There are two broad types of fish diseases that affect fish which are the infectious and non-infectious diseases. The diversity and abundance of parasites vary widely among populations of the same host species. These infection parameters are, to some extent, determined by characteristics of the host population or of its habitat.
Recent studies have supported predictions derived from epidemiological models regarding the influence of host population density: parasite abundance and parasite species richness are expected to increase with increasing host population density, at least for directly transmitted parasites.
Types of Fish Diseases
There are two broad categories of disease that affect fish, infectious and non-infectious diseases.
Infectious diseases are caused by pathogenic organisms present in the environment or carried by other fish. They are contagious diseases, and some type of treatment may be necessary to control the disease outbreak.
In contrast, non-infectious diseases are caused by environmental problems, nutritional deficiencies, or genetic anomalies; they are not contagious and usually cannot be cured by medications.
Infectious diseases: Infectious diseases are broadly categorized as parasitic, bacterial, viral, or fungal diseases. Historically, infectious diseases were the main cause of death in the world and, indeed, in some developing regions this may still be the case.
With the development of antibiotics and vaccination programs, infectious disease is no longer the leading cause of death in the western world.
Non-infectious diseases
Non-infectious diseases are disease processes that do not have a bacterial or viral origin. Compared to infectious diseases, non-infectious diseases are not contagious or communicable.
Non infectious diseases cannot be transmitted to other humans. Non-infectious diseases are those diseases that are not caused by a pathogen and cannot be shared from one person to another.
Diseases caused by these organisms are infectious diseases. There are many kinds of non-infectious diseases. Non-infectious diseases can be broadly categorized as environmental, nutritional, or genetic.
Environmental
Environmental diseases are the most important in commercial aquaculture. Environmental diseases include low dissolved oxygen, high ammonia, high nitrite or natural or man-made toxins in the aquatic environment.
Proper techniques of managing water quality will enable producers to prevent most environmental diseases.
Nutritional
Nutritional diseases can be very difficult to diagnose. A classic example of a nutritional disease of catfish is “broken back disease,” caused by vitamin C deficiency.
The lack of dietary vitamin C contributes to improper bone development, resulting in deformation of the spinal column. Another important nutritional disease of catfish is “no blood disease” which may be related to a folic acid deficiency.
Affected fish become anemic and may die. The condition seems to disappear when the deficient feed is discarded and a new feed provided.
Genetic
Genetic abnormalities include conformational oddities such as lack of a tail or presence of an extra tail. Most of these are of minimal significance; however, it is important to bring in unrelated fish for use as broodstock every few years to minimize inbreeding.
The Significance of Fish Disease to Aquaculture
Fish disease is a substantial source of monetary loss to aquaculturists. Production costs are increased by fish disease outbreaks because of the investment lost in dead fish, cost of treatment, and decreased growth during convalescence.
In nature we are less aware of fish disease problems because sick animals are quickly removed from the population by predators. In addition, fish are much less crowded in natural systems than in captivity.
Parasites and bacteria may be of minimal significance under natural conditions, but can cause substantial problems when animals are crowded and stressed under culture conditions.
Disease is rarely a simple association between a pathogen and a host fish. Usually other circumstances must be present for active disease to develop in a population. These circumstances are generally grouped under the umbrella term “Stress” (Figure 1& 2).
Management practices directed at limiting stress are likely to be most effective in preventing disease outbreaks.
Disease rarely results from simple contact between the fish and a potential pathogen. Environmental problems, such as poor water quality, or other stressors often contribute to the outbreak of disease.
Fish disease organisms are constantly present in most aquatic environments, and farm and recreational ponds are no exception. Under optimum conditions, healthy fish are able to fight off most forms of infectious diseases.
Conversely, fish subjected to stress are often unable to maintain their natural defenses against infectious diseases such as bacteria, viruses, fungi or protozoan parasites.
Stress may result from a variety of conditions, including overcrowding, handling stress, poor water quality, inadequate nutrition and weather-related environmental stress.
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These forms of stress may kill fish outright, in which case they can be considered non-infectious diseases. More often, however, they do not kill fish outright but lead to outbreaks of infectious diseases.
Ironically, a common cause of stress is chemical toxicity from disease or weed control efforts.
In summary, three factors are involved in fish disease outbreaks: infectious pathogens (viruses, bacteria, fungi or protozoan parasites) must be present and capable of attacking the fish, the fish must already be in a susceptible state, and certain environmental conditions, such as specific temperatures or poor water quality, must be present.
As mentioned, many fish disease-causing organisms are usually present in ponds, and little can be done to eliminate them or prevent them from recurring. These organisms alone, however, are usually not enough to cause disease problems.
For this reason, fish diseases and it’s related problems will usually reoccur in fish ponds unless the conditions which caused the initial fish stress can be identified and eliminated. Three main practices can minimize the possibility of disease outbreaks.
These are maintenance of good water quality, proper nutrition and elimination of contact with wild fish whenever possible. A sound fertilization program can contribute to the first two objectives.
The third can be achieved through proper pond design and water management, as well as prevention of fish introductions outside of the established stocking plan.
Common water quality stressors are low dissolved oxygen and/or a buildup of toxic nitrogenous compounds, especially ammonia and nitrite. Dissolved oxygen problems are most common in the early spring and the mid-to-late summer. Nitrogenous compounds are more of a problem in cold water during the winter.
Determining if Fish are Sick
The most obvious sign of sick fish is the presence of dead or dying animals. However, the careful observer can usually tell that fish are sick before they start dying because sick fish often stop feeding and may appear lethargic.
Healthy fish should eat aggressively if fed at regularly scheduled times. Pond fish should not be visible except at feeding time. Fish that are observed hanging listlessly in shallow water, gasping at the surface, or rubbing against objects indicate something may be wrong.
These behavioral abnormalities indicate that the fish are not feeling well or that something is irritating them. In addition to behavioral changes, there are physical signs that should alert producers to potential disease problems in their fish.
These include the presence of sores (ulcers or hemorrhages), ragged fins, or abnormal body confirmation (i.e., a distended abdomen or “dropsy” and exophthalmia or “Popeye”).
When these abnormalities are observed, the fish should be evaluated for parasitic or bacterial infections.
What to do if Fish are Sick
If you suspect that fish are getting sick, the first thing to do is check the water quality.
If you do not have a water quality test kit, contact your county extension office; some counties have been issued these kits, and your extension agent may be able to help you.
If your county is not equipped with a water quality test kit, call the aquaculture extension specialist nearest to you (see the list at the end of this publication).
Anyone contemplating commercial production of fish should invest in a water quality test kit and learn how to use it. Low oxygen is a frequent cause of fish mortality in ponds, especially in the hot weather.
High levels of ammonia are also commonly associated with disease outbreaks when fish are crowded in vats or tanks. The following parameters should be checked and compare with the available data on oxygen cycles, ammonia cycles, and management of these water quality problems.
In general, check dissolved oxygen, ammonia, nitrite, and pH, during a minimum water quality screen associated with a fish disease outbreak.
The parameters of significance include total alkalinity, total hardness, nitrate saltwater systems) and chlorine (if using tap water). Ideally, daily records should be available for immediate reference when a fish disease outbreak occurs.
These should include the dates fish were stocked, size of fish at stocking, source of fish, feeding rate, growth rate, daily mortality and water quality.
This information is needed by the aquaculture specialist working with you to solve your fish disease problem.
Good records, a description of behavioral and physical signs exhibited by sick fish, and results of water quality tests provide a complete case history for the diagnostician working on your case.
Epidemiology of parasite population in water body
Epidemiology of infectious diseases attempts to describe the patterns and processes by which diseases are distributed in the host population.
The discussion will be on what is known about the transmission of Daphnia parasites, about the factors that influence transmission, and how they work together in shaping parasite dynamics.
In a parasitological context, epidemiology is the study of infectious diseases and disease-causing agents at the population level.
It seeks to characterize the patterns of distribution and prevalence of the disease and the factors responsible for these patterns.
In a more applied context, it also strives to identify and test prevention and treatment measures. The key factor to understanding the epidemiology of diseases is to understand transmission, or the movement of parasites from one host to the next.
An important component of epidemiology is the parasite’s mode of transmission, or how it moves from one host to the next. Unfortunately, surprisingly few scientific reports include information on parasite transmission.
This parasite is transmitted between hosts via waterborne infective stages, which are released from infected hosts and are ingested by the same or other host individuals during filter feeding.
Parasitic fish diseases are most frequently caused by small microscopic organisms called protozoa which live in the aquatic environment.
There are a variety of protozoans which infest the gills and skin of fish causing irritation, weight loss, and eventually death.
Most protozoan infections are relatively easy to control using standard fishery chemicals such as copper sulfate, formalin, or potassium permanganate.
Information on specific fish diseases and proper use of fishery chemicals is available from your aquaculture extension specialist
In summary, there are many diseases of fish which can be troublesome to commercial producers as well as the recreational pond owner.
Many fish diseases outbreaks of captive fish stocks are associated with stressful conditions such as poor water quality, excessive crowding or inadequate nutrition.
There are two broad categories of disease which relate directly to selection of appropriate treatments:
Infectious diseases are contagious diseases caused by parasites, bacteria, viruses, or fungi. These often require some type of medication to help the fish recover.
Non-infectious diseases are broadly categorized as environmental, nutritional, or genetic. These problems are often corrected by changing management practices.
Fish diseases outbreaks are often complex, involving both infectious and non-infectious processes. Appropriate therapy often involves medication and changes in husbandry practices.
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