Discarded plastic and standing water can increase the risk of vector-borne infectious disease. Source: Friedrich Stark/Still Pictures

Decisions about land use can have direct and indirect impacts on infectious disease. Demand for land for agriculture and settlement has led to widespread deforestation and land cover change affecting wildlife habitat. These practices have resulted in an increase in zoonotic diseases (in which animals are the reservoirs of the infectious agent) in those areas where the populations of carrier animals have expanded or their contact with humans increased. Land use changes account for a majority of emerging and re-emerging infections, including major parasitic diseases such as Chagas disease, trypanosomiasis, leishmaniasis and onchocerciasis (Molyneux 1998), each of which has one or more animal reservoirs in the wild.

Habitat changes also alter the availability and reproductive capacity of vectors that transmit and sometimes also act as reservoirs of diseases. For example, some of the major vector-borne infectious diseases, including malaria, Japanese encephalitis, and dengue hemorrhagic fever, are transmitted by various species of mosquito (Gubler 2002). Opportunities for mosquito breeding in standing water are often increased by habitat and land-use change, by changes in natural water flows, by environmental degradation caused by human activities, and even by human-made containers such as discarded automobile tyres and non-biodegradable plastic (Gubler 1998). Environmental and public health management practices that decrease unnecessary standing water can often reduce the risk of vector-borne infectious disease.

Road building to open up wilderness for agriculture, mining, forestry, or other purposes can alter vector habitat, promoting the spread of vectors that favour more open areas. New roads can also lead to the migration of susceptible human populations to areas in which infectious disease pathogens and their vectors are present (Boxes 2 and 3).

Box 3: Bushmeat, Ebola and HIV/AIDS

Humans are susceptible to many of the same diseases that plague the great apes (chimpanzees, bonobos, gorillas and orangutans). Historically there has been little contact between people and apes, so little opportunity for diseases to transfer. But in Central Africa, the growing migration of human populations and increased access to forest habitats have allowed the trade in wild meat ('bushmeat') to flourish. Recent analyses have linked the first human cases in Ebola outbreaks to the handling of meat from infected apes (Leroy and others 2004). The Ebola virus, discovered in 1976, is fatal in a high proportion of cases in humans and great apes. Outbreaks in Central Africa have killed hundreds of people and thousands of apes in the last few years. Disease transmission is a strong argument against the consumption of primate meat. Retroviruses including HIV and simian foamy virus (SFV) have also been contracted this way (Wolfe 2004). HIV/AIDS is suspected to have originated from the fusion of two Simian Immunodeficiency Viruses, possibly acquired by humans through direct exposure to animal blood and secretions through hunting, butchering, or consumption of uncooked contaminated meat (Hahn and others 2000). Ebola outbreaks, 1976-2004 Year Country Cases Deaths Fatality (%) 1976 Sudan 284 151 53 1976-77 Zaire 319 281 88 1979 Sudan 34 22 65 1994 Gabon 52 31 60 1994 Côte d'Ivoire 1 0 0 1995 Liberia 1 0 0 1995 Democratic Republicof Congo (formerly Zaire) 315 250 81 1996-97 Gabon 97 66 68 1996 South Africa 1 1 100 2000-01 Uganda 425 224 53 2001-02 Gabon 65 53 82 2002-03 Republic of Congo 237 201 85 2004 Sudan 17 7 41 Total   1848 1287 Source: WHO 2004c Bushmeat on sale for passing motorists, Central Africa. Source: Martin Harvey/Still Pictures

The way that land is used for agriculture can also have widely divergent effects on the habitat for infectious disease vectors, depending on the prevalence of irrigation, agroforestry, prior felling of forests and so on. For example, irrigation of rice fields will create excellent breeding sites for mosquitoes. The use of insecticides however sometimes has a greater detrimental effect on natural predators of mosquitoes than on mosquitoes themselves.