Nanophyetus salmincola, the “salmon poisoning” fluke, is a small (~0.5 × 0.3 mm) oval fluke found in the small intestine of dogs, cats, and many wild fish–eating mammals in the northwestern US, southwestern Canada, and Siberia. The eggs, which pass in the feces of infected hosts, are light brown, 72–97 × 35–55 mcm, and indistinctly operculated with a small knob at one pole. The life cycle includes an extended period (3 months) of embryonation. The first intermediate hosts are snails found in endemic locations (eg, Oxytrema silicula in the US). The cercariae from these snails penetrate the skin of young salmonid fish and encyst as metacercariae in their muscles and organs. Dogs and other animals become infected by eating raw or improperly prepared infected fish.
Because these flukes embed deeply between the villi of the intestine, infection with a large number may cause enteritis. Most infections, however, are complicated via development of the salmon poisoning complex due to rickettsial organisms, which the fluke transmits ( see Salmon Poisoning Disease and Elokomin Fluke Fever in Animals Salmon Poisoning Disease and Elokomin Fluke Fever in Animals Salmon poisoning disease is an acute systemic disease of canids caused by Neorickettsia helminthoeca, gram-negative pleomorphic cocci. The disease is characterized by fever, anorexia... read more ). Praziquantel (20–30 mg/kg, PO or SC, once) and fenbendazole (50 mg/kg/day, PO, for 10–14 days) are both effective, but not approved, treatments for dogs.
Alaria alata, Alaria canis, and other Alaria spp are small (2–6 mm) flukes usually found in the small intestine of dogs, cats, foxes, mink, and wild carnivores in the western hemisphere, as well as in Europe, Australia, and Japan. The ventral part of the body is flat, and the dorsal part is conical. The eggs are oval, light brown, operculate, and fairly large (98–134 × 62–68 mcm). The life cycle includes freshwater snails (eg, Helisoma spp) as first intermediate hosts. Cercariae emerge from the snails, penetrate tadpoles, and develop into mesocercariae. Frogs, snakes, and mice then acquire infection by eating tadpoles; the mesocercariae transfer to their tissues and remain as this life-cycle stage.
Dogs and other definitive hosts become infected by feeding on these animals. The young flukes migrate via various organs of the definitive host, including the diaphragm and lungs, before reaching the small intestine. Although the flukes are generally considered to be nonpathogenic, large numbers may cause pulmonary hemorrhages during migration or enteritis when they mature in the small intestine. Infections are typically more common in feral cats and dogs than client-owned pets. These flukes may infect humans, but not directly from cats and dogs.
Infections can be treated with praziquantel using the approved cestocidal dosage ( see Table: Drugs for Intestinal Helminths of Dogs Approved in the US and UK Drugs for Intestinal Helminths of Dogs Approved in the US and UK and see Table: Drugs for Intestinal Helminths of Cats Approved in the US and UK Drugs for Intestinal Helminths of Cats Approved in the US and UK for drug dosages.). However, such treatment is extra-label.
Other species of flukes, usually not pathogenic, have been found occasionally in the intestine of dogs, cats, and other carnivores; these include Heterophyes heterophyes in some north African and Asian countries; Metagonimus yokogawai in Asia; Cryptocotyle lingua in the US, Canada, Japan, Siberia, and Europe; Apophallus venustus in North America; and Apophallus donicus in Europe and Russia. Their life cycles include snails as first intermediate hosts and fish as second intermediate hosts, in which metacercariae become encysted.
Heterobilharzia americana (the cause of canine schistosomiasis) is found in the mesenteric veins of dogs and wild animals, especially raccoons, in the southern Atlantic and Gulf Coast States, Kansas, and Oklahoma, US. The eggs (74–113 × 60–80 mcm) pass through the tissues of the intestine to the lumen and then are voided in the feces. From the snail intermediate host, cercariae escape into water and penetrate the skin of dogs and other definitive hosts, migrate to the liver, mature, and move to the mesenteric vessels. Granulomas form around the eggs in the wall of the large intestine, the liver, pancreas and other parts of the body. Lethargy, weight loss, vomiting, diarrhea, and/or hematochezia may develop in heavy infections.
“Water dermatitis” sometimes occurs when cercariae penetrate the skin. Persistent hypercalcemia and hyperglobulinemia are common findings. With abdominal ultrasound, a combination of heterogeneous layering of the small intestine wall and pinpoint hyperechoic foci in the small intestine, liver, or mesenteric lymph nodes is highly suggestive of H americana and should prompt further testing. The eggs do not readily float and, if placed in water, hatch within minutes; therefore, a sedimentation method using 0.85% saline is useful in separating eggs from ingesta. In infected dogs, eggs are passed intermittently, so on a given day eggs may not be found in feces. PCR assay is available for testing feces, liver, and intestinal biopsies.
Fenbendazole at 50 mg/kg/day, PO, for 10 days, is an effective treatment. Praziquantel at 25 mg/kg, every 8 hours, for 3–10 days, is also effective. Both are extra-label uses.
Flukes in the bile ducts and gallbladder cause mild to severe fibrosis. Many species of distome trematodes have been reported from the liver of dogs and cats in most parts of the world. Mild infections may pass unnoticed; however, in severe infection, dogs may develop progressive weakness, ending in complete exhaustion, coma, and death. The following are some of the most commonly encountered trematodes.
Opisthorchis felineus is parasitic in the bile duct, pancreatic duct, and small intestine of dogs and cats in Italy, eastern Europe, Russia, Kazakhstan, and parts of Asia. Opisthorchis viverrini is present in dogs as well as in domestic and wild cats in southeast Asia. Opisthorchis noverca is reported frequently in the bile duct of dogs in the Indian subcontinent.
All three species are small (9 × 2 mm) and elongate. Their life cycle includes certain snails (Bithynia sp) and cyprinid fishes as intermediate hosts. All three species infect humans, although not directly from dogs or cats. A related species, Clonorchis sinensis, the oriental liver fluke of people, also has been found in the bile ducts and pancreatic ducts of dogs, cats, and other animals across parts of Asia. It is larger than Opisthorchis spp. The operculated eggs of these parasites may be identified in the feces of infected animals.
Long-term presence of these flukes in the bile duct causes epithelial hyperplasia and fibrosis of the duct wall. Carcinomas in the liver or pancreas have occurred in chronic and severe cases. Treatment of Opisthorchis spp infections in dogs may be attempted with fenbendazole (200 mg/kg/day, PO, for 3 days) 1 References Nanophyetus salmincola, the “salmon poisoning” fluke, is a small (~0.5 × 0.3 mm) oval fluke found in the small intestine of dogs, cats, and many wild fish–eating mammals in the northwestern... read more or praziquantel (20 mg/kg, PO, once). For O viverrini infections in cats, praziquantel (25 mg/kg, PO, once) is the treatment of choice. Treatment of C sinensis infections in dogs may be attempted with praziquantel (30 mg/kg/day, PO, for 3 days). All these treatments are extra-label.
Platynosomum concinnum is a small fluke (6 × 2 mm) found in the bile duct and gallbladder of Felidae in the southeastern US, Puerto Rico and other Caribbean Islands, South America, parts of southeast Asia, and parts of Africa. Its life cycle includes the snail Sublima octona and a crustacean (wood louse) as intermediate hosts and certain lizards as paratenic hosts.
Cats acquire the parasite by feeding on infected lizards. In mild cases, nonspecific chronic clinical signs of unthriftiness may occur. Severe infections, however, may cause the “lizard poisoning” syndrome, which is characterized by anorexia, lethargy, depression, persistent vomiting, diarrhea, jaundice, and an enlarged abdomen, leading to death. Eggs (34–50 × 20–35 mcm) are most likely to be detected in feces using centrifugal fecal flotation with Sheather's sugar solution, or fecal sedimentation. Treatment with praziquantel (20 mg/kg/day, PO, for 3−5 days, ideally repeated 12 weeks later) has been reported; however, the drug is not approved for this use. Praziquantel appears to be the most effective agent. Bile duct surgery may also be required.
Metorchis orientalis and Metorchis conjunctus are two small flukes (5 × 1.5 mm) that have been found in the bile ducts and gallbladder of dogs, cats, and other carnivores in North America, Europe, the former USSR, and China. They seldom cause any recognizable clinical signs. Their eggs are small (24–30 × 13–16 mcm), and the life cycle includes certain freshwater snails and fish as intermediate hosts. Treatment of Metorchis spp infections in dogs may be attempted with praziquantel (30 mg/kg, PO, once), although this is an extra-label use. Additional treatments may be required. 2 References Nanophyetus salmincola, the “salmon poisoning” fluke, is a small (~0.5 × 0.3 mm) oval fluke found in the small intestine of dogs, cats, and many wild fish–eating mammals in the northwestern... read more
Eurytrema procyonis is a small fluke (2.1 × 1 mm) commonly occurring in the pancreatic duct of raccoons and foxes in the eastern US and occasionally found in the pancreatic duct, bile duct, and gallbladder of domestic cats. Infection may be associated with weight loss and intermittent vomiting. Exocrine pancreatic insufficiency and pancreatitis may be observed. The eggs are medium sized (45–53 × 29–36 mcm), and the life cycle involves a land snail and a second intermediate host thought to be an arthropod. Treatment may be attempted with fenbendazole (30 mg/kg/day, PO, for 6 days) 3 References Nanophyetus salmincola, the “salmon poisoning” fluke, is a small (~0.5 × 0.3 mm) oval fluke found in the small intestine of dogs, cats, and many wild fish–eating mammals in the northwestern... read more or praziquantel/pyrantel/febantel (praziquantel and pyrantel each at 5.8 mg/kg/day and febantel at 28.8 mg/kg/day, PO, for 5 days), although these drugs are not approved for this use.
Promod-Kumar, Srivastava AK, Pandey BB, et al. Therapeutic trial of some drugs against opisthorchiasis in experimental pups. Ind J Vet Med 1992; 12:89.
Lemetayer JD, Snead EC, Starrak GS, Wagner BA. Multiple liver abscesses in a dog secondary to the liver fluke Metorchis conjunctus treated by percutaneous transhepatic drainage and alcoholization. Can Vet J. 2016 Jun;57(6):605–609. PMID: 27247459; PMCID: PMC4866664.
Steiner J, Williams D. Feline exocrine pancreatic disease. In: Textbook of Veterinary Internal Medicine: Diseases of the Dog and Cat. 6th ed. WB Saunders; 2000:701–705.
For More Information
Deplazes P, Eckert J, Mathis A, von Samson-Himmelstjerna G, Zahner H. Parasitology in Veterinary Medicine. Wageningen Academic Publishers. 2016:197–205.
Bowman DD. Class Trematoda. In: Georgis’ Parasitology for Veterinarians. Elsevier;2021:135–151.
Bowman DD, Hendrix CM, Lindsay DS, Barr SC. The Trematodes. In: Feline Clinical Parasitology. Iowa State University; 2002:83–182.