Tilapia, like other farmed aquatic animals, may be subjected to unfavourable conditions, leading to health problems and non-infectious diseases. Unfortunately, the information that is available on non-infectious disorders of tilapia is very limited. Only nutritional disorders, gas bubble disease and the disorders caused by pollutants are reasonably well documented. The following sections briefly discuss these disorders.
8.8.1. Gas bubble disease
Excessive aeration in intensive aquaculture sys- tems may result in supersaturating culture water with dissolved gases. When these gases are absorbed by the fish during respiration, they cause a change in partial pressure, leading to gas
accumulation (gas bubbles) at different sites of the fish body. In tilapia fry, gas bubbles are concen- trated in the yolk sac, while in older fish they are found in the gills and under the skin (Roberts and Sommerville, 1982).
Outbreaks of gas bubble disease have been reported in farmed saltwater juvenile tilapia,
O. spilurus (52.5 g), in Saudi Arabia (Saeed and
Al-Thobaiti, 1997). About 50% of fish populations were affected, resulting in 30% mortality. Another outbreak occurred in adult Nile tilapia (270 g) in brackish water, affecting about 40% of the fish population, with 40% mortality. Affected Nile tilapia were heavily infected with monogenetic trematodes. In all cases, the total gas was higher than 111% saturation, nitrogen was super- saturated and oxygen undersaturated. The disease symptoms were overcome by reducing the gas pressure through splashing the water and using water with lower gas pressure.
8.8.2. Nutritional diseases
The deficiency of certain nutrients in the diets of farmed fish may lead to disease symptoms and health problems. The effects of dietary vitamin deficiency on farmed tilapia are well documented (see Chapter 6 for more details). Blue tilapia fed pantothenic acid-deficient diets exhibited poor growth, haemorrhage, sluggishness, anaemia, hyperplasia in epithelial cells of the gill lamellae and high mortality (Soliman and Wilson, 1992a). Anorexia, low growth, fin erosion, loss of body col- our, cataract and mortality were also observed in fish fed with riboflavin-deficient diets (Soliman and Wilson, 1992b). In addition, low vitamin E contents in the diets resulted in poor growth, low feed conversion ratio (FCR), skin haemorrhage, muscle dystrophy, impaired erythropoiesis and abnormal skin coloration (Roem et al., 1990). Simi- lar deficiency symptoms have been reported in tilapia hybrids fed test diets deficient in niacin, pyridoxine and thiamine (Lim and Leamaster, 1991; Shiau and Suen, 1992; Shiau and Hsieh, 1997). In the meantime, Lim and Klesius (2001) reported low growth and reduced red blood cells and total cell counts in Nile tilapia fed diets deficient in folic acid.
On the other hand, the addition of certain feed elements to tilapia feeds may play a significant protective role in the fish, especially when they are
Stress and Diseases 157
cultured at a high stocking density and under dete- riorated water quality. Schlechtriem et al. (2004) studied the effects of dietaryL-carnitine on tilapia hybrids (O. niloticus× O. aureus) reared under inten- sive pond-culture conditions. The fish were fed diets containing either 150 or 450 ppm of
L-carnitine. Histological examinations revealed that fish fed 150 ppm carnitine showed the lowest permeability to fluorescein in gills, gut and skin epithelia, the highest activity in the system of active transport for organic anions and the highest levels of the multixenobiotic resistance transporter (MXRtr) for lipophilic/amphiphilic xenobiotics, compared to the control and the 450-ppm treated fish. The MXRtr activity in the liver bile canaliculi and renal proximal tubules of this group of fish was much higher than that of the other groups except those from the indoor tank. The intralysosomal accumulation of neutral red in their head–kidney macrophages was also signifi- cantly higher. The authors suggested that even a low level ofL-carnitine enrichment can provide considerable protective effects in fish reared under intensive pond-culture conditions.
8.8.3. Disorders caused by pollutants
Aquatic environments receive a wide variety of agricultural runoffs, including pesticides, herbi- cides, fungicides, fertilizer residues, heavy metals, etc. Considerable amounts of various industrial wastes, human wastes, molluscicides and oil com- pounds are also discharged into aquatic environ- ments. These organic and inorganic compounds are very likely to cause environmental impacts such as bioaccumulation, eutrophication, environ- mental degradation and imbalance of aquatic biodiversity. Farmed and wild fish, including tilapia, are therefore expected to be adversely affected by these pollutants. Such impacts depend on fish species and size, type and concentration of pollutant, duration of exposure and environmen- tal conditions. However, little attention has been paid to the impacts of environmental pollution on farmed tilapia. Despite the fact that hundreds of chemical compounds are likely to be hazardous to these fish, even at low concentrations, only a few compounds have been considered.
Phenolic compounds are among the most serious organic pollutants in the aquatic environ- ments. These chemicals are highly toxic to aquatic
animals and can cause severe ecological and eco- nomic losses. For example, Hart et al. (1998) found that Nile tilapia subjected to the carcino- genic polycyclic aromatic hydrocarbon (PAH) 7,12-dimethylbenzanthracene (DMBA) suffered from reduced spleen, pronephros and total white blood cell counts. The fish also exhibited reduced swimming activity and feeding activity and increased skin pigmentation and mortality. In another study, Mehrim (2001) evaluated the maximum tolerable level of phenol for fingerling Nile tilapia, and the effects of chronic exposure on growth performance and feed utilization efficiency. He found that 30 ppm was the maxi- mum tolerable level, and beyond that level fish exhibited a respiratory manifestation and hyper-irritability, followed by lethargy, lesions, increased mucus secretion, skin darkness, fin ero- sion, gill and liver congestion and distension of the gall bladder. Fish exposed to the maximum tolerable level (30 ppm) had significantly reduced growth, survival and feed utilization efficiency. The addition of Biogen as a feed supplement to the diets has significantly overcome the symp- toms mentioned.
Bayluscideis another phenolic compound that is widely used as a molluscicide. It is one of the chloronitrophenol derivatives (niclosamide ethanolamine salt) (5,2-dichloro-4-nitro-salicylic- anilide). This compound is frequently used for eradication of the intermediate host snails of schistosomiasis (bilharzia) and fascioliasis in Egypt. It is very toxic to snails, but it can be toxic to fish as well. Acute toxicity of Nile tilapia intoxi- cated with Bayluscide included erratic and nervous swimming, continuous opening of mouth and gill cover, haemorrhage under the scales and at the base of the fins and degenerative and necrotic changes in the liver, kidney, spleen, heart and gills (Nafady et al., 1986; Marzouk and Bakeer, 1991; Khalil, 1998). Khalil (1998) found that 0.3 mg/l was lethal to Nile tilapia, while 0.1 mg/l was sublethal. Fish exposed to 0.15 mg/l (50% of the median lethal concentra- tion (LC50)) suffered from nervous and respira-
tory manifestations, corneal opacity, a decrease in red and white blood cell counts, haemoglobin concentration and phagocytic activity, and a high accumulation of Bayluscide in the gills, liver and muscles.
Several other organophosphorus com- pounds, such as Metrifonate, Fenthion, Diazinon,
Foschlor, Endosulfan, Malathion, Nuvalron and Dichlobenil, are widely used as insecticides, molluscicides, herbicides and therapeutic agents for many diseases. As expected, these compounds can have a wide range of impacts on cultured and wild tilapias. Rajavarthini and Michael (1996) found that exposure to Nuvalron caused dose-dependent suppression of antibody response in Mozambique tilapia. Tilapia rendalli exposed to Endosulfan suffered from hepatic lesion, encepha- litis, meningitis, oedema and inflammatory infil- trate of eosinophilic granular cells in the brain (Makhiessen and Roberts, 1982). When Nile tilapia were exposed to Diazinon, they exhibited lethargic behaviour, loss of coordination, dark red gills, a pale liver, a distended gall bladder with dilated bile ducts, dilated arterioles and lamellar capillaries (El-Kateib and Afifi, 1993). Similarly, Nile tilapia exposed to a high level of Metrifonate showed a respiratory manifestation, loss of pigmentation, dark body coloration, protru- sion of the anal opening and haemorrhage (El-Gohary, 2004). Chronic symptoms included darkness of body colour, haemorrhage, decrease in phagocytic activity and necrotic and degenera- tive changes in the kidney, liver, spleen, heart and brain.
The above results clearly indicate that water pollution can have harmful impacts on tilapia. The use of the different chemicals and drugs for controlling pests, insects, snails, herbs, etc. should be carefully regulated and managed. In addition, culture water should be analysed prior to, and continuously monitored during, culture practices to investigate whether it contains any traces of pollutants and whether the levels of these pollut- ants are hazardous to farmed tilapia. In the late 1990s, a sudden, tragic, mass mortality occurred in Nile tilapia, cage-cultured in a waterway close to El-Salam Canal in Lake Manzala, Egypt, within the ‘Cages- for-Graduates Project’, which was funded by the Social Development Fund (A.-F.M. El-Sayed, Mansoura, 1999, personal observation). This massive mortality was attrib- uted to increased levels of pesticides in the canal from land runoffs. In fact, the owners of these cages faced a serious problem because they had to pay back the loans they had taken from the Social Development Fund. Only governmental interference saved them from prison. This partic- ular example stresses the necessity of regular monitoring of culture water, especially in areas
where the water may receive pollutants from var- ious sources.
8.9. Closing Remarks
1. Stressful conditions adversely affect tilapia