Current Advice on Parasite Control:
Intestinal Parasites, Parasites of Other Systems - Trematodes
*Trematodes are commonly referred to as “flukes.”
Overview of Life Cycle
- Trematodes of dogs and cats have indirect life cycles that require one or two intermediate hosts to reach the infective stage.
- A representative trematode life cycle is that of P. kellicotti. Dogs and cats infected with adult P. kellicotti shed eggs in their feces. The eggs hatch in water, and a ciliated form, the miracidium, emerges. The miracidium penetrates its first intermediate host, a snail, where it develops to the cercarial stage. The cercaria then leaves the snail and infects the second intermediate host, a crayfish, where it develops to the infective metacercarial form. Dogs and cats become infected with P. kellicotti when they ingest these metacercariae.
- The trematode egg shows the operculum (arrow) characteristic of trematodes and pseudophyllidean cestodes.
- The ciliated trematode miracidium is the form that hatches from the egg and infects a snail as its first intermediate host.
- The trematode cercaria is the stage that infects the second intermediate host.
- Encysted metacerciae are shown in the tissues of an intermediate host.
- The adult trematode shows the suckers (arrows) characteristic of this group of helminthes.
- Disease in dogs and cats due to infection with trematodes varies and depends upon the species of trematode involved and the organs and organ systems affected.
- Paragonimus kellicotti adults develop in cysts in the lung of both dogs and cats; animals infected with P. kellicotti may be asymptomatic or can present with a variety of respiratory signs, including coughing, dyspnea, pneumothorax, bronchiectasis, and hemoptysis.
- Alaria spp. adults develop in the small intestine and are not usually associated with intestinal illness. However, migration of immature Alaria through the lungs may result in pulmonary hemorrhage and subsequent respiratory compromise.
- Nanophyetus salmincola adults are found in the small intestine of both dogs and cats. Infection with N. salmincola alone is not associated with clinical disease, but this fluke serves as the vector of Neorickettsia helminthoeca, the causative agent of salmon poisoning, a virulent canine disease in the Pacific Northwest.
- Heterobilharzia americana adults present in the mesenteric and hepatic veins, where they produce eggs that migrate directly across the intestinal wall to the lumen, a process that induces granulomatous inflammation. Clinical signs in infected dogs include diarrhea (which may be blood-tinged), vomiting, weight loss, lethargy, and occasionally, hemoperitoneum.
- Platynosomum fastosum is a liver fluke of cats in Florida, other areas of the southeastern United States, and Hawaii. Chronic infection with Pla. fastosum leads to development of enlarged bile ducts and gall bladder, biliary epithelial hyperplasia, and ultimately, liver failure.
- Trematode infection in dogs and cats is rare compared with infection rates for nematodes and cestodes. However, because of the severe disease associated with some trematodes, awareness and control of these parasites is essential in certain geographic areas, e.g., N. salmincola in the Pacific Northwest and Pla. fastosum in the coastal southeastern United States and Hawaii.
Host Associations and Transmission Between Hosts
- Trematode infections are acquired by consumption of intermediate or paratenic hosts (or, in the case of H. americana, by direct skin penetration of cercariae) and are not directly transmitted between dogs and cats.
- Both dogs and cats are susceptible to infection with P. kellicotti following ingestion of crayfish or paratenic hosts infected with metacercariae.
- Alaria spp. infections are acquired by dogs and cats via ingestion of a wide variety of intermediate or paratenic hosts (particularly frogs and snakes) that harbor immature flukes,.
- Nanophyetus salmincola metacercariae are present in the muscle of salmonid fish; dogs and cats are infected upon ingestion of uncooked fish.
- Dogs are infected with H. americana when cercariae in fresh water directly penetrate their skin.
- Platynosomum fastosum metacercariae are usually transmitted to cats via predation on lizards or other reptiles. The disease caused by this trematode in cats is commonly referred to as “lizard poisoning.”
Prepatent Period and Environmental Factors
- Prepatent periods vary among different trematode species and range in length from 3 to 5 weeks or longer. The life cycles of most trematodes of dogs and cats are dependent on water to support infection of the snail first intermediate host. Accordingly, natural infections are usually associated with an aquatic environment. An exception, Platynosomum, has a terrestrial life cycle.
Site of Infection and Pathogenesis
- Paragonimus kellicotti is found in cysts in the lung. Dogs and cats may tolerate a low number of intact cysts, but in heavy infections or when cysts rupture, severe disease may result due to pulmonary hemorrhage, pneumothorax, and/or granulomatous pneumonia.
- Adults of Alaria are present in the small intestine of infected dogs and cats. Immature Alaria migrate through the lungs prior to being swallowed and developing into adult flukes in the small intestine. When infection levels are high, pulmonary damage and hemorrhage may lead to clinical disease.
- Adults of N. salmincola are found in the small intestine of dogs and cats. This trematode does not cause disease to the infected animal directly but can serve as the vector of Neo. helminthoeca, a rickettsia that causes lymphadenopathy, diarrhea, and high fever in dogs. Disease caused by Neo. helminthoeca is referred to as “salmon poisoining” because dogs acquire infection by ingestion of raw salmonid fish. Mortality rates are high in the absence of fluid maintenance and antibiotics treatment.
- Heterobilharzia americana adults are present in the mesenteric and hepatic veins of infected dogs. The eggs produced by each adult pair migrate directly through the wall of the intestine to exit the infected dog with the feces. The eggs, which induce a pronounced inflammatory response with fibrosis, may also be carried in the circulation throughout the body, resulting in the development of disseminated visceral granulomata.
- Platynosomum fastosum infects the bile ducts and pancreatic ducts of cats, inducing epithelial hyperplasia. With chronic infections, the biliary hyperplasia can result in fibrosis, cholestasis, and hepatic failure. In cats presenting with late-stage clinical disease, jaundice, weight loss, and vomiting may be seen. Treatment of these cats is often unrewarding; trematode infections are difficult to clear, and liver pathology and associated disease persists or worsens following death of the flukes.
- Trematode eggs are less buoyant than those of nematodes or protozoa; unless high-density sucrose flotation is used, diagnosis of infection by fecal examination requires concentrating the ova present in feces by sedimentation rather than flotation.
Once identified in fecal sediment by the characteristic opercula, eggs of the different trematode species can be further differentiated by morphologic characteristics, including size.
- Pulmonary cysts may be evident on thoracic radiographs of dogs and cats infected with P. kellicotti. Characteristic eggs may be recovered on transtracheal wash.
- Abdominal ultrasound can be useful in locating the dilated bile ducts associated with Platynosomum fastosum infection and, in some cases, allows visualization of the flukes themselves. Liver biopsy also may aid in the diagnosis of Platynosomum, particularly in cases where infection has resulted in severe biliary hyperplasia and subsequent cholestasis.
- There are no products labeled for treatment of trematodes in dogs and cats; however, praziquantel, epsiprantel, and fenbendazole have been reported to be effective.
- Although albendazole has been reported to be effective in treating trematodes in dogs and cats, its use in small animals is not recommended because of potential associated side effects such as bone marrow toxicity.
- Tetracyclines are considered by many to be the treatment of choice, although other classes of antibiotics are favored by some. Short-acting corticosteroids may be used supportively. There are anecdotal reports of pet owners in high-risk areas who have dogs with exposure opportunities choosing to expose—in essence, immunize—their dogs by feeding raw fish and then treating with antibiotics at first notice of anorexia or fever. This practice appears to stimulate lifetime protection.
Control and Prevention
- Prevention of predation and scavenging activity by confining dogs on a leash or in a fenced yard and keeping cats indoors will limit the opportunity for dogs and cats to acquire infection with trematodes.
- To avoid Heterobilharzia infection, dogs should not have contact with water through such activities as swimming in canals or ponds.
Public Health Considerations
- Dogs and cats infected with trematodes do not pose a direct zoonotic infection risk to people. However, people can acquire infection with N. salmincola following ingestion of metacercariae in undercooked fish. A case of fatal disease associated with systemic infection with Alaria larvae and several cases of ocular infection have been reported following ingestion of undercooked frogs legs. Along with various avian schistosomes, Heterobilharzia cercariae from snails have been incriminated in causing dermatitis in humans (“swimmer’s itch”) following skin penetration.