Protozoal Parasites  

            The majority of the fish parasites which cause disease in fish include protozoal parasites.  Typically, these parasites are present in large numbers either on the surface of the fish, within the gills, or both.  When they are present in the gills, they cause problems with respiration and death will commonly occur when additional stressors are present in the aquatic environment.  Protozoal parasites on the skin, fins or scales only, (i.e., not affecting the gills) usually do not result in death, unless they are accompanied by a secondary bacterial infection.  The more common protozoal parasites are listed below.

Ichthyophthirius multifilis:  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 (photomicrograph) 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 (photomicrograph 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 (see gross photo) , with parasites being located beneath the hyperplastic epidermis. Photomicrograph demonstrates a single "Ich" organism with hyperplastic epidermis.  

Trichodinia sp:  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 mm 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. 

Ambiphyra sp:  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 mm 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.

Ichthyobodo sp:  This parasite is also known as Costia sp.  These are obligate flagellate parasites with a direct life cycle.  The free-swimming form is renniform, and approximately 10-20 mm long with two pairs of flagellae; whereas the attached form is pear-shaped and attaches to the gills and skin.  Disease associated with this parasite includes increased cutaneous mucous production  (hence the lay terminology of “blue slime disease”), epithelial hyperplasia of the skin and gills, ulceration and erosion of fins.  This pathogen commonly causes disease in salmonid fry, resulting in high mortality.   Diagnosis is dependent upon demonstration of the agent within affected fish by cytology or histopathology. 

Hexamita sp:  These parasites are pyriform to oval with tapering toward the posterior end.  Occasionally, rounded individuals can be identified.  The organisms are 6-8 mm wide and 10-12 mm 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).

Chilodonella sp:  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 mm in width and 30-70 mm 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 (photo).  Diagnosis is dependent upon demonstration of the organism within the affected organs by either cytology or histopathology. 

Myxozoan Parasites  

            This is a very large group of parasites which can cause disease in a wide variety of fishes.  They are obligate parasites of tissue (histozoic forms that reside in intercellular spaces or blood vessels that reside intracellularly) and organ cavities.  Key characteristics of the Myxozoa include development of a multicellular spore, presence of polar capsules in their spores and endogenous cell cleavage in both the trophozoite and sporogony stages.  The method of transmission of  myxozoans is unknown, but evidence suggests that at least some pathogenic myxozoans have an indirect life cycle.  This life cycle may require the completion of two different life cycles involving a vertebrate (fish) and an invertebrate (annelid) host with each life cycle having its own sexual and asexual stages.  Severe infestations by these parasites can result in disease and/or death of the host fish.  Each parasite is somewhat species specific as well as organ specific.  A few of the more common myxozoan parasites are discussed below. 

Myxobolus cerebralis:  This parasite is known for causing “whirling disease” in salmonids.  This is a chronic debilitating disease which is very common in the Western U. S. and is uncommon in the Midwestern and Eastern U. S., although recent “outbreaks” have been reported in New York.  The parasite feeds on cartilage of the axial skeleton and clinical signs are related to this damage.   This parasite produces spores within the cartilage which are oval to circular and approximately 7-9 mm in diameter and 6-7 mm long with a thick mucoid envelope on the posterior half of the spore (photo demonstrates spores recovered from a fish with whirling disease).  Histologically, myxozoan parasites within the cartilage of the axial skeleton, with morphological characteristics of Myxobolus cerebralis, must be observed to confirm the diagnosis.  


Aurantiactinomyxon:  This is currently thought to be the causative agent of “proliferative gill disease” of catfish.  This myxozoan parasite causes a rapid, severe, proliferation of the gill epithelium which results in impairment of respiration and osmoregulatory function.  The intermediate host is thought to be a microscopic aquatic oligochaete worm, Dero digitata,  which is found in the mud and sediment in the ponds of affected catfish.  Diagnosis is dependent upon the presence of the parasite within swollen, clubbed proliferative gill lamellae, many of which are fractured due to the associated chondrodysplasia. 

Henneguya sp:  These parasites were once thought to be responsible for the “proliferative gill disease” of catfish, but are now thought to be less pathogenic, although they can cause respiratory problems if present in large numbers.  They have a very characteristic paired, long, whip-like caudal processes giving them a spermatozoan-like appearance.  Size of spores range from 8-24 mm in length with the caudal processes being approximately 20-45 mm long.  Diagnosis is dependent upon observing the parasites within the histopathology of the gill lamellae (photomicrographs 1 and 2 demonstrate mature Henneguya cysts within the gill lamellae).

            The phylum Platyhelminthes is composed of three taxonomic classes:  Turbellaria, Trematoda, and Cestoda.  The Turbellaria are all free-living and have no association with fish diseases.  All members of the other two classes live in close relationship with host animals.  The trematodes are commonly known as flukes, whereas the cestodes are known as tapeworms. 

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.   

Dactylogyrus sp:  This parasite is approximately 0.2 to 0.5 mm in length, reaching a maximum length of 2.0 mm.  It has seven pairs of marginal hooks and usually one pair of median hooks on the opishaptor.  The dactylogrids have two to four pigmented spots (known as “eyes” or “eye spots”) in the anterior fourth of the body. All dactylogrids are oviparous with no uterus (photo).   

Gyrodactylus sp:  This parasite is smaller, rarely reaching a maximum length of over 0.4 mm.  All species are viviparous with one to three daughter generations being observable in the V-shaped uterus.  This parasite is more commonly found on the skin and less commonly present in the gills, although severe infestation will have both organs affected. 

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. 

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.

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”. (Photo demonstrates "black grub" in the fin of an affected fish).  Visible white or yellow spots in the visceral organs, usually no larger than 1 mm in diameter are often referred to as “white grubs” (photo) or “yellow grubs” (photomicrograph  demonstrates "yellow grub" in muscle tissue of affected fish) 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.   


            Cestodes are a taxonomic class of organisms in which the adult stage usually lives in the intestinal tract of vertebrates.  Intermediate stages live in a wide variety of body locations in both vertebrate and invertebrate hosts.  The bodies of most cestodes are ribbon-shaped and divided into short segments called proglottids, hence the name “tapeworm”.  Diagnosis of cestodiasis is dependent upon demonstration of the parasite within the intestinal tract of the fish.  Clinical signs of cestodiasis include emaciation, anemia, discoloration of the skin, and susceptibility to secondary infections.  Low numbers of pleurocercoids may be located in vital organs such as the brain, heart, spleen, kidney, or gonad and have a devastating effect on the fish (photo demonstrates a larval cestode within the liver of a fish).  A few of the more common cestodes are listed below.

Proteocephalus ambloplitis:  This parasite belongs to a large family of cestodes with ten recognized subfamilies.  This parasite has a complicated life cycle involving a piscine second intermediate host and a piscivorous primary host fish.  These parasites are observed within the intestines of bass.  Eggs are expelled from gravid proglottids and pass from the host in feces.  The eggs mature to an embryo which is ingested by several species of copepods as the first intermediate host.  The procercoid develops from the embryo inside the copepods.  The copepod is ingested by a forage-fish whereby the procercoid penetrates the intestinal wall of the second intermediate host.  Some encyst in the wall of the intestine, other penetrate organs in the visceral cavity and may eventually reach the musculature.  Here they develop into plerocercoids,which is then ingested by a piscivorous fish. 

 Ligula intestinalis:  This group of parasites is distinct for three reasons: (a) they are not highly host-specific, but can develop in a wide variety of second intermediate host fishes, (b) the pleurocercoid stage develops sexually in the second intermediate host, and (c) these cestodes are very broad in shape and for this reason have also been known as “beltworms”. 


            This group of parasites is comprised of worms with an anterior proboscis covered with many hooks. These parasites are often referred to as “thorny-headed worms”.  The body is composed of a presoma (proboscis and associated structures) and a cylindrical trunk.  The intestinal tract of the affected fish may contain a blood-tinged fluid, and histologically, the proboscis of these parasites will be firmly attached to the intestinal mucosa.  Photomicrograph demonstrates the proboscis (top) of an acanthocephala parasite which has burrowed into the intestinal mucosa (bottom) of an affected fish.  Affected fish will exhibit emaciation, lethargy, anemia, and possibly death with a marked infection.  We have observed salmonids from the Great Lakes with marked infestation of this parasite with literally hundreds of these parasites embedded in the intestinal mucosa (photo). 

Leeches and Copepods  

            Although not a common problem, occasionally, fish will be observed infected with either leeches or copepods.  Leeches have long, slender flexible bodies and actively swim for an attack on their prey.  Skin and underlying soft tissues are damaged and allow blood to flow into the leeches digestive tract.  Leeches are not host-specific, and the damage to the skin and gills is dependent upon the number of leeches present at any time.  Small fishes can be seriously injured or die due to excessive leech infestation. 

            Copepods include fish lice or “anchor worms”.  The more common fish lice include Lepeophtheirus and Caligus, and Argulus.  The most common genus of anchor worms includes Lernae sp (photo demonstrates a typical Lernae sp. anchor worm).  All of these are external parasites which affect the fish by imbibing blood from the host fish and causing localized skin and soft tissue damage.   They may also allow for secondary bacterial infection of the skin or musculature which may ultimately cause the demise of the fish.