The term quarantine is defined as “isolation imposed on persons or animals that have arrived from elsewhere or been exposed to, and might spread, infectious or contagious disease.” It is derived from the Italian quarantina, which means forty days. Quarantine (that is a forty-day period of isolation) can be applied to any animal, but was originally applied to humans and warm blooded animals. The expectation was that forty days (or other period of time stipulated by law) would be longer than the incubation period of serious diseases like small pox or rabies. Thus any infected animal would become identifiably unwell in that period.
Warm-blooded animals have a metabolism that keeps their body temperatures stable within a narrow range. Pathogens of these animals have adapted to cause disease within that narrow temperature range. This is why the incubation period, the time between exposure to infection and signs of disease becoming evident, of diseases in warm blooded animals can be predicted with some accuracy.
Fish are not warm blooded, and their diseases do not have incubation periods that are similar in all conditions. Fish adopt the temperature of the water that surrounds them. If their environment is temperature stable, then the incubation period of a disease may be predictable. However most fish are subject to quite wide fluctuations in temperature; Koi can be in found in water close to freezing, or up to temperatures of 300C or more, there are wide differences in incubation periods for diseases across this temperature range.
To facilitate the detection of disease in cold-blooded animals a series of “stress tests” has been developed to induce the clinical symptoms of a disease carried by an asymptomatic animal. The judicious use of these tests may be very useful in establishing the health status of a population of animals prior to their importation e.g. holding fish within the temperature range at which the disease occurs.
To a larger degree than in many other animals, stress predisposes fish to disease. Stress does not cause these diseases, as the pathogen must be present. Unless the trigger, frequently stress, is present, a disease may not develop even if the pathogen is present.