Habitats of Human Disease Vectors

Three important categories can be distinguished that provide habitat for vectors and intermediate stages of the causative agents of human diseases: (1) natural water bodies; (2) human-made water bodies, and (3) water bodies that form in human settlements and household environments. Freshwater vectors of human disease can occur in all of these habitat categories.

In terms of natural water bodies, streams and rivers are sources of the black flies that serve as vectors of onchocerciasis, and lakes and ponds provide habitats for snails that are intermediate hosts of schistosomia-sis. Although mosquito vectors do occur in natural systems as eggs, larvae, and pupae, their densities tend to be far higher when they occur in humanmade or settlement habitats, probably because of reductions in predation and competition compared to natural systems. Likewise, standing water in barrels and pots in human settlements, habitats where predation and competition are also lacking, are important habitats for mosquitoes.

The creation of human-made water bodies such as impoundments and irrigation ditches often result in hydrological changes that favor intensified vector breeding. This can result in increased Anopheles mosquito populations that may transmit malaria or in snail populations that may increase prevalence of schistosomiasis. Likewise, shifts in species composition may occur that reduce numbers of predators or competitors, and allow vectors to increase in number. Recent studies have also suggested that habitat disturbance may increase the prevalence of Buruli ulcer in ponds.

The creation of the Aswan High Dam on the Nile River in Egypt is an example of a project that resulted in an expansion of schistosomiasis. This disease was long prevalent in Lower Egypt, which is in the northern part of the country, such as in the Nile delta. Schistosomes have been found in mummies that are thousands of years old, and in the biblical story about the pharaoh's daughter finding Moses' floating cradle in the weeds, she certainly would have been attacked by cercariae when she went in to get him! In contrast, schistosomiasis was far less common in Upper Egypt (which is located in the south). However, the construction of the dam moved infected people from Lower Egypt to Upper Egypt and created favorable snail habitat because snails thrived in the slower-moving water conditions provided by the dam. Consequently, the disease became established there as well.

Potential increases in disease vectors are now generally a design consideration in development projects and this has been especially important when irrigation schemes are planned because their introduction may create new or more favorable habitats for disease vectors. A variety of hydraulic engineering approaches such as improvements in drainage, concrete lining or covering of canals, land leveling, filling in land depressions, controlling seepages, diking, and dewatering are useful additions. For example, it has been estimated that water improvements may result in disease reductions of 80% of Gambian sleeping sickness, 20% for onchocerciasis, and 10% for yellow fever. In some cases, however, pesticide applications to control insect vector and snail problems may be inevitable. Many of these habitat modifications and pesticide applications, however, may have a deleterious effect on the nontarget fauna that keeps vectors under control or is of economic or conservation value.

Tires are not a typical freshwater habitat but they are extremely important breeding sources for some mosquitoes. Tires are perfectly designed to hold water (there is no orientation from which water can drain from it), and many tree-hole occupying species use tires as breeding sites. The shipping of tires from Asia to North America for recapping and reselling (they do not recap tires in Asia) introduced the Asian tiger mosquito Aedes albopictus to North America. This mosquito is an effective transmitter of 23 human viruses, including those that cause St. Louis encephalitis, dengue, and yellow fever. Likewise, it has been speculated that reintroduction of yellow fever to Mexico was the result of shipments of

Plate 1 Top left. Anopheles gambiae: an adult female mosquito taking a blood meal and possibly vectoring malaria (WHO/TDR/ Stammers). Top right. Anophelese gambiae: fourth-instar mosquito larva; all of the immature stages of mosquitoes occupy aquatic habitats (WHO/TDR/Stammers). Bottom left. Plasmodium falciparum: Mature female gametocyte stage of the malaria pathogen (WHO/ MAP/TDR). Bottom right. Water-storage pots that Anopheles gambiae and other mosquitoes can use as breeding sites (WHO/TDR/ Service).

discarded tires from Texas to Mexico to make huarache sandals because of a shortage of tires used in making them there!

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