A great number and diversity of animal species are capable of parasitizing fish, ranging from microscopic protozoans to grossly visible crustaceans and annelids. In the wild fish, there is a large range of parasites but they are usually only present in small numbers, they can be considered a normal finding and rarely cause disease problems.

In cultured fish, there is a more limited range of parasites but they are often present in much large numbers than seen in the wild. There is always a risk of parasitic epizootic in farmed fish and this increases with the intensification of the farm system. Many factors in fish culture will favor parasitic disease; an awareness of these factors will allow remedial or preventative action to be attempted.

1.     Stocking density is usually high in fish culture systems and the propinquity of the host fish favors the transmission of parasites. This is particularly the case with parasites having a direct life-cycle, such as the ectoparasitic protozoa, which always have substantial reproductive capabilities to insure that some offspring locate a suitable host. Hosts are readily available in a farm environment and overwhelming parasitic infestation can occur.

2.     Farmed fish are more prone to physical trauma due to handling, grading, storms etc. and this gives an opportunity for parasites to colonize and feed on damaged tissue.

3.     Water in a fish farming facility is frequently sub-optimal in quality and quantity. Low flow rates allow the accumulation of infective stages within the system. High levels of ammonia irritate the gills and skin, causing an increase in mucus production and an increase in surface bacteria and organic material producing a very favorable environment in which protozoan parasites can flourish and cause further damage to the surface of the fish. At the same time high levels of nutrients from waste feed and feces will increase the local populations of bacteria and free-living protozoa, again providing food for the parasites. In this situation many free-living protozoa will use the fish as a convenient feeding platform and, while not directly parasitic to the host, may cause problems due to the sheer numbers of protozoa present. Some of the crustaceans and molluscs feeding on the waste organic material may act as intermediate hosts to some of the parasites with indirect life-cycles. Increased nutrient levels in the water may also irritate the gills and skin of the fish, again favoring parasitic invasion.

4.     Fish are often selectively bred for qualities other than disease resistance and some strains may be particularly susceptible to disease.

5.     The introduction of exotic species of fish may introduce new parasites to existing (often highly susceptible) fish stocks and expose the introduced fish to parasites already present in the facility and to which the resident fish are more resistant.

6.     The health status of the fish may be poor due to the presence of other disease, poor nutritional status, poor water quality, stress etc., so that they are more susceptible to parasitic infestation. There are several ‘debility’ parasites, which take advantage of fish compromised by other factors but cause no problems in healthy individuals. Certain husbandry practices (e.g. transportation) may cause stress and render the fish more susceptible to disease ___ it is possible to anticipate some of these procedures and consider prophylactic treatment.

7.     Environmental changes, such as a sudden rise in temperature, may favor the parasite but stress the host. Outbreaks of parasitic disease are common in over-winter fish whose disease resistance may be poor, making them very susceptible to the increase in parasite numbers that occurs when water temperatures start to rise in spring. Any change in water quality may also stress the host while favoring the parasite.

8.     The system of husbandry may be more likely to expose fish to parasites. Earth pond systems favor the completion of the life cycle of some parasites, particularly the sporozoans, and also favor the presence of intermediate hosts. Concrete systems reduce these risks but may cause more physical damage. Cage systems expose the fish directly to the parasitic fauna of the wild fish and allow the fish to feed easily on invertebrate intermediate hosts. The husbandry system also dictates the ease with which treatment and control may be administered; tank systems are easier to treat than sea cages.

9.     The parasites to which the fish are exposed vary with the source of water. River and Lake Sources will carry a much wider range and higher numbers of parasites than borehole water.

The major parasitic diseases, which face fish production are caused by:

1.     Protozoa:

a.     Ciliates:

i.    Ichthyophthirius multifiliis.

ii.    Chilodonella.

iii.    Tetrahymena.

iv.    Trichodina.

v.    Ambiphyra.

vi.    Apiosoma.

vii.    Epistylis.

viii.    Capriniana.

b.     Flagellates

i.    Hexamita.

ii.    Ichthyobodo.

iii.    Piscinoodinium.

iv.    Cryptobia.

v.    Myxozoa.

vi.    Microsporidia.

vii.    Coccidia

2.     Monogenean trematodes:

a.     Dactylogyrous sp.

b.     Gyrodactylus spp.

3.     Digenean trematodes

4.     Nematodes

5.     Cestodes

6.     Parasitic Crustacea

a.     Ergasilus.

b.     Lernaea.

c.      Argulus

7.     Leeches.

However, those mentioned above are only a few problems, but the parasitic disorders of intensive fish culture are countless.