Introduction:  For any fish producer, one of the greatest challenges is to produce wholesome healthy fish and/or fish products for the marketplace.   To accomplish this goal, the fish must be disease-free.  This text will help the fish producer: (a) identify evidence of disease in fish, (b) understand the mechanisms involved in stress as well as disease processes, and (c) provide examples of pathogens which cause disease. 

Understanding symptoms versus clinical signs:   Symptoms are a group of  processes which describe how an individual feels when they are not healthy.  These symptoms can be related to the health care professional, such as a physician or nurse to help that professional “target” specific organ systems which may establish a diagnosis.  For example, if you have a respiratory illness, such as a cold or “flu” bug, you may describe such symptoms as:  difficulty breathing, coughing, “rattling sounds in your chest”, wheezing, sneezing, etc.  Clinical signs, on the other hand, are words or phrases which describe your observations of a population of non-healthy animals.  For fish, this may include things such as: “flashing”, i.e., the turning over of the fish as they swim, exposing their white underside, “piping”, the process of coming to the surface and “breathing”.  Other common clinical signs may include: anorexia, i.e., the lost of appetite and food intake, lethargy, i.e., listlessness, failure to swim aggressively in the water.  Other clinical signs include erratic swimming patterns or a loss of fright response.  It is important to understand the difference between symptoms and clinical signs in order to “target” the possible organ systems affected when dealing with sick fish.  

Lesions:  This is another important term which must be understood in order to arrive at a proper diagnosis.  Simply stated, a lesion is “any abnormal tissue”.  Most commonly we use this term to describe diseased tissue.  Lay terminology such as “pus”, “goop” or “crud” (heaven forbid) is ambiguous and therefore has different meanings to different individuals.  Examples of lesions include: ascites, which is the accumulation of fluid within the abdominal cavity of fish, exophthalmos, which is the bulging of eyes of the fish and may involve only one eye (unilateral) or both eyes (bilateral).   Ascites and exophthalmos are probably the two most common lesions observed in sick fish.  It is important to realize that these common lesions are not diagnostic for any particular disease process, i.e., they only alert you to the fact that there is a disease process.  Other common lesions include:  hemorrhage and/or congestion, cutaneous erosions and/or ulcerations, gill necrosis, abscesses (or abscessation) and granulomas. 

 Stress:  Most fish are, by nature, very healthy animals, and in order for fish to become ill, they must first be stressed.  Stress to you and I may include: working very long hours under poor work conditions, or the in-laws “dropping in for a visit” and staying too long, etc.  Stress for a fish is a physiologic state caused by a procedure, environmental or other factor which interferes with the fish’s ability to grow to market size at the expected rate.  If the fish is stressed for an extended period of time, it may become sick or diseased due to infection by pathogens.

 Effects of stress upon fish: Stress measurements can be divided into primary, secondary and tertiary effects.  The primary effects occur instantly and include an increase in the plasma level levels of corticosteroids, as well as an adrenergic response with a resultant increase in plasma levels of catecholamines, specifically, epinephrine and norepinephrine.  The secondary effects occur more slowly as compared to the primary and include: (a) abnormal (increased or decreased) blood glucose/lactate levels, (b) abnormal (increased or decreased) plasma free fatty acids, (c) decreased numbers of circulating lymphocytes (i.e., lymphopenia), (d) decreased total water content, and (e) increased heart and gill blood flow rate.  The tertiary effects are those which are most likely to be noticed by the fish producer and include: abnormal behavior, abnormal feed consumption, decreased feed conversion efficiency, decreased growth rates, and increased disease incidence and mortality rates. 

 Stressors:  These are the processes or mechanisms by which fish become stressed.  They can be categorized into the following: (a) poor water quality, (b) environmental conditions, (c) individual factors, and (d) pathogens. 

            Poor water quality can have the effect of producing chronic stress upon fish or if the water quality changes abruptly in a short period of time, a high death loss may occur immediately.   For example, temperature fluctuations of a few degrees may go completely unnoticed in a tank situation but may provide a source of chronic stress for the fish, whereas fish in a pond which has temperature stratification may die rapidly following a thunderstorm due to the release of the dissolved oxygen from the pond.  Other water quality conditions which can cause stress include: pH changes, nitrite toxicity, ammonia poisoning and improper hardness and/or salinity.

            Environmental conditions which may cause stress include:  overstocking and/or overcrowding, and in pond situations, predation by birds or other animals, as well as environmental contaminants/pollutants, such as heavy metals, particulates, hydrocarbons and sewage.  Again, each of these factors may result in a chronic stress situation for fish, or may cause a rapid die-off of fish if the factors change rapidly.

            Individual factors are another potential source of stress for fish, but they are probably the least controllable.  Individual factors include genetic considerations of the fish, such as whether or not the fish are from an inbred line, an outbred line or hybrids.  The potential problems which should be considered linked to the genetic traits of the animals include:  temperature tolerances, genetic abnormalities, including severity and incidence of occurrence, etc.  The immune status of the fish is probably the single most important factor related to whether or not a fish will be stressed and how the individual animal handles that stress.  The immune function status is an individual factor which must be considered regarding stress.  Additionally, the individual nutritional status must also be considered as an individual factor regarding the relationship between stress and the success of the fish. 

Mortality rates:  Some insight may be gained by examining the mortality rates when evaluating causes of fish diseases (photo).  Peracute mortalities are observed only in a few circumstances, such as a catastrophic oxygen depletion of a pond or with some of the viral infections of age-susceptible fish.  Acute mortalities are observed with some of the more potent bacterial infections and chronic mortalities are most commonly associated with ecto-parasitism.  

Pathogens:  This term is used to identify infectious agents which are capable of causing disease.  Although each and every pathogen may cause disease, this disease may first be observed as a “stress” for the fish, rather than disease associated with chronic weight loss, and death (see fig 1 and fig 2.)  The following notes have been “excerpted” for use in this presentation to provide an introduction about some of the more common pathogens.

             Fish pathogens are ubiquitous to their environment.  Agents which cause disease in fish can be categorized into: (a) parasites, (b) bacteria,  (c) fungi, and (d) viruses.  Obviously, it is beyond the scope of this laboratory to discuss all of the pathogens of fish.  Therefore, the following pages discuss the most common fish pathogens observed from submitted cases to the Animal Disease Diagnostic Laboratory or the Southern Indiana Purdue Agriculture Center in Indiana.  For additional information, please consult the references identified in the Selected References listing.