Epidemiology of Fish Disease

Clinically healthy fish may be infected with potentially pathogenic organisms yet show no clinical signs of being diseased. Infection means that the organism is residing in or on the host; it does not mean that a diseased state exists. Asymptomatic carrier fishes can harbor pathogenic organisms without any adverse effects until a stress situation develops. As is the case with any other animal, stress predisposes the individual to disease.

Disease is the antithesis of health. There are an internal and external diseases, organic and functional diseases, developmental abnormalities, degenerative diseases and deficiency diseases. More specifically there are diseases caused by viruses, fungi, bacteria, protozoans, worms and crustaceans as well as diseases caused by the artificial environment of captivity itself such as metal and gaseous poisoning even poisoning by algae.

Epizootics may be caused in tanks by pathogens which are present in the wild but which do not reach any significant proportions there. Oodinium, a parasitic dinoflagellate, is one example, and Cryptocaryon, a ciliated protozoan is another.

A general reason for this higher incidence of disease in captivity is the greatly decreased ratio of fishes to volume of water more specifically a greater risk transmission occurs in the tank due to closer contact among fishes, an increase in the changes of temperature, ammonia nitrate, pH, solidity and oxygen. Dietary inadequacies lead to lowered resistance to infectious organisms already present. Further the absence of natural predators in the tank provides diseased individuals a better chance of survival which in turn spreads their disease.

Diseases can be either infectious or noninfectious. Infectious diseases are caused by organisms whereas non-infectious diseases are caused by physical anomalies, adverse environmental conditions or wounds. By definition when a diseased state exists characteristic clinical signs and pathological changes occur. Once you recognize the particular disease an appropriate treatment can be initiated.

Probably one of the most important and yet most overlooked areas of aquatic animal medicine is epidemiology. Epidemiology is that science which deals with the incidents, distribution and control of disease in a population. If you have a firm grasp of epidemiological principles many disease outbreaks can be avoided.

Rarely does a hobbyist obtain all the fishes, invertebrates and ornamental shells and corals from the same selected ocean site. If everything in the tank was obtained from one area the fishes would likely have some degree of immunity to diseases which are endemic to that location. Generally the fishes and invertebrates you put into your tank come from a variety of locations throughout the tropical areas of the world. Some of your fish may have had previous exposure to particular bacteria and developed a degree of natural resistance to these bacteria. Others may never have been exposed and in putting these two fished together those harboring the bacteria may lead to a diseased condition. Many factors are involved in whether or not the unexposed fishes will actually become sick. If they are healthy and the number of bacteria they are exposed to is small their natural body defenses may develop immunity. On the other hand if you're previously unexposed fishes are debilitated or suffering from stress when you put the carrier fishes in a diseased state will probably develop.