In eel farming, gill nephritis must be strictly prevented.

Gill nephritis is a disease that causes great harm to eels. The epidemic period of this disease is relatively long, beginning in December and lasting until May of the following year. Small-sized eel fingerlings can have a mortality rate of over 50% when affected by this disease during the overwintering period. Larger eels develop the disease somewhat later than small-sized fingerlings, generally between February and March. If the disease is severe in autumn, the harm from gill nephritis in winter will be greater. When water temperatures begin to rise in February and March, secondary infections such as water mold disease, red fin disease, and red spot disease are likely to occur. Ponds whose water contains salt generally do not develop this disease; even if the disease occurs, it is far milder than in freshwater ponds. Warm-water eel farming and recirculating filtration eel farming generally do not develop this disease.

Symptoms: The anterior gill lamellae thicken and the gaps between the posterior gill lamellae narrow. In the early stage of the disease, there is no external abnormality, but the eel gradually becomes emaciated, with a depression appearing from the center of the abdomen to the anus, the ribs becoming prominent, and the liver area bulging slightly. As the disease progresses, the pectoral fins, anal fin, and anus all become red, and water mold grows on the gills. In severe cases, the gill lamellae all adhere together, the gill filaments become club-shaped, and even turn white and rot. The gills are generally dark red, sometimes with bleeding, and blood clots cover the gills; the kidney becomes swollen.

Prevention and treatment methods: Because the cause is unknown, there is currently no ideal therapeutic drug. 1) During the autumn rearing period, other diseases should be prevented, such as using furan-type drugs to prevent red fin disease and gill rot disease, to strengthen the eels' constitution and reduce triggering factors. 2) Separate and combine ponds earlier (late September to early October), continue feeding after combining ponds, and delay the time of stopping feeding. 3) For overwintering eels, regularly splash methylene blue over the whole pond or carry out saltwater disinfection to prevent this disease from being triggered by water mold disease. 4) In the feed fed before overwintering, add trace nutrients such as vitamins and minerals to strengthen the constitution of overwintering eels. 5) Oral tetracycline, mixed into each kilogram of feed at 5.5 mg, can prevent and treat this disease. 6) Saltwater medicated bathing at a concentration of 0.5%-0.7%, from December of that year to February of the following year, gradually adding freshwater after the water temperature rises, also has a certain therapeutic effect.