Abdominal Swelling

Rainbowfishes with abdominal swelling or distended abdomen often float on the surface and are unable to feed or function normally. This may be accompanied by an abnormal pitch (i.e., head up or head down). In most cases it has a sudden onset and without any preceding traumatic event; most eventually die. Swelling of the abdomen can become very severe before it impairs normal activity in fish. The prevalence of the disease is relatively low and in most cases the cause goes undiagnosed. Some cases that have been investigated showed no consistent clinical features and the abdominal swelling may simply be a terminal clinical sign.

Possible causes are systemic bacterial infection or improper diet. Many have chronic diseases despite the sudden onset and often have granulomatous disease, usually due to mycobacterial infection. Fluid filling of the swim bladder has been associated with bacterial and fungal infection, which may then result in rupture of one of the chambers.

Abdominal distension or swelling can also be the result of swim bladder dysfunction, ascites (Dropsy), hepatic lipidosis, neoplasia (tumour), organomegaly (abnormal enlargement of organs), abscess, egg or faecal impaction. Hepatic lipidosis (Fatty liver) is common in rainbowfishes which are fed a diet high in animal fat. Hepatic neoplasia is known to occur as a result of feeding old or mouldy fish foods containing mycotoxins. Tumours, polycystic disease and large granulomatous lesions of the kidney can cause unilateral displacement of the swim bladder resulting in abnormal buoyancy. There are very few references in the scientific literature. However, there is much comment and speculation in the hobby literature and on several Internet web sites, which most call ‘swim bladder disease’.

Rainbowfishes with bloated stomachs may often recover if maintained at a higher temperature. Low temperatures will slow down their metabolic rate and reduce or stop the digestive processes. If rainbowfishes are fed a high-nitrogen feed (e.g., high-protein pellets) the food can remain undigested or half-digested in the digestive tract and the gases produced can cause the fish to become bloated, lose their appetite, and eventually die.

Magnesium sulphate (MgSO4, Epsom salt) is frequently used as a purgative: 2–5 grams/litre as a permanent bath is often physiologically beneficial to freshwater fish. Magnesium salts retain or attract water into the intestinal lumen mainly by osmosis, which distends the gut and increases peristalsis, thus producing defecation. Another mode of action of magnesium salts is causing the release of cholecystokinin, which increases peristaltic activity of the intestine. Increasing or decreasing the water temperature by a few degrees, provided it is within the fish’s tolerance range, may alter the fish’s metabolic rate and assist recovery in some cases. Starving for 2–3 days allows the bowel to empty and eliminate any gas-producing contents. Antibiotics given by injection or in the food may be effective in some cases where bacterial infections are involved.