Aeromonas hydrophila complex is probably the most commonly encountered bacterial pathogen of freshwater fishes. These bacteria are motile (capable of moving), and several different species may be responsible for Motile Aeromonad Disease (MAD). In addition to A. hydrophila, bacteria that have been implicated in MAD include A. sobria, A. caviae, and A. veronii. These are ubiquitous (found almost everywhere) organisms and opportunistic pathogens that take advantage of stressed and immunocompromised fishes. Environmental stresses such as crowding, temperature extremes, poor nutrition, and transport all may predispose fishes to MAD.
What to Watch For
Recent acquisitions to the pond or aquarium are a common finding in the clinical history. Upon physical examination, affected fish may display the following:
Systemic cases frequently result in a hemorrhagic septicemia (bacteria multiplying in the bloodstream) leading to inflammation and necrosis (cell death) of gastrointestinal tract, kidney, muscle, and spleen.
Your veterinarian can help you make a diagnosis of MAD. However, this is a difficult disease to confirm since contamination of culture samples is common. The best tissue to culture is the caudal (tail) kidney, and several fish should be sampled if possible to help confirm the diagnosis.
Clinical signs, combined with the presence of motile aeromonads in tissue samples, are usually satisfactory for a tentative diagnosis and treatment.
Differential diagnoses include aeromonas salmonicida (furunculosis), trauma, and mycobacteriosis.
When systemic disease is present the prognosis is generally poor. Fish in the early stages of infection may respond favorably to antibiotic treatment and environmental modification.
You should correct any underlying environmental problems that might result in stress on the fish. Isolate and remove any clinically infected individuals if possible.
Broad spectrum antibiotic treatment should be started without delay if MAD is suspected. Antibiotics of choice include enrofloxacin, trimethoprim-sulfamethoxazole, and amikacin. You should use these and other antibiotics only under the direction and supervision of a licensed veterinarian who has examined your fish.
The best way to prevent MAD is to quarantine new aquarium pond fish for at least one month. This practice, at the very least, should identify infected fish before they have a chance to spread the bacterial disease to your established fish population.
Other important preventative steps include maintaining excellent water quality, performing frequent water changes (at least 25% each month), not over-crowding your aquatic system, and maintaining steady temperatures and an adequate air supply.