Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide







  • Last modified
  • 14 July 2018
  • Datasheet Type(s)
  • Animal Disease
  • Preferred Scientific Name
  • epitheliocystis
  • Pathogens
  • Chlamydia-like organisms
  • Overview
  • The causative agent(s) of the disease referred to as EP are morphologically diverse and may represent a group of related organisms that produce similar pathology in varied hosts. The taxonomic placement of these diverse...

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Epitheliocystis cyst in the gill of a juvenile white sturgeon (Acipenser transmontanus). Haematoxylin and eosin stain. Bar = 100 µm.
TitleEpitheliocystis cyst in the gill of a juvenile white sturgeon
CaptionEpitheliocystis cyst in the gill of a juvenile white sturgeon (Acipenser transmontanus). Haematoxylin and eosin stain. Bar = 100 µm.
CopyrightC. N. Lannan, J. L. Bartholomew & J. L. Fryer
Epitheliocystis cyst in the gill of a juvenile white sturgeon (Acipenser transmontanus). Haematoxylin and eosin stain. Bar = 100 µm.
Epitheliocystis cyst in the gill of a juvenile white sturgeon Epitheliocystis cyst in the gill of a juvenile white sturgeon (Acipenser transmontanus). Haematoxylin and eosin stain. Bar = 100 µm.C. N. Lannan, J. L. Bartholomew & J. L. Fryer


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Preferred Scientific Name

  • epitheliocystis

International Common Names

  • English: chlamydial infections of fish; epitheliocystis disease

English acronym

  • EP


Top of page Chlamydia-like organisms


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The causative agent(s) of the disease referred to as EP are morphologically diverse and may represent a group of related organisms that produce similar pathology in varied hosts. The taxonomic placement of these diverse intracellular microorganisms is undefined but is considered to fit most appropriately in the order Chlamydiales (Moulder, 1984). Most exhibit the pleomorphic developmental cycle typical of this group of intracellular bacteria (Paperna et al., 1981).

Epitheliocystis occurs as a non-proliferative or proliferative disease, characterized by cysts in the branchial epithelia of the host. Clinical signs may include lethargy, flared opercula and rapid respiration. Cysts may appear as transparent white to yellow capsules on the gill filaments. Generally the host response is limited, and there is little or no mortality associated with infection.

[Based upon material originally published in Woo PTK, Bruno DW, eds., 1999. Fish diseases and disorders, Vol. 3 Viral, bacterial and fungal infections. Wallingford, UK: CABI Publishing.]

Host Animals

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Animal nameContextLife stageSystem
Acanthopagrus schlegelii (black porgy)
Acipenser gueldenstaedtii (russian sturgeon)
Acipenser transmontanus (white sturgeon)
Ameiurus nebulosus (brown bullhead)
Anoplopoma fimbria (sablefish)
Bidyanus bidyanus (silver perch)
Citharichthys sordidus
Cymatogaster aggregata
Cynoscion regalis
Cyprinus carpio (common carp)
Dentex dentex (common dentex)
Dicentrarchus labrax (European seabass)
Epinephelus malabaricus
Gadus morhua (Atlantic cod)
Hippoglossoides platessoides
Ictalurus punctatus (channel catfish)
Lates calcarifer (barramundi)Domesticated hostAquatic: FryEnclosed systems/Cages
Lepomis macrochirus (bluegill)
Limanda ferruginea (yellowtail flounder)Aquatic: Adult
Liza aurata (golden grey mullet)
Liza subviridis
Lutjanus kasmira (blueline snapper)
Merluccius productus
Microgadus proximus
Microstomus pacificus
Morone americana (white perch)
Morone saxatilis (striped sea-bass)
Mugil cephalus (flathead mullet)
Oncorhynchus gorbuscha (pink salmon)
Oncorhynchus keta (chum salmon)
Oncorhynchus kisutch (coho salmon)
Oncorhynchus mykiss (rainbow trout)
Oncorhynchus tshawytscha (chinook salmon)
Ophiodon elongatus
Oplegnathus punctatus
Oreochromis aureus (blue tilapia)
Oreochromis mossambicus (Mozambique tilapia)
Oreochromis niloticus (Nile tilapia)
Pagrus major (red seabream)
Paralichthys dentatus (summer flounder)
Paralichthys oblongus
Peprilus triacanthus
Phycodurus eques
Piaractus mesopotamicus (small-scales pacu)
Pleuronectes ferrugineus
Pseudopleuronectes americanus (winter flounder)
Salmo salar (Atlantic salmon)Domesticated host, Wild hostAquatic: AdultEnclosed systems/Cages
Salvelinus namaycush (lake trout)
Scophthalmus aquosus
Sebastes schlegelii
Seriola dumerili (greater amberjack)
Seriola lalandi (yellowtail amberjack)
Sparus aurata (gilthead seabream)
Syngnathus acus
Takifugu rubripes (Japanese puffer fish)Wild host
Theragra chalcogramma
Upeneus moluccensis
Urophycis regia
Zoarces americanus (ocean pout)

Hosts/Species Affected

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Epitheliocystis was first described in the bluegill sunfish (Lepomis macrochirus), by Hoffman et al. (1969), who identified the aetiological agent as a bedsonia (now termed chlamydia). Molnar and Boros (1981) verified the chlamydia-like nature of the causative agent and determined that EP was identical to the mucophilosis disease of common carp described by Plehn (1920) and believed to be caused by a unicellular alga or fungus. These descriptions were followed by reports of the disease in freshwater, marine and anadromous fish from both warm and cold water environments. The documented host range includes species from more than 20 families of fishes. Probably additional hosts have gone unreported because of the self-limiting nature of the disease.

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

Sea Areas

Atlantic, NortheastPresentPaperna et al., 1981; Paperna and Alves, 1984; Nylund et al., 1998
Atlantic, NorthwestPresentZachary and Paperna, 1977; Morrison and Shum, 1983; Lewis et al., 1992
Atlantic, SoutheastPresentPaperna and Sabnai, 1980; Paperna et al., 1981
Indian Ocean, EasternPresentLangdon et al., 1991; Anderson and Prior, 1992
Mediterranean and Black SeaPresentPaperna, 1977; Paperna and Laurencin, 1979; Paperna and Sabnai, 1980; Paperna et al., 1981; Crespo et al., 1990; Grau and Crespo, 1991; Montero et al., 2004
Pacific, Eastern CentralPresentVenizelos and Beneti, 1996
Pacific, NortheastPresentGroff et al., 1996; Kent et al., 1998
Pacific, NorthwestPresentMiyazaki et al., 1986; Egusa, 1987; Egusa et al., 1987; Kim et al., 2000
Pacific, SoutheastPresentCarvajal et al., 1990; Nowak and Clark, 1999
Pacific, Western CentralPresentWork et al., 2003


ChinaPresentWeng and Chen, 2000
IsraelPresentPaperna et al., 1978; Paperna and Sabnai, 1980; Paperna et al., 1981; Paperna and Alves, 1984
JapanPresentMiyazaki et al., 1986; Egusa, 1987; Egusa et al., 1987


South AfricaPresentPaperna and Sabnai, 1980; Paperna et al., 1981

North America

CanadaPresentDesser et al., 1988
USAPresentPaperna and Zwerner, 1976; Bradley et al., 1988
-CaliforniaPresentGroff et al., 1996
-ConnecticutPresentWolke et al., 1970
-IdahoPresentRourke et al., 1984
-OklahomaPresentZimmer et al., 1984
-OregonPresentLannan et al., 1999
-West VirginiaPresentHoffman et al., 1969

South America

BrazilPresentSzakolczai et al., 1999
-Rio de JaneiroPresentLima et al., 2001
ChilePresentCarvajal et al., 1990
EcuadorPresentVenizelos and Beneti, 1996


FrancePresentPaperna and Laurencin, 1979; Paperna et al., 1981
PortugalPresentCruz et al., 2000
Russian FederationPresentVoronin and Chernysheva, 1997
UKPresentLongshaw et al., 2004


AustraliaPresentLangdon et al., 1991; Anderson and Prior, 1992
-TasmaniaPresentFrances et al., 1997


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Epitheliocystis occurs as a non-proliferative or proliferative disease, characterized by cysts in the branchial epithelia of the host. Clinical signs may include lethargy, flared opercula, increased mucus production and rapid respiration. Cysts may appear as transparent white to yellow capsules on the gill filaments. Generally the host response is limited, and there is little or no mortality associated with infection.

Characteristic cysts are hypertrophic host cells filled with the causative bacterium. The enlarged host cells range from 10 to 400 mm in diameter and are frequently surrounded by squamous or cuboidal epithelial cells. The nature and origin of the target host cell is unclear; cysts may originate from a single cell (Zachary and Paperna, 1977) or be a coalescence of several (Wolke et al., 1970). The target cell may vary, as infection has been variously described in mucus, epithelial lining and chloride cells of an unidentified species of carp (Paperna and Alves de Matos, 1984); in capillary-forming cells of red sea bream (Pagrus major) and tiger puffer (Takifugu rubripes; Miyazaki et al., 1986); and in cells described as possible transformed macrophages in brown bullhead (Ictalurus nebulosus; Desser et al., 1988).

Preliminary diagnosis of EP is made by observation of the white to yellow cysts on the gills or skin of affected fish (Wolf, 1988). The thick capsule and granular contents, which are characteristic of EP cysts, are easily seen in wet mounts. The gill is the most frequently affected organ, but it has been suggested that the pseudobranch also be examined for cysts (Crespo et al., 1990).

No serological techniques are available for identification of the organism or for diagnosis of infection. Electron microscopy is required for definitive diagnosis of this disease. With this technique, the intracellular forms, too small to be clearly differentiated under the light microscope, can be observed and differentiated from cysts having a viral aetiology (Wolf, 1988).

List of Symptoms/Signs

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SignLife StagesType
Finfish / Air gulping - Behavioural Signs Aquatic:Adult Sign
Finfish / Cysts - Gills Aquatic:Adult Diagnosis
Finfish / Fish swimming near surface - Behavioural Signs Aquatic:Adult Sign
Finfish / Generalised lethargy - Behavioural Signs Aquatic:Adult Sign
Finfish / Mucus build up - Gills Aquatic:Adult Sign

Disease Course

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Molnar and Boros (1981) described the development of EP cysts in the gills of the common carp (Cyprinus carpio). In the early stages of the disease, infected cells are 10-15 mm in diameter and contain a central inclusion body, foamy cytoplasm and excentric nucleus. As infection progresses, infected cells increase in size to 70-80 mm in diameter and the granular inclusion fills the entire host cell, compressing or replacing both the nucleus and the cytoplasm.

In non-proliferative infections, cysts may be surrounded by a layer of squamous or cuboidal epithelial cells. Typically, no host response is apparent, even in the presence of large numbers of cysts. However, in certain cases, the organism induces an extensive host response, with unrestricted proliferation of gill epithelia and extensive mucus production, a condition referred to as hyperinfection (Paperna, 1977; Rourke et al., 1984; Bradley et al., 1988). The proliferative form of the disease has been described in Sparus aurata (Paperna, 1977). Hyperplastic epithelial cells form concentric layers around the cysts and proliferate throughout the gill filaments. Infiltrating macrophages and eosinophils combine with the hyperplastic tissue to surround and obstruct the capillary network of the gill filament. These circumstances impair both gas transfer and osmoregulatory processes, which may result in the death of infected fish.

The causes for variation in the severity of EP infections are not clear, but proliferative infections with accompanying mortality often occur in cultured fish (Paperna, 1977; Miyazaki et al., 1986; Bradley et al., 1988; Crespo et al., 1990; Groff et al., 1996; Crespo et al., 1999; Montero et al., 2004). Non-proliferative infections are more frequently found in free-ranging fish (Grau and Crespo, 1991). Progression from the chronic to the progressive form may occur when host defence mechanisms are compromised by genetics or environmental factors (Paperna and Sabnai, 1980). Because the causative organism has not been isolated, the possibility that these observations can be explained by differences in virulence of the pathogen cannot be excluded.


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Natural transmission of the chlamydia-like organisms (CLO) is not understood, but horizontal transmission apparently occurs within some host species. Hoffman et al. (1969) described experiments in the bluegill and Wolf (1988) listed experiments conducted by D.S. Wyand in goldfish (Carassius auratus) that demonstrated direct transmission. In both studies EP developed in the experimental animals 3-4 weeks after the addition of infected gill tissue to aquaria where uninfected fish were held. Paperna (1977) suggested that transmission from infected fish may occur in culture facilities through contaminated nets and other equipment. There is little information on interspecies transmission and it is not known if differences in the morphology of the agent relate to different host specificities or the CLO itself.

No information is available concerning temperature requirements for replication of the CLO or on transfer of the CLO from fish to humans or other warm-blooded animals. However, these microorganisms have only been reported in fish.

Impact Summary

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Fisheries / aquaculture Negative

Zoonoses and Food Safety

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This species is not a zoonoses.


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Anderson IG; Prior HC, 1992. Subclinical epitheliocystis in barramundi, Lates calcarifer, reared in sea cages. Australian Veterinary Journal, 69(9):226-227; 8 ref.

Bradley TM; Newcomer CE; Maxwell KO, 1988. Epitheliocystis associated with massive mortalities of cultured lake trout Salvelinus namaycush.. Diseases of Aquatic Organisms, 4(1):9-17.

Carvajal J; Ruiz G; Gonzales L, 1990. Gill histopathologies in coho salmon (Oncorhynchus kisutch) and Atlantic salmon (Salmo salar) raised under net cages in southern Chile. Medio-Ambiente, 11:53-58.

Crespo S; Grau A; Padrós F, 1990. Epitheliocystis disease in the cultured amberjack, Seriola dumerili Risso (Carangidae). Aquaculture, 90(3/4):197-207.

Crespo S; Zarza C; Padrós F, 2001. Epitheliocystis hyperinfection in sea bass, Dicentrarchus labrax (L.): light and electron microscope observations. Journal of Fish Diseases, 24(9):557-560.

Crespo S; Zarza C; Padrós F; Marín de Mateo M, 1999. Epitheliocystis agents in sea bream Sparus aurata [Pagrus aurata]: morphological evidence for two distinct chlamydia-like developmental cycles. Diseases of Aquatic Organisms, 37(1):61-72.

Cruz e Silva MP; Orge ML; Afonso-Roque MM; Grazina-Freitas MS; Carvalho-Varela M, 2000. Diplectanum aequans (Wagener, 1857) Diesing, 1858 (Monogenea, Diplectanidae) in sea bass (Dicentrarchus labrax (L.), 1758) from freshwater culture. Acta Parasitológica Portuguesa, 7(1/2):53-56.

Desser S; Paterson W; Steinhagen D, 1988. Ultrastructural observations on the causative agent of epitheliocystis in the brown bullhead , Ictalurus nebulosus Lesueur, from Ontario and a comparison with the chlamydiae of higher vertebrates. Journal of Fish Diseases, 11(6):453-460.

Draghi A; Popov VL; Kahl MM; Stanton JB; Brown CC; Tsongalis GJ; West AB; Frasca; S, 2004. Characterization of Candidatus Piscichlamydia salmonis (Order Chlamydiales), a Chlamydia-Like Bacterium Associated With Epitheliocystis in Farmed Atlantic Salmon (Salmo salar). Journal of Clinical Microbiology, 42:5286-5297.

Egusa S, 1987. Epitheliocystis disease. Fish Pathology, 22:165-171.

Egusa S; Miyazaki T; Shiomitsu T; Fujita S, 1987. Epitheliocystis-like disease among hatchery-reared fry of Oplegnathus punctatus. Fish Pathology, 22(1):33-34.

Frances J; Tennent R; Nowak BF, 1997. Epitheliocystis in silver perch, Bidyanus bidyanus (Mitchell). Journal of Fish Diseases, 20(6):453-457.

Grau A; Crespo S, 1991. Epitheliocystis in the wild and cultured amberjack, Seriola dumerili Risso: ultrastructural observations. Aquaculture, 95(1-2):1-6.

Groff JM; LaPatra SE; Munn RJ; Anderson ML; Osburn BI, 1996. Epitheliocystis infection in cultured white sturgeon (Acipenser transmontanus): antigenic and ultrastructural similarities of the causative agent to the chlamydiae. Journal of Veterinary Diagnostic Investigation, 8(2):172-180.

Hoffman GL; Dunbar CE; Wolf K; Zwillenberg LO, 1969. Epitheliocystis, a new infectious disease of the bluegill (Lepomis macrochirus). Antonie Van Leeuwenhoek Journal of Microbiology and Serology, 35:146-158.

Kent ML; Traxler GS; Kieser D; Richards J; Dawe SC; Shaw RW; Prosperi-Porta G; Ketcheson J; Evelyn TPT, 1998. Survey of salmonid pathogens in ocean-caught fishes in British Columbia, Canada. Journal of Aquatic Animal Health, 10(2):211-219.

Kim SeRa; Lee JongHwan; Son ChangHo; Kim SungHo, 2000. A case report on epitheliocystis in cultured rock fish. Korean Journal of Veterinary Clinical Medicine, 17(2):502-504.

Langdon JS; Elliott K; Mackay B, 1991. Epitheliocystis in the leafy sea-dragon. Australian Veterinary Journal, 68(7):244.

Lannan CN; Bartholomew JL; Fryer JL, 1999. Rickettsial and chlamydial infections. Fish diseases and disorders. Volume 3: viral, bacterial and fungal infections., 245-267; 5 pp. of ref.

Lewis EJ; McLaughlin SM; Bodammer JE; Sawyer TK, 1992. Epitheliocystis in ten new host species of marine fish. Journal of Fish Diseases, 15(3):267-271.

Lima FCde; Machado APG; Borges AP; Lima CHA; Andrade Cde M; Mesquita Ede FMde, 2001. Epitheliocystis disease in Tilapia nilotica (Linnaeus, 1758) from Rio de Janeiro state, Brazil. Ciência Rural, 31(3):519-520.

Longshaw M; Green MJ; Feist SW, 2004. Histopathology of parasitic infections in greater pipefish, Syngnathus acus L., from an estuary in the UK. Journal of Fish Diseases, 27:245-248.

Miyazaki T; Fujimaki Y; Hatai K, 1986. A light and electron microscopic study on epitheliocystis disease in cultured fishes. Bulletin of the Japanese Society of Scientific Fisheries, 52:199-202.

Molnar K; Boros G, 1981. A light and electron microscopic study of the agent of carp mucophilosis. Journal of Fish Diseases, 4(4):325-334.

Montero FE; Crespo S; Padrós F; Gándara Fde la; García A; Raga JA, 2004. Effects of the gill parasite Zeuxapta seriolae (Monogenea: Heteraxinidae) on the amberjack Seriola dumerili Risso (Teleostei: Carangidae). Aquaculture, 232(1/4):153-163.

Morrison C; Shum G, 1983. Epitheliocystis in American plaice, Hippoglossoides platessoides (Fabricius). Journal of Fish Diseases, 6(3):303-308.

Moulder JW, 1984. Order II. Chlamydiales Stortz and Page 1971, 334AL. In: Kreig N, ed. Bergey’s Manual of Systematic Bacteriology, Vol. 1. Baltimore, London, UK: Williams and Wilkins, 729-739.

Nowak BF; Clark A, 1999. Prevalence of epitheliocystis in Atlantic salmon, Salmo salar L., farmed in Tasmania, Australia. Journal of Fish Diseases, 22(1):73-78.

Nylund A; Kvenseth AM; Isdal E, 1998. A morphological study of the epitheliocystis agent in farmed Atlantic salmon. Journal of Aquatic Animal Health, 10(1):43-55.

Paperna I, 1977. Epitheliocystis infection in wild and cultured sea bream (Sparus aurata, Sparidae) and grey mullets (Liza ramada, Mugilidae). Aquaculture, 10:169-176.

Paperna I; Alves de Matos AP, 1984. The developmental cycle of epitheliocystis in carp, Cyprinus carpio L. Journal of Fish Diseases, 7(2):137-147.

Paperna I; Laurencin FB, 1979. Parasitic infections of sea bass, Dicentrarchus labrax, and gilt head sea bream, Sparus aurata, in mariculture facilities in France. Aquaculture, 16(2):173-175.

Paperna I; Sabnai I, 1980. Epitheliocystis disease in fishes. Fish diseases--third COPRAQ session, 228-234; [7 fig.].

Paperna I; Sabnai I; Castel M, 1978. Ultrastructural study of epitheliocystis organisms from gill epithelium of the fish Sparus aurata (L.) and Liza ramada (Risso) and their relation to the host cell. Journal of Fish Diseases, 1(2):181-189.

Paperna I; Sabnai I; Zachary A, 1981. Ultrastructural studies in piscine epitheliocystis: evidence for a pleomorphic developmental cycle. Journal of Fish Diseases, 4(6):459-472.

Paperna I; Zwerner DE, 1976. Parasites and diseases of striped bass, Morone saxatilis (Walbaum), from the lower Chesapeake Bay. Journal of Fish Biology, 9(3):267-282; [2 pl. (unpaged)].

Plehn M, 1920. Neue Parasiten im Haut und Kiemen von Fischen Ichthyochytrium und Mucophilus. Zentralblatt für Bakteriologie, Parasitenkunde de Infektionskrancheiten und Hygiene. Abt I Originale, 85:275-281.

Rourke AW; Davis RW; Bradley TM, 1984. A light and electron microscope study of epitheliocystis in juvenile steelhead trout, Salmo gairdneri Richardson. Journal of Fish Diseases, 7(4):301-309.

Szakolczai J; Vetési F; Pitz SR, 1999. Epitheliocystis disease in cultured pacu (Piaractus mesopotamicus) in Brazil. Acta Veterinaria Hungarica, 47(3):311-318.

Turnbull JF, 1993. Epitheliocystis and salmonid rickettsial septicaemia. In: Inglis V, Roberts RJ, Bomage NR (eds), Bacterial diseases of fish. Oxford, UK: Blackwell Scientific Publications, 237-254.

Venizelos A; Beneti DD, 1996. Epitheliocystis disease in cultured yellowtail Seriola mazatlana in Ecuador. Journal of the World Aquaculture Society, 27(2):223-227.

Voronin VN; Chernysheva NB, 1997. Epitheliocystis infection in common carp Cyprinus carpio L.: histopathology and pathogenicity. Bulletin of the European Association of Fish Pathologists, 17(3/4):137-139.

Weng YeouJuh; Chen ShihPing, 2000. Case report: epitheliocystis in Epinephelus malabaricus. Journal of the Chinese Society of Veterinary Science, 26(2):141-146.

Wolf K, 1988. Fish viruses and fish viral diseases. Fish viruses and fish viral diseases., xii + 476 pp.

Wolke RE; Wyand DS; Khairallah LH, 1970. A light and electron microscopic study of epitheliocystis disease in the gills of Connecticut striped bass (Morone saxatilis) and white perch (Morone americanus). Journal of Comparative Pathology, 80:559-563.

Work TM; Rameyer RA; Takata G; Kent ML, 2003. Protozoal and epitheliocystis-like infections in the introduced bluestripe snapper Lutjanus kasmira in Hawaii. Diseases of aquatic organisms, 57:59-66.

Zachary A; Paperna I, 1977. Epitheliocystis disease in the striped bass Morone saxatilis from the Chesapeake Bay. Canadian Journal of Microbiology, 23(10):1404-1414.

Zimmer MA; Ewing MS; Kocan KM, 1984. Epitheliocystis disease in the channel catfish, Ictalurus punctatus (Rafinesque). Journal of Fish Diseases, 7(5):407-410.

Links to Websites

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Chlamydiaehttp://www.chlamydiae.comThe comprehensive reference and education site to Chlamydia and the chlamydiae

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