Myxobolus cerebralis

context: en: 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.
qas: question: zh_tw: 是什麼導致了魚群的大量減少？
answers: en: text: M. cerebralis

context: en: 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.
qas: question: zh_tw: M. cerebralis 位於什麼位置？
answers: en: text: rivers in the South Island

context: en: 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 capsules and attach to the gut epithelium by polar filaments. 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 postinfection, 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 a year. The triactinomyxon spores are carried by the water currents, where they can 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, both intracellular and extracellular stages reproduce in its cartilage by asexual endogeny, meaning new cells grow from within old cells.  The final stage within the fish is the creation of the myxospore, which is formed by sporogony.  They are released into the environment when the fish decomposes or is eaten.  Some recent research indicates 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 mallards Anas platyrhynchos without loss of infectivity" to worms.  Triactinomyxons are much shorter-lived, surviving 34 days or less, depending on temperature.
qas: question: zh_tw: 那裡有多少個蠕蟲節
answers: en: text: 10

context: en: 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.
qas: question: zh_tw: 組成粘孢子蟲的六個細胞的作用是什麼？
answers: en: text: Two of these cells form polar capsules, two merge to form a binucleate sporoplasm, and two form protective valves