Human Diseases and Freshwater Vectors

A variety of human diseases are transmitted by vectors that have life cycles associated with various types of water bodies, including lakes, ponds, rivers, streams, reservoirs, and irrigated fields. Of these diseases, malaria, schistosomiasis, lymphatic filariasis, onchocerciasis, Japanese encephalitis, and dengue are generally considered now to be the most important in terms of their infection rate, morbidity (i.e., either the incidence or to prevalence of a disease), or mortality (i.e., the number of deaths resulting from a particular disease).

These diseases have had and continue to have a devastating toll on human life. For example, malaria is endemic in more than 100 developing countries and more than 2 billion humans are at risk. 80-90% of the cases occur among the poorest countries of the world in Africa, but malaria is also a major problem in parts of South America, India, and Southeast Asia.

Between 100 and 200 million people are estimated to be infected worldwide, and million deaths occur per year (even higher during epidemic years). An estimated cost of $1.7 billion in lost productivity and treatment costs applies in sub-Saharan Africa alone.

Malaria is caused by several species of Plasmodium protozoans: P. vivax, P. falciparum, P. malariae, and P. ovale. It is vectored by various species of Anopheles mosquitoes that often occur in high numbers in irrigation ditches and canals, lakes and ponds, riverine floodplains, wetland-rice cultivation areas, and human settlements. Current control measures involve personal protection (e.g., repellents, pesticide-impregnated bed nets, anti-malarial drugs), vector reduction through insecticide applications, and parasite elimination in infected humans through drug therapy.

Schistosomiasis occurs in most subtropical and tropical areas of the world, and 200 million people being affected with this debilitating disease; 20 million suffer consequent renal failure, bladder cancer, and liver fibrosis. Like malaria, the majority of the schistosomiasis cases occur in sub-saharan Africa. Children are particularly at risk; 88 million children under 15 are infected each year. The passing of blood in the urine of adolescent boys, a consequence of the disease, is so common in some parts of West Africa that it is considered normal and analogous to menar-che in adolescent girls. Certain species of snails serve as the intermediate host of the blood fluke parasites in the genus Schistosoma; S. mansoni, S. japonicum, and S. haematobium are the most important agents of this disease. Control involves snail elimination through molluscicide applications or providing drug therapy to infected individuals. The latter is complicated by the frequency of reoccurrence of infections that occurs among individuals that have already been treated.

Lymphatic filariasis (or elephantiasis), which is transmitted by a few genera of culicine mosquitoes (culicine is a subfamily of the mosquito family Culi-cidae), is a global problem with high morbidity but low mortality. It is estimated that this disease affects some 120 million people living in 80 countries, and high infection rates were noted in Egypt, sub-Saharan Africa, India, the western Pacific Islands, and parts of the Caribbean and South America. The disease is caused by two species of filarial nematodes Wucheria bancrofti and Brugia malayi. It is especially prevalent in areas with irrigation systems and in weed-infested reservoirs. Control involves mosquito reduction and drug therapy, the latter either as a prophylactic or a treatment.

Japanese encephalitis (or brain fever) is also transmitted by some genera of culicine mosquitoes and is the result of a flavovirus infection. It occurs in South, Southeast, and East Asia, and in some Pacific Islands. During epidemic outbreaks, high mortality rates occur, especially among children. Control involves mosquito reduction through habitat modification or insecticidal applications, and through vaccination.

Dengue fever (or hemorrhagic fever) occurs mostly during and shortly after the rainy season in most tropical and subtropical areas of world. More than 100 million people suffer from it each year and the number of infections is rising annually. Likewise, its distribution has spread greatly over the past three decades. It is caused by four related forms of flavo-virus and is vectored by Aedes mosquitoes, most commonly Aedes aegypti. This species of mosquito is also the infamous vector of yellow fever, a disease that has been eliminated in much of the world but which is now gaining renewed importance because of its transmission of dengue.

Adult females of many Aedes mosquitoes lay their eggs on moist soil that will later be flooded and from which the mosquitoes emerge as adults. Control is directed at mosquito reduction and biting prevention; no vaccine is available.

In contrast to diseases mentioned so far, in which the vector occurs in still or at least very slow-moving water, onchocerciasis is vectored by black flies whose immature stages occur attached to vegetation in fast-flowing streams. The parasite is the filarial nematode Onchocerca volvulus. Although most cases occur in Africa, it is also found in parts of South America and the Arabian Peninsula. It has been successfully controlled in West Africa through insecticide applications to reduce aquatic stages of the black fly vector and through the distribution of antihelminthic drugs (see later text) but it is still a problem in central and east Africa, and in parts of South America.

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