Clinostomum complanatum

Clinostomum complanatum is a species of digenean trematodes of the family Clinostomidae, first described by Rudolphi in 1814.: It's an unsegmented, leaf-shaped parasitic flatworm. All flukes are hermaphroditic and reproduce sexually and asexually at different points in its life cycle, depending on the host it is inhabiting. Its life cycle consists of two intermediate hosts and one definitive host. Intermediate hosts include mollusks and vertebrates, and its definitive hosts are piscivorous birds, but occasionally are mammals or reptiles.
This fish-borne ectoparasite is thought to be responsible for Halzoun syndrome, a rare disease transmitted after the consumption of raw or undercooked freshwater fish that are infected with metacercaria of C. complanatum. These flukes rarely impact humans, but the spread of cuisine utilizing raw or undercooked freshwater fish, as well as migratory birds, have increased the threat of zoonotic transmission.

Morphology

Mature C. complanatum are a translucent reddish yellow with body lengths ranging from about 2-6 millimeters and body widths of 0.9-2 millimeters. The body is convex dorsally and flattens toward the posterior and is widest near the gonads, narrowing at the ventral sucker. A well-developed oral collar surrounds a small oral sucker, with a muscular esophagus and prominent esophageal bulb present; a pharynx is absent. The ventral sucker is medium-sized. Two intestinal caeca extend to the posterior end and connect to a V-shaped excretory vesicle, both filled with dark brown material. Longitudinal excretory ducts run forward toward the ventral sucker. The testes are deeply lobed and triangular. A small cirrus-sac containing reproductive structures lies between the anterior testis and intestines, overlapping a small, deeply lobed ovary. The genital pore is located to the right of the body's midline. The is extensive, stretching from behind the ventral sucker to the ends of the caeca. The uterus opens near the ventral sucker, with mature eggs in the uterine sac and immature eggs between the testes.
Metacercariae vary in color from pale yellow to yellow. Length varies from 5 to 10 millimeters and are oval in shape. They are able to be seen with the naked eye.

Taxonomy

The genus Clinostomum Leidy, 1856, is made up of over 50 species. Its taxonomic history has been unstable, with reports of C. complanatum described in absence of the knowledge of other species. Species of this genus are very similar and some only differ in minor characters. Few morphological traits have been identified to reliably differentiate between species for morphological speciation, including the anterior extent of vitellarium, the presence of lateral evaginations in the uterine sac, gonad location in the body, testicular shape, genital pore location relative to gonads, and cirrus pouch location relative to other organs. Identification of species is based on characters found in adult stages, further increasing confusion and potential error in geographical distribution reportings.

''C. complanatum'' vs ''C. marginatum''

The taxonomic validity of these two common species has been disputed for over 200 years. Both names continue to be used for specimens collected across North and South America. A paper by Caffara et al. confirms the validity of Clinostomum marginatum as a junior species, with ~1% differences in its rDNA sequences. Additionally, they found reliable morphological differences in the genital complex at developmental stages of both species. This paper also proposed that C. marginatum represents the "American " species, and C. complanatum is the "European " species.

Life cycle

C. complanatum has been described as an ectoparasite of piscovorous birds and fresh and brackish water fish. Its life cycle has been studied by several authors determining that it includes a mollusk as its first intermediate host, a fish as its second intermediate host, and a fish-eating bird as its definitive, or final, host.
The cycle begins with a mollusk feeding on the feces of an infected bird and ingesting the parasite eggs. Within the freshwater gastropod, miracidia hatch from eggs and undergo several rounds of asexual reproduction, developing into sporocysts, rediae, and finally brevifurcate cercariae. Free cercariae leave the mollusk and infect a fish by penetrating the skin and encysting muscle tissue, where they develop into metacercariae. A bird feeds on the fish, and the parasite is then freed into its digestive tract, restarting the cycle. However, if a man eats the raw or undercooked meat of an infected fish, they might become an accidental host.
For more detailed information see trematode life cycle stages.

Hosts

Disclaimer: This is not an all-inclusive list of hosts; these are just a few known hosts.
First Intermediate Hosts: Mollusks

Second Intermediate Hosts: Fish and Amphibians

Definitive Hosts: Birds

Australia:
* Egretta alba
* Egretta garzetta
* Egretta intermedia plumifera
* Nycticorax caledonicus
* Ardea novaehollandiae
Brazil: On the floodplain of the high Paraná River in Brazil, it is assumed that A. cocoi and P. brasilianus are the main definitive hosts due to their feeding behavior. These species feed mainly on detritivorous fish.
* Ardea cocoi
* Phalacrocorax brasilianus
* Egretta alba
* Egretta thula
Iran
* Ardea purpurea
* Nycticorax nycticorax
* Egretta alba
* ''Egretta garzetta''

Zoonotic significance

Accidental human infections can and have occurred with the ingestion of infected raw or undercooked fish. These cases, although rare, are more likely to be reported in countries where eating raw fish is common, such as coastal regions and Asian countries.

Pathological effect

Fish

Although it is assumed that these yellow grubs have caused massive losses to natural fish resources and the worldwide aquaculture industry, there is limited research on the etiology and pathology of this disease. There are few reports of clinostome infections causing host tissue damage, but others report these fish become malnourished or lethargic, thus making them easier for birds to catch.

Birds

Adult parasites reside in buccal cavities of piscivorous birds. They attach to the esophageal mucosa, oral cavity, and lungs of these birds. Intense necrosis was observed in attachment sites, as well as inflammation going as deep as the musculature. The parasites observed in the muscular layer of Nycticorax nycticorx were surrounded by a thin zone of necrotic cells and a fibrous tissue capsule. The authors of the study suggested the damaged tissue would impact the bird's ability to eat and lead to malnutrition, weakening the bird's immune system, and making the bird more vulnerable to pathogens.

Humans

While C. complanatum commonly infects the pharynx, larynx, or upper esophagus of piscivorous birds, cases of human infection are rare. Infection in humans occur when raw or undercooked fish infected with metacercariae is ingested. The metacercariae excyst in the stomach and migrate to throat/esophagus. The parasite attaches to the esophageal mucosa by utilizing its ventral sucker, and matures; the migration through the digestive tract may take a few days. This causes acute pharyngitis or laryngitis. This infection has been called "Halzoun syndrome" or clinostomiasis. Currently infections have been reported mainly in Asian countries, where eating raw fish is traditional. One case study of an infection explained the patient suddenly developed a "violent cough and irritable sensation in the throat," alongside intermittent throat soreness. There is currently no effective antiparasitic drug. Mechanical removal of the fluke is necessary. Even after removal, it is possible that more metacercariae remain in the digestive tract; it is important to examine the throat and perform an upper gastrointestinal endoscopy to ensure there are no residual parasites.

Management efforts

As the demand for fish production rises across the world due to human consumption, strict and effective regulation should be implemented by professionals involved in the fish production chain. Fish impacted by these yellow grubs can have a large economic impact on fish farming, as the grubs appear unappealing and the fish are discarded. Discarded infected fish coupled with migratory birds can lead to the spread of C. complanatum and the expansion of its natural habitat.