30 DECEMBER 2022 • WORLD AQUACULTURE • WWW.WA S .ORG crustaceans also do so (Rusaini et al. 2013). IHHNVwas described for the first time by Don Lightner in 1983 (Lightner et al. 1983a, 1983b) infecting two species of penaeid shrimp (Penaeus vannamei and P. stylirostris) imported to Hawaii. At the time of these publications almost 40 years ago, histopathology was the technique used for detection and description of new pathogens, along with transmission electron microscopy (TEM). So, originally severe lesions in infected tissues of P. stylirostris were described as associated with the presence of this virus, although its precise origins were not known or fromwhich wild population of penaeids it originated. In subsequent years, it was discovered that the virus originated in wild populations of P. monodon (Tang et al. 2003). If sometime or somewhere in a laboratory or culture facility three species of shrimp— P. vannamei, P. stylirostris and P. monodon — shared a common water source or lived together, it is feasible that infection occurred and the virus jumped from one species to another, more susceptible species, such as P. stylirostris. IHHNV does not always produce lesions detectable by histology in P. monodon. The species most sensitive to the virus was P. stylirostris and it is the one in which tissue damage and mortalities were reported (Lightner and Bell 1987). It is interesting to note that, in the description of IHHNV by Lightner et al. (1983b), Table 4 of this paper presents a list of animals coded as 82-14 species P. monodon and the source of the sample is mentioned as Ecuador/ Tahiti. Also, in samples 82-35-2, another P. monodon, whose sample source repeats again Ecuador/Tahiti. Does this mean that an intention to introduce or an introduction of P. monodon in Ecuador had already happened? If this occurred, it is possible to conclude that this virus was introduced to the Americas before it was even discovered, as several years later it was determined that its origin was the Indo-Pacific Ocean. It exists in wild populations of P. monodon in that area and the possible origin of the viral line of Hawaii and the Americas was the Philippines (Tang et al. 2003). This brings us to pay attention to the movement of live shrimp and the risks implied by accidentally introducing a virus still not known or discovered by the scientific community (Flegel 2006). To understand specific research, there is nothing better than reading the original publication, very slowly and paying attention to all the details, including the Discussion section that is the part of a paper where authors discuss the interpretation of their results and the limitations of those. The breakthrough publication that determined and showed the association between IHHNV and Runt Deformity Syndrome (RDS) was authored by Kalagayan et al. (1991). This publication is cited several times in publications and research and can be considered an essential “must read” paper when discussing RDS and IHHNV. This excellent publication is a good example of a wellstructured discussion. The authors remind us of very important aspects of their findings and the conditions of the experiments. In the discussion section, the authors say: “While IHHNV appears to be the direct determinant of RDS in P. vannamei, additional cofactors may moderate the expression of this disease. Many of the IHHNV-positive shrimp were not runted. More than 70 percent of the “normal” growing shrimp in the IHHNV-positive group were IHHNV-positive. Yet, these animals displayed acceptable growth and were asymptomatic in terms of the runting syndrome. Thus, simple infection by IHHNV in not a complete etiologic explanation for RDS. Cofactors, in addition to the virus, are necessary to produce the disease.” The fact that they mention the need or possibility of additional cofactors to express RDS is notable as it is consistent with the basic principles and concepts of epidemiology (Thrusfield 2005) and the interaction of cofactors for the expression of the pathogen, plus a holistic vision of the relationship between host, pathogen and the environment (Sniesko 1974). Another point made by these authors is the density used in the experiments (500-1,500 shrimp/m2) represents “a 5 to 15-fold increase over commercial farm densities” used in those years. It must be mentioned that stocking densities have increased through the years and, in certain farms using super-intensive systems, stocking densities are close to the ones used by Kalagayan et al. (1991) of 300-500 shrimp/m2, with projected production of 30-70 t/ha (Nisar et al. 2022). This approach still has to overcome the challenges of testing this technology at a sustained and massive commercial scale. Finally, this iconic paper ends with a comment that should be used to understand the context of the findings and described results: “Studies to determine the role of host (particularly genetic aspects), the environment and management cofactors to the expression of IHHNV in P. vannamei are needed.” It seems as if they were anticipating the results obtained in culture ponds 30 years later and the effects of IHHNVwith certain populations of genetically selected shrimp that apparently have adapted and developed resistance and tolerance to the virus, reflected in good production levels with IHHNV-positive populations. In 1995, based on the evidence available at the time, IHHNV was included in the list of Obligatory Notifiable Diseases by the OIE (nowWorld Organization of Animal Health). The negative effects of IHHNV on aquacultured shrimp and the mortality caused in P. stylirostris were mentioned in a book used as the standard reference about diseases in penaeid shrimp (Lightner 1996). During the 1990s, 2000s and 2010s, several articles about Histology of IHHNV expression in an infected cell: the Cowdry Type A inclusion (CAI) in the cuticular epithelium of the stomach in farmed Penaeus vannamei. Tissue damage and expression of this virus is nowadays rarely observed while doing histopathology in farmed P. vannamei, a situation that has changed from two decades ago, when it was more common to observe the CAI in several tissues.
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