World Aquaculture 15 ing IMN outbreaks (Nash et al. 1987). During the outbreak described here, shrimp stocking was reduced (from 25 to 16 individuals/m2), but no sign of recovery was noticed. This suggests that in addition to the persistent mortality that characterizes IMN through the culture cycle, the gregarines and bacterial infection have also contributed to the mortality. Although gregarines were not taxonomically identified in this study, it is likely that the species involved was either in the genus Nematopsis or Cephalobus. These two genera of gregarines have global distribution in penaeid shrimp aquaculture (Lotz and Overstreet 1990). Generally, gregarines are linked to impaired shrimp health and bacterial infections, such as hemocytic enteritis, that cause reduced growth and causes mortality in cultured pacific white shrimp in Ecuador (Jiménez et al. 2002) and Mexico (Fajer–Avila et al. 2005). The high abundance of gregarines and intestinal bacteria might have exacerbated the reaction of the shrimp to the viral agent and enhanced mortality. The sporozoites, gamonts and gametocysts of several species of Nematopsis (N. penaeus, N vannamei, N. marinus) have been found infecting L. vannamei cultured in Ecuador (Jiménez 1992, Lightner 1993, Jiménez et al. 2002), where infections have been mitigated by removing such intermediate carriers as the polychaete Polydora cirrhosa, which were common benthic pond dwellers (Lightner 1993). Recently, food medicated with sodium monensin (Elancobank) and sulfachloropyrazine (Avimix-STk) has been shown to remove and control Nematopsis gametocysts from the intestine of naturally infested cultured Pacific white shrimp (Fajer–Avila et al. 2005). Best management practices such as maintaining a high dissolved-oxygen concentration; keeping stable temperature, pH and salinity levels; and controlling shrimp feeding are important strategies in pond management. Moreover, according to Horowitz and Horowitz (2001), minimization of waste, removal of sludge and organic matter and maintaining optimal water quality, including the reduction of excess ammonia and nitrite, the generation of both specific pathogen free and specific pathogen-resistant shrimp stocks, stimulation of shrimp immune systems with stimulants and enhancement of immnotolerance to viruses by using tolerins are priority issues of biosecurity and shrimp health management that will help the manager avoid the occurrence of various shrimp diseases. Notes 1Department of Environmental Health Sciences, Arnold School of Public Health, University of South Carolina, 800 Sumter Street, Columbia, SC 29208, USA. E-mail: gusadoca@gmail.com 2Center for Coastal Environmental Health and Biomolecular Research (CCEHBR)/ National Oceanic and Atmospheric Administration (NOAA)/National Ocean Service (NOS)/NCCOS; 219 Ft. Johnson Road, Charleston, South Carolina 29412-9110, USA. *Current address: Environmental Toxicology Research Group, School of Resource & Environmental Management, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia V5A 1S6, CANADA. E-mail: jalavasa@sfu.ca 3Graf, Ch., N. Gervais, M.C. Fernandes, and J.C Ayala.2003. Transmissão da Síndrome da Necrose Idiopática Muscular (NIM) em Litopenaeus vannamei. (Technical manuscript unpublished).5p. (Available from the first author). Acknowledgments The author thanks Dr. Victoria Otton for her valuable review and corrections on this contribution. References Anderson, I.G., G. Nash and M. Shariff. 1990. 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