Nematodes infect freshwater, marine and brackish water fish species. Pathology normally occurs within the intestines but can affect all organs and substantial damage to the host can occur. Some nematodes cause high levels of fish mortality in wild fish populations. Nematodes can infect fish as adults but larval stages of nematodes infecting piscivorous birds, mammals or reptiles, or less frequently predatory fish, can also infect fish species.
Some nematodes are zoonotic and include species such as Anguillicola, Philometra, Skrjabillanus, and Anisakis. Ingestion of uncooked infected fish meat poses a zoonotic threat to humans. Following ingestion by humans the larvae are activated by high temperatures, acidic pH and pepsinogen within the stomach.
Nematodes also cause an economic threat to the market value of fish, through consumer attitudes towards the presence of these parasites within food products. Infected fillets are rejected and can increase production costs. Disease is mainly found within wild fish populations as captive fish are fed non-infected feed.
Fish within niche habitats tend to have a narrow host range compared to fish species found all over the world. Severe disease can arise when a nematode colonises a new host species, for example nematodes of Japanese eel, A. crassus, causes mild disease but in European and American eel species it causes more severe pathology. Skrjabillanid genera (Skrjabillanus tincae, Skrjabillanus cyprini, Molnaria intestinalis, Sinoichthyonema amuri), infect only one host, whereas Capillaria species can colonise numerous fish species.
Adult stages of Anisakis spp., Pseudoterranova spp., Phocascaris spp. and Contracaecum spp. occur in whales and seals, whereas their third-stage larvae occur in fish musculature.
Nematodes can affect a wide range of species including eels, sturgeons, bream, pike, cod, trout, carp, bass, rays, skate, bleaks, arapaima, perches, piranha zander, lingcod, viviparous blenny (European eelpout) and aquarium species such as guppy (millionfish) and discus fish. They can also infect mammals such as Cetacea (whales, dolphins and porpoises) and Phocidae (seals).
Fish nematodes can also infect humans and they penetrate the gastric or intestinal mucosa, resulting in stomach cramps and abdominal pain. The worms can also migrate to other areas of the body.
Sexual maturity of the nematodes is reached through a complicated multi-host life cycle. Any disruptions to these cycles prevent the development of the adult nematodes, therefore cultured fish taken from their natural environment are less likely to develop nematode infections. Nematodes do, however, affect aquarium fish species.
Immune reactions in the host to live and dead worms accentuate the severity of the disease by the production of immunoglobulin E (IgE), mast-cell degranulation, eosinophilia, causing oedema and urticaria. Nematodes have two main types of antigens soluble excretory and secretory (E/S) that elicit both humoral (antibody production) and cellular host reactions.
Nematodes have been shown in fish stock in Africa, North America, South America, Europe and Russia.
The nematodes deprive their host of food and can feed on host tissues, sera and blood causing emaciation and anaemia. Clinical signs of nematode infestation vary and can range from deformed body shape, haemorrhage, mortality, traumatic enteritis, loss of balance through damage to their swim bladder, reduced swimming performance, lethargy, reduced sexual display rate, ulceration of gill cover, fraying of fins, large nodules on the ventral surface of the skin and fish can be seen swimming or floating on their sides. Infected fish can be more susceptible to decreased oxygen content in the water. Pathogen such as Philonema and Philometra can cause atrophy or destruction of gonads, ascites and distension of the abdomen.
Nemotodes cause an array of different pathogenicity within fish and can cause damage to the GI tract, swim bladder, gonads, internal organs (especially the liver), gills , eye and skin. Mechanical damage to the mucosa and submucosa by nematode migration and proteolytic damage from nematode enzymes are common. Larger parasites can cause deformation of organs and body shape, mesenteric and visceral adhesions, granulomas, haemorrhage, deep nodules within the stomach wall and general inflammation. Some nematodes can cause blockages to the GI tract .
In some nematode infections accumulation of granule cells, leucocytes and macrophages and epithelial fibroblasts around third-stage larvae can be seen. With Schulmanela (Hepaticola) petruschewskiies and other liver damaging pathogens a greyish discoloration of the liver, with pinhead and larger nodules can be seen along with hyperaemia, petechial haemorrhages and icterus. Histologically, worms, fibrinous-serous exudate, haemorrhage and hyperaemia of the liver capillaries can be seen with leucocytic infiltration, epithelioid cell proliferation and giant cells.
Swim bladder damage can be caused by species such as Cystidicola and Anguillicola, and epithelial hyperplasia, hyperaemia and necrotic areas around migrating larvae can be seen in the swim bladder wall, which can give rise to secondary bacterial infections. Continual damage by migrating larvae cause fibrosis and thickening of the swim bladder. Swelling of the scale sacs and inflammation and haemorrhaging under the scales can be seen with nematodes such as Philometra and Philometroides which are known to infect the skin, fins and gill epithelium.
Larger skin parasites such as Philometra and C. acipenseris species can be easily seen as, red parasites in the opercula and on the fins and from raised nodules surrounding migrating larvae respectively. Gut and organ inhabiting nematodes will need to be found on post-mortem and microscopic examination of infected tissue. Histozoic species such as skrjabillanid can be identified under the microsope from scrapings of intestinal serosa. Squash preparations of infected organs between two slides can be used for larval nematodes. Radiographs have been used to diagnose Anguillicola within the swim bladder.
PCR can be used for accurate diagnosis of nematode species.
There is no effective drug against fish nematodes but drugs such as flubendazol, levamisole, mebendazole, trichlorphon and triclabendazole have been used.
Freezing at -20°C for 24 hours, heating above 60°C or salting will kill Anisakis larvae. Pseudoterranova larvae can be killed at 30°C for 15 h or at 20°C for 7 days, but dead nematodes can trigger immune reactions in human hosts as they contain immunogenic molecules.
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