Myxobolus cerebralis
Encyclopedia
Myxobolus cerebralis is a myxosporea
n parasite of salmonid
s (salmon
, trout
, and their allies) that causes whirling disease in farmed
salmon and trout and also in wild fish populations. It was first described in rainbow trout
in Germany a century ago, but its range has spread and it has appeared in most of Europe (including Russia), the United States, South Africa and other countries. In the 1980s, it was discovered that M. cerebralis needs to infect a tubificid
oligochaete (a kind of segmented worm) to complete its life-cycle. The parasite infects its hosts with its cells after piercing them with polar filament
s ejected from nematocyst-like capsules.
Whirling disease afflicts juvenile fish (fingerlings and fry) and causes skeletal
deformation and neurological damage. Fish "whirl" forward in an awkward corkscrew-like pattern instead of swimming normally, find feeding difficult, and are more vulnerable to predators. The mortality rate is high for fingerlings, up to 90% of infected populations, and those that do survive are deformed by the parasite residing in their cartilage
and bone
. They act as a reservoir for the parasite, which is released into water following the fish's death. M. cerebralis is one of the most economically important myxozoans in fish as well as one of the most pathogenic. It was the first myxosporean whose pathology and symptoms were described scientifically. The parasite is not transmissible to humans.
and naming of both M. cerebralis and of myxozoans in general have complicated histories. It was originally thought that this parasite infected fish brains (hence the specific epithet cerebralis), however it quickly became apparent that while it can be found in the nervous system
, it primarily infects cartilage
and skeletal tissue
. Attempts to change the name to Myxobolus chondrophagus, which would more accurately describe the organism, failed because of nomenclature rules
. Later, it became apparent that organisms previously called Triactinomyxon dubium and T. gyrosalmo (class
Actinosporea) were in fact triactinomyxon stages of M. cerebralis, whose life cycle was expanded to include the triactinomyxon stage. Similarly, other actinosporeans were folded into the life cycles of various myxosporeans.
Today, the myxozoans, previously thought to be multicellular protozoans are considered animal
s by many scientists, though their status has not officially changed. Recent molecular studies suggest that they are related to Bilateria
or Cnidaria
, with Cnidaria being closer morphologically because both groups have extrusive filaments, but with Bilateria being somewhat closer in some genetic studies.
M. cerebralis has many diverse stages ranging from single cells
to relatively large spores, not all of which have been studied in detail.
(µm) long and three processes or "tails" that are each about 200 micrometers long. A sporoplasm packet at the end of the style contains 64 germ cells surrounded by a cellular envelope. There are also three polar capsule
s, each of which contains a coiled polar filament
between 170 and 180 µm long. Polar filaments in both this stage and in the myxospore stage (see picture above) rapidly shoot into the body of the host, creating an opening through which the sporoplasm can enter.
to produce more amoeboid
cells, which migrate into deeper tissue layers, in order to reach the cerebral cartilage.
. Though M. cerebralis is the only myxosporean ever found in salmonid cartilage, other visually similar species may be present in the skin, nervous system
, or muscle
.
.jpg)
Myxobolus cerebralis has a two-host life-cycle involving a salmonid fish and a tubificid oligochaete. So far, the only worm known to be susceptible to M. cerebralis infection is Tubifex tubifex
, though what scientists currently call T. tubifex may in fact be more than one species. First, myxospores are ingested by tubificid worms. In the gut
lumen
of the worm, the spores extrude their polar capsule
s and attach to the gut epithelium
by polar filament
s. The shell valves then open along the suture line and the binucleate germ cell penetrates between the intestinal epithelial cells of the worm. This cell multiplies, producing many amoeboid
cells by an asexual cell fission process called merogony. As a result of the multiplication process, the intercellular space of the epithelial cells in more than 10 neighbouring worm segments may become infected.
Around 60–90 days post-infection, sexual cell stages of the parasite undergo sporogenesis
, and develop into pansporocysts, each of which contains eight triactinomyxon-stage spores. These spores are released from the oligochaete anus into the water. Alternatively, a fish can become infected by eating an infected oligochaete. Infected tubificids can release triactinomyxons for at least 1 year. The triactinomyxon spores swim through the water to infect a salmonid through the skin. Penetration of the fish by these spores takes only a few seconds. Within five minutes, a sac of germ cells called a sporoplasm has entered the fish epidermis, and within a few hours, the sporoplasm splits into individual cells that will spread through the fish.
Within the fish, there are both intracellular and extracellular stages that reproduce in its cartilage by asexual endogeny, meaning that new cells grow from within old cells. The final stage within fish is the myxospore, which is formed by sporogony. They are released into the environment when the fish decomposes or is eaten. Some recent research indicates that some fish may expel viable myxospores while still alive.
Myxospores are extremely tough: "it was shown that Myxobolus cerebralis spores can tolerate freezing at −20°C for at least 3 months, aging in mud at 13°C for at least 5 months, and passage through the guts of northern pike
Esox lucius or mallard
s Anas platyrhynchos without loss of infectivity" to worms. Triactinomyxons are much shorter lived, surviving 34 days or less, depending on temperature.
M. cerebralis infections have been reported from a wide range of salmonid species: 8 species of "Atlantic" salmonids, Salmo
; 4 species of "Pacific" salmonids, Oncorhynchus
; 4 species of Char, Salvelinus
; the Grayling, Thymallus thymallus; and the Huchen, Hucho hucho. M. cerebralis causes damage to its fish hosts through attachment of triactinomyxon spores and the migrations of various stages through tissues and along nerves, as well as by digesting cartilage. The fish's tail may darken, but aside from lesions on cartilage, internal organs generally appear healthy. Other symptoms include skeletal deformities and "whirling" behavior (tail-chasing) in young fish, which was thought to have been caused by a loss of equilibrium, but is actually caused by damage to the spinal cord and lower brain stem. Experiments have shown that fish can kill Myxobolus in their skin (possibly using antibodies), but that the fish do not attack the parasites once they have migrated to the central nervous system. This response varies from species to species.
In T. tubifex, the release of triactinomyxon spores from the intestinal
wall damages the worm’s mucosa; this may happen thousands of times in a single worm, and it is believed that this can impair nutrient absorption. Also, infected worms have lower body mass and may be discolored. Spores are released from the worm almost exclusively when the temperature is between 10°C and 15°C, so fish in warmer or cooler waters are less likely to be infected, and infection rates vary seasonally.
. This makes young fish more susceptible to deformities and provides M. cerebralis more cartilage on which to feed. In one study of seven species of many strains, brook trout
and rainbow trout
(except one strain) were far more heavily affected by M. cerebralis after two hours of exposure than other species were, while bull trout
, Chinook salmon
, brown trout
, and arctic grayling
were least severely affected. While brown trout may harbor the parasite, they typically do not show any symptoms, and this species may have been M. cerebralis original host. This lack of symptoms in brown trout meant that the parasite was not discovered until after nonnative rainbow trout were introduced in Europe. The susceptibility of various salmonids is listed in Salmonid susceptibility to whirling disease
.
Moderate or heavy clinical infection of fish with whirling disease can be presumptively diagnosed on the basis of changes in behavior and appearance about 35 to 80 days after initial infection, though "injury or deficiency in dietary tryptophan
and ascorbic acid
can evoke similar signs," so conclusive diagnosis may require finding myxospores in the fish's cartilage. In heavy infections, examining cartilage microscopically may be all that is needed to find spores. In less severe infections the most common test involves digestion of the cranial cartilage with the protease
s pepsin
and trypsin
(the pepsin-trypsin digest—PTD) before looking for spores. The head and other tissues can be further examined using histopathology
to confirm that the location and morphology of the spores matches what is known for M. cerebralis. Serological identification of spores in tissue sections using an antibody
raised against the spores is also possible. Parasite identity can also be confirmed using the polymerase chain reaction
to amplify the 415 base pair 18S rRNA gene
from M. cerebralis. Fish should be screened at the life stage most susceptible to the parasite. Routine screening using these techniques is carried out in countries where the parasite occurs and in countries like Australia and Canada that are not known to have the parasite but where its introduction could threaten local fish.
Although originally a mild pathogen of Salmo trutta
in central Europe and other salmonids in north east Asia, the spread of the Rainbow trout
(Oncorhynchus mykiss) has greatly increased the impact of this parasite. Having no innate immunity to M. cerebralis, rainbow trout are particularly susceptible, and can release so many spores that even more resistant species in the same area, like S. trutta, can become overloaded with parasites and incur 80%–90% mortalities. Where M. cerebralis has become well-established, it has caused decline or even elimination of whole cohorts of fish.
. Rainbow trout
, the most susceptible species to this parasite, are not native to Europe; successfully reproducing feral
populations are rare, so there are few wild rainbow trout that are young enough to be susceptible to infection. On the other hand, they are widely reared for restocking sport-fishing waters and for aquaculture
, where this parasite has its greatest impact. Hatching and rearing methods designed to prevent infection of Rainbow trout fry have proved successful in Europe. These techniques include hatching eggs in spore-free water and rearing fry to the "ossification" stage in tanks or raceways. These methods give particular attention to the quality of water sources in order to guard against spore introduction during water exchanges. Fry are moved to earthen ponds only when they are considered to be clinically resistant to the parasite, after skeletal ossification occurs.
restriction placed on export
s of salmon products to Australia.
]
M. cerebralis was first recorded in North America in 1956 in Pennsylvania
, having been introduced via infected trout imported from Europe, and has spread steadily south and westwards. Until the 1990s, whirling disease was considered a manageable problem affecting rainbow trout in hatcheries. However, it has recently become established in natural waters of the Rocky Mountain states (Colorado
, Wyoming
, Utah
, Montana
, Idaho
, New Mexico
) where it is causing heavy mortalities in several sportfishing rivers. Some streams in the western United States have lost 90% of their trout. In addition, whirling disease threatens recreational fishing, which is important for the tourism industry, a key component of the economies of some U.S. western states. For example, "the Montana
Whirling Disease Task Force estimated that trout fishing generated US $300,000,000 in recreational expenditures in Montana alone". Making matters worse, some of the fishes that M. cerebralis infects (bull trout
, cutthroat trout
, and steelhead) are already threatened or endangered
, and the parasite could worsen their already precarious situations. For reasons that are poorly understood, but probably have to do with environmental conditions, the impact on infected fish has been greatest in Colorado and Montana and least in California
, Michigan
, and New York.
s, and trout mucous were ineffective, as were anesthetized or dead fish. If spores could be disarmed, they would be unable to infect fish, but it is unclear whether any of the methods that worked in the laboratory could be employed in the wild.
Some strains of fish are more resistant than others, even within species; using resistant strains may help reduce the incidence and severity of whirling disease in aquaculture. There is also some circumstantial evidence that fish populations can develop resistance to the disease over time. Additionally, aquaculturists
may avoid M. cerebralis infections by not using earthen ponds for raising young fish; this keeps them away from possibly infected tubificids and makes it easier to eliminate spores and oligochaetes through filtration, chlorination, and ultraviolet bombardment. To minimise tubificid population techniques include periodic disinfection of the hatchery or aquaculture ponds and the rearing of small trout indoors in pathogen-free water. Smooth-faced concrete or plastic-lined raceways that are kept clean and free of contaminated water keep aquaculture facilities free of the disease.
Lastly, some drugs such as furazolidone
, furoxone, benomyl
, fumagillin
, proguanil
and clamoxyquine
have been shown to impede spore development, which reduces infection rates. For example, one study showed that feeding Fumagillin to Oncorhynchus mykiss reduced the number of infected fish from between 73% and 100% to between 10% and 20%. Unfortunately, this treatment is considered unsuitable for wild trout populations, and no drug treatment has ever been shown to be effective in the studies required for United States Food and Drug Administration
approval.
Recreational and sports fishers can help to prevent the spread of the parasite in a number of ways. Cleaning fishing equipment between fishing trips and never transporting fish from one body of water to another should protect against cross contamination of waterways. Spores are particularly persistent in felt soled wading shoes, which can be treated with 10% chlorine bleach
and water for at least 15 minutes and then rinsed thoroughly. Fish bone
s or entrails should never be disposed of in any body of water, since spores from the carcass will be released into the waterway. Salmon and trout should not be used as bait
.
Myxosporea
The Myxosporea are a class of microscopic parasites, belonging to the Myxozoa. They have a complex life cycle which comprises vegetative forms in two hosts, an aquatic invertebrate and an ectothermic vertebrate, usually a fish. Each host releases a different type of spore...
n parasite of salmonid
Salmonidae
Salmonidae is a family of ray-finned fish, the only living family currently placed in the order Salmoniformes. It includes salmon, trout, chars, freshwater whitefishes and graylings...
s (salmon
Salmon
Salmon is the common name for several species of fish in the family Salmonidae. Several other fish in the same family are called trout; the difference is often said to be that salmon migrate and trout are resident, but this distinction does not strictly hold true...
, trout
Trout
Trout is the name for a number of species of freshwater and saltwater fish belonging to the Salmoninae subfamily of the family Salmonidae. Salmon belong to the same family as trout. Most salmon species spend almost all their lives in salt water...
, and their allies) that causes whirling disease in farmed
Fish farming
Fish farming is the principal form of aquaculture, while other methods may fall under mariculture. Fish farming involves raising fish commercially in tanks or enclosures, usually for food. A facility that releases young fish into the wild for recreational fishing or to supplement a species'...
salmon and trout and also in wild fish populations. It was first described in rainbow trout
Rainbow trout
The rainbow trout is a species of salmonid native to tributaries of the Pacific Ocean in Asia and North America. The steelhead is a sea run rainbow trout usually returning to freshwater to spawn after 2 to 3 years at sea. In other words, rainbow trout and steelhead trout are the same species....
in Germany a century ago, but its range has spread and it has appeared in most of Europe (including Russia), the United States, South Africa and other countries. In the 1980s, it was discovered that M. cerebralis needs to infect a tubificid
Tubificidae
The Naididae are a family of clitellate oligochaete worms like the sludge worm, Tubifex tubifex. They are key components of the benthic communities of many freshwater and marine ecosystems....
oligochaete (a kind of segmented worm) to complete its life-cycle. The parasite infects its hosts with its cells after piercing them with polar filament
Polar filament
A polar filament is a structure found in the polar capsule of myxosporean organisms, and is analogous to the "penetrant" structure found in cnidocytes....
s ejected from nematocyst-like capsules.
Whirling disease afflicts juvenile fish (fingerlings and fry) and causes skeletal
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
deformation and neurological damage. Fish "whirl" forward in an awkward corkscrew-like pattern instead of swimming normally, find feeding difficult, and are more vulnerable to predators. The mortality rate is high for fingerlings, up to 90% of infected populations, and those that do survive are deformed by the parasite residing in their cartilage
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
and bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
. They act as a reservoir for the parasite, which is released into water following the fish's death. M. cerebralis is one of the most economically important myxozoans in fish as well as one of the most pathogenic. It was the first myxosporean whose pathology and symptoms were described scientifically. The parasite is not transmissible to humans.
Taxonomy
The taxonomyTaxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
and naming of both M. cerebralis and of myxozoans in general have complicated histories. It was originally thought that this parasite infected fish brains (hence the specific epithet cerebralis), however it quickly became apparent that while it can be found in the nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
, it primarily infects cartilage
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
and skeletal tissue
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
. Attempts to change the name to Myxobolus chondrophagus, which would more accurately describe the organism, failed because of nomenclature rules
Binomial nomenclature
Binomial nomenclature is a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin grammatical forms, although they can be based on words from other languages...
. Later, it became apparent that organisms previously called Triactinomyxon dubium and T. gyrosalmo (class
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
Actinosporea) were in fact triactinomyxon stages of M. cerebralis, whose life cycle was expanded to include the triactinomyxon stage. Similarly, other actinosporeans were folded into the life cycles of various myxosporeans.
Today, the myxozoans, previously thought to be multicellular protozoans are considered animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s by many scientists, though their status has not officially changed. Recent molecular studies suggest that they are related to Bilateria
Bilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
or Cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
, with Cnidaria being closer morphologically because both groups have extrusive filaments, but with Bilateria being somewhat closer in some genetic studies.
Morphology
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
to relatively large spores, not all of which have been studied in detail.
Triactinomyxon stage
The stages that infect fish, called triactinomyxon spores, are made of a single style that is about 150 micrometersMicrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
(µm) long and three processes or "tails" that are each about 200 micrometers long. A sporoplasm packet at the end of the style contains 64 germ cells surrounded by a cellular envelope. There are also three polar capsule
Polar capsule
Polar capsules are structures found in the valves of Myxosporean parasites, which contain the polar filament. The polar capsule is constructed of a proteinaceous and a polysaccharide layer, both layers of which continue into the polar filament....
s, each of which contains a coiled polar filament
Polar filament
A polar filament is a structure found in the polar capsule of myxosporean organisms, and is analogous to the "penetrant" structure found in cnidocytes....
between 170 and 180 µm long. Polar filaments in both this stage and in the myxospore stage (see picture above) rapidly shoot into the body of the host, creating an opening through which the sporoplasm can enter.
Sporoplasm stage
Upon contact with fish hosts and firing of the polar capsules, the sporoplasm contained within the central style of the triactinomyxon migrates into the epithelium or gut lining. Firstly, this sporoplasm undergoes mitosisMitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
to produce more amoeboid
Amoeboid
Amoeboids are single-celled life-forms characterized by an irregular shape."Amoeboid" and "amœba" are often used interchangeably even by biologists, and especially refer to a creature moving by using pseudopodia. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to...
cells, which migrate into deeper tissue layers, in order to reach the cerebral cartilage.
Myxosporean stage
Myxospores, which develop from sporogonic cell stages inside fish hosts, are lenticular. They have a diameter of about 10 micrometers and are made of six cells. Two of these cells form polar capsules, two merge to form a binucleate sporoplasm, and two form protective valves. Myxospores are infective to oligochaetes, and are found among the remains of digested fish cartilage. They are often difficult to distinguish from related species because of morphological similarities across generaGenus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
. Though M. cerebralis is the only myxosporean ever found in salmonid cartilage, other visually similar species may be present in the skin, nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
, or muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
.
Life cycle
Tubifex tubifex
Tubifex tubifex, also called the sludge worm, or sewage worm, is a species of tubificid segmented worm that inhabits the sediments of lakes and rivers on several continents...
, though what scientists currently call T. tubifex may in fact be more than one species. First, myxospores are ingested by tubificid worms. In the gut
Gut (zoology)
In zoology, the gut, also known as the alimentary canal or alimentary tract, is a tube by which bilaterian animals transfer food to the digestion organs. In large bilaterians the gut generally also has an exit, the anus, by which the animal disposes of solid wastes...
lumen
Lumen (anatomy)
A lumen in biology is the inside space of a tubular structure, such as an artery or intestine...
of the worm, the spores extrude their polar capsule
Polar capsule
Polar capsules are structures found in the valves of Myxosporean parasites, which contain the polar filament. The polar capsule is constructed of a proteinaceous and a polysaccharide layer, both layers of which continue into the polar filament....
s and attach to the gut epithelium
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
by polar filament
Polar filament
A polar filament is a structure found in the polar capsule of myxosporean organisms, and is analogous to the "penetrant" structure found in cnidocytes....
s. The shell valves then open along the suture line and the binucleate germ cell penetrates between the intestinal epithelial cells of the worm. This cell multiplies, producing many amoeboid
Amoeboid
Amoeboids are single-celled life-forms characterized by an irregular shape."Amoeboid" and "amœba" are often used interchangeably even by biologists, and especially refer to a creature moving by using pseudopodia. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to...
cells by an asexual cell fission process called merogony. As a result of the multiplication process, the intercellular space of the epithelial cells in more than 10 neighbouring worm segments may become infected.
Around 60–90 days post-infection, sexual cell stages of the parasite undergo sporogenesis
Sporogenesis
Sporogenesis is the production of spores in biology. The term is also used to refer to the process of reproduction via spores. Reproductive spores are formed in many eukaryotic organisms, such as plants, algae and fungi, during their normal reproductive life cycle...
, and develop into pansporocysts, each of which contains eight triactinomyxon-stage spores. These spores are released from the oligochaete anus into the water. Alternatively, a fish can become infected by eating an infected oligochaete. Infected tubificids can release triactinomyxons for at least 1 year. The triactinomyxon spores swim through the water to infect a salmonid through the skin. Penetration of the fish by these spores takes only a few seconds. Within five minutes, a sac of germ cells called a sporoplasm has entered the fish epidermis, and within a few hours, the sporoplasm splits into individual cells that will spread through the fish.
Within the fish, there are both intracellular and extracellular stages that reproduce in its cartilage by asexual endogeny, meaning that new cells grow from within old cells. The final stage within fish is the myxospore, which is formed by sporogony. They are released into the environment when the fish decomposes or is eaten. Some recent research indicates that some fish may expel viable myxospores while still alive.
Myxospores are extremely tough: "it was shown that Myxobolus cerebralis spores can tolerate freezing at −20°C for at least 3 months, aging in mud at 13°C for at least 5 months, and passage through the guts of northern pike
Northern Pike
The northern pike , is a species of carnivorous fish of the genus Esox...
Esox lucius or mallard
Mallard
The Mallard , or Wild Duck , is a dabbling duck which breeds throughout the temperate and subtropical Americas, Europe, Asia, and North Africa, and has been introduced to New Zealand and Australia....
s Anas platyrhynchos without loss of infectivity" to worms. Triactinomyxons are much shorter lived, surviving 34 days or less, depending on temperature.
Pathology
Salmo
Salmo is a genus of fish in the salmon family that includes the familiar species Atlantic salmon and brown trout . The natural distribution of the genus is chiefly European. Only the range of the Atlantic salmon extends to northern North America...
; 4 species of "Pacific" salmonids, Oncorhynchus
Oncorhynchus
Oncorhynchus is a genus of fish in the family Salmonidae; it contains the Pacific salmons and Pacific trouts. The name of the genus is derived from the Greek onkos and rynchos , in reference to the hooked jaws of males in the mating season .-Range:Salmon and trout with ranges generally in waters...
; 4 species of Char, Salvelinus
Salvelinus
Salvelinus is a genus of salmonid fish often called char or charr; some species are called "trout". Salvelinus is a member of the Salmoninae subfamily of the Salmonidae family. Charr may be identified by light cream pink or red spots over a darker body. Scales tend to be small, with 115-200 along...
; the Grayling, Thymallus thymallus; and the Huchen, Hucho hucho. M. cerebralis causes damage to its fish hosts through attachment of triactinomyxon spores and the migrations of various stages through tissues and along nerves, as well as by digesting cartilage. The fish's tail may darken, but aside from lesions on cartilage, internal organs generally appear healthy. Other symptoms include skeletal deformities and "whirling" behavior (tail-chasing) in young fish, which was thought to have been caused by a loss of equilibrium, but is actually caused by damage to the spinal cord and lower brain stem. Experiments have shown that fish can kill Myxobolus in their skin (possibly using antibodies), but that the fish do not attack the parasites once they have migrated to the central nervous system. This response varies from species to species.
In T. tubifex, the release of triactinomyxon spores from the intestinal
Intestine
In human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...
wall damages the worm’s mucosa; this may happen thousands of times in a single worm, and it is believed that this can impair nutrient absorption. Also, infected worms have lower body mass and may be discolored. Spores are released from the worm almost exclusively when the temperature is between 10°C and 15°C, so fish in warmer or cooler waters are less likely to be infected, and infection rates vary seasonally.
Susceptibility
Fish size, age, concentration of triactinomyxon spores, and water temperature all affect infection rates in fish, as does the species of the fish in question. The disease has the biggest impact on fish less than five months old because their skeleton has not ossifiedOssification
Ossification is the process of laying down new bone material by cells called osteoblasts. It is synonymous with bone tissue formation...
. This makes young fish more susceptible to deformities and provides M. cerebralis more cartilage on which to feed. In one study of seven species of many strains, brook trout
Brook trout
The brook trout, Salvelinus fontinalis, is a species of fish in the salmon family of order Salmoniformes. In many parts of its range, it is known as the speckled trout or squaretail. A potamodromous population in Lake Superior are known as coaster trout or, simply, as coasters...
and rainbow trout
Rainbow trout
The rainbow trout is a species of salmonid native to tributaries of the Pacific Ocean in Asia and North America. The steelhead is a sea run rainbow trout usually returning to freshwater to spawn after 2 to 3 years at sea. In other words, rainbow trout and steelhead trout are the same species....
(except one strain) were far more heavily affected by M. cerebralis after two hours of exposure than other species were, while bull trout
Bull trout
The bull trout, Salvelinus confluentus, is a char of the family Salmonidae native to northwestern North America. Historically, S. confluentus has been known as the "Dolly Varden" , but was re-classified as a separate species in 1980. Bull trout are listed as a threatened species under the U.S....
, Chinook salmon
Chinook salmon
The Chinook salmon, Oncorhynchus tshawytscha, is the largest species in the pacific salmon family. Other commonly used names for the species include King salmon, Quinnat salmon, Spring salmon and Tyee salmon...
, brown trout
Brown trout
The brown trout and the sea trout are fish of the same species....
, and arctic grayling
Arctic grayling
Arctic grayling is a species of freshwater fish in the salmon family of order Salmoniformes. It comprises five subspecies native to the Nearctic and Palearctic ecozones. T. a. arcticus is widespread throughout the Arctic and Pacific drainages in Canada, Alaska, and Siberia, as well as the upper...
were least severely affected. While brown trout may harbor the parasite, they typically do not show any symptoms, and this species may have been M. cerebralis original host. This lack of symptoms in brown trout meant that the parasite was not discovered until after nonnative rainbow trout were introduced in Europe. The susceptibility of various salmonids is listed in Salmonid susceptibility to whirling disease
Salmonid susceptibility to whirling disease
This page tabulates susceptibility of various salmonids to whirling disease.-Reference:MacConnell, E., and E. R. Vincent. 2002. Review: The effects of Myxobolus cerebralis on the salmonid host. Pages 95-107 in J. L. Bartholomew and J. C. Wilson, editors. Whirling disease: Reviews and current...
.
Diagnosis
Tryptophan
Tryptophan is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG...
and ascorbic acid
Ascorbic acid
Ascorbic acid is a naturally occurring organic compound with antioxidant properties. It is a white solid, but impure samples can appear yellowish. It dissolves well in water to give mildly acidic solutions. Ascorbic acid is one form of vitamin C. The name is derived from a- and scorbutus , the...
can evoke similar signs," so conclusive diagnosis may require finding myxospores in the fish's cartilage. In heavy infections, examining cartilage microscopically may be all that is needed to find spores. In less severe infections the most common test involves digestion of the cranial cartilage with the protease
Protease
A protease is any enzyme that conducts proteolysis, that is, begins protein catabolism by hydrolysis of the peptide bonds that link amino acids together in the polypeptide chain forming the protein....
s pepsin
Pepsin
Pepsin is an enzyme whose precursor form is released by the chief cells in the stomach and that degrades food proteins into peptides. It was discovered in 1836 by Theodor Schwann who also coined its name from the Greek word pepsis, meaning digestion...
and trypsin
Trypsin
Trypsin is a serine protease found in the digestive system of many vertebrates, where it hydrolyses proteins. Trypsin is produced in the pancreas as the inactive proenzyme trypsinogen. Trypsin cleaves peptide chains mainly at the carboxyl side of the amino acids lysine or arginine, except when...
(the pepsin-trypsin digest—PTD) before looking for spores. The head and other tissues can be further examined using histopathology
Histopathology
Histopathology refers to the microscopic examination of tissue in order to study the manifestations of disease...
to confirm that the location and morphology of the spores matches what is known for M. cerebralis. Serological identification of spores in tissue sections using an antibody
Antibody
An antibody, also known as an immunoglobulin, is a large Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, termed an antigen...
raised against the spores is also possible. Parasite identity can also be confirmed using the polymerase chain reaction
Polymerase chain reaction
The polymerase chain reaction is a scientific technique in molecular biology to amplify a single or a few copies of a piece of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence....
to amplify the 415 base pair 18S rRNA gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
from M. cerebralis. Fish should be screened at the life stage most susceptible to the parasite. Routine screening using these techniques is carried out in countries where the parasite occurs and in countries like Australia and Canada that are not known to have the parasite but where its introduction could threaten local fish.
Impact
Brown trout
The brown trout and the sea trout are fish of the same species....
in central Europe and other salmonids in north east Asia, the spread of the Rainbow trout
Rainbow trout
The rainbow trout is a species of salmonid native to tributaries of the Pacific Ocean in Asia and North America. The steelhead is a sea run rainbow trout usually returning to freshwater to spawn after 2 to 3 years at sea. In other words, rainbow trout and steelhead trout are the same species....
(Oncorhynchus mykiss) has greatly increased the impact of this parasite. Having no innate immunity to M. cerebralis, rainbow trout are particularly susceptible, and can release so many spores that even more resistant species in the same area, like S. trutta, can become overloaded with parasites and incur 80%–90% mortalities. Where M. cerebralis has become well-established, it has caused decline or even elimination of whole cohorts of fish.
Impact in Europe
The impact of M. cerebralis in Europe is somewhat lessened by the fact that the species is endemic to this region, giving native fish stocks a degree of immunityImmunity (medical)
Immunity is a biological term that describes a state of having sufficient biological defenses to avoid infection, disease, or other unwanted biological invasion. Immunity involves both specific and non-specific components. The non-specific components act either as barriers or as eliminators of wide...
. Rainbow trout
Rainbow trout
The rainbow trout is a species of salmonid native to tributaries of the Pacific Ocean in Asia and North America. The steelhead is a sea run rainbow trout usually returning to freshwater to spawn after 2 to 3 years at sea. In other words, rainbow trout and steelhead trout are the same species....
, the most susceptible species to this parasite, are not native to Europe; successfully reproducing feral
Feral
A feral organism is one that has changed from being domesticated to being wild or untamed. In the case of plants it is a movement from cultivated to uncultivated or controlled to volunteer. The introduction of feral animals or plants to their non-native regions, like any introduced species, may...
populations are rare, so there are few wild rainbow trout that are young enough to be susceptible to infection. On the other hand, they are widely reared for restocking sport-fishing waters and for aquaculture
Aquaculture
Aquaculture, also known as aquafarming, is the farming of aquatic organisms such as fish, crustaceans, molluscs and aquatic plants. Aquaculture involves cultivating freshwater and saltwater populations under controlled conditions, and can be contrasted with commercial fishing, which is the...
, where this parasite has its greatest impact. Hatching and rearing methods designed to prevent infection of Rainbow trout fry have proved successful in Europe. These techniques include hatching eggs in spore-free water and rearing fry to the "ossification" stage in tanks or raceways. These methods give particular attention to the quality of water sources in order to guard against spore introduction during water exchanges. Fry are moved to earthen ponds only when they are considered to be clinically resistant to the parasite, after skeletal ossification occurs.
Impact in New Zealand
M. cerebralis was first found in New Zealand in 1971. The parasite has only been found in rivers in the South Island, away from the most important aquaculture sites. Additionally, salmonid species commercially aquacultured in New Zealand have low susceptibility to whirling disease, and the parasite has also not been shown to affect native salmonids. An important indirect effect of the parasites presence is quarantineQuarantine
Quarantine is compulsory isolation, typically to contain the spread of something considered dangerous, often but not always disease. The word comes from the Italian quarantena, meaning forty-day period....
restriction placed on export
Export
The term export is derived from the conceptual meaning as to ship the goods and services out of the port of a country. The seller of such goods and services is referred to as an "exporter" who is based in the country of export whereas the overseas based buyer is referred to as an "importer"...
s of salmon products to Australia.
Impact in the United States
M. cerebralis was first recorded in North America in 1956 in Pennsylvania
Pennsylvania
The Commonwealth of Pennsylvania is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to...
, having been introduced via infected trout imported from Europe, and has spread steadily south and westwards. Until the 1990s, whirling disease was considered a manageable problem affecting rainbow trout in hatcheries. However, it has recently become established in natural waters of the Rocky Mountain states (Colorado
Colorado
Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
, Wyoming
Wyoming
Wyoming is a state in the mountain region of the Western United States. The western two thirds of the state is covered mostly with the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High...
, Utah
Utah
Utah is a state in the Western United States. It was the 45th state to join the Union, on January 4, 1896. Approximately 80% of Utah's 2,763,885 people live along the Wasatch Front, centering on Salt Lake City. This leaves vast expanses of the state nearly uninhabited, making the population the...
, Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
, Idaho
Idaho
Idaho is a state in the Rocky Mountain area of the United States. The state's largest city and capital is Boise. Residents are called "Idahoans". Idaho was admitted to the Union on July 3, 1890, as the 43rd state....
, New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...
) where it is causing heavy mortalities in several sportfishing rivers. Some streams in the western United States have lost 90% of their trout. In addition, whirling disease threatens recreational fishing, which is important for the tourism industry, a key component of the economies of some U.S. western states. For example, "the Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
Whirling Disease Task Force estimated that trout fishing generated US $300,000,000 in recreational expenditures in Montana alone". Making matters worse, some of the fishes that M. cerebralis infects (bull trout
Bull trout
The bull trout, Salvelinus confluentus, is a char of the family Salmonidae native to northwestern North America. Historically, S. confluentus has been known as the "Dolly Varden" , but was re-classified as a separate species in 1980. Bull trout are listed as a threatened species under the U.S....
, cutthroat trout
Cutthroat trout
The cutthroat trout is a species of freshwater fish in the salmon family of order Salmoniformes. It is one of the many fish species colloquially known as trout...
, and steelhead) are already threatened or endangered
Endangered species
An endangered species is a population of organisms which is at risk of becoming extinct because it is either few in numbers, or threatened by changing environmental or predation parameters...
, and the parasite could worsen their already precarious situations. For reasons that are poorly understood, but probably have to do with environmental conditions, the impact on infected fish has been greatest in Colorado and Montana and least in California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
, Michigan
Michigan
Michigan is a U.S. state located in the Great Lakes Region of the United States of America. The name Michigan is the French form of the Ojibwa word mishigamaa, meaning "large water" or "large lake"....
, and New York.
Prevention and control
Some biologists have attempted to disarm triactinomyxon spores by making them fire prematurely. In the laboratory, only extreme acidity or basicity, moderate to high concentrations of salts, or electrical current caused premature filament discharge; neurochemicals, cnidarian chemosensitizerChemosensitizer
A chemosensitizer is a drug that makes tumor cells more sensitive to the effects of chemotherapy.- External links :* entry in the public domain NCI Dictionary of Cancer Terms...
s, and trout mucous were ineffective, as were anesthetized or dead fish. If spores could be disarmed, they would be unable to infect fish, but it is unclear whether any of the methods that worked in the laboratory could be employed in the wild.
Some strains of fish are more resistant than others, even within species; using resistant strains may help reduce the incidence and severity of whirling disease in aquaculture. There is also some circumstantial evidence that fish populations can develop resistance to the disease over time. Additionally, aquaculturists
Aquaculture
Aquaculture, also known as aquafarming, is the farming of aquatic organisms such as fish, crustaceans, molluscs and aquatic plants. Aquaculture involves cultivating freshwater and saltwater populations under controlled conditions, and can be contrasted with commercial fishing, which is the...
may avoid M. cerebralis infections by not using earthen ponds for raising young fish; this keeps them away from possibly infected tubificids and makes it easier to eliminate spores and oligochaetes through filtration, chlorination, and ultraviolet bombardment. To minimise tubificid population techniques include periodic disinfection of the hatchery or aquaculture ponds and the rearing of small trout indoors in pathogen-free water. Smooth-faced concrete or plastic-lined raceways that are kept clean and free of contaminated water keep aquaculture facilities free of the disease.
Lastly, some drugs such as furazolidone
Furazolidone
Furazolidone is a nitrofuran antibacterial. It is marketed by Roberts Laboratories under the brand name Furoxone and by GlaxoSmithKline as Dependal-M.-Uses:It is used to treat diarrhoea and enteritis caused by bacteria or protozoan infections....
, furoxone, benomyl
Benomyl
Benomyl is a fungicide which was introduced in 1968 by DuPont. It is a systemic benzimidazole fungicide that is selectively toxic to microorganisms and to invertebrates, especially earthworms. Benomyl binds to microtubules, interfering with cell functions such as meiosis and intracellular...
, fumagillin
Fumagillin
Fumagillin is a complex biomolecule and used as an antimicrobial agent. It was isolated in 1949 from the microbial organism Aspergillus fumigatus.-In animals:...
, proguanil
Proguanil
Proguanil is a prophylactic antimalarial drug.Proguanil is effective against sporozoites.Proguanil hydrochloride is marketed as Paludrine by AstraZeneca.-Mechanism:...
and clamoxyquine
Clamoxyquine
Clamoxyquine or clamoxyquin , as the pamoate or hydrochloride salt, is an antiamebic and antidiarrheal drug.It has been used as a veterinary medicine to treat salmonids for infection with the myxozoan parasite, Myxobolus cerebralis....
have been shown to impede spore development, which reduces infection rates. For example, one study showed that feeding Fumagillin to Oncorhynchus mykiss reduced the number of infected fish from between 73% and 100% to between 10% and 20%. Unfortunately, this treatment is considered unsuitable for wild trout populations, and no drug treatment has ever been shown to be effective in the studies required for United States Food and Drug Administration
Food and Drug Administration
The Food and Drug Administration is an agency of the United States Department of Health and Human Services, one of the United States federal executive departments...
approval.
Recreational and sports fishers can help to prevent the spread of the parasite in a number of ways. Cleaning fishing equipment between fishing trips and never transporting fish from one body of water to another should protect against cross contamination of waterways. Spores are particularly persistent in felt soled wading shoes, which can be treated with 10% chlorine bleach
Bleach
Bleach refers to a number of chemicals that remove color, whiten, or disinfect, often via oxidation. Common chemical bleaches include household chlorine bleach , lye, oxygen bleach , and bleaching powder...
and water for at least 15 minutes and then rinsed thoroughly. Fish bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
s or entrails should never be disposed of in any body of water, since spores from the carcass will be released into the waterway. Salmon and trout should not be used as bait
Bait (luring substance)
Bait is any substance used to attract prey, e.g. in a mousetrap.-In Australia:Baiting in Australia refers to specific campaigns to control foxes, wild dogs and dingos by poisoning in areas where they are a problem...
.
See also
- Ceratomyxa shastaCeratomyxa shastaCeratomyxa shasta is a myxosporean parasite that infects salmonid fish on the Pacific coast of North America. It was first observed at the Crystal Lake Hatchery, Shasta County, California, and has now been reported from Idaho, Oregon, Washington, British Columbia and Alaska .-Life history:In...
– another pathogenic myxosporean parasite of salmonids. - Infectious salmon anemiaInfectious salmon anemiaInfectious salmon anemia or anaemia is a viral disease of Atlantic Salmon that affects fish farms in Canada, Norway, Scotland and Chile, causing severe losses to infected farms...
(ISA) – a viral infection of Atlantic Salmon. - Kudoa thyrsitesKudoa thyrsitesKudoa thrysites is a myxosporean parasite of marine fishes. It has a worldwide distribution, and infects a wide range of host species. This parasite is responsible for causing economic losses to the fisheries sector, by causing post-mortem "myoliquefaction", a softening of the flesh to such an...
– a myxosporean parasite of many species, which causes fish tissues to liquefy on death. - Tetracapsuloides bryosalmonaeTetracapsuloides bryosalmonaeTetracapsuloides bryosalmonae is a myxozoan parasite of salmonid fishes, which causes Proliferative kidney disease , one of the most serious parasitic diseases of salmonid populations in Europe and North America, which causes losses of up to 90% in infected populations.-Taxonomy:Until the late...
– the enigmatic myxosporean which causes "Proliferative Kidney Disease" in salmonids. - Salmonid susceptibility to whirling diseaseSalmonid susceptibility to whirling diseaseThis page tabulates susceptibility of various salmonids to whirling disease.-Reference:MacConnell, E., and E. R. Vincent. 2002. Review: The effects of Myxobolus cerebralis on the salmonid host. Pages 95-107 in J. L. Bartholomew and J. C. Wilson, editors. Whirling disease: Reviews and current...
External links
- Report of the World Trade Organization on Australian restrictions on salmon imports
- The Whirling Disease Foundation
- The Whirling Disease Initiative
- Whirling Disease Maps & Data
- Species Profile- Whirling Disease (Myxobolus cerebralis), National Invasive Species Information Center, United States National Agricultural LibraryUnited States National Agricultural LibraryThe United States National Agricultural Library is one of the world's largest agricultural research libraries, and serves as a National Library of the United States and as the library of the United States Department of Agriculture...
. Lists general information and resources for Whirling Disease.