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Flukes in Small Animals

By Andrew S. Peregrine, BVMS, PhD, DVM, DEVPC, DACVM, Department of Pathobiology, Ontario Veterinary College, University of Guelph, Guelph, Ontario, Canada

Intestinal Flukes

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 USA, southwestern Canada, and Siberia. The eggs, which pass in the feces of infected hosts, are light brown, 72–97 × 35–55 μm, and indistinctly operculated with a small knob at one pole. The life cycle includes an extended period (3 mo) of embryonation. The first intermediate hosts are snails found in endemic locations (eg, Oxytrema silicula in the USA). The cercariae from these snails penetrate the skin of young salmonid fishes 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 through development of the salmon poisoning complex caused by rickettsial organisms, which the fluke transmits (see Salmon Poisoning Disease and Elokomin Fluke Fever). 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, A 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 anterior part of the body is flat, and the posterior part is conical. The eggs are oval, light brown, and fairly large (98–134 × 62–68 μm). 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 through 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. These flukes may infect people. Infections can be treated with praziquantel using the approved cestocidal dosage (see Table: Drugs for Intestinal Helminths of Dogs Approved in the USA and UK and Drugs for Intestinal Helminths of Cats Approved in the USA 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 USA, Canada, Japan, Siberia, and Europe; and Apophallus donicum in North America and eastern Europe. Their life cycles include snails as first intermediate hosts and fish as second intermediate hosts, in which metacercariae become encysted.

Heterobilharzia americana is found in the mesenteric veins of dogs and wild animals, especially raccoons, in the southeastern USA. The eggs 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 intestine, the liver, and other parts of the body. Lethargy, weight loss, vomiting, and/or diarrhea may develop in heavy infections. “Water dermatitis” is sometimes seen when cercariae penetrate the skin. 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. Fenbendazole at 50 mg/kg/day, PO, for 10 days, is an effective treatment. Praziquantel at 25 mg/kg, tid, for 2 days is also effective. Both are extra-label uses.

Hepatic Flukes

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, and parts of Asia. O viverrini is seen in dogs as well as in domestic and wild cats in southeast Asia. They are small (9 × 2 mm) and elongate. Their life cycle includes certain snails (Bithynia sp) and cyprinid fishes as intermediate hosts. 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. It is larger than Opisthorchis spp. The operculated eggs of these parasites may be identified in the feces of infected animals.

Longterm presence of these flukes in the bile duct causes epithelial hyperplasia and fibrosis of the duct wall. Carcinomas in the liver or pancreas have been seen 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) or praziquantel (20 mg/kg, PO, once). 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 and pancreatic ducts of Felidae in southeastern USA, Puerto Rico and other Caribbean Islands, South America, some of the Pacific islands, 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 signs of unthriftiness may be seen. 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. Treatment with praziquantel (20 mg/kg/day, PO, for 3−5 days, ideally, repeated 12 wk later), or nitroscanate (100 mg/kg, PO, once) has been successful, although these drugs are not approved for this use. Praziquantel appears to be the most effective agent. Bile duct surgery may also be required.

Metorchis albidus and M 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, and the former USSR. They seldom cause any recognizable clinical signs. Their eggs are small (24–30 × 13–16 μm), 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 (20 mg/kg, PO, once), although this is an extra-label use.

Eurytrema procyonis is a small fluke (2.1 × 1 mm) commonly seen in the pancreatic duct of raccoons in the eastern USA and occasionally found in the pancreatic duct, bile duct, and gallbladder of domestic cats. Infection may be associated with weight loss and intermittent vomiting. The eggs are medium sized (45–53 × 29–36 μm), 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) 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.