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Fish helminths

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Introduction

Infections with helminths are quite common in both feral and cultured fish. There are several groups of worms that parasitize on various fish species: flatworms (monogenean and digenean trematodes or flukes, tapeworms), roundworms (nematodes), and opportunistic free-living turbellarians.

In addition to the negative impact on health of feral and cultured fish, some fish helminths can also be of medical importance. Medical problems are caused mainly by those parasites, which infective stages (third-stage larvae of nematodes, metacercariae of trematodes, plerocercoids of tapeworms) are localized in the flesh of fish. Humans might acquire infection after consuming raw or poorly-cooked fish meat. As an example, anisakid nematodes (Anisakis, Pseudoterranova), the spirurid nematodes of the genus Gnathostoma, small liver and intestinal flukes (Opisthorchis, Clonorchis, Heterophyes, Haplorchis, Stellantchasmus) or Diphyllobothrium tapeworms can be mentioned.

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Monogenean flatworms
(Platyhelminthes, Monogenea)

Gyrodactylus spp. (gyrodactylosis fluke, skin fluke)

Taxonomy

cellular organisms - Eukaryota - Opisthokonta - Metazoa Eumetazoa - Bilateria - Platyhelminthes - Monogenea - Monopisthocotylea - Gyrodactylidae - Gyrodactylus

Brief facts

Gyrodactylus spp. is a monogenean (with direct life cycle), viviparous (live-bearing free-swimming larvae) parasitic flatworm (fluke) (about 0.5 mm long) of fishes in freshwater rivers and lakes.

The free-swimming larva (oncomiracidium) migrates to the body surface and attach to the host using their opisthaptor (an organ located at the posterior end of the worm) which is usually equipped with large hooks, clamps and/or suckers.

Gyrodactylids have no eyespots, and often an embryo is visible inside the fluke. Fluke anchored on skin appears to be "bobbing" or stretching and compressing its body.

Differentiation of Gyrodactylus spp. into species is difficult and, consequently, some species remain unidentified or incorrectly classified. Differentiation of the ten species of Gyrodactylus spp. described from carp and the five from goldfish is particularly difficult.

Signs

Clinical signs include flashing, rubbing, gasping, lethargy, "yawning", clamped fins, excess mucus production, secondary cutaneous ulcerations, scale loss, and deaths in severe infestations. Affected fish appear pale, with desquamation of the skin epithelium, focal hemorrhagic lesions around the heavily infested skin zones, and excessive mucus on the rest of the skin. Gyrodactylus spp. cause morbidity and mortality in fish because of their high intensities, and significant damage at the point of attachment (with considerable host reactions) through their opisthaptor and by grazing on exposed structures and vulnerable integument.

Treatment

Treatment methods include the use of praziquantel as a bath or prolonged immersion treatment (2–10 mg/L). Other treatment options include organophosphates (to which some fish species are very sensitive), mebendazole, formalin, and potassium permanganate.

Gyrodactylus salaris

The most famous monogenean parasite in the last years is undoubtedly Gyrodactylus salaris, which eliminated several salmon populations in Norwegian rivers and caused significant losses in farms. On susceptible hosts the parasite has a high reproductive rate which rapidly increases the intensity of infection, and this also promotes efficient transmission between fish especially when fish are under crowded conditions. The parasite is mainly a pathogen of Atlantic salmon, Salmo salar, in freshwater rivers and lakes in Norway. Artic charr, Salvelinus alpinus, and rainbow trout, Oncorhynchus mykiss, are known reservoir hosts, and the parasite is found in Norway, Sweden, Finland, Denmark, Russia and Germany. The pathogen is normally attached to the fins and to a lesser degree to the body surface, cornea and nostrils, and very rarely on the gill apparatus.

Gyrodactylus salaris is a major problem in farmed and wild salmon in Norway. It is assumed that it was introduced to Norwegian waters recently, probably in the 1970s, with salmon parr from a hatchery in Sweden where the parasite had occurred. In most localities in Norway, G. salaris spread quickly infecting in two months about 90% of the salmon parr population. Migration of salmon pre-smolt between rivers via fjords and possibility of spreading of G. salaris by fish eggs probably facilitated rapid spreading of the parasite as well. The presence of the parasite resulted in considerable decline in salmon parr density—to about 50% two years after G. salaris introduction and to 2–4% after 5–7 years.

Control measures involve replacing populations of salmon with parasite-free stocks after application of rotenone which kills all the fish in the river. Using this drastic method, the parasite has been eradicated from more than half of the localities where it previously occurred. However, the most important aim for veterinarians and fish farmers at present should be to take all possible measures to prevent introduction of the parasite to new regions. Laboratory experiments with Scottish stocks of Atlantic salmon have indicated possibly disastrous consequences of introducing G. salaris into new areas.

Gyrodactylus in PubMed

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Dactylogyrus spp. (gill fluke)

Taxonomy

cellular organisms - Eukaryota - Opisthokonta - Metazoa Eumetazoa - Bilateria - Platyhelminthes - Monogenea - Monopisthocotylea - Dactylogyridae - Dactylogyrus

Brief facts

Dactylogyrus species are a group of monogenean gill parasites these parasites are highly host specific to freshwater fish of family Cyprinidae.

Dactylogyrids are egg layers (oviparous) and are often found on imported fancy goldfish, angelfish, and discus. Dactylogyrus release eggs into the water, which hatch into a free-swimming stage (oncomiracidium) that seeks out a fish host and attach to the gills using their opisthaptor (an organ located at the posterior end of the worm). Dactylogyrus can be recognized by the prominent two to four anterior eyespots and a four-pointed anterior end.

Signs

Clinical signs include flashing, rubbing, gasping, lethargy, "yawning", clamped fins, excess mucus production, secondary cutaneous ulcerations, scale loss, and deaths in severe infestations. Flukes can predispose fish to secondary ulcers, bacterial diseases, and eventual osmoregulatory compromise due to epithelial damage created by their attachment and feeding behavior.

Treatment

Treatment methods include the use of praziquantel as a bath or prolonged immersion treatment (2–10 mg/L). The eggs of Dactylogyrus spp. are not susceptible to treatment and multiple treatments are required. Other treatment options include organophosphates (to which some fish species are very sensitive), mebendazole, formalin, and potassium permanganate. An egg-killing mixture of closantel and mebendazole has been used to treat monogeneans in koi (Cyprinus carpio). However these drugs has been shown to be very toxic to goldfish (Carassius auratus). Formalin is the only US Food and Drug Administration (FDA)-approved treatment for finfish.

Dactylogyrus
Dactylogyrus difformis (C). Dactylogyrus sphyrna (D).
Pazooki J et al. Iran J Parasitol. Aug 2011.

Dactylogyrus in PubMed

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Capsalidae (Neobenedenia and Benedenia)

Taxonomy

cellular organisms - Eukaryota - Opisthokonta - Metazoa Eumetazoa - Bilateria - Platyhelminthes - Monogenea - Monopisthocotylea - Capsalidae -

Brief facts

Neobenedenia and Benedenia are large, and are characterized by a large circular opisthohaptor on the posterior end and two smaller suckers on the anterior end. Benedenia has two pairs of tightly apposed, curved anchors, whereas Neobenedenia has three pairs. These two parasites adhere to the skin, gills, and eyes of susceptible fish, causing significant mechanical damage in large numbers.

Neobenedenia

Neobenedenia, large (3-10 mm), flat parasites of marine fish of the integument, move actively and consume the cellular substance of the integument. Heavy infestations cause deep lesions in the integument exposing the dermis. Worms often settle on the eyes and damage the cornea, resulting in destruction of the eyes. Lesions are often inflamed, hemorrhagic, and secondarily contaminated with bacteria. Infestations are, therefore, deadly to fish in culture and aquaria. The first record of such an infestation comes from the New York Aquarium, where heavy infestations by Neobenedenia melleni (originally described as Epibdella melleni Maccallum, 1927), spread among numerous species of fish kept in warm waters. Only a few fish species were resistant. Fish surviving infestation developed an acquired resistance to reinfection

Benedenia

Benedenia monticelli is a common parasite of grey mullet in the Mediterranean and the North Red Sea. Parasites demonstrate a preference for the mouth integument; however, with increasing infestation, worms settle on lips, mandibular, gular (behind chin and between the sides of the lower jaws), and preopercular surfaces of the head, eyes, and ultimately on the entire body surface. The feeding worm perforates the membranous connections between the cranium and the jawbones, the gular arch and the membranous extensions of the operculum. Once introduced into stocks of cultured mullets (of all sizes), infestations take on epizootic proportions with lethal consequences. Benedenia spp., a parasite of juvenile pompano, Trachinotus carolinus, have been reported in Florida.

Treatment

Treatment methods include the use of praziquantel as a bath or prolonged immersion treatment. The eggs of marine capsalids are not susceptible to treatment and multiple treatments are required. In addition the eggs can be long and have attachments that can adhere to anything in the tank, making treatment and management of these parasites difficult. Other treatment options include organophosphates (to which some fish species are very sensitive), mebendazole, formalin, and potassium permanganate. Formalin is the only US Food and Drug Administration (FDA)-approved treatment for finfish. For marine infections, a freshwater dip in quarantine may be helpful to both diagnose and remove monogeneans. This can be done using a dark, five-gallon bucket for freshwater dips and dipping marine fish for 1 to 3 minutes in the bucket. When dips are completed, water in the bucket can be swirled and dislodged capsalid parasites will look like snowflakes swirling in the water. Biologic control has been reported using cleaner fish (French angelfish, neon gobies, and Pacific cleaner wrasse) in marine systems.

Neobenedenia in PubMed

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Digenean trematodes
(Trematoda, Digenea)

Brief facts

Digenetic trematodes are parasites with an indirect life cycle with larval forms (metacercariae) that can cause unsightly lesions on fish but are generally not pathogenic. Severe infections can cause deaths, larval migration may lead to secondary bacterial infections, and ocular lesions may lead to feeding problems. Examples include Clinostomum ("yellow grub"), Neascus ("black spot"), Diplostomum ("eye fluke"), and Posthodiplostomum ("white grub"). The problem appears in outdoor-raised pond fish such as koi and goldfish, but can be seen occasionally in tropical pet fish kept in aquaria. The life cycle does not typically continue in indoor aquaria unless snails (or other intermediate hosts) are present.

In tropical and subtropical natural habitats, trematode metacercariae constitute the most abundant parasitic infections in fish, particularly juvenile fish. Few species of metacercariae have been identified and the taxonomic identity of their vast majority remains obscure. Among juvenile fish, infections are particularly abundant in freshwater cichlids and in grey mullet found in both brackish water and marine habitats.

Heavily infected, market-size fish, especially those with conspicuously large worms or cysts in the muscles, will be rejected by the food control authority as well as the consumer. Another, more serious, aspect of metacercarial infestations in marketable fish is their possible risk to human health. Many heterophyiid trematodes can cause mild to severe pathology in humans who consume infected fish. However, infection in humans occurs only when fish are consumed raw, lightly salted, or insufficiently grilled.

Ecological aspects of transmission

Prerequisites for the establishment of metacercarial infection in the habitat are (a) the presence of the snail host until the trematode completes development from the larval stage to the cercarial stage and (b) accessibility to the definitive host, usually a piscivorous bird or mammal, which shed the trematode eggs. In modern, intensive, indoor holding-tank farms or in most intensively used earth ponds, these prerequisites are rarely met. Although potential vector species of snails (such as Bulinus truncatus, Lymnaea sp., and Melanoides tuberculata in freshwater and Pirenella conica in sea water) are often found in large numbers in indoor systems, these systems are usually inaccessible to wild birds and mammals.

Digenean trematodes in fish
Metacercaria of Posthodiplostomum cuticula (E) from skin and fins. Rhaphidocotyle sp. from gills (F). Metacercaria of Diplostomum spataceum from the eyes (G).
Pazooki J et al. Iran J Parasitol. Aug 2011.

Digenean trematodes in fish in PubMed

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Cestode tapeworms
(Cestoda)

Brief facts

There are few important cestodes of fish. The most detrimental, the Asian tapeworm, Bothriocephalus acheilognathi, has been found in koi and other cyprinids (excluding goldfish), channel catfish, and aquarium fish such as discus, livebearers, killifishes, and angelfish.

Signs

Clinical signs range from none to lethargy, anorexia, weight loss, chronic intestinal inflammation, intestinal obstruction, and severe mucosal damage.

Treatment

Oral praziquantel (not FDA approved) can be used to treat at 50 mg/kg by mouth for one dose, or 5 to 12 gm/kg of feed every 24h  2 to 3 days. Treatment is recommended to be given in a separate tank to prevent the dispersion of eggs when the cestode dies.

Bothriocephalus acheilognathi (Asian tapeworm)

The Asian tapeworm Bothriocephalus acheilognathii originates from grass carp in the Amur River, southern Siberia. From 1954 to 1982, Asian tapeworm infections spread to farmed and native cyprinids and a few non-cyprinids via the introduction of grass and common carp to Europe, Asia, and North America. Subsequently, the worms were spread to a multitude of locations via the exportation of infected fish from China, Germany, and South Africa.

Infection may reach 30-156 mature and gravid worms per 90-160-mm-long pond-reared carp, and <l0-20 worms per fully grown grass carp.

B. acheilognathii causes up to 90% mortality among heavily infected grass carp and juvenile carp. Pathological changes include: pressure lesions, mild-to-severe catarrhal enteritis that is sometimes hemorrhagic at the parasite attachment point, and connective tissue proliferation. Intestines of small infected fish, such as golden shiner, can be plugged by the worms and, in some instances, perforated.

The cosmopolitan copepod crustacean Mesocyclops leukarti is a frequent intermediate host of the Asian tapeworm, particularly in large bodies of water. Other genera of copepods, such as Thermocyclops, Ectocyclops, and Paracyclops, found in fish ponds in China and Korea and Cyclops abyssorum in northern Germany are also compatible intermediate hosts. Distribution and seasonality of Asian tapeworm infections largely depends on the abundance of compatible copepods.

The life cycle of the Asian tapeworm, a thermophilic species, can be delayed or even interrupted by low temperatures. At 28-30 °C, 77% of the eggs hatch within 24 h after release, and the remainder during the following five days. If the temperature drops to 14-15 °C, the incubation period ranges from l0-28 days; development does not occur below 12 °C. The completion of procercoid growth in the copepod intermediate host occurs 10-17 days after infection at 25 °C. In fish, worms mature within 21-23 days at 28-29 °C and 1.5-2 months at 15-22 °C.

Tapeworms in fish in PubMed

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Roundworms
(Nematoda)

Brief facts

Fish can be either intermediate or final hosts of nematodes. Nematodes in fish are similar to other animal species and appear as smooth, long worms.

Species

Most important are parasitic roundworms of fish species that affect the intestinal tract; these include Camallanus, Capillaria, and Capillostrongyloides. Other species, such as Eustrongyloides, can cause cysts to form in the liver, muscles, and peritoneum. Camallanus is an ovoviviparous nematode that affects cichlids, guppies and other live bearers. In Camallanus infections, the owner may a report a "red worm" protruding from the vent of their fish. Capillaria sp. is most often diagnosed in angelfish, discus, and other cichlids, but can be found in many other tropical freshwater fish. It has a direct life cycle. In a closed aquarium, an infestation of Capillaria can spread very rapidly among susceptible fish.

Signs

Clinical signs can range from none to anemia, lethargy, poor weight gain, failure to thrive, and reproductive problems.

Treatment

Treatment of nematodes can be accomplished by the use of fenbendazole or levamisole. Neither drug is FDA-approved for use in food fish.

Nematodes in fish in PubMed

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References

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