Fish health and disease eel

Eels can be cultured at very high densities, making them efficient users of space and culture facilities. However, careful management of the culture system in terms of water quality and temperature is essential if disease problems are to be avoided.

Eels appear to be more susceptible to diseases and resultant mortality than many other aquaculture species. Unstable temperature conditions and the accumulation of uneaten feed are direct or indirect causes of mortality in elvers and adult eels. Temperature changes affect their feeding activity and reduce their resistance to disease. Eels are also very susceptible to low dissolved oxygen concentrations. Table 18 lists the principal pathogens of cultured eels.
Saprolegnia fungal infection, or “cotton cap disease” as it is known in Japan, is a common cause of mortality in Japanese ponds. However, this infection is only a secondary condition; the primary cause of disease is a pathogenic bacterium. This infection occurs in spring and autumn, when temperatures are 15-20°C. The symptoms include white patches on the body of the eels.

 

Table 18. Specific pathogens of cultured eels
VIRUSES
? Rhabdoviral dermatitis
? Herpesvirus anguillae (EHVF: “Eel herpesvirus in Formosa”) ? Infectious Pancreatic Necrosis Virus (IPNV)
BACTERIA
? Pseudomonas anguilliseptica (red spot disease) ? Edwardsiella anguillimortiferum (red disease) ? Edwardsiella tarda (liver-kidney disease)
? Vibrio furnissii
? Vibrio anguillarum
? red-fin disease
? common enteritis
PARASITES
Protozoa
? Trichodina sp.
? Ichthyophthirius multifiliis (Ich)
Copepoda
? Ergasilus celestis
? Lernaea cyprinacea
Microspora
? Pleistophora sp. (white spot disease)
? Pleistophora anguillarum (beko disease)
? Myxidium sp. (dermatitis)
? Mixobolus sp. (white spot disease)
Monogenea
? Pseudodactylogyrus anguillae
? Pseudodactylogyrus bini
? Gyrodactylus sp.
Crustacea
? Argulus giordani
Nematoda
? Anguillicola crassus
? Anguillicola globiceps
FUNGI
? Saprolegnia sp.(cotton cap disease)
? Branchiomycosis

The “red disease” in pond-cultured Japanese eels, caused by Edwardsiella anguillimortiferum, is generally reported in the summer when temperatures exceed 28°C but it also occurs in the spring. This pathogen causes macroscopic putrefactive lesions in the kidney or liver, resulting in high mortality rates. The eels that survive develop a strong immunity and are medicated with food containing chloromycetin or sulphadiazine. In Europe, the same disease is said to be produced by Vibrio anguillarum, a common pathogen of many marine fish (Pillay 1995).
Edwardsiella tarda is the pathogenic bacteria of the liver-kidney disease of eels, and has caused mortalities in pond culture in China. This disease occurs mainly during the periods from February to May and from October to December each year, and causes serious damage to the eel culturing industry in Chaozhou, Guangdong Province (Quanzhang and Xinling 1994). In Japan, Edwardsiella tarda causes mortalities during all stages of eel culture, and particularly at the glass eel (Anguilla japonica) (post-larval) and elver (young eel) stages (Salati, Ono and Kusuda 1991). Pseudomonas anguilliseptica is the aetiological agent of “red spot disease” or “Sekiten-byo” in the Japanese eel. This bacterium, although less common in European eels, has been found in The Netherlands by Haenen and Davidse (2001) causing low levels of mortality at high temperatures (15-25°C in
A. anguilla). The symptoms include a pale body, petechial haemorrhage in the back and tail, liver haemorrhage and congested posterior kidney.
A toxigenic vibrio (Vibrio furnissii), detected in A. anguilla, is also pathogenic for eels, causing necrotic areas, fluid accumulation in the body cavity and a swollen intestinal tract in elvers (Amaro, Biosca and Garay 1995). Japanese eels are also affected by rhabdoviral dermatitis, caused by a virus that makes fish moribund, with cutaneous lesions, necrosis of the dermal strata, haemorrhage and inflammatory cellular infiltration; it occurs at temperatures between 15°C and 20°C but not at 25°C; for this reason this kind of disease never appears in warm water environments (Kobayashi, Shiino and Miyazaki 1999).
Trichodina sp., is a parasite found in European eels cultured in re-circulation systems in Denmark. Production there is based on imported glass eels (from France) or elvers caught in river estuaries along the European Atlantic coast (Madsen, Buchmann and Mellergaard 2000); this is thought to be the likely source of the infection. The “white spot disease” is caused by parasitic sporozoa-like species (Pleistophora, Myxidium and Mixobolus) and affects mostly elvers but sometimes also adults. This disease affects the kidneys and muscles: the bodies become black, with disappearing patches of pigmentation and the animals swim vertically (Pillay 1995). Anguillicola crassus is a parasite affecting Japanese and European eels; these species seem to have not developed any antibody response in laboratory experiments (Haenen et al. 1996). Between 30 and 100 percent of European eel populations are infected by this nematode. In Europe, it was first recorded in 1982 from eels from the Weser-Ems region in northern Germany, but soon it occurred in many other German localities (river basins of the Elbe, Weser and Rhine, water bodies of Berlin) as well as in Holland, Belgium, Denmark, northern Italy and England (Moravec 1992). In 1998 it was found in the river Erne in Ireland (Evans, Matthews and McClintock 2001).
The parasite Anguillicola crassus, together with the swim-bladder nematode (A. globiceps), affects farmed and wild populations of both Anguilla japonica and A. anguilla. Anguillicola crassus is found in Japan, the Republic of Korea, Taiwan Province of China, and China, while A. globiceps is reported only in Japan and China. These nematodes use cyclopoid copepods as intermediate hosts. Other known intermediate hosts of A. crassus are Eucyclops serrulatus (Japan) and Thermocyclops hyalinus (Republic of Korea), and Mesocyclops leuckarti, T. hyalinus, T. taihokuensis, E. serrulatus, Acanthocyclops viridis and Cyclops strenuus (China). A. globiceps and A. crassus show a seasonal occurrence in T. hyalinus with a high prevalence in summer. Paratenic hosts are as yet unknown in Asia. A. crassus is relatively common in farmed and wild populations of Japanese eels in Asia, but A. globiceps is usually found only in wild populations of Japanese eels in Japan and China. In culture ponds, A. crassus is more prevalent and abundant in European eels than in Japanese eels. Although A. globiceps merely induces the thickening of the host’s swim bladder wall, A. crassus has severe pathological effects on European eels and heavy infection leads to host mortality. The prevalence of A. crassus in Japanese eels cultured in Japan and the Republic of Korea is relatively low in winter, whereas the prevalence of A. globiceps in wild populations of Japanese eels from Japan is high in winter (Nagasawa, Kim and Hirose 1994).
Other infections are caused by the crustacean parasite Argulus giordani, found frequently in “valli” culture (Italy); the anchor worm Lernaea cyprinacea in Japanese ponds (which can now be simply prevented by repeated troclorform baths, or “cured”, for example by removal by hand); and Ichthyophthirius multifiliis, which causes “ich” (Pillay 1995).
A branchial kidney disease (Branchionephritis) has been identified in Japan, but the causes remain uncertain. It has caused considerable economic losses in eel farms. This disease makes the skin of the gill lamella swell, causing adhesion, and the inflamed kidneys show signs of bleeding (Pillay, 1995) In 1988, a new virus was isolated from both the Japanese eel (Anguilla japonica) and the European eel (A. anguilla) cultivated in an intensive culture system in Japan. This virus was designated Herpesvirus anguillae (eel herpes virus).
The microsporean Pleistophora anguillarum is the etiologic agent of “beko disease”, which affects Anguilla japonica in Taiwan Province of China. This parasite attacks the muscle tissues, prominently the skeletal muscle (Kou and Lo 1994). Other eel diseases reported in Taiwan Province of China are parasitic diseases, enteritis type bacterial diseases (red-fin disease, red-spot disease, vibriosis, edwardsiellosis and common enteritis), fungal diseases (branchiomycosis and saprolegniasis), gill diseases (gas disease and gill ulcer) and diseases caused by water quality, deformity, nutritional disease, and drug injuries which evolve into diseases (Shih, Lu and Chen 1993). IPNV (Infectious Pancreatic Necrosis Virus) (Hsu, Chen and Wu 1993) and EHVF (Eel Herpesvirus in Formosa) are found in Taiwan Province of China (Shih, Lu and Chen 1993) in cultured Japanese eels.
A study carried out in three Canadian grow-out facilities (Nova Scotia, Newfoundland, and New Brunswick) identified two common gill parasites Pseudodactylogyrus anguillae (Monogenea) and Ergasilus celestis (copepoda) on wild Anguilla rostrata (Barker and Cone 2000). Pseudodactylogyrus species (P. anguillae and P. bini) are also found in A. anguilla (Buchmann 1993).
Brief accounts are provided of other infectious and parasitic diseases of eels, particularly in Australian eels. Included are: aeromoniasis, or red fin disease; Myxidium dermatitis; Myxobolus infection; Trichodina infection; white spot disease, or Ich; Pseudodactylogyrus and Gyrodactylus gill parasites; Anguillicola roundworm; saprolegniasis; and a leech, Zeylancobdella (Gosper 1995). Excessive levels of oxygen and nitrogen in water can cause bubble-like tumours in elvers; in some cases gas can be found in muscles and blood vessels. The symptoms can be cured by introducing clean water at lower temperatures (Pillay 1995).
Although eels are extremely susceptible to many fish diseases, most can be minimized by careful control of imported glass eels and elvers, and by good management and husbandry systems that manage water quality, temperature and oxygen levels. Any stress can lead to the development of a disease.