Academic journal article Bulletin of the World Health Organization

Malaria in the African Highlands: Past, Present and Future

Academic journal article Bulletin of the World Health Organization

Malaria in the African Highlands: Past, Present and Future

Article excerpt

Introduction

Altitude is one of the oldest defences against malaria. As early as the sixteenth century the Spanish recognized there was little or no malaria at high altitudes in the New World (1). The protection afforded by elevation was also described in many early textbooks on tropical medicine, including that of the pioneer James Lind (2). Even before the mechanism of malaria transmission had been elucidated, Hirsch recognized that protection against the disease was related to low temperatures and high altitudes (3). He noticed that epidemics in the highlands were always located in a valley with a small declivity or a basin-like depression in a plateau -- places where water collects and malaria mosquitos breed. The salubrious effects of higher altitude were well recognized by early European settlers, and hill stations became a common feature of expatriate life in various parts of the tropics. Even today in Ethiopia many lowland farmers shelter in the highlands until the malaria season has ended (T. Ghebreyesus, personal communication, 1997). Hills and mountains have therefore been recognized as a natural shelter against the heat and diseases of the lowlands for at least several centuries.

The current upper height limit for malaria in the African highlands is difficult to define precisely, and is likely to rise, as discussed below. In many countries this boundary was thought to occur around 2000m; for example, in Burundi (4), Ethiopia (5, 6), Kenya (7), Morocco (8), and Rwanda (4). Malaria epidemics have occasionally been reported at higher altitudes (up to 2550m (7)) but are rare. In other parts of Africa the upper limit is slightly lower: at around 1700-1800m in Zaire (9) and at 1200m in Zimbabwe (10). Generally we considered that areas higher than 1500m (Fig. 1) have little or no malaria.

[Figure 1 ILLUSTRATION OMITTED]

Most malaria epidemics in the African highlands are due to Plasmodium falciparum, the most lethal and dominant species found in the continent. While epidemics of vivax malaria have also been reported, for example, in the Atlas Mountains in the 1930s (8), they are unusual. In most highland areas, local communities have little or no immunity against malarial parasites and thus the disease affects both adults and children. This contrasts with the lowlands where immunity is high among most adults and malaria morbidity is confined largely to young children and primigravidae. As a consequence of the low immunity in highland communities, epidemics in the mountains are characterized by high morbidity and mortality among both children and adults, as illustrated by the outbreaks experienced in the Ethiopian highlands, where there were approximately 7000 deaths in 1953 (12) and 150000 in 1958 (13). The highlands are thus areas of unstable malaria patterns primarily because of the low and fluctuating levels of transmission experienced by local communities. Consequently many of these semi-immune populations experience severe outbreaks every few years.

Introduction of malaria into the African highlands

How malaria became established in the African highlands can be illustrated using the example of Nandi Plateau in Kenya. This plateau lies to the north-east of Lake Victoria at an altitude of over 1500m. At the turn of the last century there were few mosquitos in the area and, according to local residents, there was no malaria in the hills (14). Sir Harry Johnston, a local resident, believed that malaria did not exist on the plateau in 1901, and the principal medical officer at that time wrote that Nandi was "one of the healthiest stations in the Protectorate". Also, malaria was not mentioned by the many officials, traders, soldiers or doctors who lived there. According to Nandi elders, malaria was introduced into the highlands by soldiers returning from the First World War in 1918 and 1919, which resulted in around 25% of the indigenous population contracting the disease. …

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