Myxobolus cerebralis
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Myxobolus cerebralis | |
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Triactinomyxon stage of Myxobolus cerebralis - note the three "tails" | |
Scientific classification ![]() | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Cnidaria |
Class: | Myxosporea |
Order: | Bivalvulida |
Family: | Myxobolidae |
Genus: | Myxobolus |
Species: | M. cerebralis
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Binomial name | |
Myxobolus cerebralis Hofer, 1903
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Synonyms | |
Myxosoma cerebralis |
Myxobolus cerebralis is a
Whirling disease affects
The
Taxonomy
M. cerebralis is one of the 1,350 known myxozoan parasites known to infect fish.[1] Once thought to be a species of Protozoa, taxonomists noticed characteristics that more closely related M. cerebralis to the phylum Cnidaria. These features included cnidocysts, which are tentacles that are used to hold onto and prey upon the host. M. cerebralis has many diverse stages ranging from single cells to relatively large spores, not all of which have been studied in detail. This complex lifecycle involves two different hosts and numerous developmental stages. These stages happen through mitosis, endogeny, plasmotomy, or possibly meiosis. In the first part of its lifecycle, M. cerebralis is attached to its salmonid host externally. They then use their stinging tentacles to infect the host, causing the skeletal tissues and nervous system to become deformed.
Today, the myxozoans, previously thought to be multicellular protozoans, are considered animals by most scientists, though their status has not officially changed. Recent molecular studies suggest they are related to Bilateria or Cnidaria, with Cnidaria being closer morphologically because both groups have extrusive filaments. Bilateria were somewhat closer in some genetic studies, but those were found to have used samples that were contaminated by material from the host organism, and a 2015 study confirms they are cnidarians.[citation needed]
Morphology
M. cerebralis has many diverse stages ranging from single cells to relatively large spores, not all of which have been studied in detail.
Triactinomyxon stage
The stages that infect fish, called
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
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 genera. 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.
Life cycle
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
Around 60–90 days postinfection, sexual cell stages of the parasite undergo
Within the fish, both intracellular and extracellular stages reproduce in its cartilage by asexual
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 mallards Anas platyrhynchos without loss of infectivity" to worms.[citation needed] Triactinomyxons are much shorter-lived, surviving 34 days or less, depending on temperature.[citation needed]
Pathology
Skeletal deformation in a mature brook trout caused by M. cerebralis infection.
M. cerebralis infections have been reported from a wide range of salmonid species: eight species of "Atlantic" salmonids,
In T. tubifex, the release of triactinomyxon spores from the
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 most impact on fish less than five months old because their skeletons have not ossified. 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 only discovered after nonnative rainbow trouts were introduced in Europe.
Diagnosis
The normally uniform trout cartilage is scarred with lesions in which M. cerebralis spores develop, weakening and deforming the connective tissues.
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
Impact
Although originally a mild pathogen of Salmo trutta in central Europe and other salmonids in northeast Asia, the introduction 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, such as 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 because the species is endemic to this region, giving native
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 quarantine restriction placed on exports of salmon products to Australia.
Impact in the United States
M. cerebralis has been reported in nearly two dozen (green) states in the United States, according to the Whirling Disease Initiative 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 trout fishing generated US $300,000,000 in recreational expenditures in Montana alone". Making matters worse, some of the fish species 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.
Impact in Canada
Whirling disease was first confirmed in fish in Johnson Lake in Banff National Park in August, 2016.[2] CFIA Labs confirmed in August and Parks Canada announced the outbreak August 23, 2016. Although it was first discovered in Banff, it is not necessarily where the disease originated and spread. The Government of Alberta is currently sampling and testing fish in 6 different watersheds (Peace River, Athabasca, North Saskatchewan, Red Deer, Bow and Oldman) to see where the disease has spread. Initial sample fish were collected in 2016, and are currently being processed by the Government of Alberta and CFIA labs. Since testing began, it has been detected in the Upper Bow River, and in May 2017 it was confirmed that whirling disease had also been detected in the Oldman River Basin. The declaration does not mean that every susceptible finfish population within the Bow and Oldman River watersheds are infected with the disease.[citation needed]
The parasite was first detected in the adjacent province of British Columbia in January, 2024.[3]
As a result of the new declaration, a domestic movement permit will be required from the CFIA for susceptible species and end uses identified in the Domestic Movement Control Program, the vector Tubifex tubifex, the disease causing agent Myxobolus cerebralis, and/or related things out of the infected and buffer areas of Alberta. Recreational and sport fishing, including fishing led by a professional guide, will not require a CFIA permit.
Prevention and control
Some biologists have attempted to disarm triactinomyxon spores by making them fire prematurely. In the laboratory, only extreme
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 populations, 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
Recreational and sports fishers can help to prevent the spread of the parasite by not transporting fish from one body of water to another, not disposing of fish bones or entrails in any body of water, and ensuring boots and shoes are clean before moving between different bodies of water. Federal, state, provincial, and local regulations on the use of bait should be followed.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/4/44/Deformed_Brook_Trout.jpg/250px-Deformed_Brook_Trout.jpg)
![](http://upload.wikimedia.org/wikipedia/commons/thumb/b/b7/Whirling_disease_pathology.jpg/220px-Whirling_disease_pathology.jpg)
See also
- Ceratomyxa shasta– another pathogenic myxosporean parasite of salmonids
- Infectious salmon anemia(ISA) – a viral infection of Atlantic salmon
- Kudoa thyrsites – a myxosporean parasite of many species, which causes fish tissues to liquefy on death
- Tetracapsuloides bryosalmonae– the enigmatic myxosporean which causes "proliferative kidney disease" in salmonids
- Salmonid susceptibility to whirling disease
Notes
- ^ Hedrick, Ronald (20 November 2022). "Recent Advances with Taxonomy, Life Cycle, and Development of Myxobolus cerebralis in the Fish and Oligochaete Hosts". American Fisheries Society Symposium.
- ^ "First case of whirling disease in Canada" (Press release). Government of Canada. Canadian Food Inspection Agency. 2016-08-25. Retrieved 2024-03-20.
- ^ Ball, David P. (2024-01-19). "Whirling disease confirmed in B.C. for 1st time, sparking fears for fish populations". CBC News.
External links
![](http://upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/30px-Commons-logo.svg.png)
- Report of the World Trade Organization on Australian restrictions on salmon imports
- The Whirling Disease Initiative
- Whirling disease maps and data
- Species Profile- Whirling Disease (Myxobolus cerebralis), National Invasive Species Information Center, United States National Agricultural Library, lists general information and resources for whirling disease
- Whirling Disease - Yellowstone National Park (U.S. National Park Service)
- Whirling Disease - Stop Aquatic Hitchhikers
- Whirling disease - Utah Division of Wildlife Resources
- Colorado Parks & Wildlife - Whirling Disease and Colorado's Trout
- What is Whirling Disease? - North Central Regional Aquaculture Center - Mohamed Faisal - Donald Garling
- Whirling disease | Alberta.ca