Historic emergence, impact and current status of shrimp pathogens in the Americas.

Shrimp farming in the Americas began to develop in the late 1970s into a significant industry. In its first decade of development, the technology used was simple and postlarvae (PLs) produced from wild adults and wild caught PLs were used for stocking farms. Prior to 1990, there were no World Animal Health Organization (OIE) listed diseases, but that changed rapidly commensurate with the phenomenal growth of the global shrimp farming industry. There was relatively little international trade of live or frozen commodity shrimp between Asia and the Americas in those early years, and with a few exceptions, most of the diseases known before 1980 were due to disease agents that were opportunistic or part of the shrimps' local environment. Tetrahedral baculovirosis, caused by Baculovirus penaei (BP), and necrotizing hepatopancreatitis (NHP) and its bacterial agent Hepatobacterium penaei, were among the "American" diseases that eventually became OIE listed and have not become established outside of the Americas. As the industry grew after 1980, a number of new diseases that soon became OIE listed, emerged in the Americas or were introduced from Asia. Spherical baculovirus, caused by MBV, although discovered in the Americas in imported live Penaeus monodon, was subsequently found to be common in wild and farmed Asian, Australian and African penaeids. Infectious hypodermal and hematopoietic necrosis virus (IHHNV) was introduced from the Philippines in the mid 1970s with live P. monodon and was eventually found throughout the Americas and subsequently in much of the shrimp farming industry in the eastern hemisphere. Taura syndrome emerged in Penaeus vannamei farms in 1991-1992 in Ecuador and was transferred to SE Asia with live shrimp by 1999 where it also caused severe losses. White Spot Disease (WSD) caused by White spot syndrome virus (WSSV) emerged in East Asia in ∼1992, and spread throughout most of the Asian shrimp farming industry by 1994. By 1995, WSSV reached the eastern USA via frozen commodity products and it reached the main shrimp farming countries of the Americas located on the Pacific side of the continents by the same mechanism in 1999. As is the case in Asia, WSD is the dominant disease problem of farmed shrimp in the Americas. The most recent disease to emerge in the Americas was infectious myonecrosis caused by IMN virus. As had happened before, within 3years of its discovery, the disease had been transferred to SE Asia with live P. vannamei, and because of its impact on the industry and potential for further spread in was listed by the OIE in 2005. Despite the huge negative impact of disease on the shrimp farming industry in the Americas, the industry has continued to grow and mature into a more sustainable industry. In marked contrast to 15-20years ago when PLs produced from wild adults and wild PLs were used to stock farms in the Americas, the industry now relies on domesticated lines of broodstock that have undergone selection for desirable characteristics including disease resistance.

Application of molecular diagnostic methods to penaeid shrimp diseases: advances of the past 10 years for control of viral diseases in farmed shrimp.

The most important diseases of farmed penaeid shrimp have infectious aetiologies. Among these are diseases with viral, rickettsial, bacterial, fungal and parasitic aetiologies. Diagnostic methods for these pathogens include the traditional methods of gross pathology, histopathology, classical microbiology, animal bioassay, antibody-based methods, and molecular methods using DNA probes and DNA amplification. While methods using clinical chemistry and tissue culture are standard methods in veterinary and human diagnostic laboratories, the former has not been routinely applied to the diagnosis of penaeid shrimp diseases and the latter has yet to be developed, despite considerable research and development efforts that have spanned the past 40 years. No continuous shrimp cell lines, or lines from other crustaceans, have been developed. Hence, when molecular methods began to be routinely applied to the diagnosis of infectious diseases in humans and domestic animals in the mid- to late 1980s, the technology was applied to the diagnosis of certain important diseases of penaeid shrimp for which only classical diagnostic methods were previously available. A DNA hybridization assay for the parvovirus IHHNV was the first molecular test developed for a shrimp disease. This was followed within a year by the first PCR test for MBV, an important baculovirus disease of shrimp. Today, shrimp disease diagnostic laboratories routinely use molecular tests for diagnostic and surveillance purposes for most of the important penaeid shrimp diseases.