The medical diagnosis is established by finding eggs of "Opisthorchis viverrini" in feces using the Kato technique.

An antigen 89 kDa of "Opisthorchis viverrini" can be detected by ELISA test.

A PCR test capable of amplifying a segment of the internal transcribed spacer region of ribosomal DNA for the opisthorchiid and heterophyid flukes eggs taken directly from faeces was developed and evaluated in a rural community in central Thailand. The lowest quantity of DNA that could be amplified from individual adults of "Opisthorchis viverrini" was estimated to 0.6 pg.

Effective prevention could be readily achieved by persuading people to consume cooked fish (via education programs), but the ancient cultural custom to consume raw, undercooked or freshly pickled fish persists in endemic areas. One community health program, known as the "Lawa" model, has achieved success in the Lawa Lakes region south of Khon Kaen. Currently, there is no effective chemotherapy to combat cholangiocarcinoma, such that intervention strategies need to rely on the prevention or treatment of liver fluke infection/disease.

Cooking or deep-freezing (-20 °C for 7 days) of food made of fish is sure method of prevention. Methods for prevention of "Opisthorchis viverrini" in aquaculture fish ponds were proposed by Khamboonruang et al. (1997).

Most immunodiagnostic tests will detect infection and have a sensitivity above 90% during all stages of the diseases. In addition antibody concentration quickly drops post treatment and no antibodies are present one year after treatment, which makes it a very good diagnostic method. In humans, diagnosis of fasciolosis is usually achieved parasitologically by findings the fluke eggs in stool, and immunologically by ELISA and Western blot. Coprological examinations of stool alone are generally not adequate because infected humans have important clinical presentations long before eggs are found in the stools.

Moreover, in many human infections, the fluke eggs are often not found in the faeces, even after multiple faecal examinations. Furthermore, eggs of "F. hepatica", "F. gigantica" and "Fasciolopsis buski" are morphologically indistinguishable. Therefore, immunonological methods such ELISA and enzyme-linked immunoelectrotransfer blot, also called Western blot, are the most important methods in diagnosis of "F. hepatica" infection. These immunological tests are based on detection of species-specific antibodies from sera. The antigenic preparations used have been primarily derived from extracts of excretory/secretory products from adult worms, or with partially purified fractions. Recently, purified native and recombinant antigens have been used, e.g. recombinant "F. hepatica" cathepsin L-like protease.

Methods based on antigen detection (circulating in serum or in faeces) are less frequent. In addition, biochemical and haematological examinations of human sera support the exact diagnosis (eosinophilia, elevation of liver enzymes). Ultrasonography and xray of the abdominal cavity, biopsy of liver, and gallbladder punctuate can also be used (ref: US-guided gallbladder aspiration:

a new diagnostic method for biliary fascioliasis. A. Kabaalioglu, A. Apaydin, T. Sindel, E. Lüleci. Eur. Radiol. 9, 880±882 (1999) . False fasciolosis (pseudofasciolosis) refers to the presence of eggs in the stool resulting not from an actual infection but from recent ingestion of infected livers containing eggs. This situation (with its potential for misdiagnosis) can be avoided by having the patient follow a liver-free diet several days before a repeat stool examination.

In animals, intravital diagnosis is based predominantly on faeces examinations and immunological methods. However, clinical signs, biochemical and haematological profile, season, climate conditions, epidemiology situation, and examinations of snails must be considered. Similarly to humans, faeces examinations are not reliable. Moreover, the fluke eggs are detectable in faeces 8–12 weeks post-infection. In spite of that fact, faecal examination is still the only used diagnostic tool in some countries. While coprological diagnosis of fasciolosis is possible from 8- to 12-week post-infection (WPI), "F. hepatica" specific-antibodies are recognized using ELISA or Western blot after 2-4 week post-infection. Therefore, these methods provide early detection of the infection.

Liver fluke is a collective name of a polyphyletic group of parasitic trematodes under the phylum Platyhelminthes.

They are principally parasites of the liver of various mammals, including humans. Capable of moving along the blood circulation, they can occur also in bile ducts, gallbladder, and liver parenchyma. In these organs, they produce pathological lesions leading to parasitic diseases. They have complex life cycles requiring two or three different hosts, with free-living larval stages in water.

Metagonimiasis is diagnosed by eggs seen in feces. Only after antihelminthic treatment will adult worms be seen in the feces, and then can be used as part of a diagnostic procedure. A 1993 analysis of the efficacy of ELISA tests to diagnose metagonimiasis implied that simultaneous screening of specific antibodies to several parasite agents are important in serological diagnosis of acute parasitic disease and more research should be done on the efficacy of these methods of diagnosis.

Diagnosis may be difficult because the egg-laying capacity of heterophyids is limited, and therefore sedimentation concentration procedures may be needed to demonstrate eggs in lighter infections. Accurate species identification is also difficult because eggs of most flukes are similar in size and morphology, especially those of "Heterophyes heterophyes", "Clonorchis" and "Opisthorchis". It is important to ask where the person may have contracted the disease, find out if they have been to en endemic area, and check for signs and symptoms that would lead to metagonimiasis.

The body of liver flukes is leaf-like, and flattened. The body is covered with a tegument. They are hermaphrodites having complete sets of both male and female reproductive systems. They have simple digestive systems, and primarily feed on blood. The anterior end is the oral sucker opening into the mouth. Inside, mouth lead to a small pharynx which is followed by an extended intestine that runs through the entire length of the body. The intestine is heavily branched and anus is absent. Instead the intestine runs along an excretory canal that opens at the posterior end. Adult flukes produce eggs which are passed out through the excretory pore. The eggs infect different species of snails (as intermediate hosts) in which they grow into larvae. The larvae are released into the environment from where the definitive hosts (humans and other mammals) get the infection. In some species, another intermediate host is required, generally a cyprinid fish. In this case, the definitive hosts are infected from eating infected fish. Hence, they are food-borne parasites.

Several public health prevention strategies could help lower the rates of metagonimiasis. One is to control the intermediate host (snails). This can be done through use of molluscidals. Another is to use education to ensure all people, especially in areas were the disease regularly occurs, fully cook all fish. This could potentially be problematic and not as effective as hoped as many of the people affected by metagonimiasis eat raw or pickled fish as part of a traditional, long-seated dietary practice. Additionally, implementing more sanitary water conditions would reduce the continual reintroduction of eggs to water sources, thus restarting the lifecycle. Complete control of metagonimiasis presents several potential problems because it does have several reservoir hosts, thus eradication is unlikely.

The acute phase (invasion and migration) may be marked by diarrhea, abdominal pain, fever, cough, urticaria, hepatosplenomegaly, pulmonary abnormalities, and eosinophilia. During the chronic phase, pulmonary manifestations include cough, expectoration of discolored sputum containing clumps of eggs, hemoptysis, and chest radiographic abnormalities. Extrapulmonary locations of the adult worms result in more severe manifestations, especially when the brain is involved." "Diagnosis is based on microscopic demonstration of eggs in stool or sputum, but these are not present until 2 to 3 months after infection. (Eggs are also occasionally encountered in effusion fluid or biopsy material.) Concentration techniques may be necessary in patients with light infections. Biopsy may allow diagnostic confirmation and species identification when an adult or developing fluke is recovered.

Paragonimiasis can commonly be misdiagnosed as tuberculosis.

Under most situations, infection is hard to recognize because the symptoms are mild or even absent. In humans and wild animals, infection is not easily identified. Especially the adult flukes, even if in large number, generally do not cause complications. There is not yet a standard diagnostic test. Therefore, manual diagnosis is done at many levels. Diagnosis basically relies on a combination of postmortem analyses, clinical signs displayed by the animals, and response to drenching. In heavy infection, symptoms are easily observed in sheep and cattle as they become severely anorexic or inefficiently digest food, and become unthrifty. Copious fetid diarrhea is an obvious indication, as the soiling of hind legs and tails with fluid feces are readily noticeable. Even though it not always the case, immature flukes can be identified from the fluid excrement. On rare occasions, eggs can be identified from stools of suspected animals. In developing countries diagnosis and prognosis is often hindered by multiple infections with other trematodes, such as "Fasciola hepatica" and schistosomes, because these flukes are given primary importance due to their pervasive nature.

Several drugs are effective for fascioliasis, both in humans and in domestic animals. The drug of choice in the treatment of fasciolosis is triclabendazole, a member of the benzimidazole family of anthelmintics. The drug works by preventing the polymerization of the molecule tubulin into the cytoskeletal structures, microtubules. Resistance of "F. hepatica" to triclabendazole has been recorded in Australia in 1995 and Ireland in 1998.

Praziquantel treatment is ineffective.

There are case reports of nitazoxanide being successfully used in human fasciolosis treatment in Mexico. There are also reports of bithionol being used successfully.

More recently, Mirazid, an Egyptian drug made from myrrh, has been investigated as an oral treatment of trematode-caused ailments including fascioliasis.

Nitazoxanide has been found effective in trials, but is currently not recommended. The life cycle includes freshwater snails as an intermediate host of the parasite.

Infection can be prevented by immersing vegetables in boiling water for a few seconds to kill the infective metacercariae, avoiding the use of untreated feces ("nightsoil") as a fertilizer, and maintenance of proper sanitation and good hygiene. Additionally, snail control should be attempted.

The drug of choice to treat paragonimiasis is praziquantel, although bithionol may also be used.

Clonorchiasis is an infectious disease caused by the Chinese liver fluke, "Clonorchis sinensis", and two related species.

Clonorchiasis is a known risk factor for the development of cholangiocarcinoma, a neoplasm of the biliary system.

Symptoms of opisthorchiasis caused by "Opisthorchis viverrini" and by "Opisthorchis felineus" are indistinguishable from clonorchiasis caused by "Clonorchis sinensis", so the disease by these three parasites should be referred as clonorchiasis.

Praziquantel is the drug of choice for treatment. Treatment is effective in early or light infections. Heavy infections are more difficult to treat. Studies of the effectiveness of various drugs for treatment of children with "F. buski" have shown tetrachloroethylene as capable of reducing faecal egg counts by up to 99%. Other anthelmintics that can be used include thiabendazole, mebendazole, levamisole and pyrantel pamoate. Oxyclozanide, hexachlorophene and nitroxynil are also highly effective.

Amphistomiasis is considered a neglected tropical disease, with no prescription drug for treatment and control. Therefore, management of infestation is based mainly on control of the snail population, which transmit the infective larvae of the flukes. However, there are now drugs shown to be effective including resorantel, oxyclozanide, clorsulon, ivermectin, niclosamide, bithional and levamisole. An in vitro demonstration shows that plumbagin exhibits high efficacy on adult flukes. Since the juvenile flukes are the causative individuals of the disease, effective treatment means control of the immature fluke population. Prophylaxis is therefore based on disruption of the environment (such as proper drainage) where the carrier snails inhabit, or more drastic action of using molluscicides to eradicate the entire population. For treatment of the infection, drugs effective against the immature flukes are recommended for drenching. For this reason oxyclozanide is advocated as the drug of choice. It effectively kills the flukes within a few hours and it effective against the flukes resistant to other drugs. The commercially prescribed dosage is 5 mg/kg body weight or 18.7 mg/kg body weight in two divided dose within 72 hours. Niclosamide is also extensively used in mass drenching of sheep. Successfully treated sheep regain appetite within a week, diarrhoea stops in about three days, and physiological indicators (such as plasma protein and albumin levels) return to normal in a month.

"Clonorchiasis sinensis" is a trematode (fluke) which is part of the phylum Platyhelminthes. It is a hermaphroditic fluke that requires two intermediate hosts. The parasitic worm is as long as 10 to 25mm and lives in the bile ducts of the liver. The eggs of the worms are passed through fecal matter which are then ingested by mollusks. One becomes infected by eating undercooked, smoked, pickled salted freshwater fish. Freshwater fish are a second intermediate host for the parasitic worm. They become infected when the larvae (cercaria) of the worm penetrates the flesh of the fish. The water snail is the first intermediate host in which a miracidium (an embryonated egg discharged in stool) goes through its developmental stages (sporocyst, rediae and cercariae). Clonorchiasis is endemic in the Far East, especially in Korea, Japan, Taiwan, and Southern China. Clonorchiasis has been reported in non endemic areas (including the United States). In such cases, the infection follows the ingestion of imported, undercooked or pickled freshwater fish containing metacercariae.

Trematodiases refers to a number of different trematode infections, many of which are spread by other animals. In 2015 the food born ones affected about 71 million people.

- Food-borne trematodiases as listed by the WHO:

- Clonorchiasis

- Opisthorchiasis

- Fascioliasis

- Paragonimiasis

- Others:

- Metagonimiasis

- Fasciolopsiasis

- Metorchiasis, caused by the Canadian liver fluke

- Dicrocoeliasis

In order to control for the disease, the "Lymnaea" spp snails, which are the intermediate host for the liver flukes, need to be controlled. There are three ways that have proven most effective when controlling the snail populations:

- The first is by treating pastures and water channels with copper sulfate. This method is not always practical, because it is too expensive to treat in large areas. Lack of cooperation between neighbors is also a problem, snails are easily transported, and treated pastures become re-infested by neighboring fields and streams.

- Drenching the sheep with carbon tetra-chloride in paraffin oil has proven to be an alternative. However, drenching in more than recommended doses can be fatal, by causing liver damage, which could initiate the disease in sheep carrying "B. oedematiens" spores.

- Drainage is an effective option to eliminate the snails. However, draining the places where the grass grows eliminates a source of food for the sheep and creates other unwanted problems.

Modern imaging techniques allow the diagnosis to be made more easily and without invasive imaging of the biliary tree. Commonly, the disease is limited to the left lobe of the liver. Images taken by CT scan, X-ray, or MRI show enlarged intrahepatic (in the liver) bile ducts due to ectasia. Using an ultrasound, tubular dilation of the bile ducts can be seen. On a CT scan, Caroli disease can be observed by noting the many fluid-filled, tubular structures extending to the liver. A high-contrast CT must be used to distinguish the difference between stones and widened ducts. Bowel gas and digestive habits make it difficult to obtain a clear sonogram, so a CT scan is a good substitution. When the intrahepatic bile duct wall has protrusions, it is clearly seen as central dots or a linear streak. Caroli disease is commonly diagnosed after this “central dot sign” is detected on a CT scan or ultrasound. However, cholangiography is the best, and final, approach to show the enlarged bile ducts as a result of Caroli disease.

The capture, transportation and culture of bait fish can spread damaging organisms between ecosystems, endangering them. In 2007, several American states, including Michigan, enacted regulations designed to slow the spread of fish diseases, including viral hemorrhagic septicemia, by bait fish. Because of the risk of transmitting "Myxobolus cerebralis" (whirling disease), trout and salmon should not be used as bait. Anglers may increase the possibility of contamination by emptying bait buckets into fishing venues and collecting or using bait improperly. The transportation of fish from one location to another can break the law and cause the introduction of fish and parasites alien to the ecosystem.

Peritoneal and thoracic cavities contains a great quantity of fluids, as does the pericardial sac.

The liver is sometimes swollen with perihepatitis. There appear a great number of necrotic foci, 3 to 4 mm diameter, which extend deeply in the organ. Hepatic lymph nodes are enlarged. The gallbladder is full.

Subcutaneous tissues are full of cyanotic venous blood. They may darken the hide, hence the name "black disease".

Though not a health concern in thoroughly cooked fish, parasites are a concern when human consumers eat raw or lightly preserved fish such as sashimi, sushi, ceviche, and gravlax. The popularity of such raw fish dishes makes it important for consumers to be aware of this risk. Raw fish should be frozen to an internal temperature of −20 °C (−4 °F) for at least 7 days to kill parasites. It is important to be aware that home freezers may not be cold enough to kill parasites.

Traditionally, fish that live all or part of their lives in fresh water were considered unsuitable for sashimi due to the possibility of parasites (see Sashimi article). Parasitic infections from freshwater fish are a serious problem in some parts of the world, particularly Southeast Asia. Fish that spend part of their life cycle in salt water, like salmon, can also be a problem. A study in Seattle, Washington showed that 100% of wild salmon had roundworm larvae capable of infecting people. In the same study farm raised salmon did not have any roundworm larvae.

Parasite infection by raw fish is rare in the developed world (fewer than 40 cases per year in the U.S.), and involves mainly three kinds of parasites: Clonorchis sinensis (a trematode/fluke), Anisakis (a nematode/roundworm) and Diphyllobothrium (a cestode/tapeworm). Infection by the fish tapeworm "Diphyllobothrium latum" is seen in countries where people eat raw or undercooked fish, such as some countries in Asia, Eastern Europe, Scandinavia, Africa, and North and South America. Infection risk of anisakis is particularly higher in fishes which may live in a river such as salmon ("shake") in Salmonidae, mackerel ("saba"). Such parasite infections can generally be avoided by boiling, burning, preserving in salt or vinegar, or freezing overnight. Even Japanese people never eat raw salmon or ikura (salmon roe), and even if they seem raw, these foods are not raw but are frozen overnight to prevent infections from parasites, particularly anisakis.

Below are some life cycles of fish parasites that can infect humans:

Chronic liver disease takes several years to develop and the condition may not be recognised unless there is clinical awareness of subtle signs and investigation of abnormal liver function tests.

Testing for chronic liver disease involves blood tests, imaging including ultrasound and a biopsy of the liver. The liver biopsy is a simple procedure done with a fine thin needle under local anaesthesia. The tissue sample is sent to a laboratory where it is examined underneath a microscope.

Feline hepatic lipidosis shares similar symptoms to other problems, including liver disease, renal failure, feline leukemia, Feline infectious peritonitis and some cancers. Diagnosis requires tests that target the liver to make an accurate diagnosis. Jaundice is highly indicative of the disease. Blood tests and a liver biopsy will confirm the presence of the disease.

PSC is generally diagnosed on the basis of having at least two of three clinical criteria after secondary causes of sclerosing cholangitis have been ruled out:

- serum alkaline phosphatase (ALP) > 1.5x the upper limit of normal for longer than 6 months;

- cholangiography demonstrating biliary strictures or irregularity consistent with PSC; and,

- liver biopsy consistent with PSC (if available).

Historically, a cholangiogram would be obtained via endoscopic retrograde cholangiopancreatography (ERCP), which typically reveals "beading" (alternating strictures and dilation) of the bile ducts inside and/or outside the liver. Currently, the preferred option for diagnostic cholangiography, given its non-invasive yet highly accurate nature, is magnetic resonance cholangiopancreatography (MRCP), a magnetic resonance imaging technique. MRCP has unique strengths, including high spatial resolution, and can even be used to visualize the biliary tract of small animal models of PSC.

Most people with PSC have evidence of autoantibodies and abnormal immunoglobulin levels. For example, approximately 80% of people with PSC have perinuclear anti-neutrophil cytoplasmic antibodies; however, this and other immunoglobulin findings are not specific to those with PSC and are of unclear clinical significance/consequence. Antinuclear antibodies and anti-smooth muscle antibody are found in 20%-50% of PSC patients and, likewise, are not specific for the disease but may identify a subgroup of PSC patients who also have autoimmune hepatitis (i.e. PSC-AIH overlap syndrome).

Other markers which may be measured and monitored are a complete blood count, serum liver enzymes, bilirubin levels (usually grossly elevated), kidney function, and electrolytes. Fecal fat measurement is occasionally ordered when symptoms of malabsorption (e.g., gross steatorrhea) are prominent.

The differential diagnosis can include primary biliary cholangitis (formerly referred to as primary biliary cirrhosis), drug-induced cholestasis, cholangiocarcinoma, IgG4-related disease, post-liver transplantation non-anastomotic biliary strictures, and HIV-associated cholangiopathy. Primary sclerosing cholangitis and primary biliary cholangitis are distinct entities and exhibit important differences, including the site of tissue damage within the liver, associations with inflammatory bowel disease (IBD), which includes ulcerative colitis and Crohn's disease, response to treatment, and risks of disease progression.