Bacterial infections and diseases

Where do bacterial infections come from?

It is important to our understanding of bacterial disease to realise that there are essentially two types of pathogenic (disease-causing) bacteria

• Primary or obligate pathogens – These are pathogens that are not part of the normal aquatic flora and are capable of causing disease in healthy individuals. Typically, Aeromonas salmonicida

• Opportunistic pathogens. These are normally free-living, either in the water on the fish, but are capable of adopting a pathogenic role under certain circumstances. Many of these are saprophytes, normally living on dead organic matter such as plant and animal remains or faeces. Typically, Aeromonas hydrophilia, Pseudomonas and Vibrio. In organically-rich water there may be millions of these opportunistic bacteria in every litre of water, but under normal circumstances, they do not pose a threat to healthy fish. This situation can be directly compared to the normal microbial flora of the human mouth and skin which are teeming with potentially harmful ‘germs’ but rarely cause any problems unless the bugs can get past our body defences.

As you might have guessed, in general most of the bacterial diseases that affect ornamental fish are caused by opportunistic bacteria. Which means that most bacterial infections result because of changes in the bacteria / fish relationship. This is a very delicate relationship, but generally when an opportunistic bacterium tries to establish an infection the fish host is able to resist by a variety of defence mechanisms. However, if the numbers or virulence of the pathogen increases or the host resistance is reduced the pathogen may be able to succeed

Host / parasite relationship: The balance between susceptibility and resistance determines either good health or disease

How do opportunistic bacteria become pathogenic?

I should stress that we are taking a somewhat over-simplified view of a complex subject, but in general there seems to be three major ways that infections occur

• Damage to the integument (skin) resulting from an abrasion, parasite activity or chemical damage (for example from ammonia, nitrite or high/low pH). My own experience is that small cuts or abrasions on healthy fish rarely become infected – in much the same way that we do not get infections every time we cut ourselves. I mention this because there is often a tendency to diagnose obvious bacterial infections as “cuts” or scrapes and hope they get better.

• The fish’s normal defences are depressed as a result of environmental stress or disease. This enables opportunistic bacteria to take advantage. This is similar to the situation when we catch colds and other diseases if we are run-down. Fish can become stressed for a variety of reasons, the most common reasons being poor water quality, parasites, overcrowding, thermal stress.

• A substantial increase in the numbers of opportunistic bacteria such as we would find in systems with other infected fish and /or high levels of decomposing organic matter. This is typically the situation when the disease starts to spread and several fish are affected. Under these conditions, especially if fish are still stressed, it seems that some opportunistic bacteria can become primary pathogens through weight of numbers.

It is because of these complications, together with tremendous reproductive potential of most bacteria, that a seemingly minor problem can rapidly snowball out of control leading to high numbers of infected fish and possibly many losses.

Treating bacterial infections

It is hard to lay down hard and fast rules about treating bacterial diseases. It mostly comes down to the experience of the person on the spot in deciding how severe the situation is and what course of action is required. However, before any treatment is started it is important to try and establish the underlying cause – particularly if several fish are affected, as this would suggest some form of environmental stress.

The reason why this is important is that few, if any, treatments completely destroy all the pathogenic bacteria. What they do is knock the bacteria back for a short time, giving the fish’s own defences chance to start fighting back and the wound-healing process chance to start.

Clearly, the fish will only be able to fight back if conditions are improved and all efforts are made to keep the numbers of bacteria as low as possible. So really any anti-bacterial treatments should be seen as supportive treatment for the fish’s own immune system and what we are trying to do is to tilt the balance of health back in the fish’s favour.  

There are broadly four treatment methods for bacterial infections. They can be used individually or in combination for an enhanced effect

Baths:

• Short-term baths for a few hours each day using anti-bacterial products such as chloramine-T, potassium permanganate or antibiotics

• Long-term pond or tank treatments usually using a proprietary anti-bacterial or anti-ulcer product

On their own baths are only likely to be effective for relatively minor bacterial infections such as fin rot or situations when just one or two fish have minor ulcerations. This supposes that any underlying problems are resolved or treated. Baths are unlikely to resolved severe cases.

Topical treatments:

This involves cleaning the lesion, usually with an antiseptic solution and applying a waterproof ointment or cream such as Orabase or similar. Topical treatments, if used properly can be very effective in dealing with minor to moderate cases – providing that the infection is not systemic. I would normally expect to see signs of improvement – that is a reduction in inflammation or the first visible signs of skin growth – within seven days of the start of treatment. If there is no sign of healing within a week to ten days a change of treatment is vital.

Medicated food

These are specially prepared foods containing antibiotics. In the UK they are only available with a veterinary prescription. While the use of medicated food has obvious advantages, the disadvantages are the slight delay while the food is ordered and processed, very sick fish may not be eating and there is widespread resistance to the most commonly used antibiotics. It is possible to prepare your own medicated food by top-dressing ordinary food with an appropriate antibiotic.

Antibiotics (injectable)

This usually involves a course of five injections over a ten-day period in conjunction with topical treatments. It is important to use the appropriate antibiotic (established by laboratory antibiotic sensitivity tests). This procedure does require a degree of knowledge and skill. As it usually involves sedating the fish for every treatment it can be very time-consuming, particularly if large numbers of fish are involved. The positive side is that there is a very high success rate (typically > 90%)– even in the most severe cases.