White tail disease occurs in farmed giant freshwater prawns (Macrobrachium rosenbergii) and is named after the obvious clinical signs in diseased postlarvae. Two viruses, Macrobrachium rosenbergii nodavirus (MrNV) and a very small virus-like particle named XSV (extra small virus), are each found in diseased prawns. The disease was first observed in a hatchery in Guadeloupe Island in French West Indies where abnormal and sudden mortalities had been recorded since 1994. Losses were variable in intensity (5-90% cumulative mortality), depending on the broodstock used to generate the postlarvae. The first gross sign ofthe disease is the presence ofwhitish postlarvae 2-3 days after emergence. The prevalence of opaque and milky postlarvae increases dramatically 1-2 days later and changes are particularly obvious in abdomen (tail). The mortality rate reaches a maximum 5 days after the first observation of gross signs.
In diseased prawns, the most affected tissues are the striated muscles in the abdomen and cephalothorax and the connective tissues of all organs. The cytoplasm of infected cells contains discrete, pale to darkly basophilic inclusions, ranging from <1 to 40 mm in diameter. Muscles exhibit multifocal areas of hyaline necrosis of the fibers, with moderate edema.
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