12 March 2009 Observing external clinical signs of the idiopathic Myonecrosis (IMN) during production of Pacific white shrimp (Litopenaeus vannamei) in Brazil Gustavo Dominguez1 and Juan José Alava2* In Brazil, shrimp aquaculture has been developed in the last decade and shows signs of growth. For example, the output of the shrimp production in northeastern Brazil increased from 40,000 to 60,128 t during the period 2001-2002 and the demand for Pacific white shrimp (Litopenaeus vannamei) postlarvae went from 0.5 to 11.4 billion in 1994 compared with 2002 (Camara et al. 2004). This productive activity is associated with increased demand for land, hatchery stocks and feed. At the same time, shrimp viral diseases have emerged on Brazilian shrimp farms and threaten the country’s outstanding production. The importation of non-native shrimps (L. vannamei and L. stylirostris) during early 1980s introduced several viruses, such as the Infectious Hypodermal and Haematopoietic Necrosis (IHHNV), Taura Syndrome Virus (TSV) and Necrotizing Hepatopancreatitis (NHP) into Brazil (Briggs et al. 2004). Another viral disease that has appeared there and, which is causing severe shrimp mortality, is the Idiopathic Muscle Necrosis, recently renamed Infectious Myonecrosis (IMN; Lightner et al. 2004). Other names used to describe this disease are white muscle disease, muscle necrosis, spontaneous muscle necrosis, muscle opacity, idiopathic myopathy, white syndrome and milky prawn disease (Rigdon and Baxter 1970, Lakshmim et al. 1978, Nash et al. 1987, Tonguthai 1992, Flegel et al.1992). The disease first appeared in a shrimp farm located in the Municipality of Parnaíba, state of Piaui, northeastern Brazil, in September 2002 and it has been identified in other countries where Pacific white shrimp are cultured (Lightner et al. 2004). In Brazil, the economic losses in shrimp production because of the IMN were calculated to be US$20 million in 2003 (Nunes et al. 2004). Infectious myonecrosis has been found elsewhere in the world in other penaeid shrimp species (Penaeus aztecus, P. japonicus and P. monodon; Rigdon and Baxter 1970, Lakshmim et al. 1978, Momoyama and Matsuzato 1987, Flegel et al. 1992), giant freshwater shrimp (Macrobrachium rosenbergii; Nash et al. 1987, Andserson et al.1990), freshwater crayfish (Cherax terminatus; Evans et al. 1999), swamp crayfish (Procambarus clarkii; Lindqvist and Mikkola 1978) and Norway lobsters (Nephrops norvegicus; Stentiford and Neil 2000). Lightner et al. (2004) recently reported that the etiologic pathogen is a spherical RNA-virus 40 nm in diameter. Viral IMN (IMNV) has been cataloged as a new disease for cultured Pacific white shrimp, causing necrosis of the skeletal muscle (Lightner et al. 2004, Tang et al. 2005). This information is similar to the description previously made in black tiger shrimp (Penaeus monodon; Flegel et al. 1992). More recently, bioassay studies have demonstrated that both L. stylirostris and P. monodon are also susceptible to IMNV infection (Tang et al. 2005). Infectious myonecrosis is characterized as a disease with an acute display of gross signs and high mortalities, followed by a chronic phase with persistent low-level mortality, affecting postlarvae, juveniles and subadult cultured stocks of Pacific white shrimp (Lightner et al. 2004). This particular species of shrimp has been found to be the most susceptible to INMV infection when compared to infected L. stylirostris and P. monodon during bioassays (Tang et al. 2005). In this article, we present an original description of a set of several clinical signs found in Pacific white shrimp from ponds on a shrimp farm located in Camocim, a village located 360 km from Fortaleza, which is the capital city of the state of Ceará, northeastern Brazil, as well as notes on the shrimp culture production during the outbreak of this viral disease in 2003 and 2004. Shrimp Production during the Epidemic Before IMN appeared, at the beginning of 2003, shrimp production on the farm was generally considered to range from 2,500-3,000 kg/ha, with a stocking density of about 25-30 individuals/m2. During the period August-December 2003, total shrimp production area was 30.1 ha, divided into five ponds ranging 4.7-8.9 ha, with an average size of 6.0 ha. Routinely, feeding was done four times daily, twice in the morning (0700 and 1030) and in the afternoon (1330 and 1630). In accordance with consumption strategies, feed trays were used to provide food and to monitor feed consumption for feeding rate adjustments and biomass es-
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