovale have stages ("hypnozoites") that can remain dormant in the liver cells for extended periods of time (months to years) before reactivating and invading the blood. Such relapses can result in resumption of transmission after apparently successful control efforts, or can introduce malaria in an area that was malaria-free
• Various strains of parasites can occur within the same species. Variant strains of P. falciparum are considered one cause for repeated episodes of malaria because immunity built against one strain is not necessarily protective against another
• P. falciparum (and to a lesser extent P. vivax) have developed strains that are resistant to antimalarial drugs. Such strains are not uniformly distributed. Constant monitoring of the susceptibility of these two parasite species to drugs used locally is critical to ensure effective treatment and successful control efforts. Travelers to malaria-risk areas should use for prevention only those drugs that will be protective in the areas to be visited.

Areas Where Malaria Is No Longer Endemic

Malaria transmission has been eliminated in many countries of the world, including the United States and countries of western Europe. However, cases of malaria still occur in these countries, mostly in returning travelers or immigrants ("imported malaria"). All patients must be diagnosed and treated promptly for their own benefit but also to prevent the reintroduction of malaria.

more: Areas Where Malaria Is No Longer Endemic