Bacterial diseases of coldwater fish

This article covers bacterial disease in cold and cool water species farmed in temperate climates — principally rainbow trout and cyprinids. Diseases of tropical species are outside its scope.

The three components of disease in fish

In aquaculture, disease is even more tightly bound to the environment than in terrestrial species. What matters is the intersection of three things: the agent, the host and water quality.

Most bacteria regarded as fish pathogens are already present in the water and on the fish’s surface — they are largely opportunists. Disease appears when the fish’s resistance falls. The main things that lower it are:

• Sudden shifts in water temperature — fish are ectotherms, and the immune response is temperature-dependent.
• Low dissolved oxygen.
• High ammonia and nitrite — usually a consequence of overfeeding or insufficient water exchange.
• High stocking density.
• Handling and transport — damage to the mucus layer opens the first line of defence.

This is why the first place to look in a fish disease investigation is almost always the water quality records.

The main agents

Aeromonas species

Aeromonas hydrophila and A. salmonicida are among the most common bacterial agents.

A. hydrophila is a classic opportunist, widespread in fresh water, producing superficial haemorrhages, skin lesions and abdominal distension in stressed fish. It is seen more often as temperature rises.

A. salmonicida is the agent of furunculosis, a more specific disease of salmonids. It takes its name from the abscess-like foci that form in muscle and can open to the surface. Carrier fish can harbour the organism without signs, which is why the source of incoming stock matters.

Yersinia ruckeri — enteric redmouth (ERM)

Named for the haemorrhage around the mouth and throat, although that sign is not present in every case. Exophthalmos, periocular haemorrhage and darkened skin are other findings.

ERM is a classic problem of trout farming. The organism can establish carriers among recovered fish and reappear during periods of stress. Vaccines are available in many countries and are widely used in production.

Flavobacterium species

This group is notable for producing different pictures at different temperatures.

Flavobacterium psychrophilum acts at low water temperatures and particularly affects young fish, causing erosion of the tail and fins and skin lesions. It can cause serious losses in the fry stage.

Flavobacterium columnare, by contrast, comes to the fore at higher temperatures. It produces yellowish-white, flat lesions on skin and gills; where gills are involved the course is rapid.

Their opposite temperature preferences are a useful guide to which agent to expect as the seasons turn.

Detecting it early

Signs of disease in fish are noticed later than in terrestrial animals. The main things to watch:

• Falling feed intake — usually the earliest and most reliable sign.
• Behavioural change — sitting near the surface, gathering at the edge of the pond, disordered swimming.
• Appearance — darkening skin, lesions, fin erosion, exophthalmos.
• A rise in daily mortality — hard to notice without consistent records.

Recording daily feed intake and mortality is the most practical early-warning tool in aquaculture.

Prevention

Water quality management. Regular monitoring of oxygen, temperature, ammonia and nitrite is the foundation of disease control.

Density and feeding. Overcrowding raises stress and overfeeding raises organic load; both translate directly into disease risk.

Reduce handling. Grading and transport damage the mucus layer. Do them only when necessary and allow the fish time to recover.

Biosecurity. The source of incoming fish, a quarantine practice, and not sharing equipment between ponds all limit the introduction of agents.

Vaccination. Vaccines exist for some agents — ERM and furunculosis among them — and are part of routine practice on many farms. Suitability varies by species and country.

Diagnosis requires sampling affected fish for laboratory examination. Authorisation and withdrawal-period requirements for medicines in aquaculture differ markedly between countries; plan treatment with the veterinarian who knows the site and within local regulations.