It is difficult to determine just how many of the diseases are actually bacterial because mixed infections of both bacteria and parasites are usually present. To complicate the problem further, bacterial infections often go undiagnosed or misdiagnosed because the laboratory techniques used to identify bacteria accurately are not readily available to the hobbyist, or the aquarium fish industry for that matter. Bacterial isolation and identification require a working knowledge of techniques in microbiology, a variety of growth media and numerous test reagents. Identification can be quite costly if done by a veterinarian or microbiology lab.

Over the years, many bacterial pathogens (disease producers) have been identified. Of these, the most important in freshwater aquarium fish is a bacterium names Aeromonas. Both as a primary infection in wounds or a secondary problem as the result of stresses, such as rapid temperature changes, handling, poor water quality or parasites, Aeromonas has emerged as the most prevalent bacterial pathogen. This is true not only for freshwater tropicals but also for bait fish and warmwater food fish, such as channel catfish.

Most hobbyists don to realize that there are several types (species) of Aeromonas, all of which can cause infection in aquarium fish. In a recent study funded by the Western world Pet Supplies Association, our laboratory identified three different species of Aeromonas present in kidney and wound samples from fish imported from Singapore fish farms. Each species was able to cause infection, and often two species could be isolated from the same sample.

Of the species of Aeromonas, the one most familiar to the hobbyist is Aeromonas hyrdophila. There are many names for the infection produced by this bacterium, ranging from the scientific version, known as hemorrhagic septicemia, to the layman's versions, called redsore and dropsy. These names describe the main features of an infection that can take at least three physical forms. One of the most obvious is the external hemorrhagic (bloody) ulcer. These open wounds appear as localized ulcers on the body of the fish, often with areas of dead tissue and sloughing scales. Hemorrhages also may appear at the base of the fins. Some researchers postulate that Gyrodactylus (body flukes) may act as carriers for the bacteria, which then enter the wounds made by the flukes. These bacteria produce substances that allow them to adhere to the surfaces of cells. Other types of abrasions or tissue damage may also play a role in establishing infection.

Another easily identifiable form of the disease is dropsy. This condition, in which the abdomen fills with fluid and appears swollen, is due to an internal bacterial infection in the kidney, liver or intestinal tract. The resulting disruption of normal circulation of blood and other fluids causes a backup in the system. The fluids accumulate in the intestine and abdominal cavity, resulting in the distention of the abdomen. In severe cases, the scales may stand out from the body, displaying the condition often called pinecone disease. Sometimes the eyes may appear to bulge, known in scientific terms as exopthalmia. This type of infection does occur often in tropical fish but seems to be more commonly associated with goldfish.

The third type of infection produced by Aeromonas is by no means the least. This form is often the most virulent (potent) and can be rapidly fatal. This insidious form does not produce any noticeable external ulcers. The most notable feature is a behavioral change in which the infected fish generally become listless and lethargic. Some fish may lose color or appear darker. The fish do not feed, frequently seem off balance and may sink to the bottom laying on their sides. The course of the disease is rapid, and by the time the behavioral changes are noticed, the entire population of a tank may succumb, sometimes in a day or so. This is an internal infection, with the bacteria being found in the kidneys and blood of these fish. The bacteria produce potent toxins that account for the severity of the disease.

There is a fourth species of Aeromonas that is also responsible for disease in ornamental fish. This bacterium, known as Aeromonas salmonicida, causes what is referred to as goldfish ulcer disease of GUD. This bacterium has long been a scourge in salmon and trout, causing the disease of furunculosis. In salmonid fish, the acute form of this disease commonly appears in growing fish and adults. The classic features of furunculosis include darkening of the body, lethargy and hemorrhages at the base of the fins. The infection gets its name from the typical development of swellings on the body surface, which rupture to release a bloody fluid. This surface ulcer is the furuncle. Affected fish usually die within two or three days.

In goldfish and carp, the infection is produced by an atypical, or what is known as an aberrant, salmonicida. This disease, recognized in European carp culture over a century ago, was given the name Carp Erythrodermatitis and was thought to be part of the disease process of "carp dropsy syndrome." It wasn't until as recently as 30 years ago that this syndrome was found to be comprised of two diseases, one produced by a virus and the other produced by the aberrant bacteria from salmonids. The disease is now known to infect members of the carp family, including goldfish and koi, and has also been indentified in bass and other types of sunfish.

The classic feature of GUD in goldfish is the development of a large circular, very shallow hemorrhagic ulcer, generally occurring on the sides of the fish. Infected fish are depressed and lethargic, lose their appetite and become darker in color. The ulcer gradually develops a center of dead tissue that is very susceptible to secondary fungal infection. The outcome of this disease is variable and depends on the immune response of the infected fish. Complete healing of the ulcer can occur, resulting in an extensive darkly pigmented scar. In other cases, the disease become systemic, resulting in the death of the fish.

Although considered to be freshwater bacteria, Aeromonas can live in saltwater and infect marine and estuarine fish. A recently completed survey has identified Aeromonas existing in a wide range of salinities except hypersaline waters, such as the Great Salt Lake. The results from this survey also showed that some saline habitats actually had a much higher density of Aeromonas than did freshwater habitats. This new information indicates that this organism can occur naturally in marine systems that interface with freshwater.

Despite these findings, the primary bacterial pathogen of marine fish is considered to be Vibro. Vibro is to marine fish what Aeromonas is to freshwater fish, and many parallels exist between these two diseases. For example, several species of both can produce disease, stress plays a critical role in initiation of infection, and both bacteria can produce either external ulcers of internal infections (septicemia). Many of the clinical signs are also similar. As in infection with Aeromonas, fish infected with Vibrio show bloody lesions on their surface. Some may have exopthalmia, hemorrhages at the base of the fins and a fluid-filled abdomen. The darkened color and behavioral changes associated with Aeromonas infection are also present.

Vibriosis has become an increasingly important problem with the increase in the mariculture (marine fish farm) industry. Over the past decade, not only are more species of fish being raised in intensive culture, but invertebrates as well. Striped bass, redfish, dorado and various flounders and soles have been introduced to mariculture. Shellfish, including shrimp, oyster, lobster and abalone, are farmed worldwide. The problems encountered in culture, such as poor water quality, nutritional deficiencies and crowding, have led to stress-induced Vibrio infections, resulting in heavy economic losses from some of these species. Shrimp, for example, are susceptible to an infection in which the bacteria produce a chemical compound or enzyme that can dissolve the external shell. This condition leads either to the death of the infected animal or an unsightly, unsalable product.

Aeromonas and Vibrio are probably the most familiar bacteria known to the hobbyist. Certainly they are the most common bacterial pathogens in aquarium fish. However, nearly two dozen other bacteria have been identified from infections in fish. Some of these bacteria are uncommon and their significance as potential problems is unknown. Others infect a particular group of fish. Renibacterium, for example, causes bacterial kidney disease only in salmonids. What is known for certain is that bacterial problems for the most part are stress related, and the outcome of an encounter with bacteria depends upon the status of the immune response. Factors such as nutrition, water quality, temperature and many others directly influence whether the fish or the bacteria will win the battle.

COLUMNARIS DISEASE

Columnaris disease is known by many common names that describe its external appearance. Terms such as cottonmouth, mouth fungus, fin rot and saddle back are all used to describe the forms that this infection may take. The disease is actually caused by the bacterium named Flexibacter columnaris and not a fungus, as the common names imply. This bacterial infection was first identified almost 70 years ago in fish from the Mississippi River. Since that time, columnaris infections have been described in both freshwater and marine species from around the world.

The bacterium Flexibacter is a long, slender rod-shaped organism that moves with a gliding motion. Under microscopic examination, these bacteria are very motile, gliding rapidly across the field, and can be identified readily by their characteristic movement. When large numbers of the bacteria are present, they tend to stack up, forming a characteristic haystack that is easily recognizable even by the untrained observer. These haystacks are found on fish at the tips of infected gill and fin tissue or on the body itself.

As in other bacterial infections, columnaris disease is frequently associated with stress conditions. Factors such as high water temperatures, crowding and poor environmental conditions can predispose fish to infection. External injury, particularly in smooth-skinned catfish such as Synodontis, can make fish very susceptible to infection. Once established, this disease can be highly communicable, causing severe outbreaks that are often associated with high mortalities among young fish.

The physical presence of columnaris disease is expressed by a variety of external signs that are related to the varying virulence or ability to cause disease among different strains. In the case of saddle back disease, the infection begins on the dorsal (top) surface of the fish and rapidly progresses down and around the body, resulting in what looks like a saddle around the middle of the fish. The bacteria have a yellow color that can give the fish a patchy yellowish appearance. These bacteria are also noted for their ability to produce enzymes that can erode the skin completely, exposing the underlying musculature. Less virulent bacteria may produce localized shallow ulcers. gill tissue is a common site of infection and may be, in some cases, the only affected area. Damage to the gills is generally associated with brownish dead patches of tissue that are readily invaded by opportunistic fungi, such as Saprolegnia.

Columnaris disease is not just a problem in tropical fish. A coldwater form called peduncle disease predominantely infects young salmonids. The bacteria responsible for this disease predominantely infects young salmonids. The bacteria responsible for this disease is Cytophaga psychrophila. This bacterium is quite similar to Flexibacter. It also is a long flexible rod that moves with a gilding motion. Many other similarities are present, but the major difference is that peduncle disease usually does not occur in water temperatures above 13 degrees Celsius (55 degrees Fahrenheit).

Beverly Dixon is a microbiologist and teaches biology at a university in California. She has done extensive research on the immunology, diseases and treatment of fish.