Bacterial infections in aquarium fish - the silent threat in aquariums

Types of Bacterial Infections (for information and understanding of complexity)

Most bacteria that cause fish diseases are actually common inhabitants of water and usually do not cause problems. However, fish that are stressed by one or more factors (such as temperature changes, poor water quality, recent transport, or handling) may have a weakened (less effective) immune system, making them more susceptible to bacterial infections. Moreover, stress factors that weaken the fish's immune system can actually promote the growth of bacteria, further increasing the risk of disease.

Bacterial pathogens affecting fish can roughly be classified into two groups based on their characteristics during so-called Gram staining (see below):

• Gram-positive bacteria: These bacteria stain blue during Gram staining. An example of a gram-positive bacterium commonly found in fish is Streptococcus.
• Gram-negative bacteria: These bacteria stain pink and are more common in the aquatic environment. Examples include Aeromonas hydrophila, Aeromonas salmonicida, Flavobacterium columnare, Vibrio, and Pseudomonas species.

Gram Staining is a crucial initial test that helps in categorizing the type of bacteria, which is informative for subsequent treatment decisions. Further, isolating the bacteria and conducting culture and sensitivity tests are essential. These tests involve growing the bacteria in a lab to identify the most effective antibiotics, thereby preventing the use of ineffective or overly broad treatments.

Columnaris: The Treacherous Killer of Aquarium Fish

One of the most common bacterial diseases in fish is caused by Flavobacterium columnare, commonly referred to as columnaris. Although it often deceives with its appearance, which many consider to be fungal infections due to its hairy, cottony texture, columnaris is a bacterial condition. The name itself comes from the column-like strands the bacteria form on fish, contributing to this misconception.

Columnaris is an aggressive pathogen, known for its rapid progression and high mortality. Infected fish may initially exhibit behaviors such as "flashing," where they rub against objects in the tank—a signal that can be mistakenly interpreted as an indication of various diseases and is not specific to columnaris.

As the disease progresses, it manifests more on the gills, where red or brown changes occur that are indicative of bacterial gill disease. Infected fish may have difficulty breathing, often gasping for oxygen-rich water. These symptoms, although expressive, can be difficult to recognize, complicating early diagnosis.

Progression and Symptoms

The disease can further develop into mouth rot or saddleback conditions, where the fish's spine may bend, and their tail may hang in the water. Such development not only signals the severity of the infection but also the urgency of intervention.

Other Bacterial Threats

While columnaris is the main problem, other gram-negative bacteria also represent common threats in aquatic environments. For example, fin rot is another common bacterial disease, typically caused by other pathogens but equally devastating.

On the internal front of diseases, hemorrhagic septicemia, often signaled by a "red spot," is primarily attributed to Aeromonas salmonicida. This condition indicates a serious infection where bacteria have extensively damaged internal organs, leading to advanced stages of infection where treatment options are limited and often drastic.

Silent Killers

A critical aspect that complicates the management of bacterial diseases in fish is their ability to quickly kill without visible symptoms. These pathogens can attack the internal organs of fish and kill them—organ failure. Such cases require immediate and robust response, as the death of several fish in quick succession usually signals a bacterial epidemic.

Prevention and Elimination of Bacterial Diseases in Aquarium Fish

Prevention and elimination of bacterial diseases in aquariums involve a combination of good hygiene, regular monitoring, and prompt treatment.

Aquarium owners should therefore:

1. Maintain clean water conditions and proper filtration to reduce the number of pathogens.
2. Quarantine isolate new fish before introducing them to existing tanks to prevent the spread of diseases.
3. Monitor subtle changes in fish behavior and appearance, taking measures before diseases fully manifest.
4. In case of occurrence, consult with a veterinarian specializing in aquatic animals or an experienced aquarist to adjust a specific treatment strategy.

Treatment and Prevention of Aquarium Fish Infections

Management and Treatment

Effective management of bacterial infections revolves around several key strategies:

1. Environmental Management:
   • Water Quality: Maintaining optimal water conditions is paramount. Poor water quality can stress fish, weakening their immune systems and making them more susceptible to infections.
   • Stress Reduction: Handling, transportation, and environmental stresses should be minimized as stressed fish are more prone to infections.
2. Antibiotic Treatment:
   • It's crucial to choose the right antibiotic based on the type of bacteria (gram-positive or gram-negative) and the results from sensitivity tests. Misuse of antibiotics can lead to resistance, making future treatments more difficult.
   • Administration Methods: 
   • Bacterial infections can only be effectively treated with antibiotics in feed. Many people believe (and the instructions on antibiotics claim!) that antibiotics need to be added to the water. This is simply incorrect.
   • Preparing medicated feed. First, heat 1/4 cup of water (about 58 milliliters) in the microwave. Then, while continuously stirring, mix in seven grams of plain animal-derived gelatin. Take two tablespoons of dry commercial fish feed (pellets or flakes) and mix them with a little hot water/gelatin mixture. Add hot water/gelatin until you achieve a pasty consistency. If the mixture is too watery, add more feed.
   • Then add just a "pinch" (approximately 1/16 teaspoon, thus adding 1% to 2%) of the drugs to the mixture. If you are using more than one drug, mix the drugs together, then use just a "pinch" of this mixture. If you are using a sachet of medication, take just a "pinch" of the sachet's content. Thoroughly mix and crush the mixture. Spread it into a patty about 3 mm thick on plastic wrap or a plate. Then place in the refrigerator. If you plan to store the feed for longer than two weeks, put it in a small plastic bag and freeze it.
   • All fish in the aquarium should be fed a diet of medicated feed for at least ten days. Know that the exact amount of drug that gets into the feed is not very important. Antibiotics can only be overdosed in large doses over several months.
3. Monitoring and Follow-Up:
   • After initiating treatment, continuous monitoring is necessary to assess the effectiveness and make adjustments as needed. This includes observing the fish for signs of recovery or any adverse reactions to the treatment.

Conclusion

Elimination and treatment of bacterial infections in fish require a comprehensive approach that includes proper diagnosis, appropriate use of antibiotics, and a healthy and clean environment.

Source: Roy P. E. Yanong, Use of Antibiotics in Ornamental Fish Aquaculture. https://aquariumscience.org/index.php/10-3-bacterial/ Illustration: Bacterial Infections in Fish. Source: https://aquariumscience.org/index.php/10-3-bacterial/