| Aquaculture and schistosomiasis |
The idea of mixing aquaculture and schistosomiasis researchers in a single meeting was prompted by the need to emphasize the importance of the interface between these two fields. A network meeting was held in Manila, Philippines, from August 6 to 10, 1991, and involved U.S. Agency for International Development (USAID) grantees working in aquaculture or schistosomiasis. The meeting was organized and sponsored jointly by the Board on Science and Technology for International Development (BOSTID), National Research Council, Washington, D.C., and the International Center for Living Aquatic Resources Management (ICLARM), Manila, and was funded by the Office of Research at USAID. The participants' grants were funded through USAID's Program in Science and Technology Cooperation (PSTC) or the U.S.-Israel Cooperative Development Research (CDR) Program. The meeting consisted of two days of grantee presentations of individual research projects and two days of visits to field research sites in nearby areas.
The 25 papers presented during the meeting are included here. One additional paper is included from a grantee who could not attend the meeting. The 15 aquaculture papers cover a wide field of topics, ranging from three on technology, four on propagation, two on nutrition, and four on DNA and genetics, to two on ecology and the environment. Likewise, the 11 papers on schistosomiasis cover diverse topics, including four on immunology, two on epidemiology or biology, and four on biocontrol. None of the papers address both aquaculture and schistosomiasis (except for Loker et al., in passing), because the grants did not overlap both study areas. However, the participants formulated and compiled a list of 35 recommendations relevant to future research, including four that specifically propose work involving both study areas.
Aquaculture, an ancient human practice, has become an increasingly important farmer-oriented industry for protein production in many countries, both developing and developed. The technologies required for large- and small-scale farm aquaculture of major or supplemental crops of various fish species, frogs, crayfish and prawns, and mollusks are now in place and are being widely distributed. The development of these technologies has not come about spontaneously, but as a result of three basic trends: (1) the need for more protein production to feed the growing human population; (2) increased pollution in, or destruction of, many of the world's natural fishery habitats and the subsequent decline in fish populations; and (3) poorly managed or unmanaged harvest systems that have permitted the overexploitation of natural fisheries. Realistically, a slowing and reversal of these trends will require considerable education, policy and managerial changes, effort, and time. Thus, aquaculture technology will continue to grow at a rapid rate and it should be anticipated that many other aquatic species will be "domesticated" and disseminated for mass production of protein.
One of the main organizations involved in the enhancement of fisheries techniques and aquaculture is the International Center for Living Aquatic Resources Management. Currently, there are several ICLARM regional centers in the Eastern Hemisphere that have been very successful in initiating small-farmer-oriented aquaculture programs based primarily on various cichlid species. One of these centers is located in Manila. Accordingly, the participants of this network meeting had the opportunity to visit the Freshwater Aquaculture Center and the Freshwater Fish Hatchery and Extension Training Center at Central Luzon State University and to learn about the project in Genetic Improvement of Farmed Tilapias.
The parasitism of humans by schistosomes is almost certainly more ancient than the development of aquaculture and is still a major public health problem in tropical and subtropical areas of the world. Schistosomiasis is one of the six priority vector-borne diseases targeted for control by the World Health Organization. The disease (also called bilharziasis) is caused by any one of five species of the genus Schistosoma that parasitize humans through direct contact in water. At present, it affects an estimated 200-250 million people in approximately 75 countries, primarily in Africa, Asia, Latin America, and the Middle East. Schistosomiasis normally has a low mortality rate; however, it is typically chronic, extremely debilitating, and causes a very significant loss in work productivity. Thus, the annual impact of lost work in some developing countries with high prevalence rates can amount to hundreds of millions of U.S. dollars.
Human schistosomes develop through a complex life cycle involving freshwater snail intermediate hosts. The stage infecting humans develops in the snail and is released into the water, where it swims until it dies or comes in contact with, and penetrates, human skin. Once in the human the parasite migrates, matures, and develops large numbers of eggs, some of which pass out in the urine and/or feces. If the eggs are deposited in fresh water, another stage of the parasite hatches and swims around until it locates and infects the snail intermediate host. Only a select few species of snails actually function in the intermediate host role, and most are specifically susceptible to only one or two of the schistosome species. Thus, the schistosome cycles are dependent on: (1) an infected human (in Asia other animals can also serve as vertebrate hosts for the parasite, and these enhance the transmission of parasites to humans); (2) the depositing of eggs into fresh water and their hatching; (3) the location and infection of the correct snail species and further development of the parasite for release into the water; and (4) human skin exposure in water to the infective stage of the parasite that came from the infected snails, and a subsequent infection.
Why the concern about schistosomiasis when efforts to develop a vaccine are progressing well and a medication, praziquantel, is now available for treatment of the disease? Although these developments are promising, and in the case of praziquantel very successful, the disease is continuing to spread and case numbers are increasing annually. There is also some evidence of praziquantel resistance developing in the parasite. Furthermore, there is no realistic expectation that the eventual vaccine will be available, costwise, to many infected rural populations in tropical countries. It is imperative, therefore, that non-vaccine control methods and programs be continued and public awareness and education enhanced through local sources.
As mentioned earlier, schistosomiasis is a tropical and subtropical disease that primarily affects rural populations. In this scenario the focus for infection is continued contact with water through activities such as fishing, farming (for example, rice culture), bathing, washing clothes and cooking utensils, personal hygiene, and recreation. Thus, water development projects in developing countries where the disease occurs offer likely sources for new foci or epidemics of the disease. The need for increased education and local awareness about schistosomiasis is particularly important when considering the current water resources situation. Many of the countries with high prevalence rates of this disease are the same countries experiencing serious water shortages. And, wherever water occurs there are usually large concentrations of humans, which encourages the transmission of waterborne disease. Recently, a number of the countries experiencing such water shortages have embarked on large-scale dam projects and irrigation schemes. Sadly, most have resulted in large-scale schistosomiasis epidemics in the settlers who come to work the newly habitable lands. These water development schemes are also the very same areas where aquaculture efforts are likely to be initiated; often the persons involved in these projects, like the other settlers, are unaware of the risk of schistosomiasis.
When considering this problem, there are at least four future trends that seem certain: (1) human populations in most developing countries will continue to increase at a rapid rate; (2) freshwater resources will become increasingly scarce, which will result in more water development projects in developing countries; (3) aquaculture will become increasingly important in developing countries as a source for protein and will be linked to the water development projects; and (4) waterborne diseases such as schistosomiasis will continue to spread so that large-scale epidemics will be a common public health problem in humans associated with water development projects. Therefore, the meshing of aquaculture and schistosomiasis researchers in a network meeting seems timely and has resulted in an increased awareness on both sides of the interrelatedness of the two fields. Hopefully, this meeting will inspire future research efforts that encompass both aquaculture and the study of schistosomes and schistosomiasis.