Unidentified Kidney Coccidia and Others

bivalve mollusc Protozoa Unidentified Kidney Coccidia and Others

I. Causative Agent and Disease Mediterranean Sea, England and Scot- Coccidiasina is a subclass within land, Pectin scallops and the European the class Conoidasida in the phylum flat oyster in France, cockles in Spain, Apicomplexa. These protozoa are blacklipped oysters in Australia and in intracellular parasites of vertebrates as blue mussels, bay scallops and the eastern well as invertebrates. Some members of oyster on the Atlantic coast of North two suborders, Adeleorina and Eimeri- America and on the Pacific coast in the orina, parasitize bivalve molluscs in the native littleneck clam in Washington State kidneys (Pseudoklossia glomerata, P. and in blue mussels, native littleneck and pectinis, P. pelseneeri, Klossia telli- Japanese littleneck (Manila) clams in nae), germ cells of the ovarian tubules British Columbia, Canada. In Alaska, an (Merocystis tellinovum), developing unidentified kidney coccidian (possibly ova (unidentified coccidia), connective multiple species) has been found in 12-33% tissues (unidentified coccidian sporo- of examined blue mussels, basket cockles cysts) and sometimes visceral ganglia (P. and native littleneck clams. Also, varying glomerata). Coccidia of bivalve molluscs prevalences of littleneck clams from all are of two types: those that complete part Alaskan locations (total prevalence ~ 62%) of their life cycle in bivalves and presum- have been parasitized by a sporocyst stage ably require an alternate host because of another unidentified coccidian dissemi- certain developmental stages have not nated throughout all connective tissues. A been observed; and those organisms that light intensity of similar sporocysts have complete their entire life cycle within also been found in the connective tissues their bivalve hosts where all develop- surrounding the gut of a single butter clam. mental stages have been described. De- pending on the species of bivalve host, III. Clinical Signs tissue degeneration and necrosis from the The kidney coccidian in native intracellular development of these para- littleneck clams from Washington State sites ranges from minor in developing reportedly causes behavioral modifica- ova to severe kidney damage or parasitic tion such that live parasitized clams castration of females when infecting are found on the surface of the sand germ cells. Severe kidney tubule infesta- substrate (“kick-outs”) rather than buried tion of an unidentified coccidian in Ca- underneath. Otherwise there are no other nadian bay scallops reportedly resulted gross clinical signs of disease or mortal- in systemic dissemination of the parasite ity in the other reported infestations. into various other tissues. No mortality Routine histological examination detects directly related to coccidian parasitism these coccidia within their respective in bivalves has been documented in feral target host tissues with varying degrees populations but has been reported in of cell damage or host response ranging scallops during at least one situation of from none to significant in the case of recirculating hatchery culture. certain kidney coccidia.

II. Host Species IV. Transmission Coccidia have been described Transmission of these coccidia is infecting Tapes and Tellina clams in the likely horizontal via ambient seawater

38 39 bivalve mollusc Protozoa

but many may require at least one alter- connective tissues of Alaskan littleneck nate host to complete their life cycles. clams typically contain elongate sporo- The typical life cycle producing the zoites (28 µm by 4 µm). Generic and spe- developmental stages of coccidia can cies identification of coccidia are based be divided into three phases: asexual primarily on the structure of oocysts merogony (schizogony) where multi- and sporocysts as well as the number of plication occurs inside the specific host sporocysts contained by the oocyst. target cells producing trophozoites giv- ing rise to schizonts containing mero- VI. Prognosis for Host zoites that infect other cells to continue The effects of coccidia on host merogony or to begin gamogony; sexual bivalve molluscs is variable ranging from gamogony (anisogomy) where merozo- no effect to significant kidney necrosis ites, upon entering host cells, transform and parasitic castration in females. In into gamonts of macrogametes (female) Alaska, moderate host inflammatory and biflagellated microgametes (male) infiltration with minor kidney necrosis that join to form a zygote or oocyst; and caused by infestation has been observed asexual sporogony where oocyst cyto- in two basket cockles. However, gener- plasm divides to contain sporoblasts that ally these parasites have appeared harm- develop into sporocysts each containing less in wild as well as cultured bivalve sporozoites that, when liberated from species. the oocyst, infect cells to start the entire cycle anew. For most coccidia in bivalve VII. Human Health Significance molluscs gamogony and sporogony oc- There are no zoonotic human health cur in the bivalve host with merogony concerns regarding these coccidian para- presumably occurring in other hosts sites in bivalve mollusc tissues. that have not been identified. The few remaining reported coccidia apparently complete all developmental phases in the bivalve host.

V. Diagnosis Large mature macrogamonts may be observed in squash preparations of kidney tissues while all intracellular developmental stages may be observed by histological examination. The kidney coccidian of the Alaskan littleneck clam appears identical to one reported in the same clam species in Washing- ton State and is likely an undescribed species based on: the apparent comple- tion of its entire life cycle in the clam; the production of a thick-walled spore in addition to a thin-walled oocyst; and the presence of tetrazoic (4 sporozoites) sporocysts instead of dizoic sporocysts. Large sporocysts of another unidentified coccidian disseminated throughout the

38 39 bivalve mollusc Protozoa Unidentified Kidney Coccidia

and Others 

Histological section of gamonts (arrow) of a Pseudoklossia-like coccidian in the kidney of native littleneck clam 

Microgamont (arrow) of a Pseudoklossia-like coccidian in the kidney of native littleneck clam

40 41

bivalve mollusc Protozoa 

Resting spore (arrow) of Pseudoklossia-like coccidian in the kidney of native littleneck clam



Oocyst (arrow) of Pseudoklossia-like coccidian in the kidney of basket cockle

40 41 bivalve mollusc Protozoa Unidentified Kidney Coccidia

and Others 

Histological section of sporocyst containing elongate sporozoites (arrow) of an

unidentified coccidian in the connective tissue of native littleneck clam  

Ultrastructural detail of littleneck clam sporozoites (arrow) and sporocyst wall (arrowhead), TEM

42 43 bivalve mollusc Protozoa Typical Coccidia Life Cycle

Stages occur in bivalve host

Asexual Sporogony oocysts



sporoblasts



sporocysts Stages occur in other host

sporozoites 

Asexual Merogony  sporozoites



trophozoites

schizonts 

merozoites

Sexual Gamogony merozoites

 

male female microgametes macrogametes 

oocysts

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