bivalve mollusc Helminths Trematode Metacercariae and Sporocysts

I. Causative Agent and Disease mussels and razor clams while unidenti- Trematodes or flukes are members fied encysted metacercariae have been of the phylum Platyhelminthes and the observed in blue mussels, basket cockles, class . Adult worms of the littleneck clams and weathervane scal- subclass are endoparasites oc- lops. curring in all classes of vertebrates and use invertebrates as the first and second III. Clinical Signs intermediate host and rarely as the final Clinical signs of trematode sporocyst host. A metacercaria is the encysted ju- infestation may include increased flesh venile trematode usually occurring in the yield regarding initial sporocyst devel- second intermediate host that requires opment that ultimately results in poor ingestion by the final host to become growth and debilitation in later stages, an adult worm. A sporocyst is formed weakness in valve closure, reduced as part of the trematode developmen- byssal thread production in mussels, tal cycle in the first intermediate host. infertility (parasitic castration), chalky Depending on the trematode species, shell deposits, induction of pearl forma- sporocysts and metacercariae are found tion reducing marketability and bivalve in the tissues of a variety of intermediate mortality. Encysted metacercariae hosts including bivalve molluscs. Low most often are benign depending on the numbers of encysted metacercariae in trematode species and intensity. How- most molluscs generally cause no overt ever, some investigations have shown disease but when present in significant metacercariae to cause shell gaping and numbers can cause general debilitation, deformities, production of pearls and behavioral changes favoring preda- chalky shell deposits, behavioral changes tion, tissue destruction and host death. that favor predation, reduced tolerance Sporocyst stages are overtly destructive to stress, and host mortality in heavy in- causing parasitic castration, weakness festations. Metacercariae and developing and gaping of valves with gradual de- sporocysts may be observed in various struction and replacement of molluscan bivalve tissues during routine histologi- tissues resulting in eventual mortality cal examination. of the host. There are several different trematode families utilizing marine mol- IV. Transmission luscs as intermediate hosts, most notably Trematode life cycles involving the family and the genus marine molluscs generally include two . intermediate hosts and one definitive host (only two hosts for Sanguinicolidae) II. Host Species with several possible combinations of Various species of digenetic trema- pathways that include gastropods and tode sporocyst and metacercarial stages bivalves (or other invertebrates) for inter- occur in many marine bivalve mollusc mediate hosts and , birds or bivalves species worldwide including oysters, (or other invertebrates) as final hosts. clams, scallops, mussels and cockles. Trematodes of the family Bucephalidae In Alaska, sporocysts of unidentified are of great importance because their trematodes have been observed in blue larvae affect numerous commercially

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important mussels and oysters and food as small foci having varying degrees of such as flatfishes and cods. The encapsulating host fibrous tissues and pathway for Bucephalus sp. typifies the inflammatory cells. family and begins in a bivalve followed by a fish then on to a second fish as the VI. Prognosis for Host final host. Eggs in the alimentary tract of Effects of encysted metacercariae on the final predatory fish host are depos- the bivalve host vary from none to being ited with feces in seawater and hatch responsible for killing young spat. In the into ciliated miracidia that parasitize the Pacific Northwest and Alaska, metacer- first intermediate bivalve host. Once in cariae are considered relatively harmless the bivalve a miracidium develops into to bivalve hosts except when encountered a mother sporocyst that produces more in significant numbers. Sporocysts may sporocysts that eventually produce cer- cause severe tissue damage and debilita- cariae released from the host. These cer- tion resulting in parasitic castration and cariae drift with the currents and parasit- bivalve mortality. ize the second intermediate fish host and encyst as metacercariae. The encysted VII. Human Health Significance metacercariae find their way to the gut of Trematodes in the family Echinos- the final host fish when the second inter- tomatidae parasitize birds as the final mediate fish host is eaten. Metacercariae host. Echinostome metacercariae, such become adult trematodes in the gut of the as Himasthla sp., found in bivalve tissues fish where eggs are produced to begin consumed raw have been implicated in the cycle again. Encysted metacercariae human gastrointestinal disturbances. found within bivalve host tissues indicate members of other trematode families that may follow different life cycle pathways of having a bivalve mollusc as a second intermediate host rather than a fish.

V. Diagnosis Wet mounts of fresh tissues may show the encysted metacercariae (Gym- nophallidae are unencysted) which can be extremely difficult to find if in low prevalences. Sporocysts of some trema- todes may be grossly orange in color within the bivalve tissues and differenti- ated by species based on the morphology of the cercariae. Sporocysts and encysted metacercariae can also be observed in tissue sections during routine histo- logical examination. Sporocysts may extensively infiltrate host tissues, exhibit dichotomous branching and are strongly basophilic containing germ balls and cercariae. Metacercariae may exhibit trematode features such as oral and ven- tral suckers and appear in various tissues

48 49 bivalve mollusc Helminths Trematode Metacercariae and Sporocysts

Histological section of encysted trematode metacercaria (arrow) encapsulated by host inflammatory cells and fibrous tissue within the connective tissues of blue mussel

 Histological section of a Bucephalus-like trematode sporocyst containing germ balls (arrow) in the connective tissues of blue mussel

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 Histological section of a different type of sporocyst containing cercariae (arrow) in razor clam

Bucephalus sp. Life Cycle

Predatory fish final host for adult trematode

Mature egg passes in feces

Intermediate fish host with encysted metacercaria Ciliated – miracidium

Free swimming cercaria Infected bivalve where sporocysts develop

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