Factors affecting fish health
Fish health or the health of aquaculture organisms has to be conceived as a state of physical well-being. The importance of proper nutrition for rapid growth and the prevention of nutritional deficiencies have been discussed. Adequate nutrition is also vital for the overall health and vigor needed to cope with a variety of disease agents. Nutritional deficiency symptoms associated with vitamin imbalances are well documented . However, imbalances in vitamin content of fish diets are not the only causes of nutritional diseases. Thyroid tumours, liver degeneration, visceral granuloma, anaemia and pigmentation impairment can be caused by other forms of nutritional imbalances. High levels of starch may give rise to symptoms of diabetes in trout and enlarged liver in channel catfish. Freedom from disease is an essential element of physical well-being, but physical and environmental stress have also significant roles in the maintenance of healthy conditions. Many of the potential pathogens of aquaculture species are normally found in the aquatic environment, but in spite of their presence disease may not occur. Obviously, disease is essentially the result of interaction between the species, the disease agent and the environment. So the three major factors of significance are the susceptibility of the species to the pathogens present, the virulence of the pathogenic agent and the environmental conditions that may trigger epizootics.
Despite the individual importance of each one of these factors in the maintenance of good health or avoidance of disease, it should be emphasized that it is the balance between these factors that determines the state of health. Even in the presence of all three factors, the interaction may be such that no disease occurs. But a disturbance in any of the factors, leading to disruption of the relationship, can give rise to disease.
The susceptibility or the resistance of the culture species to the action of the disease agent is governed by its physical barriers, its exposure experience and its age. Among the physical barriers are the skin, scales, exoskeleton or shells and mucous membranes which limit the entry of toxic, infectious and parasitic agents. The physiological defences that keep the body from being overrun include the white blood cells that engulf pathogens, avoidance mechanisms, detoxification of chemicals from water or diet by the liver, storage of certain metals by the bones and local tissue reactions. The overall nutritional well-being is the source of the host’s physiological ability to defend itself. The immune system and its specific activity against biological agents such as viruses, bacteria and parasites forms an important means of disease resistance. Populations with previous exposure to specific disease agents will generally not be as readily susceptible as those on a first encounter. For this reason and also because of the fragility of their defence system, young ones are more susceptible to diseases than older ones, except that the spawners may experience additional stress because of their reproductive functions. The species specificity of certain disease agents is also a factor of importance in understanding health hazards.
Once the pathogen has established itself within or on the host under favourable condi-tion, the infection may take one of three routes:
a) the pathogen proliferates, eventually causing mortality of the host;
b) the defences of the host surmount the infection and eliminate the pathogen from its system; or
c) a carrier state develops, whereby a balance between the host and the pathogen may persist generally, with no evident disease symptoms.
From an aquaculture point of view, the greatest concern is the rapid multiplication of the pathogen within the host and the danger of transfer to other individuals of the host population, which may result in an uncontrollable epizootic. During the incubation period (which is the interval between the penetration or establishment of the pathogen in the host and the appearance of the first symptoms of the disease), the host will often be shedding the pathogen. If the host recovers after this initial stage, or after any of the later stages of the infection, without entirely eliminating the pathogen, a carrier condition exists. A carriercan disseminate the pathogen into the surrounding environment or can harbour it in a latent state without shedding. So, even after the clinical stage of infection, some individuals recovering from the disease continue to disseminate pathogens in a manner similar to those that are chronically ill.
As the transfer of infection can occur without the manifestation of disease symptoms, the infections may often be difficult to identify and can be passed unnoticed from individual to individual or even generation to generation.
Until the population experiences particularly stressful conditions, which exacerbate disease symptoms, no infection may be suspected. The problem of carrier states in aquaculture species remains one of the most crucial ones for the aquaculturist.
When a disease outbreak is encountered, the pattern of losses, the size of hosts affected and the duration of the epizootic provide valuable information. Sudden, explosive mortalities often implicate acute environmental problems, such as oxygen deficiency, the presence of lethal concentrations of toxicants or lethal levels of temperature. The appearance of a few sick individuals, unusual behaviour or loss of appetite can indicate the beginnings of infectious disease. A disease is generally due to the inability of the host to adjust adequately to environmental stress and consequent dominance of the pathogen, and so the aquaculturist should act quickly when losses occur in typical patterns. A balance between the host and the pathogen should be restored by resolving environmental problems and by effective therapeutic treatment. Timely action is the essence of success in controlling epidemics of mortality in aquaculture, but it needs considerable skill to correct adverse environmental conditions in time to prevent major losses.
The type of aquacultural practice adopted has a decisive role on the susceptibility of the culture species. As indicated earlier, a high density of stocks and the use of restricted spaces like cages, tanks and raceways lead to closer contact between individuals as well as environmental stress. Higher stock densities also mean the use of larger quantities of concentrated feeds and/or fertilizers. This leads to denser growth of plankton and benthos which may include intermediate hosts of disease
agents. The environmental and disease risks related to overloading of ponds and enclosures with organic manures are very considerable. The use of heated water effluents from industries and sewage effluents also has builtin-risks; so an aquaculturist has to be prepared for quick and effective action, when an adverse situation develops.
While some of the aquaculture practices are conducive to diseases, there are others that are effective in controlling them. For example, the practice of regular drying of fish ponds and application of lime on the pond bottom helps to kill parasites and many other infectious disease agents.
The following transboundary aquatic animal diseases had a devastating influence on aquaculture, especially in the Asia-Pacific region. The losses incurred by the industry have led to closure of many farms. A global estimate of the losses made by the World Bank in 1997 was in the range of US $3 billion per annum for epizootic ulcerative syndrome (EUS). First reported from Japan as a mycotic granulomatosis (MG) in freshwater ayu, EUS now occurs, though not in a virulent form, in many Asian countries, affecting over 100 species of wild and cultured fish in fresh water and to some extent in brackish waters. The causative agent has been confirmed as a fungus,
White spot syndrome virus (WSSV), first reported in Taiwan province of China, was found in Japan in 1993 and later in almost all shrimp-producing countries in Asia and the
Americas. It has been officially confirmed in at least nine of those countries. In 1997 losses were in the range of US $600 000 in Thailand alone.
Viral nervous necrosis (VNN) causes serious mortality among groupers cultivated in the Asia-Pacific region. This disease was first reported from Japan and has since been reported from Indonesia, Korea, Singapore and Thailand. Expanding grouper aquaculture increases the risk of introducing the pathogen in new localities and environments.
Neobenedenia girellae, one of the commonlyreported parasites of the grouper and other marine fishes, was introduced into Japan along with amberjacks from China and Hong Kong. This caused heavy infection among cultured flounder, one-year-old amberjacks and over 15 cultured marine fishes including brackish-water tilapias. The parasite is a serious threat to grouper culture in several countries of South-east Asia.
Biological agents are probably the most common cause of disease initiation and are the primary focus of attention in infectious diseases. As mentioned earlier, potential pathogens are always present in the aquatic environment. They may include viruses, bacteria, fungi, protozoans, parasitic crustaceans, helminths and other worms. The virulence or pathogenicity of the agent is the relevant factor in the determination of health hazards. It depends upon the physical or biochemical attributes of the agent. Bacteria with flagella or with capsules are generally better equipped to invade the host and resist adverse conditions. Some bacteria are able to elaborate toxins, which cause haemorrhage or affect the nervous system of the host. Enzymes such as chitinase enable bacteria to erode chitinous membranes. Parasites, on the other hand, attach themselves to the host through special organs of attachment, such as suckers.
Penetration into the host is the first step for a microbial agent to multiply and invade the vital organs of its host. This normally happens through ingestion, rupture of the skin, transgression of gill lamellae or penetration of the egg membrane. The specific point of entry may have a decisive role on the virulence of the microbe. Wounds in the skin are common entrance points for some of the bacterial and viral infections, which in turn invite fungal secondary invaders such as Saprolegnia sp. Other routes of entry are usually (i) the gills, where the pathogens can either enter the body through the delicate and thin epithelium, or establish themselves on them as in the case of protozoan infection with Schizamoeba salmonis or Ichtyobodo necator (Costia necatrix), and (ii) the digestive tract, where protozoans like Ceratomyxa shasta may become numerousenough to weaken the fish. Some bacteria may penetrate the intestinal lining under certain conditions. Eventually the pathogen may return to the aquatic medium when shed by the host.The host/pathogen relationship generally undergoes several stages of development. The incubation period is when the pathogen multiplies but the host does not yet show clinical signs of disease. The incubation period may range from a day or two for virulent pathogens to prolonged periods of several months. After this asymptomatic period, specific and nonspecific signs of disease become evident. Whether the host dies or survives will depend on its ability to resist the infection. During an epidemic, some of the infected animals may not exhibit clinical signs at all and become carriers, capable of transmitting the disease agent or initiating a future epizootic. Animals that recover from a disease may be completely free of the disease agent or continue to be asymptomatic carriers. In many instances, a disease condition may involve more than one pathogen or the infection by one primary agent may create conditions suitable for a secondary agent to gain access. Bacterial infections often follow the establishment of a parasite or of a virus. It is not uncommon to find a wide array of disease and parasitic problems occurring simultaneously.
The environment plays a crucial role in disrupting the balance between the host and the pathogen. In many situations, the culture animals live a healthy normal life in the presence of pathogens; but when environmental stresses occur and the balance tips in favour of the disease, the pathogen gets the upper hand and disease conditions ensue.
As the primary environmental parameters required would have been adequately considered in selection of the site and species, the relevant stress factor would normally be environmental disturbances that extend the adaptive responses of the animal beyond the normal range or affect the normal functioning to such an extent that chances of survival are significantly reduced. Morphological, biochemical and physiological disturbances occur in different stages and are characterized by a variety of metabolic conditions, such as anoxia, fright, forced exertion, anaesthesia, temperature changes and injury. Though the effect of stress is the alteration of host biochemistry in order to increase the probability of survival of the host, some of the resulting metabolic changes contribute also to increased susceptibility to infection.
Of the physical factors, temperature is one that has an effect on a number of other variables in the environment. Temperatures above or below the tolerance limits of the host animal create stress. Increased metabolic rate caused by high temperature results in higher oxygen demand. However, dissolved gases, including oxygen, generally decrease in solubility with increasing temperature. Also the solubility of toxic compounds increases with increasing temperature, creating unfavourable conditions.
As well as the environmental effect on the host, the effect of temperature on the pathogen is also an important factor to be considered. For example, a rise in temperature generally accelerates to a certain limit all the biological processes of the causative agent, lowering its viability and sometimes causing its death. Similarly, lowering of temperature decreases the biological processes to a certain minimum below which the organism may not survive. Pathogenic organisms of the same genus in the same host may react differently to a change in temperature.
The minimum water quality conditions necessary to maintain fish health are:
dissolved oxygen 5 ppm
pH range 6.7–8.6 (extremes 6.0–9.0)
free total CO2 3 ppm or less
ammonia 0.02 ppm or less
alkalinity at least 20 ppm (as CaCO3)
Obviously there are differences in the tolerance limits of different species, but these values provide a general guideline. Levels of tolerance of other elements are chlorine: 0.003 ppm; hydrogen sulphide: 0.001 ppm; nitrite (NO2): 100 ppb in soft water, 200 ppb in hard water; and total suspended and settleable solids: 80 ppm or less.
Pesticide pollution is one of the common causes of environmental stress in aquaculture situations. The maximum pesticide concentrations that may be tolerated by fish, without noticeable effects, and recognized by the Environmental Protection Agency of the USA, are listed in Table 9.1.
Even though it is not a hazard to the aquaculture species itself, the development of off-flavour is a phenomenon that seriously affects the economics of culture. The earthy or musty taste of fish grown in affected ponds would make them unmarketable. The cause of off-flavour is reported to be a compound called geosmin produced by actinomycetes and a number of blue-green algae of the genus Oscil-latoria (such as O. princeps, O. agardhi, O. tenuis, O. prolifica, O. limosa, and O. muscorum). All these organisms grow on mud that ishigh in organic matter. The organic matter decomposes, causing the reduction of the mud.
These organisms grow well on the interface between the reduced mud and the oxidized water layer above it. The off-flavour generally disappears when the fish are held in clean water (preferably running water) for one to two weeks.
Another source of off-flavour in fish is industrial wastes. The odour and taste of these wastes are usually concentrated in the fat deposits of the fish’s body. The most important chemicals that impart off-flavours are phenols, tars and mineral oils. Chlorinated phenols, such as o-chlorophenol and p-chlorophenol, impart a distinct flavour to carp even in low concentrations of 0.015 and 0.06 mg/l respectively. Eels and oysters are even more sensitive and develop off-flavour when the water contains as little as 0.001 mg/l o-chlorophenol. A concentration of 5–14 mg/l mineral oil, or less if in suspension, also imparts a distinct flavour.
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