The physical environment

Topography The nature of the physical surroundings can influence the diseases that are found there. In much of Asia, a complex interaction termed 'forest fringe malaria' describes the greater likelihood of developing malaria at the forest margin. The man enters the forest to fell timber, often illegally, while the woman goes there to collect firewood, bringing them into range of mosquitoes that live within the forest cover. A similar cycle of transmission occurs with yellow fever as illustrated in Fig. 16.3. Destruction of primary forest, to be replaced by secondary growth, makes ideal conditions for the development of 'mite islands', which are important in scrub typhus (Section 16.2).

Human activity not only destroys the natural balance of nature, but also often changes the landscape to make it more suitable for the transmission of communicable diseases. The growing of rice in paddy fields provides suitable conditions for Culex mosquitoes that transmit Japanese encephalitis and Anopheles sinensis, the vector of malaria in much of China. The construction of dams and irrigation canals has encouraged the proliferation of intermediate host snails of schistosomiasis. However, the Simulium fly that transmits onchocerciasis breeds in fast-flowing oxygenated streams that are often destroyed when dams are built, depriving them of their breeding place. All major construction projects should, therefore, have a health evaluation to determine how the health risk can be minimized.

Climate can be divided into different components of temperature, rainfall (humidity) and less importantly, wind. These attributes of the climate have a marked influence on where diseases are found and the ways in which they are to be controlled.

Temperature varies by distance from the equator, altitude, prevailing winds and the size of land masses. A number of diseases are found only in the tropics, which is the main area for communicable diseases. Temperature decreases with altitude so that malaria will be found at the lower hot altitudes, while respiratory diseases are common in the colder hills. At the fringe of the mosquitoes' range, exceptional conditions of temperature and humidity can produce epidemic malaria.

Temperature not only affects the presence or absence of disease, but also often regulates the extent. The malaria parasite has a shorter developmental cycle as the temperature rises, thereby permitting an increased rate of transmission. Many insect vectors have a more rapid development in the tropics, making them difficult to control. The life cycle of a number of parasites are directly related to temperature.

Rainfall is perhaps the most essential element in human livelihood. Rainfall must be sufficient and regular (Fig. 1.4) allowing people to plant crops and ensure that they come to fruition. An irregular rainfall can be as disastrous as a low rainfall, leading to failed crops, malnutrition and a reduction of resistance to infection.

Rainfall also has a direct effect on certain diseases. Moderate rainfall creates fresh breeding sites for Anopheles mosquitoes, but excessive rain can wash out larvae and cause a reduction in the number of mosquitoes. Some diseases, such as trachoma, favour dry arid regions.

Wind produces local alterations to the weather. A major wind system is the monsoon, which brings rainfall to the Indian sub-continent and Southeast Asia. In West Africa, the hot dry Harmattan blows down from the Sahara, reducing humidity and increasing dust. It is these secondary effects on rainfall and temperature that determine the disease patterns.

The winds are appreciated by man to improve his living conditions in the warm moist areas of the world and avoided in the hot dry zones. However, excess wind in hurricane areas or the localized tornado cause destruction and loss of life (Fig. 1.5). Natural disasters disrupt the normal pattern of life, destroy water supplies and provide ideal conditions for epidemics to occur.

Seasonality Temperature and rainfall together determine the best time to grow crops and the seasonal patterns of a number of diseases. In areas of almost constant rain, there is very little seasonal variation, but in the drier regions, seasonality can be quite marked. These areas are illustrated in Fig. 1.4.

The pattern of life determined by sea-sonality can be generalized as follows:

• Food stores are low or absent during the rains as it is the longest time since the harvest.

• During the rains, people are required to work their hardest when they have the least amount of food.

• The rains bring seasonal illnesses, especially diarrhoea and malaria, which debilitate just when complete fitness is required.

• The time of the rains often coincides with late pregnancy for the woman, conception having taken place during harvest. Since all members of the family are required to work in the fields and much of the burden of cultivating falls on the woman, the

Fig. 1.4. The tropics - rainfall and seasonality.----,The tropics, Cancer to Capricorn;-, developing country zone. Seasonality within the tropical region: H, rainfall in every season; H, heavy seasonal rainfall; H, variable seasonal rainfall; □, arid.

Fig. 1.4. The tropics - rainfall and seasonality.----,The tropics, Cancer to Capricorn;-, developing country zone. Seasonality within the tropical region: H, rainfall in every season; H, heavy seasonal rainfall; H, variable seasonal rainfall; □, arid.

increased strain threatens her pregnancy, while her physical reserves are stretched even further. • Once harvest comes, then body weight is restored, excess crops are stored or sold and some respite taken before the cycle repeats itself.

This pattern leads to the following observations:

1. Attendance for treatment at medical institutions and admission to hospital often follow a cyclical pattern. This is illustrated in Fig. 1.6 where it will be seen that the reporting of ill health is least during the dry months and increases with the rains.

2. Knowledge of the seasonality of a disease can be used in health planning, the deployment of manpower, the ordering of supplies, the best time to take preventive action, etc.

3. Many illnesses show a marked seasonal pattern. Mosquitoes require water to breed, so rainfall will determine a seasonal pattern for many of the vector-borne diseases. The massive contamination of rivers caused by the first rains washing in accumulated pol lutants from the many dry months, makes this a period of diarrhoeal diseases. The seasonality of cholera, allows a warning system to be implemented and prevention initiated (see Section 8.6).

4. A different pattern of seasonal diseases occurs with the viral infections, where measles (see Fig. 1.7) serves as a good example. As measles confers life-long immunity, the only way that sufficient susceptibles can accumulate for another epidemic to occur is by immigration or reproduction. If the birth rate is high, a critical number of susceptibles will soon be produced and annual epidemics will occur. If the birth rate is low, then the interval may be every 2-3 years.

5. Knowledge of the seasonality of a disease allows planned preventive services. If a mobile or mass vaccination campaign is used to combat measles, then timing it in the few months before an expected epidemic is the most cost-effective. In Tanzania, measles outbreaks often occur in the rainy season (Fig. 1.7), a time of shortages, malnutrition and difficult communications - the worst possible time to have to do emergency vaccination to contain the epidemic. Just a

"D

Aug Sep Oct Nov Dec

Jan Feb Mar

Apr May Jun Jul Month

Aug Sep Oct Nov Dec

Fig. 1.6. Seasonality of admissions to Mbeya hospital, Tanzania, 1980-1983.

few months before, there was little ill health, nutritional status was high, road conditions good and medical staff were at their slackest. This would have been the best time to ensure that every child was vaccinated.

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