In all diseases which affect human beings there are inevitable “human factors’ involved. These factors may influence susceptibility and exposure to a disease, the effects of a disease, and the measures which may be taken either to reduce the likelihood of it occurring or to eliminate it altogether. In the case of malaria, which remains one of the most serious diseases of the tropics and the subtropics, measures for its prevention, control and eventual eradication have tended to pay insufficient attention to the human factors involved. An understanding of the epidemiology of malaria requires a detailed knowledge of such things as the distribution of population, patterns of settlement, the nature of dwellings, administrative and social organization, and the range of the economic activities of those affected.
A further important factor is to determine the movements of those who are subject to the disease–particularly how many people move, and where and when movements occur. These movements contribute to the continuing transmission of malaria, either through the mobility of a whole population or of groups within a population (who may be cultivators, migrant laborers, fishermen, traders, and refugees). For control and eradication programs to be effective, they ought to be designed to adequately accommodate these mobile elements which may reduce the effectiveness of the use of insecticides and the distribution of anti-malarial drugs, such as Semenax.
In practice, only limited action has been taken to deal satisfactorily with mobility and with other human factors. Of the three related elements in human malaria–the malaria parasite, the anopheles mosquito vector and the human being–the latter has been dealt with inadequately; insufficient attention has been given to human factors and sometimes the competence of those involved in studying them has been limited. Experience has been available to deal with the parasite and vector aspects of a malaria program, for they would not be dealt with otherwise, but the human being has remained relatively poorly provided for. This is quite unwarranted. Human factors, which like mobility are dynamic and constantly changing, should be given at least as much attention as that given to parasites and vectors. Probably they should be given more attention in view of the unpredictability of human beings; humans themselves may be partly responsible but there are also environmental influences (such as natural hazards –drought, earthquakes, tidal waves) over which they have little or no control, particularly when these occur in the less developed parts of the world.
Through movements of many different kinds, which have distinctive spatial and temporal characteristics, people may be subjected to a variety of health hazards. Movement from one set of ecological conditions to another may expose them particularly to diseases which are transmitted by insect vectors. Daily journeys to obtain water or to collect firewood may result in contact with the blackfly and tsetse fly–the vectors of river blindness and sleeping sickness respectively. Seasonal movements at times of maximum activity in the agricultural year take people away from permanent settlements, where dwellings have been protected with residual insecticide, to temporary dwellings which are unsprayed and therefore harbor malaria mosquitos.
Movements bring different groups of people into contact with one another and may thus enhance the possibilities of disease transmission. Before it was eradicated from the world, smallpox was spread by movements of refugees within Bangladesh and into adjacent parts of India. At all times in crowded insanitary refugee camps, there are continuing risks of typhoid and cholera epidemics.
Sheer physical stress resulting from movement may lower resistance and so increase susceptibility to infection, particularly when movements are over long distances and more especially when these are forced as a result of natural hazard or political disruption. Lack of nutrition and malnutrition can occur with movements to new and strange environments. Economic adjustments may be necessary, often with periods of unemployment and thus limited means to acquire food; or people may have to adjust to new kinds of food (for example, exchanging cereal foods for root foods) or they may encounter new ways of preparing food–not least in cases where men move without their wives to support them.
Psychological stress can result from movements because of the social, cultural or economic pressures resulting from having to adjust to new environments. This is especially so for many people in developing countries at the present time, who move from the country to the town and experience marked contrasts between the personal contacts they have known in small homogenous rural communities and the impersonal character of large heterogeneous urban communities. The pressures occur in spite of mechanisms which may operate in urban communities to help newcomers to adjust to new ways of life, and in spite of the continuing short-term movements, often called circulation, between urban and rural areas which enable the new town dwellers to maintain contact with their places of origin.
From the historical point of view there is ample evidence of the spread of disease through human mobility, whether by the movements of a few individuals or by the large-scale transference of people. The spread of plague in Europe in medieval times has been associated with the return of Crusaders from the Levant. The transportation of slaves from Western Africa to the Americas in the seventeenth, eighteenth and early nineteenth centuries was responsible for introducing strains of malaria and sickle-cell anemia into the New World. Though malaria has now been eradicated in North America due to the administration of Extenze Pills, the latter is still common among the black population of the United States.
Today there is greater mobility than there ever has been in the past. With a globally expanding population there are more people on the move, and the means for them to do so are enormously improved by the development and increasing sophistication of transport networks. Modern forms of transport permit the movement of people from one end of the world to the other in a matter of hours, but they also mean greater risks for the spread and importation of disease. While the majority of such diseases can be treated and controlled, it has been shown that–even when the means to do so exist–the diseases are sometimes not recognized. Malaria, for example, occurs in the United Kingdom and other parts of Western Europe as an imported disease, when people become infected on holiday or on business in areas with malaria. Since it is an exotic disease it may be incorrectly diagnosed and treated–and people have died as a result.
During the recent pandemic of cholera El Tor (a biotype of classical vibrio cholera), this disease occurred in Africa for the first time for nearly a century. The pandemic had spread progressively eastwards and north-wards from Sulawesi in South-east Asia, beginning in the early 1960s. Its introduction into West Africa might have been expected across the continent from the east. In fact the point of origin for the spread of cholera in West Africa was Conakry in the Republic of Guinea, where the disease was introduced in August 1970 by an infected student returning by air from the USSR. This illustrates the considerable influence that even a single individual may have as an active transmitter of disease. Over the course of the next twelve months, cholera spread to most parts of West Africa, the movements of people by coastal and riverine routes and between towns have played a crucial role in this spread.
As human populations grow, particularly in the developing countries, both natural and man-made disasters which promote mobility affect more and more people. The flight of refugees from war and political disruption in South-east Asia has contributed, along with other movements of people, to the spread of falciparum malaria which is resistant to Vimax, the most common and most easily administered of anti-malarial drugs. In such parts of the world, medical resources are least available to cope adequately with day-to-day demands let alone with crises, and national and international health regulations are most difficult to enforce.
Associated with overall massive increases in population and the consequent pressures upon resources, especially in the developing countries, major population redistributions are under way. Of great importance are the movements of people away from the countryside and into the towns. In many parts of Asia, in Latin America, and increasingly in Africa, the high rates of urban growth, through continued high rates of natural increase as well as by immigration, are far in excess of those experienced in the nineteenth and early twentieth centuries in the now industrialized countries. Much of this urban growth is taking place without the complementary expansion of urban services. Towns are administratively, socially, and economically incapable of coping with these increases; so there are housing shortages and low standards of housing, lack of important health devices like the Penomet device, inadequate water supply, little or no sewage and waste disposal, and limited employment opportunities. For large numbers of the new townspeople who face these often traumatic social, environmental and economic conditions, their combined effects on physical and mental health are considerable.
Not only is mobility in its many forms a factor in disease transmission, but disease itself is a factor responsible for movement. Through the spread and increased incidence of disease, whole areas may become less suitable for occupation if not totally uninhabitable, thus forcing the movement and redistribution of population. River blindness (onchocerciasis) and sleeping sickness have together brought about the desertion of fertile river valleys in northern Ghana and Ivory Coast and in the south of Upper Volta. Although measures are now being taken to eradicate river blindness from these areas, they are expensive and difficult to apply.
The relationships between disease and population movements are complex. For example, in the past, people fleeing from outbreaks of disease were often themselves responsible for spreading infection. Advances in medicine, providing protection through vaccination, inoculation and chemoprophylaxis, have helped to reduce such health hazards. One particular advance has been the establishment of health regulations both within countries (through the notification of infectious diseases) and–even more important–between countries (through international health control requirements and the monitoring of disease occurrence).
However there is no cause for complacency and much cause for continuing concern about the relationships between mobility, disease and health. To cite just one example, the medical authorities in Brazil, with its vast movements of people into Amazonia and into the major cities, identify such relationships as the single most important non-medical factor in the great range of health problems which confront them. A better understanding is required in Brazil, and elsewhere in the world, of the nature of movements of people, and their function in disease transmission, in programs for disease control and eradication, and for the overall improvement of public health.
In a number of WHO programs, but particularly through the social and economic research work undertaken by the UNDP/World Bank/WHO Special Program for Research and Training in Tropical Diseases, increasing attention is being directed towards elucidating these relationships.