Scuticociliatosis is a severe and often fatal

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The ent ...

infection An infection is the invasion of tissue (biology), tissues by pathogens, their multiplication, and the reaction of host (biology), host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmis ...

of several groups of marine organisms. Species known to be susceptible include a broad range of

teleost Teleostei (; Ancient Greek, Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts (), is, by far, the largest group of ray-finned fishes (class Actinopterygii), with 96% of all neontology, extant species of f ...

seahorse A seahorse (also written ''sea-horse'' and ''sea horse'') is any of 46 species of small marine Osteichthyes, bony fish in the genus ''Hippocampus''. The genus name comes from the Ancient Greek (), itself from () meaning "horse" and () meanin ...

shark Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...

s, and some

crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

s. The disease can be caused by any one of about 20 distinct species of

unicellular A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and ...

eukaryote The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

s known as

scuticociliate Scuticociliatia is a subclass of ciliates in the class Oligohymenophorea. Its members are called scuticociliates. These unicellular eukaryotes are microorganisms that are usually free-living and can be found in freshwater, marine, and soil habita ...

s, which are free-living marine microorganisms that are opportunistic or

facultative Facultative means "optional" or "discretionary" (antonym ''obligate''), used mainly in biology in phrases such as: * Facultative (FAC), facultative wetland (FACW), or facultative upland (FACU): wetland indicator statuses for plants * Facultative ...

parasites. Scuticociliatosis has been described in the wild, in captive animals in

aquarium An aquarium (: aquariums or aquaria) is a vivarium of any size having at least one transparent side in which aquatic plants or animals are kept and displayed. fishkeeping, Fishkeepers use aquaria to keep fish, invertebrates, amphibians, aquati ...

s, and in

aquaculture Aquaculture (less commonly spelled aquiculture), also known as aquafarming, is the controlled cultivation ("farming") of aquatic organisms such as fish, crustaceans, mollusks, algae and other organisms of value such as aquatic plants (e.g. Nelu ...

. It is best studied in fish species that are commonly farmed, in which typical effects of infection include skin

ulceration An ulcer is a discontinuity or break in a bodily membrane that impedes normal function of the affected Organ (biology), organ. According to Robbins's pathology, "ulcer is the breach of the continuity of skin, epithelium or mucous membrane caus ...

hemorrhage Bleeding, hemorrhage, haemorrhage or blood loss, is blood escaping from the circulatory system from damaged blood vessels. Bleeding can occur internally, or externally either through a natural opening such as the mouth, nose, ear, urethra, ...

, and

necrosis Necrosis () is a form of cell injury which results in the premature death of cells in living tissue by autolysis. The term "necrosis" came about in the mid-19th century and is commonly attributed to German pathologist Rudolf Virchow, who i ...

, with post-mortem examination identifying ciliates in the skin,

gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...

s, blood, and internal organs including the brain.

Host range

Scuticociliatosis has been reported in a broad range of

s; it is best studied in those teleost fish commonly raised in

, where the disease is of significant economic impact. It has also been reported in

s, and

s. The mortality rate is particularly high among

flatfish A flatfish is a member of the Ray-finned fish, ray-finned demersal fish Order (biology), suborder Pleuronectoidei, also called the Heterosomata. In many species, both eyes lie on one side of the head, one or the other migrating through or around ...

, possibly due to their sedentary lifestyle involving skin-to-skin contact between individuals. Since 2022 there have also been several reports that scuticociliates might be responsible for mass die-off events of

sea urchin Sea urchins or urchins () are echinoderms in the class (biology), class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal zone to deep seas of . They typically have a globular body cove ...

s across the world's oceans.

Disease mechanism

Scuticociliatosis consists of overwhelming infection of an animal's body by any one of around 20 species of

. These unicellular organisms are free-living in marine environments but are opportunistic

parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...

s with a diverse host range. It is unclear what triggers infection, although infection rates are known to be higher, in both experimental and aquaculture conditions, in warmer water. Low

salinity Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...

has also been reported to reduce disease rates. Under some conditions, ciliates have been reported to successfully infect healthy fish, likely through the gills; other reports suggest abrasions or skin damage may be required. Scuticociliates are histophagous (tissue-eating) and extensively degrade body tissues.

Histological Histology, also known as microscopic anatomy or microanatomy, is the branch of biology that studies the microscopic anatomy of biological tissue (biology), tissues. Histology is the microscopic counterpart to gross anatomy, which looks at large ...

postmortem examination of affected fish usually reveals ciliates in the skin and gills, blood, and internal organs, with significant damage to the brain and nervous system, which is likely responsible for behaviors such as abnormal swimming in infected individuals.

Causative agents

The exact species of scuticociliate responsible for a given outbreak is often not identified. As a result, differences in virulence and disease course among different scuticociliates are not well characterized. In one study, infection by '' Miamiensis avidus'' was reported to have a higher mortality rate than '' Pseudocohnilembus persalinus'', '' Pseudocohnilembus hargisi'' and '' Uronema marinum''. Infections by ''U. marinum'' show a less severe disease course, possibly restricted to the skin surface; it has been suggested that this ciliate may be only a secondary pathogen. ''M. avidus'', ''P. persalinus'', ''U. marinum'', and ''U. nigricans'' have all been reported in aquaculture settings.

Outbreaks

Scuticociliatosis outbreaks are recurring problems in

fisheries Fishery can mean either the enterprise of raising or harvesting fish and other aquatic life or, more commonly, the site where such enterprise takes place ( a.k.a., fishing grounds). Commercial fisheries include wild fisheries and fish farm ...

, especially of

olive flounder The olive flounder (''Paralichthys olivaceus''), bastard halibut, Japanese flounder or Korean halibut is a temperate marine species of large-tooth flounder native to the North-western Pacific Ocean. It is the highest valued finfish in the world, ...

. Outbreaks in both aquaculture and natural settings are more common in the spring and summer, when water temperature is higher. Outbreaks have also been identified in captive settings; in one

an outbreak of ''M. avidus'' was responsible for the deaths of six sharks. Scuticociliatosis due to '' Miamiensis avidus'' infection is believed to be responsible for a 2017 die-off of fish and

leopard shark The leopard shark (''Triakis semifasciata'') is a species of houndshark, in the family (biology), family Houndshark, Triakidae. It is found along the Pacific coast of North America, from the U.S. state of Oregon to Mazatlán in Mexico. Typically ...

s found in the

San Francisco Bay San Francisco Bay (Chochenyo language, Chochenyo: 'ommu) is a large tidal estuary in the United States, U.S. state of California, and gives its name to the San Francisco Bay Area. It is dominated by the cities of San Francisco, California, San ...

Treatment and prevention

There is no treatment for scuticociliatosis once parasites have infected an animal's internal organs. In aquaculture settings, chemical treatment of the water to kill ciliates can be used in conjunction with

antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy ...

References

{{reflist, 30em Fish diseases