Environmental Considerations for Selected Infectious Diseases of Fish

Mycobacterium spp is an important infectious pathogen of fish and other aquatic organisms. There have been many species of Mycobacterium identified in fish; however, all are nontuberculous and most are environmental. Typically, environmental conditions common in aquaculture favor growth of Mycobacterium. These conditions include low DO concentrations, accumulation of organic debris and particularly biofilms, as well as acidic pH. Mycobacteria are hydrophobic because of their thick, waxy cell walls and tend to adhere to surfaces and each other. Once infected fish are present in a system, these animals may continually shed the organism in feces and cells released from external lesions. Ultraviolet light (ie, UV filtration) may decrease the number of these bacteria in the water column, which may help decrease transmission; correction of these environmental factors can also enhance control efforts to control the disease. In aquaculture, infected fish are almost always culled. In display aquaria, however, the decision may be made to maintain infected fish. In these cases, environmental management can be an important part of the protocol developed for the care of affected animals. Maintaining oxygen concentration near saturation and avoiding acidic and organically rich environments in the presence of some type of disinfection on the system (eg, UV or ozone) can greatly enhance the quality of an exhibit populated with mycobacteria-positive fish. In this situation, the exhibit should be clearly labeled as mycobacteria-positive, and employees should be properly trained to minimize risk of disease transmission from the contaminated exhibit to personnel working in the area or to other systems or animals.

Bacterial gill disease is an important disease of cultured salmonids and has been reported in other species, including hybrid striped bass. It is rare in aquarium fish. It is caused by bacteria in the genus Flavobacterium and other closely related genera and is manifest by gill pathology (hyperplasia, clubbing of gill filaments) and subsequent mortality. However, bacterial gill disease has an important environmental component, and mechanical damage to gill epithelium, which may be caused by excessive crowding and debris in the water column, is believed to be an important predisposing factor. Cleaning the environment, removing debris, and decreasing stocking density all contribute to a positive treatment outcome. Correcting these underlying problems also prevents recurrence of the condition. Potassium permanganate can be useful in removing organic detritus from an affected freshwater system. However, caution is advised, because this treatment may cause mortality in fish already compromised with significant gill damage.

Many infectious diseases of fish occur in a specific “temperature window,” and adjusting temperature may be an important part of successful treatment. For example, Fusarium solani is a fungus that has been linked to suboptimal environmental temperatures in bonnethead sharks. Infected sharks must have environmental temperatures raised above 80°F (27°C) to control or prevent the infection. Saprolegnia, a common fungus-like oomycete of freshwater and brackish water fish, is also often associated with suboptimal environmental temperature. Raising water temperature a few degrees may significantly improve response to treatment. Clinicians are encouraged to determine optimal environmental temperature when fish are observed with this condition and to make appropriate corrections if needed.

Temperature is also critical in control of most viral diseases of fish. Two important diseases of koi, spring viremia of carp (SVC) and koi herpes virus (KHV), have strong temperature predilections. This should always be kept in mind when working with these popular pets. It should be remembered that KHV is a warm water disease. Infected koi typically develop disease when water temperatures are in the range of 64°–81°F (18°–27°C). Fish with KHV typically exhibit significant gill lesions and signs of hypoxia. In contrast, SVC is a cool water disease, typically occurring at water temperatures of 41°–64°F (5°–18°C). Both of these diseases are reportable at the state and federal levels. Although KHV is considered endemic in the USA, SVC is a foreign animal disease, and the USDA will impose quarantine and forced destruction of fish if an infection is confirmed. Consideration of water temperature is an important first step when confronted with a disease in this species.

Almost all viral diseases of fish have specific temperature ranges at which they cause active disease. Clinicians must familiarize themselves with diseases of concern and the temperatures at which they occur in the species at hand.

Organic debris can accumulate because of poor husbandry or poor system design. Detection of accumulated detritus can be surprisingly difficult when areas of “dead space” exist in a system, because these areas are often not directly visible. Environmental parameters that suggest this problem include an inability to maintain DO concentrations at or near saturation. Further, total alkalinity and pH may be lower than that in source water. All of these changes suggest a substantial amount of decomposition of organic matter is occurring somewhere in the system. Pet owners or aquarists may need to break the system down to locate and remove the accumulated material.

In these systems, a presenting disease problem may be infestation of external surfaces of the fish with Trichodina, a common ciliate. Trichodina is often considered a bio-indicator of suboptimal conditions that may include overcrowding, overfeeding, and excessive organic debris (ie, a dirty system). Thoroughly cleaning the system, removing detritus, and correcting design flaws when possible are important to correct these problems. Treating the parasite without correcting system problems will result in continual recurrence and frustration. Other infectious agents that may be observed in the presence of poor sanitation include sessile ciliates such as Epistylis, Heteropolaria, Ambiphyra, and Apiosoma. Saprophytic fungi and fungi-like diseases (eg, the oomycete Saprolegnia) may also be seen when hygiene is a concern.

Fish that survive a severe environmental insult, such as an acute hypoxic event, are at increased risk of opportunistic bacterial infections. Common infections that may occur 1–2 wk after an insult could include infections with Aeromonas hydrophila and Pseudomonas fluorescens in freshwater systems, and Vibrio spp in marine systems. Given this possibility, it may be prudent to treat valuable fish with a broad-spectrum antibiotic with general efficacy against gram-negative organisms after survival of a significant insult. In many cases, good water and a quiet environment will be sufficient for recovery, but when in doubt, the use of antibiotics may be justified, especially if they can be provided in feed. Once fish start to break with an infection, they often go off-feed, making treatment much more difficult.