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Natural habitats

The incidence of onchocerciasis (river blindness) can be affected by land use change.
Source: Mark Edwards/Still Pictures

Intact ecosystems can help control diseases by providing a balance of species potentially involved in the life cycle of infectious diseases, along with predators and other agents that control or limit the animal reservoirs, vectors and pathogens. Disease agents that live much of their life cycle outside the human host, such as those responsible for water- and vector-borne diseases, are highly susceptible to environmental conditions. It is these diseases for which the greatest linkages to surrounding ecology have been found.

Anopheline mosquito species occupy a variety of ecological niches that can be altered by environmental changes (Keating and others 2003). For example, partial deforestation, with subsequent changes in land use and human settlement patterns, has coincided with an upsurge of malaria and its Anopheline mosquito vectors in Africa, Asia, and Latin America (Walsh and others 1993). In eastern and southern Africa, the proportion of under-five deaths due to malaria doubled between 1982-89 and 1990-98 (Figure 2). Climate change, resistance to drugs, and the spread of HIV/AIDS causing depressed immune function, are also factors in the increased incidence of malaria (WHO 2003b).

Forest destruction can lead to a decrease or increase in onchocerciasis (river blindness, caused by the filarial worm Onchocerca volvulus), depending upon the impact of such factors as remaining forest cover and new stream flow regimes on the habitat of the black fly which transmits the larvae (Walsh and others 1993). On the other hand, reforestation can also take its toll. In northeastern United States it has enhanced the spread of Lyme disease (Box 4).



Box 4: Reforestation, biodiversity loss, and Lyme disease

The female deer tick, Ixodes dammini, is the vector for Lyme disease.
Source: Kent Wood/Still Pictures

Lyme disease is a bacterial disease occurring in North America, Europe, and Asia that is transmitted by the bite of infected deer ticks. It was first named in 1977, but was recognized earlier. The major reservoir hosts for the bacteria are rodents, while deer are the major host for the tick vectors (Steere and others 2004).

Patchy reforestation of the northeastern United States led to a dramatic increase in the deer population, which in turn increased the tick population. Habitat changes also decreased rodent predators, resulting in an expansion of rodent hosts for the Lyme disease pathogen. Wet conditions in late spring and early summer were associated with an increase in Lyme disease incidence in the northeast of the country possibly by increasing tick survival and activity (McCabe and Bunnell 2004).

These environmental changes have been combined with increased human use of this habitat for homes and recreation. Because new homes are often built in wooded areas, transmission of Lyme disease near homes has become an important problem. Dutchess County, a semi-rural peri-urban county north of New York City, has one of the highest incidences of Lyme disease in the United States, with a crude mean annual incidence rate of 400 cases per 100 000 persons per year during the period 1992-2000 (Chow and others 2003). Specific strategies such as clearing leaf litter, and brush- and wood-piles in gardens can reduce deer, mouse and tick habitat thereby reducing the tick population and likelihood of disease (CDC 2004a, CDC 2004b).


Figure 2: Malaria resurgence in eastern and southern Africa

Source: WHO 2003b

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