Infez Med. Dec;9(4) [Relationship between malaria, environment, people and civilisation in central Italy. Reclamation of the Fucino plain]. [Article in . Clinical malaria has proven an elusive burden to enumerate. There is a need, therefore, to define a relationship between clinical incidence. Correction for Martin et al., Evolution of human–chimpanzee differences in malaria susceptibility: Relationship to human genetic loss of.
In terms of spatial dimensions, the movements to and from malarious areas are of epidemiologic importance. People who move can be categorized as either active transmitters or passive acquirers 2. Active transmitters harbor the parasite and transmit the disease when they move to areas of low or sporadic transmission. Passive acquirers are exposed to the disease through movement from one environment to another; they may have low-level immunity or may be nonimmune, which increases their risk for disease.
Based on the above definitions, a typology can be devised that identifies categories of population movement. Different activities can be associated with these categories, and these activities, in turn, can be associated with differing risks for malaria transmission. All types of population movement can be accommodated in this typology, and people may exhibit more than one type of mobility.
Population Movement and Malaria In developing countries, activities involving population movement include urbanization, colonization, labor related to agriculture and mining, and conflict. In industrialized countries, the impact of population movements on malaria risk is mainly related to intercontinental travel.
In some regions, malaria risk may increase as a result of a combination of different forms of mobility, as well as other factors unrelated to population movements.
For example, in the African highlands, many of the issues described below act concurrently The categorization of the various examples below simplifies a complex issue but is useful for indicating major processes related to mobility and malaria risk.
Urbanization The world's urban population is growing at four times the rate of the rural population Urban pull is prevalent throughout the developing world, with rural-to-urban migration taking place faster than ever before Sub-Saharan Africa is the most rapidly urbanizing region in the world 13and the urban population in India has doubled in the last 2 decades When accompanied by adequate housing and sanitation, urbanization can lead to a decrease in malaria through reductions in human-vector contact and vector breeding sites.
Malaria parasite interactions with the human host.
However, in developing countries, rapid, unregulated urbanization often leads to an increase in or resumption of malaria transmission because of poor housing and sanitation, lack of proper drainage of surface water, and use of unprotected water reservoirs that increase human-vector contact and vector breeding.
Although water pollution in urban areas usually leads to decreases in vector populations, some vectors, such as An.
In urban areas of India, water is not supplied regularly and is stored in houses, providing extensive breeding places for An. In India, the fact that the National Malaria Eradication Program concentrated only on rural areas, ignoring the problem of urban malaria, was one of the factors leading to a resurgence of malaria in the s Several types of population movement contributed to malaria transmission in India.
First, circulation from stable rural malaria areas to unstable urban areas had firmly established malaria transmission in urban areas. Then, after the National Malaria Eradication Program, rural areas became free of endemic malaria but were receptive, so circulation from urban areas back to rural areas reintroduced malaria transmission.
Changes in vector behavior exophilic and exophagic behavior limiting the effectiveness of sprayingvector resistance to insecticides, and increasing drug resistance, especially in Plasmodium falciparum 14also played a role. Population movement also contributed to drug resistance, with people of different immune status moving from endemic- to nonendemic-disease areas, accelerating transmission of resistant strains. Colonization of New Territory Through the interaction of a number of factors, the colonization of unpopulated or sparsely populated areas may be accompanied by an increase in malaria.
Settlers, who have low-level immunity or are nonimmune, may migrate into a disease-endemic area, spreading the disease. Initially, housing tends to be basic, leading to close human-vector contact. Moreover, housing is often near rivers or lakes to facilitate water collection, increasing the exposure of humans to mosquitoes.
Activities to develop an area, such as deforestation and irrigation, can increase the number of vector breeding sites, contributing to an increase in malaria. Colonization may be accompanied by major building projects, such as dams, canals, highways, or mining activities--referred to as the tropical aggregation of labor--which can further enhance malaria transmission.
Reemergence of malaria through mobility related to colonization occurred in Brazil. Malaria had been practically eradicated from most areas of the Amazon region by the national malaria campaign in the s and s Since the s, however, the incidence of malaria has increased dramatically because of massive population movements to colonize new territory.
New highways were built in the s, linking the Amazon region to the rest of the country and attracting laborers to work on road construction. In the s, many more people were attracted to the region by agricultural settlements and hydroelectric projects. Finally, in the s, the discovery of gold led to a greater influx of people, along with the establishment of hundreds of mines throughout the region.
The types of population movement involved in the colonization of the Amazons are migration and long-term circulation from malaria-free areas of Brazil to the malaria-endemic Amazon region.
The people involved are nonimmune passive acquirers who on becoming infected can become active transmitters.
Malaria parasite interactions with the human host.
If these active transmitters return to their initial place of residence in a malaria-free but highly receptive area, they can reintroduce the parasite and initiate an outbreak of malaria. For example, in26 new active foci of malaria were recorded in Brazilian states outside the Amazon region Settlers in the Amazon region are highly mobile, moving with daily, periodic, and seasonal circulation from settlements in unstable disease-endemic regions to hyperendemic-disease regions of the rainforests.
This mobility keeps settlements unstable and at high risk for epidemics through the constant flow of parasitemic laborers 17 Agricultural Labor Swaziland provides an example of how agricultural labor has changed the spread of malaria. In the s, control measures DDT spraying were successfully implemented in the lowveld so that agricultural development could take place, and bymalaria had been all but eradicated from Swaziland However, agricultural developments in the s involving an irrigation project for the cultivation of sugar cane created conditions favorable for malaria.
Vector density increased, along with a high frequency of feeding on humans, as no domestic or wild animals were around the project area to serve as alternative hosts. This resurgence of malaria was catalyzed by the reintroduction of parasite carriers in the form of migrant workers from disease-endemic areas of Mozambique, who were involved in migration or long-term circulation to work on the sugar estates in the s and early s In Colombia, the annual parasite index defined as the ratio between the number of cases reported and the population at risk has increased threefold since the s This increase seems to be related to the migration of nonimmune people to areas such as the Naya basin, where malaria is endemic, and to the circulation of groups within the Naya basin.
The circulation is predominantly seasonal, related to agriculture. People descend from hills and terraces, where malaria risk is minimal, to the malarious delta zone to cultivate and harvest their crops.
In doing so, they are exposed to the anopheline population of the area and are at high risk for malaria. In addition, an estimated 25 million people have fled their homes but remain internally displaced in their countries of origin The displacement of large numbers of people and their circulation can favor malaria transmission. If refugees are nonimmune, they could travel through or to malarious regions and acquire the infection, and if they are infectious, they could disseminate the disease to other areas.
Malaria is one of the most commonly reported causes of death among refugees and has caused high rates of both illness and death among refugees and displaced persons in disease-endemic countries, such as Thailand, Sudan, Somalia, Burundi, Rwanda, and the Democratic Republic of Congo In recent outbreak among Burundian refugees at a refugee camp in northwestern Tanzania, deaths from malaria and anemia in children under 5 years of age have increased fold since the outbreak, reflecting the lack of immunity in this age group In the Sahel region of Africa, where civil wars and conflicts have occurred for many decades and large numbers of displaced people live in resettlement or refugee camps often located in lowland disease-endemic areas, epidemics are common As a result of 15 years of continuous war, which displaced hundreds of thousands of people, Luanda, the capital of Angola, underwent an unprecedented population increase in the s.
This population movement resulted in a shift in malaria endemicity in Luanda from hypoendemic to mesoendemic level within 5 years As a cause of child deaths, malaria moved from sixth to first place.
Increasing parasite resistance to chloroquine also became a major problem. This situation arose because of the enormous influx of displaced people of low socioeconomic status into an environment with stagnant water reservoirs.
The population movements that increased malaria transmission in Luanda were long-term circulation and migration from stable rural areas to an unstable urban area. Besides movements of large numbers of people, wars and civil unrest tend to favor malaria transmission. The disruptive effect of war on agriculture and water management can increase vector breeding sites; the destruction of housing can increase human-vector contact; the destruction of cattle can prompt zoophilic vectors to become anthropophilic if their usual food supply is disrupted 27 ; and control measures can be seriously diminished if health-care facilities are reduced or unavailable.
Intercontinental Travel The intercontinental transfer of malaria can occur through the introduction of an infective vector into a nonendemic-disease area, as in so-called airport malaria, or through the movement of a parasitemic person to a nonendemic-disease area, as in imported malaria. Although the incidence of these cases is low, they account for most malaria transmission in industrialized countries.
Airport Malaria Airport malaria is defined as malaria acquired through the bite of an infected tropical anopheline mosquito by persons whose geographic history excludes exposure to this vector in its natural habitat The vector is usually introduced into a nonendemic-disease country on an international flight. For example, random searches of airplanes at Gatwick Airport London found that 12 of 67 airplanes from tropical countries contained mosquitoes After a mosquito leaves the aircraft, it may survive long enough to take a blood meal and transmit the disease, usually in the vicinity of an airport.
In temperate climates, temperature and humidity can be favorable in the summer for the mosquito not only to survive but also to move around and perhaps lay eggs.
With the enormous and continuing increase in air traffic, cases of airport malaria may increase. Several such cases are described below. During a hot summer insix cases of airport malaria were identified in and around Roissy-Charles-de-Gaulle Airport Four of the patients were airport workers, and the others lived in Villeparisis, approximately 7.
Anopheline mosquitoes were thought to have traveled in the cars of airport workers who lived next door to two of the patients. Intwo cases of P. Another five cases of airport malaria were reported in Geneva in the summer of High minimum temperatures were thought to have allowed the survival of infected anophelines introduced by aircraft.
In Britain, two cases of P. Hot, humid weather in Britain may have facilitated the survival of an imported mosquito. Imported cases of malaria in Europe 3435 Throughout the world, many countries are reporting an increasing number of cases of imported malaria Figure because of the great increase in long-distance travel in recent decades.
For example, cases imported from Africa to the United Kingdom rose from in to 1, inand the ratio of all imported cases of falciparum to vivax malaria rose from 0. Recently, a woman in Italy was infected with malaria through a bite from a local species, An. This species was a common malaria vector in Italy until the country was declared malaria free in Local breeding sites, including isolated pools in dried-up irrigation channels, were identified, and the mosquito responsible is thought to have acquired the parasite after biting a parasitemic girl who had acquired malaria in India.
Airport malaria was ruled out because of the distance from the nearest airport. This may be the first case of malaria introduced to Europe in 20 years and demonstrates the hazards of population movement the parasite had been introduced from India combined with human activities providing vector breeding sites.
In the United States, recent outbreaks of presumed local mosquito-borne transmission have been reported in California, with migration from a disease-endemic area The people involved were migrant workers from malaria-endemic areas. An outbreak in involved 28 cases 26 in Mexican migrant workers of P. The epidemic curve indicated secondary spread, which confirmed local mosquito-borne transmission. In the early s, outbreaks were identified in neighborhoods of Houston with many immigrants from countries with malaria transmission.
These outbreaks occurred when the weather was hot and humid and thus conducive to the completion of the sporogonic cycle and the survival of female anophelines Given that climate change could lead to more favorable conditions for vector survival in Europe and the United States 40the increase in incidence in both airport and imported malaria is cause for concern.
Many other types of infections are associated with at least a transient increase in HIV viral load. Hence, it is logical to expect malaria to do the same and potentially to accelerate HIV disease progression.
On the other hand, the control of malaria parasitemia is immune mediated, and this prevents most malarial infections from becoming clinically apparent in semi-immune adults in endemic areas.
The immune deficiency caused by HIV infection should, in theory, reduce the immune response to malaria parasitemia and therefore increase the frequency of clinical attacks of malaria.
However, as research evidence emerged from sub-Saharan Africa in the s and s, it soon became clear that malaria is not a typical opportunistic infection.
In fact, the interaction between HIV and malaria has proved to be remarkably subtle, and it is only in the past few years that a clearer picture of this association has begun to emerge. The Association between HIV and Malaria Ina review of clinical studies concluded that the numerous studies published to that date had failed to show any convincing and consistent link between HIV and malaria, with the exception of an increased rate of placental malaria in HIV-infected pregnant women.
In addition, they did not take into account the wide variation in immunosuppression found at different stages of HIV-1 infection. Infection with HIV-1 causes progressive cellular immunosuppression, and any resulting impairment in the immune response to malaria might be associated with failure to prevent infection or to suppress parasitemia and clinical disease. This study suggests that malaria may speed the progression of HIV disease, and this is supported by a study from Uganda showing increased CD4 cell decline associated with episodes of malaria despite prompt treatment.
Placental HIV-1 viral load is increased in women with placental malaria, especially those with high parasite densities. That is, immunocompromised mothers have deranged chemokine and cytokine profiles, less protective immune responses, and consequently higher parasite densities and viral loads, leading to an increased risk of mother-to-child transmission of HIV. In areas of stable malaria, transmission is intense and continuous, although seasonal variations may occur.
Immunity develops early in life, and young children and pregnant women are at greatest risk of morbidity and mortality from malaria. In these areas, HIV-related immunosuppression may increase rates of malaria infection and clinical malaria disease, but does not increase the rates of severe or complicated malaria. These findings suggest that HIV infection not only may interfere with parasite control, but also, perhaps more important, may cause the loss of antitoxic immunity, which protects persons with parasitemia from clinical disease.
In regions of unstable malaria, transmission is intermittent and less predictable, and epidemics may occur. The disease burden is similar in all age groups because preexisting antimalarial immunity is limited. As a result, malarial fever rates are a direct function of parasite transmission rates.
Malaria on the Move: Human Population Movement and Malaria Transmission
Thus, HIV coinfection has its impact on disease presentation, with an increased risk of complicated and severe malaria and death. Response to Treatment and Drug Interactions Antimalarial therapy is most effective in individuals who have acquired some immunity to malaria. One would predict, therefore, that the response to therapy would be decreased in immunosuppressed HIV-infected individuals living in regions of stable transmission.
Although 2 early studies in the Democratic Republic of Congo formerly Zaire found no difference in responses to antimalarial treatment in HIV-infected children compared with uninfected children, 28,29 more recent studies suggested that treatment with artemisinin, sulfadoxine-pyrimethamine SPand artemether-lumefantrine was less effective in HIV-infected than in uninfected men and nonpregnant women.
The antimalarial drugs halofantrine, artemether, and lumefantrine should not be given to patients receiving PIs or the NNRTI delavirdine because of excessive risk of toxicity.
For patients receiving other NNRTIs nevirapine or efavirenzdrug-drug interactions may reduce the concentrations of lumefantrine and artemether, thereby increasing the risk of treatment failure. However, the magnitude and clinical significance of these potential interactions need further research.
Malaria and HIV-1 are 2 of the most common infections in sub-Saharan Africa and, to a lesser extent, in other developing countries. It is estimated that 38 million Africans are infected with HIV-1, 40 whereas million to million suffer from malaria each year.