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Disease Caused By Different Parasites In Fishes And Their Preventive Measures

Sajid Maqsood*, Prabjeet Singh, Munir Hassan Samoon, Gohar Bilal Wani and Ubair Nisar.

Faculty of Fisheries, Sher-e-Kashmir University of Agricultural Science and Technology-Kashmir.

*Corresponding author E-mail :


A. Protozoan Parasites

1. Ichthyophthirius multifiliis:

This is probably the most common parasite of all fishes.  The common name for this parasite and disease is "Ich" or "white spot". The mature parasite reaches approximately 1 mm in diameter and is commonly observed in the gills and/or skin as coalescing white spots, hence the common name.  The trophont or mature stage of the parasite has a large "horseshoe" shaped nucleus, and the entire surface of the parasite is covered in cilia.  The life cycle of this parasite is direct, but is spent, in part, off of the host.  The trophont is within the epidermis of the host, until it leaves the fish, encysts demonstrates mature cyst of this parasite) and divides to produce many host-seeking tomites.  The tomites penetrate the skin and gills of the fish to complete the life cycle.  The life cycle is temperature dependent with a shorter life cycle occurring at warmer water temperatures.

            Fish with a cutaneous infection  will "flash", i.e., turn over and expose their white underside, whereas fish with a gill infection will "pipe", i.e., come to the surface of the water and "breathe" through their mouth.  Gill lesions include epithelial hyperplasia with the presence of mature trophonts within the gills.  Cutaneous lesions also exhibit focal epidermal hyperplasia, with parasites being located beneath the hyperplastic epidermis.  

2. Chilodonella:

This is a motile ciliated protozoal parasite which causes disease in the skin and gills of fish.  It is typically heart-shaped with the posterior end being broader and slightly notched.  It measures approximately 20-40 m in width and 30-70 m in length and its surface is covered with cilia.   There is a large macronucleus in the posterior portion of this organism and a smaller micronucleus is near or within the macronucleus.   This parasite has been attributed to death of fish due to respiratory and osmoregulatory imbalances associated with severe gill parasitism.  Diagnosis is dependent upon demonstration of the organism within the affected organs by either cytology or histopathology. 

3. Cryptocaryosis:

Cryptocaryon irritans, parasite of gills and skin, is the causative agent of this disease. External signs consist of white spots and mucous excess or ulcers on the skin and impairment of respiratory function. Gill histopathology consist of inflammation, haemorrhages, hyperplasia and lamellar destruction. This ciliate is a typical marine fish parasite affecting commercial and ornamental fish and producing high mortality in culture conditions. Outbreaks appeared mainly at high temperatures. Some treatment and control measures are similar to those recommended for ichthyophthiriasis, though quinine derivatives and low salinity baths have also been used. Diagnosis is based on macroscopical examination followed by microscopical examination for confirming the presence of the ciliate. The parasite can also be found in histopathological studies.

4. Trichodina:

There are three genera which form the Trichodina complex: Trichodina, Trichodonella and Tripartiella, however, all three are commonly referred to as "Trichodina". All are approximately 100 m in diameter and have a saucer to "frisbee" shape and are ringed with cilia around its entire surface. They have a circular arrangement of tooth-like structures (denticular ring) within the body which provides them a characteristic appearance in fresh gill and skin cytology preparations (photo). Fish with severe gill infections of trichodina will have respiratory and osmoregulatory difficulty and may "pipe" as well as "flash" if there is cutaneous involvement. Fin erosions and/or ulcerations can be observed in chronic cutaneous infections. Diagnosis of this parasitic disease is dependent upon identification of the parasite within the skin or gill cytologic preparations or histopathology.

Fish trichodinids include mainly Trichodina spp., Trichodinella spp. and Tripartiella spp. These peritrichid ciliates are more commensals than genuine ectoparasites, but can produce different damages in massive infections. The fish show a grey-blue turbid layer on the skin. Respiratory function can be impaired in gill infections. Trichodinids parasitize a lot of freshwater and marine fish species. Diagnosis is mainly based on microscopical examination of fish or gill scraping preparations. Hygiene in hatcheries and quarantine for ornamental fish are recommended for prevention. This ciliatosis can be treated with formaldehyde in baths. In freshawter ornamental fish and fry, baths of salt solutions can be applied, with variable success.

5. Ambiphyra:

 These are ciliated protozoal organisms which are thought to be free-living, but have been known to parasitize fish.  They are sessile organisms with a cylindrical to conical body with oral cilia and a permanent motionless equatorial ciliary fringe.  They range in size from approximately 60-100 m and adhere to the epithelium of the skin and/or gills. Disease and death of fish have been associated with chronic infections of the gills due to mechanical blockage of respiratory epithelium. Diagnosis of this parasite is dependent upon identification of this organism within the skin or gill scrapings or histopathology.

6. Scuticociliatida:

Several species of the genera Uronema, Phylasterides and Miamiensis have been recorded as facultative parasites of different fish. Clinical signs of scuticociliatosis depend on the parasite location. External signs include skin lesions or ulcers and pigmentation changes, but the parasite frequently invade the body muscle and the internal organs, which become destroyed by this histophagous parasite. Nervous system can also be colonised, which can be accompanied by erratic swimming, equilibrium loss or lethargy. The disease cause severe infections and outbreaks in some cultured fish and mortalities can reach 100% of some affected stocks. Different fish species can be affected, but the most severe cases reported deals with turbot, Mediterranean seabass, several marine aquarium fish, Australian tunnids and Japanese flounder.

Diagnosis is based on the finding of ciliates in ascitic fluid or scrapings of different organs. The typical morphology of Scuticociliatida can be easily observed in fresh preparations at light microscope, in stained smears or histological sections. However, the identification at the genus and species level requires specific staining evidencing the somatic and oral infraciliature and the scutica. There are no efficacious treatment for this parasitosis. Formalin baths have been assayed with certain success only in the initial phase of infection.

B. Endoparasitic Protozoans

1. Coccidiosis:

Different coccidia (Apicomplexa) are known from among freshwater and marine parasites, but their pathological significance for the cultures is very variable. The genera Eimeria, Goussia and Cryptosporidium include the species more frequently reported from cultured fish. In freshwater fish, G. carpelli parasitizes different cyprinids and E. anguillae is typical of eels. In marine fish, E. sparis and G. sparis have been reported from Sparus aurata and E. dicentrarchi and E. bouixi from Dicentrarchus labrax. Fish Cryptosporidium spp. include species from seabream, seabass, turbot, and aquarium fish, affecting mainly larvae and juvenile, with deleterious effects not always very evident, but resulting in poor condition. C. molnari is more frequent in seabream than in seabass. The species of turbot is probably a new one. Diagnosis of fish coccidia is mainly based on histopathology and/or on fresh examination at microscope. Immunodiagnostic methods are available for some human and animal species, but not for fish species. Control of animal coccidians is based on the use of differentcoccidiostatics or coccidiocides, but information regarding fish coccidia is very scarce. Furazolidone, amprolium chloride and furanace, among others, have been tried to treat different fish cocidia.

2. Microsporidiosis:

Microsporea are represented in fish by different genera, mainly Enterocytozoon, Glugea, Loma, Pleistophora and Tetramicra. In freshwater fish, Pleistophora and Loma are relatively frequent. Among cultured marine fish, there have been several reports of Plesitophora senegalensis in gilthead seabream, whereas Glugea sp. and Tetramicra brevifillum have been found in turbot. Pathological concern of microsporidiosis in fish is dependent on location and infection intensity. Variable loses in turbot cultures have been related to Tetramicra infections. Diagnosis is based on the direct detection of the parasite at microscope, mainly the spores, but ultrastructural studies are necessary for identification at the specific (or even generic) level. A PCR based assay has been recently developed for T. brevifillum. Among chemicals tested for treatment, toltrazuril has apparently given better results than fumagillin and amprolium.

3. Myxosporea (myxosporidiosis):

The class Myxosporea (phylum Myxozoa) include numerous genera and species, most of them parasites of fish. Some species are well know pathogens for freshwater fish. In the last years Myxosporea have been increasingly reported in cultured marine fish. They are characterised by a spore with one to several valves, one or more infective sporoplasms and one to several polar capsules with a coiled polar filament inside.

The most pathogenic species belong to the genera Ceratomyxa, Myxobolus, Myxidium, Spaherospora, Enteromyxum, Kudoa, Tetracapsuloides and Sphaerospora.

In freshwater fish the most significant diseases are whirling disease, PKD, sphaerosporosis and ceratomyxosis (produced by Ceratomyxa shasta). Myxosporea reported from cultured marine fish include species of the genera Ceratomyxa, Enteromyxum, Kudoa, Lepthoteca, Sphaerospora and Sinuolinea. Other myxosporeans reported ocassionally in the survey are Leptotheca sp. and Polysporoplasma sparis from the kidney of S. aurata, and Sinoulinea sp. from the urinary bladder of turbot, the three reported only by 1 laboratory from Spain, mainly in routine or occasional samplings. The species of Leptotheca has been described as L. sparidarum from seabream an dentex.

3. Myxobolus cerebralis (Whirling disease)

The causative agent is Myxobolus cerebralis (synonym Myxosoma cerebralis). Clinical signs include dark coloration of the posterior part of the body and abnormal swimming in spiral, followed by skull deformation and spinal curvature. Almost all salmonid species can be infected, but susceptibility is very variable according to the species. Oncorhynchus mykiss and other Oncorhynchus spp. Are very susceptible, while Salmo trutta is rather resistant. The involvement of an intermediate oligochaete host in the life cycle of this myxosporean was demonstrated 18 years ago. This knowledge has facilitated preventive measures, consisting of the use of concrete or plastic ponds or tanks and their frequent cleaning for avoiding the presence of oligochaetes and thus the transmission of the disease.

This is very important, considering the limited efficacy of treatments assayed till now (fumagillin, toltrazuril) for this myxosporea and other species. In some European countries, the incidence of whirling disease has clearly decreased, whereas in USA whirling disease is widely distributed and is still an important pathological problem. Diagnosis is based on the histological examination of skull cartilage, or their enzymatic digestion followed by microscopical observation of the typical spores. A PCR assay has also been developed.

Hexamita / Spironucleus

These parasites are pyriform to oval with tapering toward the posterior end.  Occasionally, rounded individuals can be identified.  The organisms are 6-8 m wide and 10-12 m long.  They have three pairs of anterior flagella which are approximately one and one-half times the length of the body.  The flagella originate from the blepharoplast at the anterior end of the axostyles.  These organisms can reproduce by longitudinal binary fission as well as undergo schizogony within the epithelial cells of the ceca or intestine.  This parasite causes disease within the gastrointestinal tract of fish and affected fish will have clinical signs related to malnutrition and emaciation. Diagnosis is dependent upon finding the parasite from cytologic scrapings of the ceca or intestinal tract or histopathology of these organs.  A poorly understood parasite, thought to be Hexamita-like is thought to be responsible for Freshwater Hole-in-the-Head and Lateral Line Erosion (FHLLE).

C. Ectoparasitic flagellates

1. Ichthyobodo spp.(Costiasis)

Ichthyobodo sp. (also known as Costia) are the agents of this disease of the gills and skin. I. necator is the species parasitizing salmonids in freshwater, but a different species is considered to be present in marine fish. Affected fish appear thin and lethargic, and may show a grey-whitish pellicle on skin, epidermic erosion or even haemorhages or ulcers, as well as gill hyperplasy and edema. Costiasis is widely distributed in different fish species, mainly in larval and juvenile stages, and mortality can occur in fry or ornamental fish with moderate to severe infections. Besides direct mortalities, indirect damage due to decreased health condition and gill lesions must be considered.

Diagnosis is based on microscopical examination and histopathology. Prevention relies on hygienic measures.

Costiasis can be treated with formaline 1:4000 or 1:6000 in baths with a good aeration.

2. Cryptobiasis

The other ectoparasitic flagellates found in the survey are the gill Cryptobia spp., with a direct life cycle. Marine ectozoic species include C. branchialis, from different coastal fish and C. eilatica, described from the gills of Sparus aurata and Diplodus noct in the Red Sea. In heavy infections, the parasites produce gill hyperplasia and epithelial destruction, with subsequent respiratory impairment. External signs are anorexia and skin darkness. In the Mediterranean it is relatively common in cultured seabass and seabream. The infection can produce trickling but persistent mortalities, so loses can reach 10% after several weeks.

Diagnosis is based on microscopic fresh and histological examination.

Formalin baths (150 mg/l) can be effective for treatment.

D. Endoparasitic flagellates

1. Cryptobia spp., Trypanoplasma spp., Trypanosoma spp.:

Some species of these genera parasitize internal organs of fish. Cryptobia iubilans is the only pathogenic intestinal species, common in aquaria cichlid fish. Trypanoplasma spp. and Trypanosoma spp. include parasites of the bloodstream and of tissues, with indirect life cycles (leeches are the main vectors). The best known is Trypanoplasma salmositica (frequently referred as Cryptobia samositica) producing cryptobiasis of salmonids. Clinical signs consist of exophthalmia, splenomegaly, hepatomegaly, abdominal distension with ascites, anemia and anorexia. Mortality is dependent on fish stocks and species, but may be high in juveniles. The disease has severe impact in salmonid cultures in North America. An experimental protective vaccine has been developed. Other pathogenic species, Trypanoplasma borreli, parasitizes mainly cyprinids in Europe and North America. The genus Trypanosoma includes numerous species of both freshwater and marine fish. Some freshwater species are pathogenic for cyprinids.

2. Hexamitiasis

Hexamita spp. are parasites of the intestine and gall bladder of freshwater fish, mainly salmonids but also cyprinids and ornamental fish. Hexamitiasis, typical of weak fish, is frequent as a secondary infection. Affected fish can show nervous behaviour, and internally the intestine may appear pale. Mortalities can occur in fry and ornamental fish. Diagnosis is manly based on the direct observation of the flagellate in fresh intestinal scrapings or histopathology study.

E. Dinoflagellates

1. Amyloodiniosis

Also known as "velvet disease", the causative agent is Amyloodinium ocelatum, an ectoparasite on the skin and gills of different fish species. Appart from the velvet appearance, clinical signs consist of anorexia and scratching. Histopathological lesions include gill inflammation, haemorrhages and hyperplasy. Massive infections are frequently associated to mortalities, both in mariculture and sea aquaria, mainly at high temperatures. The infection is very common in Mediterranean fish, though other fish species are affected, including tropical and aquarium fish. Other dinoflagellates (as Piscinoodinium spp.) parasitize different freshwater fish.

Diagnosis is mainly based on microscopic fresh and histological examination, though an ELISA test is available. No effective control measures are known for Mediterranean fish.

Freshwater (2-4 minutes) or copper sulphate (0.75 mg/l, 12-14 days) baths have been suggested as an aid to control the trophonts or dinospores, respectively. In Pacific threadfin (Polydactylus sexfilis) recent findings suggest the suitability of hydrogen peroxide as treatment in juvenile fish. Some evidences suggest the development of immunity against re-infections, and specific antibodies have been demonstrated in the sera of infected fish.

2. Argulus

  • Argulus — Also known as Fish lice and is easy to detect with the naked eye especially against the background of fins.

  • Size varies from between 1mm and 5mm.

  •   Attaching themselves to the fish by suckers which damages the skin, they also inject a poison into the body of the fish which causes inflammation, bleeding and potentially secondary bacterial infection

  • Chemical treatments recommended to eradicate these parasites are either Masoten, Dimilin or Paradex.

3. Mongenetic Trematodes  

            This is a group of trematodes which complete their entire life cycle on the host.  The adults attach to the host by a haptor or opishaptor  which is a specially adapted structure on the posterior end of the parasite.  This organ has hooks which allow the parasite to attach firmly to the host fish.   These parasites usually cause minimal damage to fish, but will infest the skin, fin and gills of pond fishes.  Severe infestations may be responsible for poor respiration and/or emaciation.  The two most common monogenetic trematodes include:  Dactylogyrus and Gyrodactylus. 

3.1 Gyrodactylus

  • Gyrodactylus — Also known as Skin Fluke. Fish suffering from infestations of gill flukes may suffer respiratory problems as the flukes begin to damage the delicate gill tissues. Secondary bacterial infection often occurs in fish left suffering from these parasites, due to the physical damage caused by the anchors.

  • Chemical control of both types of fluke can be achieved with Chloramine T, Malachite Green Formalin and Masoten, or Potassium Permanganate.

  • In order to kill all generations, repeat treatments may be necessary, the frequency being dependent on temperature and chemical

    1. Dactylogyrus - The Gill Fluke

  • Gill and Skin flukes are two of the family of monogenetic trematode genera, all of which are characterized by the large grappling hooks which are used to attach themselves to their victims.

  • Flukes are parasite affecting fish. They range from 0.05 to 3.00mm long and there are actually a huge number of species in the genus

3.3 Lernaea - Anchor worm

  • Lernaea is a common parasite which is clearly visible to the naked eye and can reach 10 to 12mm.

  • The parasite burrows its head into the fish tissue, under a scale and only the body and tail are normally visible.

  • Lernaea lay eggs which can lay undetected in the pond and can hatch when conditions and water temperatures are right.

  • Chemical treatments will not affect the viability of eggs so repeat treatments may be required to kill all generations.

4. Digenetic Trematodes  

            This group of parasites has a complex life cycle with several successive larval generations, alternating sexual and asexual generations and changes of hosts to develop into the adult in its primary host.  The life cycles of trematodes involving fishes may either use fishes as the primary hosts or as intermediate hosts.  Adult trematodes may infest the intestine or gall bladder of fishes.  A few of the more common digenetic trematodes are listed below. 

4.1 Diplostomum spathaceum

The life cycle of this parasite begins as an adult trematode in the intestine of gulls or other fish-eating birds.  The body of the adult is 0.3-0.5 cm in length and distinctly divided into a flattened anterior forebody and a cylindrical and narrower hindbody.  Eggs are shed and passed in the feces of the bird to the water.  The eggs hatch in approximately 21 days into free-swimming ciliated miracidia.  The miracidia infest aquatic snails as the first intermediate host by penetration of the snail’s hepatopancreas.  The miracidia then become a mother sporocyst, followed by one or more daughter sporocysts.  Each daughter sporocyst produces many cercariae which are released into the water.  These cercariae  seek a second intermediate host by penetrating the fins, skin, gills or cornea of small fishes.  Primary host fish which ingest the initially infected fish (second intermediate host) become infected and the life-cycle is completed when the host fish are ingested by fish-eating birds.

4.2 Posthodiplostomum minimum:  

This trematode has several synonyms including:  Neodiplostomum minimum, Neodiplostomum orchilongum and Postodiplostomum orchilongum.  The life cycle of this trematode is very similar to that of D. spathaceum  above, although, infectivity of cercariae to fishes lasts no more than 24 hours after release from the snail.  Each cercaria actively raises a scale and enters under the scale pocket, causing irritation to the fish. Blood, congestion and hemorrhage occur at the bases of fins or other places of cercarial penetration.  The trematodes migrate from the point of entry to visceral organs of the fishes, usually within one to three hours after penetration.  Metacercariae are located in any organ of the fishes' body, but are generally more numerous in the liver, kidney, heart, spleen and other organs of abdominal viscera.   With many of the digenetic trematodes, the metacercariae within the skin results in increased melanin deposition, hence the term "black spot disease". Visible white or yellow spots in the visceral organs, usually no larger than 1 mm in diameter are often referred to as "white grubs" or "yellow grubs" and could be caused by several trematode species. Diagnosis of digenetic trematode infections is dependent upon identification of the genus and species of the trematode within infected fish.   

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