8.6 Common Fish Diseases and Their Treatment

Parasites

Ich (white spot disease): Ich is caused by the parasite Ichthyophthirius multifiliis (Ich). Ich appears on infected fish as small white specks on their skin and/or gills (Figure 21a). Fish may exhibit “flashing” behavior, characterized by a quick rubbing or scratching movements against the tank bottom, wall, or surface of the water (Durborow et al. 2000). Excess mucus is commonly present; however, the only clear sign may be a dead or dying fish. Treatment for Ich is difficult; however, elevating water temperature to above 85°F can kill Ich by disrupting its life cycle. Chemical treatments for quarantine tanks or decoupled systems include multiple treatments of formalin, copper sulfate (CuSO~4~), or potassium permanganate (KMnO~4~). Check appropriate dose rates before administering. These chemicals should not come into contact with plant components and must be administered in an isolated tank. Simply harvesting the fish may be the simplest solution.

Whirling disease: Caused by Myxobolus cerebralis, whirling disease primarily infects salmonids (trout and salmon) and can enter the aquaculture system through affected fish. Symptoms include abnormal swimming, darkening of posterior part, and skeletal deformation (Idowu et al. 2017). There is no true effective treatment for whirling disease. Producers should only purchase salmonid fingerling from hatchery that are certified whirling disease free and use treated water or ground water for production.

Bacterial Infections

Columnaris: Infections from Flavobacterium columnare are common in aquaculture-reared fish. Common symptoms include red or pale ulcers on the skin; yellowish mucus on the skin, gills, and/or mouth; and necrosis/erosion of the gills. Saddleback is a common lesion caused by columnaris and appears as a pale white saddle-like band encircling the body (Figure 21b). The bacteria can cause disease under normal culture conditions, but more likely when fish are stressed by low oxygen, high ammonia, high nitrite, high water temperatures, rough handling, mechanical injury, and crowding. (Durborow et al. 1998). Columnaris is typically treated with chemical treatment of the water using KMnO~4~ or by using Terramycin® (oxytetracyline HCl). Medicated feed that contains the antibiotics Aquaflor®, Terramycin® or Romet® may be effective. Chemical treatments or antibiotic feed should not come into contact with plant components and must be administered in an isolated tank.

Aeromonas: Aeromonas is a genus of bacteria that is widespread and is commonly isolated from freshwater culture environments. The disease caused by these bacteria in fish is called Motile Aeromonas Septicemia (MAS) (Hanson et al. 2019). Aeromonas infections are probably the most common bacterial disease diagnosed in cultured warmwater fish. Fish with septicemia often have hemorrhages (red areas or spots) on the skin, eyes, and fins; a dis¬tended abdomen; flared scales due to edema in the scale pockets (dropsy); and/or a red, inflamed anus (Figure 21c). Internally, the muscle and visceral tissue are often red, and the body cavity may contain bloody fluid. Typical MAS can be attributed to a predisposing factor, such as a handling event, temperature shock, water quality stressor, spawning, or aggression. Treatment is currently limited to three antibiotics: Aquaflor®, Terramycin® and Romet®-30. Proper withdrawal times for each antibiotic must be observed before treated fish can be processed/harvested. Chemical treatments or antibiotic feed should not come into contact with plant components and must be administered in an isolated tank.

Enteric Septicemia of Catfish (ESC): ESC is also known as “Hole-in-Head Disease” and is caused by the bacteria Edwardsiella ictaluri. It most commonly affects catfish species and is accountable for one-third of reported fish diseases in the southeastern U.S. Behavioral signs of infection include head-chasing-tail or whirling rather than swimming, as well as “star gazing.” External signs include red or white shallow ulcers, a hole appearing in the top of the head, and fluid buildup in the abdomen, causing severe distension. Treatment is typically administering medicated feed containing the antibiotics Aquaflor®, Romet®, or Terramycin®. Chemical treatments or antibiotic feed should not come into contact with plant components and must be administered in an isolated tank.

Viral Infections

Tilapia Lake Virus (TiLV): TiLV is one of the only significant viruses that affect tilapia in both wild and cultured situations. It is caused by Tilapia tilapinevirus and has been seen in Asia, Africa, and South America. It is transferred quickly through infected populations, and there is no treatment at the time of this publication.

Source: Janelle Hager, Leigh Ann Bright, Josh Dusci, James Tidwell. 2021. Kentucky State University. Aquaponics Production Manual: A Practical Handbook for Growers.