# Liver Worm in Dogs and Cats: Signs and Treatment A liver worm in a dog or cat is almost always a trematode, also called a liver fluke, not a roundworm or a tapeworm. The flukes that matter most in small animal practice are *Fasciola hepatica*, *Opisthorchis felineus*, *Clonorchis sinensis*, and a group of related biliary flukes such as *Metorchis conjunctus*, *Amphimerus* species, *Platynosomum illiciens*, and *Paropisthorchis caninus*. These parasites live in the bile ducts or liver tissue, and they reach a dog or cat only after passing through a snail and then a second intermediate host such as a fish or an aquatic plant. This article explains the biology behind that route, the clinical signs owners notice, the diagnostic tests that actually find fluke eggs, and the drugs used to clear the infection. It also covers the zoonotic risk that raw fish and raw water plants carry for the people in the household. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## What "Liver Worm" Really Means in Dogs and Cats Owners often use "liver worm" to describe any worm they believe is living in the liver. In dogs and cats, that label is misleading. The true liver worms of veterinary importance are flatworms in the class Trematoda. They are leaf-shaped, not cylindrical, and they do not look like the spaghetti-shaped roundworms owners picture. Two broad groups appear in the veterinary literature. The first group is the bile duct flukes, which include *Opisthorchis*, *Clonorchis*, *Metorchis*, *Amphimerus*, *Platynosomum*, and *Paropisthorchis*. These flukes settle in the biliary tree and provoke inflammation of the bile ducts. The second group is *Fasciola* species, which can migrate through liver tissue before settling in the bile ducts and cause more direct damage to the liver parenchyma. A case report from Canada described a one-year-old German shepherd and husky cross dog with multiple liver abscesses and severe cholangitis caused by the fluke *Metorchis conjunctus* [1]. That single case shows how much damage a biliary fluke can do when it blocks or irritates the bile ducts. A separate report from India recovered a small fluke, *Paropisthorchis caninus*, from the liver of a street dog and found degeneration of liver tissue, hyperplasia of the bile duct lining, fibrous tissue proliferation, and chronic proliferative inflammation [2]. These are the tissue changes that produce the clinical signs owners eventually see. ### Fluke Species, Hosts, Regions, and Drugs | Fluke species | Main definitive hosts | Geographic region | Drug reported in the literature | |--|--|--|--| | *Metorchis conjunctus* | Dogs, cats, wild carnivores | North America | Praziquantel [1] | | *Amphimerus* sp. | Cats, dogs, humans | Ecuador, Latin America | Praziquantel and triclabendazole are used in human and animal care | | *Opisthorchis felineus* | Cats, dogs, humans | Europe, Russia, Central Asia | Praziquantel | | *Opisthorchis viverrini* | Dogs, cats, humans | Southeast Asia | Praziquantel | | *Clonorchis sinensis* | Dogs, cats, humans | China, East Asia | Praziquantel | | *Platynosomum illiciens* | Cats | Southeast Asia, Americas | Praziquantel | | *Paropisthorchis caninus* | Dogs | India | Not established in the source | | *Fasciola hepatica* | Sheep, cattle, dogs, cats, humans | Worldwide | Triclabendazole | The table is a starting point, not a prescription. Drug choice depends on the species, the country, the label status of the drug, and the individual animal. ## The Life Cycle of a Liver Fluke

A liver fluke cannot pass directly from one dog or cat to another. It needs at least two intermediate hosts, and sometimes three, before it becomes infective. That is why fluke infections cluster in specific places, usually near freshwater where snails and fish live. ### Step 1: Eggs Leave the Definitive Host Adult flukes in the bile ducts lay eggs that pass down the bile duct, into the intestine, and out in the feces. A study of *Amphimerus* in Ecuador found eggs in the stool of 71.4 percent of cats and 38.7 percent of dogs living in affected communities, which confirms that dogs and cats are definitive hosts and reservoir hosts for this fluke [3]. A definitive host is the animal in which the adult worm lives and reproduces. ### Step 2: The Snail Host Once the egg reaches fresh water, it hatches into a free-swimming stage that must find a snail. Inside the snail, the parasite multiplies through several larval stages. This step is slow and depends on water temperature, which is why fluke risk is seasonal in temperate regions and year-round in the tropics. ### Step 3: The Second Intermediate Host The parasite leaves the snail and seeks a second host. For *Opisthorchis*, *Clonorchis*, and *Metorchis*, that host is a freshwater fish or shrimp. A 2024 survey of fish in East Lake in Wuhan, China found *Clonorchis sinensis* metacercariae in five fish species, with an overall prevalence of 32.5 percent and the highest prevalence of 57.9 percent in *Pseudorasbora parva* [4]. Metacercariae are the dormant, infective cysts that sit in fish muscle. For *Fasciola hepatica*, the second intermediate host is not a fish. The infective metacercariae attach to aquatic plants such as watercress and water dropwort, and a dog or cat becomes infected by eating those plants. ### Step 4: Infection of the Dog or Cat A dog or cat becomes infected by eating raw or undercooked fish, shrimp, or aquatic plants that carry metacercariae. The cyst dissolves in the intestine, the young fluke migrates to the liver, and it matures in the bile ducts over several weeks. The study from Wuhan noted that feeding cats small fish caught by anglers creates a potential infection risk, alongside human consumption of undercooked fish [4]. That is the practical route for most pets. ```mermaid flowchart TD [Adult fluke in bile ducts] --> [Eggs pass in feces] [Eggs pass in feces] --> [Egg hatches in fresh water] [Egg hatches in fresh water] --> [Snail host] [Snail host] --> [Free swimming larvae] [Free swimming larvae] --> [Fish shrimp or aquatic plants] [Fish shrimp or aquatic plants] --> [Metacercariae eaten by dog or cat] [Metacercariae eaten by dog or cat] --> [Young fluke migrates to liver] [Young fluke migrates to liver] --> [Adult fluke in bile ducts] ``` ## Clinical Signs of Liver Fluke Infection The signs of a liver worm in a dog or cat reflect damage to the bile ducts and liver tissue. They develop slowly, and early infection can be silent. ### Early and Mild Infection Many infected animals show no obvious signs at first. A study of cats with elevated ALT found that infected cats had significantly higher eosinophil counts than uninfected cats, but their ALT and ALP values were not significantly different from uninfected cats [5]. Eosinophils are a type of white blood cell that rises with parasite infections. This means a fluke can be present and causing an immune response before routine liver enzymes look abnormal. ### Progressive Signs As the flukes multiply and the bile ducts thicken, owners may notice: - Weight loss despite a normal appetite - Reduced appetite or intermittent vomiting - Jaundice, which is yellowing of the gums, the whites of the eyes, or the skin - Ascites, which is fluid accumulation in the abdomen that makes the belly look bloated - Lethargy and reduced activity - Diarrhea, sometimes pale or greasy Ascites and jaundice are signs of advanced biliary disease. A five-year-old cat with abdominal enlargement was found to have large hepatic cysts, and the treating team initially considered a liver fluke before histopathology revealed cyst-like lymphocytic cholangiohepatitis [6]. That case is a reminder that a bloated abdomen in a cat has several possible causes, and a fluke is only one of them. ### Laboratory Signs The classic laboratory finding is eosinophilia. The cat study found this association directly [5]. In a dog with *Metorchis conjunctus*, the infection produced multiple liver abscesses and severe cholangitis, which means the fluke can cause bacterial complications on top of the parasitic disease [1]. In experimental *[Fasciola gigantica](/knowledge/parasites/livestock-parasites/fasciola-gigantica)* infection in rabbits, researchers recorded increased markers of oxidative stress and a rise in the liver enzymes AST and ALT [7]. Those findings support the general principle that fluke infection damages liver cells and triggers an inflammatory response. ## How Liver Fluke Infection Is Diagnosed Diagnosis of a liver worm requires the right test. A standard fecal flotation often misses fluke eggs. ### Fecal Sedimentation Fluke eggs are heavy and sink in water, so the preferred method is fecal sedimentation, not flotation. A 2024 study of cats used both zinc sulfate flotation and formalin-ether sedimentation to detect *Platynosomum*, then confirmed the species with PCR and [DNA sequencing](/blog/guides/dna-sequencing) of the ITS2 region and the cox1 gene [5]. The combination of sedimentation and molecular confirmation is the most reliable approach when the species matters. A negative fecal test does not rule out fluke infection. Flukes do not shed eggs continuously, and light infections can be missed. The cat with hepatic cysts had a negative fecal examination by both simple flotation and formalin-ether sedimentation, and the final diagnosis came from surgery and histopathology [6]. That case shows why a negative stool test is not the end of the diagnostic path. ### Imaging Ultrasound is the main imaging tool. It can show thickened bile duct walls, dilated ducts, and in some cases cysts or abscesses in the liver. The dog with *Metorchis conjunctus* had multiple liver abscesses that were treated with percutaneous transhepatic drainage and alcoholization, which means imaging played a central role in both diagnosis and treatment planning [1]. The cat with hepatic cysts had large multiple cysts with irregular walls and internal septations visible on ultrasound [6]. ### Serology and Molecular Testing Serology detects antibodies against fluke proteins. A human case report described a liver abscess caused by co-infection with *[Toxocara canis](/knowledge/parasites/pet-parasites/toxocara-canis-roundworm-dogs-puppies-visceral-larva-migrans-human)* and *Fasciola hepatica* that was confirmed by serological tests [8]. In veterinary practice, serology is used less often than fecal sedimentation and imaging, but it can help when eggs are not found. Molecular methods such as PCR are increasingly used to identify the exact species, as shown in the *Platynosomum* study [5]. ### A Practical Diagnostic Sequence 1. Collect a fecal sample and request sedimentation, not flotation. 2. If the animal has elevated liver enzymes or eosinophilia, add abdominal ultrasound. 3. If eggs are not found but imaging is suspicious, discuss serology or molecular testing with your veterinarian. 4. If the animal has a bloated abdomen or jaundice, treat it as urgent. ## Treatment of Liver Flukes in Dogs and Cats Treatment has two parts: killing the fluke and managing the liver damage it caused. ### Praziquantel Praziquantel is the most commonly reported drug for biliary flukes. The dog with *Metorchis conjunctus* was treated with a prolonged course of praziquantel alongside antibiotics and drainage procedures [1]. Praziquantel is effective against many trematodes and is widely available in veterinary practice. The dose and duration depend on the species and the label directions, and a veterinarian should set them. ### Triclabendazole Triclabendazole is the drug of choice for *Fasciola* species in many settings. A human case of *Fasciola hepatica* and *Toxocara canis* co-infection improved after sequential treatment with albendazole and triclabendazole [8]. In [veterinary medicine](/blog/careers/veterinary-medicine-careers-from-clinical-practice-to-public-health), triclabendazole is used for fascioliasis in livestock and has been used in dogs and cats when the diagnosis is confirmed. It is not approved for all species in all countries, so a veterinarian must confirm the legal and label status before prescribing it. ### Supportive Care Flukes that block bile ducts can cause cholangitis and abscesses. The dog with *Metorchis conjunctus* needed percutaneous transhepatic drainage and alcoholization of the abscesses in addition to praziquantel and antibiotics [1]. Supportive care may include fluids, antibiotics for secondary bacterial infection, and liver-supportive measures. A cat with cyst-like lymphocytic cholangiohepatitis improved after twelve weeks of oral prednisolone, which shows that not every liver cyst is parasitic and that immune-mediated disease needs a different treatment [6]. ### Follow-Up After treatment, a veterinarian will typically repeat the fecal sedimentation test to confirm that eggs are gone. The timing depends on the drug and the species, and it is usually several weeks after the last dose. Repeat ultrasound may be needed if the animal had bile duct thickening or abscesses. The cat study that used PCR for *Platynosomum* showed that molecular testing can confirm clearance when microscopy is uncertain [5]. ## Zoonotic Risk and Prevention Liver flukes that infect dogs and cats can also infect people. *Amphimerus* infection in cats and dogs in Ecuador was described as a zoonotic disease, with these animals acting as definitive and reservoir hosts [3]. *Clonorchis sinensis* is a food-borne parasite that causes serious disease in humans, and the Wuhan fish survey identified urban lakes as potential sources of infection [4]. *Opisthorchis viverrini* is a carcinogenic liver fluke in Southeast Asia, and a large study in Thailand found that eating raw fish was a significant risk factor for human infection [9]. ### How People Get Infected People become infected the same way dogs and cats do, by eating raw or undercooked freshwater fish, shrimp, or aquatic plants that carry metacercariae. A study in Thailand found that households with a cat were seven times more at risk than households without a cat, and eating raw fish dishes prepared by family members increased risk by 4.74 times [10]. The cat itself is not the direct source. The shared food habit is the source. ### Practical Prevention for Pet Owners - Cook all freshwater fish and shrimp thoroughly before feeding them to a dog or cat. - Do not feed raw fish scraps, even as a treat. - Do not let pets scavenge fish waste at fishing sites. - Wash aquatic plants thoroughly and cook them before eating. - Do not use untreated freshwater from ponds or canals as drinking water for pets. - Deworm pets on a schedule recommended by a veterinarian. The *Platynosomum* study found that cats without a deworming program had 16 times higher odds of infection than regularly dewormed cats [5]. ### Environmental Control Fluke transmission depends on snails and freshwater. Reducing snail habitat and preventing pets from accessing slow-moving freshwater reduces risk. The mathematical modeling study of *Opisthorchis viverrini* found that a combination of drug distribution at medium coverage and high coverage of education and sanitation was the best strategy for reducing transmission [11]. The same logic applies to household pets: treat the infected animal, remove the source of infection, and prevent re-exposure. ## What Is Still Uncertain Several questions about liver flukes in dogs and cats remain open. The true prevalence in most countries is unknown because fecal sedimentation is not performed routinely and many infections are subclinical. The *Amphimerus* study found high prevalence in one region of Ecuador, but that does not tell us the rate in other regions [3]. The *Platynosomum* study found an 11.63 percent infection rate in a small group of cats with elevated ALT, which is a selected population, not a general estimate [5]. The optimal drug and dose for each fluke species in each host species is not fully established. Praziquantel is widely used, but the exact protocol varies. Triclabendazole is effective for *Fasciola*, but its use in dogs and cats is not standardized in every country. The role of dogs and cats as reservoir hosts for human infection is debated. The mathematical model of *Opisthorchis viverrini* suggested that humans are necessary to maintain transmission and can sustain it without additional reservoir hosts, which means treating humans could be enough to interrupt transmission in some settings [12]. That does not mean treating pets is unnecessary, but it changes how we think about control. ## Limitations and When to Contact a Veterinarian This article is educational and is not a substitute for veterinary diagnosis or treatment. Individual cases vary, and only a veterinarian who examines your pet can make a diagnosis and prescribe treatment. Contact a veterinarian promptly if your dog or cat has any of the following: - Yellowing of the gums, eyes, or skin - A bloated or distended abdomen - Weight loss that continues despite normal eating - Persistent vomiting or diarrhea - Lethargy or collapse - Known exposure to raw freshwater fish or aquatic plants If your pet has a confirmed fluke infection, follow the treatment plan exactly and return for the scheduled recheck. Do not stop treatment early, even if the signs improve. Do not use over-the-counter dewormers for a suspected fluke infection, because most of them do not treat trematodes. ## Frequently Asked Questions ### What is a liver worm in dogs and cats? A liver worm in dogs and cats is usually a trematode, also called a liver fluke, that lives in the bile ducts or liver tissue. Common species include *Fasciola hepatica*, *Opisthorchis felineus*, and *Clonorchis sinensis*. ### Can dogs and cats get liver flukes from eating raw fish? Yes. Dogs and cats get liver flukes by eating raw or undercooked freshwater fish, shrimp, or aquatic plants that carry infective metacercariae. Cooking fish thoroughly prevents infection. ### What are the signs of liver fluke infection in pets? Signs include weight loss, jaundice, ascites, vomiting, diarrhea, and lethargy. Eosinophilia on a blood test is a common laboratory finding even when liver enzymes are normal. ### How is liver fluke infection diagnosed? Diagnosis uses fecal sedimentation to find fluke eggs, not flotation. Ultrasound, serology, and molecular testing can support the diagnosis when eggs are not found. ### What is the treatment for liver flukes in dogs and cats? Praziquantel is the most commonly reported drug for biliary flukes. Triclabendazole is used for *Fasciola* species. A veterinarian should set the dose and duration based on the species and label directions. ### Can people get liver flukes from their pets? People get liver flukes from eating raw or undercooked freshwater fish or aquatic plants, not directly from touching a pet. However, sharing raw fish dishes with pets increases the risk for everyone in the household. ### Do I need to treat my pet if the fecal test is negative? A negative fecal test does not rule out fluke infection. Flukes do not shed eggs continuously, and light infections can be missed. If your pet has signs or a history of exposure, discuss further testing with your veterinarian. ### How can I prevent liver fluke infection in my pet? Cook all freshwater fish and shrimp before feeding them to pets. Prevent access to raw fish scraps and untreated freshwater. Keep your pet on a veterinarian-recommended deworming schedule. ## Related Articles - [Signs Of Worms In A Dog](/knowledge/veterinary-medicine/parasitic-diseases/signs-of-worms-in-a-dog) - [Signs My Dog Has Worms](/knowledge/veterinary-medicine/parasitic-diseases/signs-my-dog-has-worms) - [What Are The Signs And Symptoms Of A Dog With Worms](/knowledge/veterinary-medicine/parasitic-diseases/what-are-the-signs-and-symptoms-of-a-dog-with-worms) - [Worm Infections in Sheep: Clinical Signs, Diagnosis, and Control](/knowledge/parasites/livestock-parasites/worm-infections-in-sheep) - [Cat Diarrhea with Worms: Causes and Treatment Options](/knowledge/veterinary-medicine/digestive-and-urinary-care/cat-diarrhea-worms-causes-treatment) - [Aquarium Fish Parasitic Worms: Identification and Treatment](/knowledge/veterinary-medicine/aquarium-fish-care/aquarium-fish-parasitic-worms-identification-treatment) - [Cat Anal Gland Issues: Signs and Treatment](/knowledge/veterinary-medicine/digestive-and-urinary-care/cat-anal-gland-issues-signs-and-treatment) - [Dog Eyelid Infection: Causes, Signs, and Treatment](/knowledge/veterinary-medicine/symptom-guides/dog-eyelid-infection-causes-signs-and-treatment) - [Pannus in Dogs: Causes, Signs, and Treatment](/knowledge/veterinary-medicine/skin-allergy-and-ear-care/pannus-in-dogs-causes-signs-and-treatment) ## Sources 1. 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