Trematode infection: background, pathophysiology, epidemiology (2023)

The trematode life cycle is completed in two distinct host classes: definitive (ie, humans, domestic animals, wildlife) and intermediate (ie, freshwater snails). See the pictures below.

Slugs that act as intermediate hosts for trematodes of medical importance are listed in Table 1. A list of these hosts for different trematodes and sources of infection are summarized in Table 2.

Table 1. Vectors and geographic areas associated with specific trematode species (Open the table in a new window)

Table 2. List of definitive and intermediate hosts and sources of infection of large trematodes (Open the table in a new window)

Blood flukes (Schistosoma species)

Schistosomiasis, or bilharzia, is a tropical parasitic disease caused by fluke worms that live in the blood of relativesschistosoma,from the Greek forskhistos(divide) isoma(body). Originally thought to be a single organism with a divided body, it was eventually recognized that the parasite has a male and female form. The main schistosomes that infect humans includeShematoby(transmitsBulinussnails and causing urinary schistosomiasis in Africa and the Arabian Peninsula),S mansoni(transmitsBiomphalariasnails and causing intestinal and hepatic schistosomiasis in Africa, the Arabian Peninsula and South America), andS japonicum(transmitted by an amphibian snailOnmelaniaand the cause of intestinal and hepatosplenic schistosomiasis in China, the Philippines and Indonesia).

S intercalatumiS Mekongthey are only of local importance.S japonicumis a zoonotic parasite that infects a wide range of animals, including cattle, dogs, pigs and rodents.S mansoniit also infects rodents and primates, but humans are the main hosts. About a dozen other types of schistosomes are animal parasites, some of which occasionally infect humans.

Unlike other trematodes, schistosomes have separate sexes, but males and females are found together. The male is short and stocky and holds a relatively long female worm in his gynecophoric canal, a groove-like structure. WITHhematobee,both man and woman live together in the veins that drain the bladder, pelvis and ureter, whileS japonicumiS mansonithey live in the inferior and superior mesenteric veins. Therefore, these flukes are known as blood flukes. These species are distinguished from other schistosome species based on the morphology of their eggs and their adult and cercarial forms.Shematopoieticeggs have a terminal spine, whileSmansoniiSjaponicumthe eggs have lateral and central spines.

Humans become infected by free-swimming forked cercariae in fresh water by penetrating the skin. The cercaria lose their tail and the outer layer of the glycocalyces, transform into a schistosomula (larva form) and travel through the venous circulation to the heart, lungs and portal circulation. The larvae mature and develop into adult worms in about 3 weeks and reach the vessels that drain the bladder (Shematopoietic) or mesentery (Sjaponicum, Smansoni). They live and lay eggs in these venous places during the life of the host.

The eggs penetrate through the vascular endothelium, enter the lumen of the bladder or intestine and are excreted in the urine (Shematopoietic) or chair (Sjaponicum, Smansoni). If these secreted eggs gain access to fresh water, the miracidium emerges from the egg and swims freely until it finds a suitable snail. In snail hosts, after 2 generations of asexual reproduction (sporocysts), forked cercariae emerge into the water to infect other susceptible human hosts. A single miracidium can multiply in a snail and produce almost 100,000 cercariae.

Table 3. Comparative features of Major HumanSchistosomaType (Open the table in a new window)

Lung flukes

RodLet's comparecontains more than 30 species that have been reported to cause infections in animals and humans. Among them, approximately 10 species have been reported to cause human infections, of whichPWestermansis the most important.PWestermans, also known as the oriental lung fluke, is the most widespread species in Africa, South America and parts of Asia. Among other speciesLet's comparewhich are reported to cause human diseases from all over the worldParagonimus heterotremus,which has been recorded from the northeastern parts of the Indian subcontinent.^[3]

P westermaniis a thick, fleshy, reddish-brown egg-shaped worm (7.5-12 mm long, 4-6 mm wide and 3.5-5 mm thick). It inhabits the lung parenchyma near the bronchioles in humans, foxes, wolves and various feline hosts (eg lions, leopards, tigers, cats).

Infection is typically transmitted by ingestion of metacercariae contained in raw freshwater crayfish or crustaceans. Additionally, consumption of raw meat from paratenic hosts (eg, omnivorous mammals) may also contribute to human infection. Freshwater snails and crustaceans are the first and second intermediate hostsLet's comparespecies, that is. In the duodenum, the cyst wall dissolves and the metacercariae are released. Metacercariae migrate through the intestinal wall, the peritoneal cavity, and finally through the abdominal wall and diaphragm into the lungs. There, the immature worms finally settle near the bronchi, grow and develop to become sexually mature hermaphrodite worms.

Adult worms begin to lay eggs that are not embryonated and are expelled with the sputum. However, if ingested, they are excreted in the feces. The eggs continue to develop in water. A ciliated miracidium develops in each egg over a period of 2-3 weeks. The miracidium escapes from the egg and penetrates the corresponding type of snail (the first intermediate host), in which it passes through a sporocyst generation and 2 radium generations to form cercariae. Cercariae emerge from the snail, attack freshwater crustaceans (crabs or crayfish) and cyst to form metacercariae. When ingested, they cause infection and the cycle repeats itself. Pay attention to the picture below.

Liver flukes (CsinensisiFLIVER)

C sinensis

C sinensisis a widespread parasite found in Southeast Asia that infects the biliary tract of humans. The panicle is oblong, flat, transparent and relatively small (10-25 mm long and 3-5 mm wide). It has a pointed front end and a rounded back end. Humans become infected by eating raw or partially cooked freshwater fish or dried, salted or pickled fish infected with metacercariae. In the duodenum, the cyst is digested and the immature larva is released. The larva enters the bile duct, where it develops and matures into an adult worm. The adult worm feeds on mucosal secretions and begins to lay fully embryonated eggs that are excreted in the feces.

Once they reach fresh water and are ingested by the appropriate species of percolate snail (the first intermediate host), the eggs hatch and produce a miracidium. Inside the snail, the miracidia reproduce asexually through one sporocyst generation and radium generations to forked cercariae.

See the picture below.

Cercariae escape from the snail into the water and penetrate under the shells of freshwater cyprinids (another intermediate host). In fish, the cercariae lose their tails and cyst in the scales or muscles of the fish to become metacercariae, which are infectious to humans. When ingested, infected fish cause infection in humans.

F hepatic

Fascioliasis, a zoonosis caused by infection withFLIVER(digenetic trematode), is a major livestock disease associated with high economic losses due to mortality; condemnation of the liver; reduced production of meat, milk and wool; and expenditure on anthelmintics. The disease has a cosmopolitan distribution, with cases reported from Scandinavia to New Zealand and southern Argentina to Mexico.

Fhepatic,also known as sheep liver fluke, is a large liver fluke. This fluke primarily causes zoonosis in sheep and other domestic animals. Humans become infected by eating watercress and other aquatic plants contaminated with metacercariae that enter the duodenum and excyst. They then penetrate the intestinal wall, the peritoneal cavity and the liver capsule (Glisson's capsule) to reach the bile duct of the liver, where they develop and mature into adult worms.

Adult worms begin to lay unembryonated eggs, which are excreted in the stool. They continue to develop in fresh water. The miracidium hatches from the egg and attacks the appropriate snail host. Inside the host snail, the larva reproduces asexually through one generation of sporocysts and 2 generations of radii to finally develop into cercariae. After exiting the snail, cercariae attach to aquatic plants and form metacercariae. When people and sheep eat these plants, they become infected, repeating the life cycle.

D dendritic,D hospes

Dicroceliasis is a parasitic disease caused by small liver flukesD dendriticiDhospes. The disease is a worldwide and widespread problem in grazing cattle, and sometimes (although rarely) it is also found in humans. However, due to its unusual method of transmission through the ingestion of infected ants, human dicroceliasis remains a relatively rare occurrence in humans. Human cases of dicroceliasis have been reported throughout Eastern Europe, Western Europe, Africa, Australia, India and Saudi Arabia.

Pancreatic flukes (Epancreaticum, EColomatic, Eovis)

These flukes have thick bodies and are 8-16 mm long and 6 mm wide. They parasitize the pancreatic ducts and occasionally the bile ducts of sheep, pigs and cattle in Brazil and Asia. Three types,Epancreas, EColomatic, iFrom sheepare recognized.

The first intermediate hosts are land snails (Bradybaenaspecies) and cercariae encyst in grasshoppers (metephalusspecies), which are the second intermediate host. After a grasshopper with cercariae is ingested by a suitable host animal, the immature flukes are released and migrate into the pancreatic duct, where they mature and produce eggs within approximately 11-14 weeks.

There are no obvious clinical signs of infection with these parasites.Dicrocoelius- similar eggs can be proven in feces. Mild infections cause proliferative inflammation of the pancreatic duct, which can become enlarged and blocked. Fibrotic, necrotic and degenerative lesions develop in severe infections. Losses due to pancreatic dementia have been reported, but the pathogenesis suggests an additional loss of production.

Intestinal flukes (FBuški, Hheterophyes, Myokogawai, G from man)

F buskiis the most common intestinal nematode that causes infections in humans. TrematodesHheterophys,Myokogawai, iG from manthe causes of human infection are less common.

F buski,known as the giant intestinal fluke, it is found in the duodenum and jejunum of pigs and humans and is the largest intestinal fluke that parasitizes humans. Humans become infected by eating freshwater aquatic plants such as water chestnut, water chestnut and water bamboo, which may contain metacercariae. In the intestines, the metacercaria cyst attaches to the duodenum or jejunum, develops and grows into adult worms. They lay unembryonated eggs, which are excreted in the feces.

In the water, inside the egg, a ciliated miracidium develops, comes out and pierces the corresponding snail host. Inside the snail, after several stages of asexual reproduction, a large number of cercariae are produced. The latter emerge from snails and cysts on the surface of aquatic plants to metacercariae. Ingestion of these plants causes infection in humans, and the cycle repeats itself.