Conversion of land to agriculture is still proceeding in many developing countries and is likely to continue. The United Nations Environment Programme (UNEP) predicts that agricultural land area could nearly double in Africa and West Asia by 2050 and climb by 25 percent in the Asia-Pacific region, although greater investment in agricultural management and technology on existing cropland could lower these numbers considerably (109). Much of this conversion will target forested areas. Indeed, conversion of forestland to agriculture is already a prime force driving forest loss in the tropics and even in some temperate areas, including China (110). One fifth of the world’s remaining large blocks of forest may well become cropland and pasture (111).
Unfortunately, land conversion especially forest conversion is associated with a higher incidence of certain infectious diseases, including malaria and leishmaniasis. Forest clearance often creates new habitats, such as depressions where water can collect and mosquitoes, ticks, and fleas can breed. Often, the forest fringe at the edge of converted forestland provides a potent contact point between disease organisms from the forest and human populations. For example, the mosquito Anopheles dirus, which breeds in sunlit pools along the partially cleared forest margin, is a very effective malaria vector. Intense malaria transmission by this mosquito occurs along forest fringes in large portions of South and Southeast Asia (112). The clearing of forests for cattle grazing in parts of southern India’s Shimoga district caused a different problem. It led to an upsurge in the local tick population and an outbreak of Kyasanur forest disease in the 1980s, the product of a rare virus from the forest (113).
In Africa, deforestation favors malaria transmission by the mosquito Anopheles gambiae, which prefers to breed in the open rather than in the dense forest. The rise in surface temperatures that clearing of forests brings can also help in spreading malaria by speeding both the life cycle of the mosquito and the development of the malaria parasite it harbors. In the Usambara mountains in northeastern Tanzania, forest-clearing activities along the mountaintops are considered one cause of the introduction and spread of malaria (114). A combination of altered natural conditions plus migration has created a particularly deadly situation in Brazil’s Amazon basin. (See “Box 2.4. Malaria in the Brazilian Amazon”.)
At times, the influence of agricultural conversion on disease can be complex and can extend over large regions. In southern Honduras, conversion of forest to cotton and sugarcane culture and cattle pasture altered the region’s hydrological cycle, making it drier and hotter and less hospitable to the mosquito species responsible for malaria in the area. Malaria incidence declined accordingly. However, the semidesertification in the south prompted the population’s migration to the newer factories and plantations of the north; many relocated on cleared forestland where malaria was still present. The migrants carried no immunity to malaria, and so a sharp rise of the disease in the north began in 1987. Heavy pesticide use on export crops also played a part in the outbreak by encouraging widespread pesticide resistance among the anopheline mosquitoes (115).
As with irrigation-related illnesses, it is difficult to quantify the additional burden of illness associated with land conversion. The complex relationships among habitat modification, the functioning of ecosystems, and the transmission of disease mean that it is difficult to predict exactly how land use changes will affect disease rates, especially when the vulnerability of exposed populations varies so widely with income, access to health care, and proper nutrition.