Public health and aquaculture
Even though the main concern of the aquaculturist is the health of the species being cultured, the implications of aquaculture activities for human health and well-being are of equal importance, as continuation of his enterprise and public support for it will depend on its safety levels. Needless to say, his own health and that of his family and the community are directly involved. The hazards to public health are much greater in tropical areas, where there is greater prevalence of water-borne diseases. In fact in many tropical areas, particularly in Africa, conflicts between public health and fish culture interests have been one of the reasons for the abandonment of early endeavours to develop rural homestead fish farming. The large number of small, shallow, poorly managed fish ponds were considered to provide additional breeding grounds for vectors of water-borne diseases, even though their importance in the production of much-needed animal proteins was well recognized.
The major consideration relates to fish ponds becoming breeding grounds for mosquitoes which transmit malaria. The human malaria infection, brought about by four different parasite species of the genus Plasmodium, is transmitted through about 60 or so different species of Anopheles mosquitoes. The parasite is introduced into the human bloodstream as a sporozoite through the bite of an infective mosquito. In the human host the parasite develops and multiplies, first in the liver and then in the bloodstream, where it invades the red blood cells. Some of the parasites develop into sexual forms called gametocytes, which are eventually taken up by the bite of an Anopheles mosquito. In the mosquito the parasite undergoes various stages of development leading to the formation of sporozoites. These accumulate in the salivary glands and are injected into man during a blood meal. As all the carrier species of mosquitoes are aquatic breeders, it is necessary to ensure that pond farms do not become breeding grounds for the vectors and contribute to the spread of malaria.
Generally, pond farms are built on low-lying swampy areas that are favoured places for mosquito breeding. Properly constructed pond farms, with adequate drainage facilities, on these sites are expected to reduced the risk of mosquito breeding and thereby the prevalence of malaria. However, when shallow ponds are constructed and remain weed-infested without proper water management, they become equally inviting for mosquito breeding as the swamps on which the ponds were built. It is generally recommended that ponds should be at least 0.61 m (2 ft) deep and preferably 0.914 m (3 ft). The inside of pond dikes should be as steep as possible to avoid too shallow margins. The ponds should be kept free from all weeds, particularly floating ones, and all emergent vegetation cut back so as to avoid mosquito breeding. Any seepage from the pond should be drained off through seepage channels. Cattle should be prevented from grazing on the embankments, as hoof prints are notorious breeding grounds for mosquitoes in humid tropics. Most fish species, particularly in the fry and fingerling stages, will feed on mosquito larvae if available, and many feed on the algal vegetation that shelters them. So well-stocked ponds may be able to control the growth of algae as well as mosquito larvae. It has also been recommended that in mosquito-affected areas the ponds may be stocked with larvicidal fish, such as Haplocheilus panchax and Gambusia affinis.
Bilharizia (schistosomiasis) is another human disease transmitted through contact with water. It ranks among the most important public health problems of the tropics and sub-tropics and is probably only second to malaria as a parasitic disease. Human schistosomiasis is caused by at least three species of blood flukes
(trematodes), namely Schistosoma haematobium (genito-urinary bilharziasis), S. mansoni and S. japonicum (intestinal bilharziasis). The adult forms mature in the blood of humans (or other hosts) and the eggs are laid in the plexus round the colon. The eggs escape through the bladder or intestinal wall into the urine or faeces and, on contact with water, hatch and produce free-swimming larval forms called miracidia, which must find suitable snail hostswithin a day or perish. When the larva finds a suitable intermediate host it penetrates the skin and establishes itself in the snail, passing through several generations and multiplying to form further free-living larval forms called cercaria. These emerge from the snail and swimabout until they find a human host. They bore through the unbroken skin of the host (or through the buccal mucosa if the water is swallowed), make their way to the liver, mature and mate. The two sexes then make their way to the terminal blood vessel near the bladder or colon, where the eggs are laid, and the cycle begins all over again.
It is thus evident that water plays a significant role in the transmission of bilharziasis. It must be contaminated by infected people, and environmental conditions (water temperature, water flow, vegetation and the presence of organic foods) should be suitable for the growth of specific snail populations that serve as intermediate hosts. Evidently, the most effective and logical means of preventing pond farms and enclosures becoming a source of infection is adherence to personal hygiene by the farmhands and the neighbouring community. The use of contaminated water sources for the farm should be avoided, as far as possible. Another important measure is the control of snail populations. There are several genera of water snails that are known to be vectors. The clearance of emergent and submerged vegetation would greatly help in the control of snail infestation. There are a number of culturable fish species that feed on snails and could be used as the main or subsidiary species for culture. There are also smaller snail-feeding fish such as Astatereochromis alluaudi and Haplochromus spp., which could be introduced intoponds for the sole purpose of controlling snail populations. The system of combining duck raising with fish farming, practised in a number of countries now, could be an effective means of control, as ducks feed on snails as well as algal vegetation in ponds. The best method of avoiding snail infestation is reported to be the periodic draining and drying of ponds, followed by liming of the pond bottom. Parasitized snails are generally unable to survive prolonged desiccation and so periodic drying of the pond will be an effective means of eliminating snails carrying Schistosoma.
Besides the hazard of spreading water-borne diseases, possible public health problems that can be created by specific aquaculture practices also need to be considered. The most susceptible practices may be the use of large quantities of animal manures and the use of waste-water including domestic sewage in pond farming, as well as the culture of mollusks in contaminated waters. There appears to be no conclusive proof that the use of waste water, including sewage effluents, has caused human diseases. The high pH and oxygen levels in waste-water fish ponds could actually be producing quite disease-free environmental conditions, in contrast to expectations that such systems encourage fish parasites, disease and pathologies (Allen and Hepher, 1979). Bacteriological studies have shown that although a number of bacterial groups (Aeromonas, Pseudomonas, Streptococcus and Klebsiella) were isolated from the intestinal tracts of Pacific salmon and rainbow trout grown in waste water in experimental studies, none of them were isolated from other visceral organs (Allen et al., 1979). There was no evidence that any of the potential pathogens had become systemic under fish-culture conditions.
Though the zoonotic aspects of fish disease are undeniably important, diseases of fish that are definitely transmissible to man are few and are limited to certain pathogenic helminthes for which fish form the intermediate host. Transmission takes place through the consumption by man of raw or improperly processed fish flesh. The more important parasites are the tapeworm Diphyllobothrium latum and the flukes Clonorchis sinensis, Opisthorchis spp. and Metagonimus yokagawaii. But these have not been isolated from farmed fish.
Filter-feeding molluscs, like oysters and mussels, which are eaten raw could form greater public health risks if grown in contaminated waters. Oyster and mussel culturists should certainly avoid such areas or take the necessary action to prevent discharge of contaminants into the farming areas. The process of depuration that is adopted in organized shell-fish farming and the certification of the quality of shellfish products are additional measures that prevent public health hazards caused by aquaculture.