Ulcer Disease, Dropsy, Fin Rot and Lice in Pond Fish: Signs and Prevention

Disease, a major problem for fish farmers, is more common in fish raised in closed waters such as ponds than in open rivers and wetlands. In Bangladesh the diseases farmers know best are ulcer disease, dropsy, fin and tail rot, fish lice and disorders caused by poor nutrition. According to the Department of Fisheries, the country's aquaculture produced 2.58 million metric tons of fish in 2019-20, and Bangladesh ranks fifth in the world in inland aquaculture; spotting disease early protects farm incomes.

In a 2020 survey of 90 farms, hatcheries, nurseries and drug shops in Narsingdi district by Sylhet Agricultural University researchers (Heliyon, 2022), farmers named ulcer disease, fin and tail rot, dropsy, white spot and argulosis (fish lice) among the most common diseases; 90 percent of respondents reported ulcer disease.

Ulcer disease is known to scientists as epizootic ulcerative syndrome, or EUS. According to the 2001 diagnostic guide of the Food and Agriculture Organization (FAO), it is caused by the fungus-like water mould Aphanomyces invadans. Red spots appear first and deepen into ulcers that reach the muscle. The wounds often pick up secondary bacterial infections, and outbreaks usually kill many fish.

A study published in PLoS One in 2023 says the disease affects more than 160 freshwater and estuarine species; it was first recorded in Japan in 1971 and has since been reported in more than 26 countries. FAO notes major outbreaks across South and Southeast Asia, including Bangladesh and India, and links outbreaks with low temperatures. Snakeheads (Channa), the group that includes shol, gajar and taki, are especially susceptible, and a survey in Mymensingh found the disease in mrigal, rohu and catla. Tilapia and Chinese carps, by contrast, are generally resistant.

Control in wild waters is almost impossible; for farms, FAO lists these measures:

• drying and liming ponds before stocking
• keeping wild fish out of the pond
• stocking hatchery-reared fry that have been treated as a precaution
• salt bath treatments
• disinfecting nets and equipment

Dropsy is a symptom rather than a disease: fluid builds up in the body cavity or tissues. It can follow bacterial or parasitic infection, liver problems, poor or suddenly changed water, or long stress. The belly swells, the eyes bulge, the scales stand out, the gills turn pale and the fish become sluggish. The Narsingdi survey found it mostly in tilapia and catfish, and the Mymensingh survey in pangas, tilapia and pabda.

Fin and tail rot begins at the edge of the fins and works towards the base; once it reaches the base, the lost tissue does not grow back. It is caused by bacterial or fungal infection, usually set off by poor water, injury, poor diet or stress. In Narsingdi it was reported in tilapia, carp and catfish. FAO advises keeping sick fish as far as possible from healthy ones and, if deaths continue, sending samples to a laboratory to establish the cause.

Fish lice are mainly parasites of the genus Argulus, which has about 140 species. They cling tightly to the fish; young lice feed on mucus and skin cells, and adults feed on blood. The Narsingdi survey found lice on carp, and the Mymensingh survey on pangas and rohu.

Feed short of protein, vitamins or minerals slows growth and invites disease. FAO's 1992 report on nutritional fish pathology lists, for common carp short of vitamin A, loss of appetite, faded colour, bleeding in the fins and skin and bulging eyes. A bent spine is linked mainly to a lack of vitamin C, while a lack of vitamin K slows blood clotting and causes anaemia and bleeding. Growing natural food in the pond and feeding a balanced diet regularly are therefore important.

The Narsingdi survey found about 140 drugs and chemicals and 30 antibiotics in use. The researchers reported that many farmers used them without knowing the right dose, method of application or withdrawal period before sale, a concern for human health too, and recommended proper training.

Photo: Khalid Mahmood / Wikimedia Commons (CC BY-SA 4.0)

Sources: Jagonews24, FAO (Asia Diagnostic Guide to Aquatic Animal Diseases, 2001; Nutritional Fish Pathology, 1992), Heliyon (Kawsar et al., 2022), Veterinary Medicine and Science (Rasul et al., 2025), PLoS One (2023), Veterinary World (2015), Wikipedia