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close this bookEpidemiologic Surveillance after Natural Disaster (PAHO - OPS; 1982; 105 pages) [FR] [ES]
close this folderPart 1 : Epidemiologic surveillance and disease control after natural disaster
close this folderChapter 1 : Risk factors for communicable diseases after disasters
View the documentEpidemiologic factors that determine the potential of communicable disease transmission
View the documentThe relative risk of communicable disease after natural versus manmade disasters
View the documentPostdisaster experience with communicable disease

Epidemiologic factors that determine the potential of communicable disease transmission

The potential risk of communicable diseases after disaster is influenced by six types of adverse change. These are changes in preexistent levels of disease; ecological changes which are the result of the disaster; population displacement; changes in population density; disruption of public utilities; and interruption of basic public health services.

Changes in preexistent levels of disease

Usually the risk of a communicable disease in a Community affected by disaster is proportional to the endemic level. There is generally no risk of a given disease when the organism causes it is not present beforehand. Developing countries frequently have such poor systems for reporting communicable disease, however, that their national authorities lack adequate information about levels of specific organisms. Political pressure is nonetheless sometimes exerted for taking public health measures against diseases such as smallpox, cholera, yellow fever or other vector-borne diseases in geographic areas considered free of them by communicable disease specialists.

Relief workers can conceivably introduce communicable disease into areas affected by disaster. Diseases potentially introduced include new strains of influenza, foot-and-mouth disease, and those borne by insect vectors, particularly by Aedes aegypti. Also, nonimmune relief workers may be susceptible to endemic diseases to which the local population is tolerant or immune, and they may become ill.

Ecological changes caused by the disaster

Natural disasters, particularly droughts, floods and hurricanes, frequently produce ecological changes in the environment which increase or reduce the risk of communicable disease. Vector-borne and water-borne diseases are the most significantly affected. A hurricane with heavy rains which strikes the Caribbean coastal area of Central America may, for example, reduce the number of Anopheles aquasalis hatched, since the vectors prefer brackish tidal swamps and increase A. albimanus and A. darlingi, which breed easily in fresh, clear water and overflows. The net effect of the hurricane on human malaria, of which both mosquitoes are vectors, would be difficult to predict. Rain from such a hurricane would also cause flooding of streams and canals which in rural areas are often the source of drinking water. Under some circumstances, a water-borne zoonotic disease, such as leptospirosis, may become more widely disseminated via water-contact or drinking from contaminated sources. There is evidence that the short term effect of diluting supplies of already contaminated drinking water with rain may, however, reduce the level of disease (1). The population may, moreover, avoid drinking water contaminated by flooding for a cultural/psychological reason such as the presence of animal carcasses.

Population displacement

Movement of populations away from the areas affected by a disaster can affect the relative risk from communicable diseases in three ways. If the population moves nearby, the existing facilities and services in the receiving community will be strained. When resettlement occurs at some distance, the chances increase that the displaced population will encounter diseases not prevalent in their own community, to which they are susceptible. For example, nonimmunized, rural Andean populations brought together in camps after an earthquake may then be exposed to measles. Alternatively, displaced populations may bring the agents or vectors of communicable diseases with them. The latter concern frequently occurs when populations from low-lying coastal areas with malaria are evacuated further inland before a hurricane.

Population density

Population density is a critical factor in the transmission of diseases spread by the respiratory route and through person-to-person contact. Because of the destruction of houses, natural disasters almost invariably contribute to increased population density. Survivors of severe disaster seek shelter, food and water in less affected areas. When the damage is less severe, crowding may occur when people move in with other families and congregate in such public facilities as schools and churches. The resulting problems most commonly mentioned are acute respiratory illness, and include influenza and non-specific diarrheas.

Disruption of public utilities

Electricity, water, sewage disposal and other public utilities may be interrupted after a disaster. In a village with no electric power and where there are promiscuous defecation habits and contaminated sources of water in normal times, very little (if any) additional risk from communicable diseases follows the disaster. However, in economically more developed areas the extended disruption of basic services increases the risks of food-borne and water-borne disease. Insufficient water for washing hands and bathing also promotes the spread of diseases transmitted by contact.

Interruption of basic public health services

The interruption of basic public health services like vaccination, ambulatory treatment of tuberculosis and programs for the control of malaria and vectors are frequent, but often overlooked factors that increase the probability of disease transmission after disaster in a developing country. The risk of transmission increases proportionally to the extent and the duration of the disruption. An outbreak of communicable disease may, therefore, occur months or years after a drought, a famine or a civil disturbance. The interruption causing such an occurrence is usually the result of the diversion of staff and financial resources to the relief effort, beyond the critical period. In addition or in conjunction with this, the failure to reestablish resources at sufficient levels contributes to the interruption.

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