WWW.WA S .ORG • WORLD AQUACULTURE • DECEMBER 2022 31 IHHNVwere published, including a proposal related to the possible negative effects in the populations of wild shrimp in the Americas (MoralesCotarrubias et al. 1999) and the origins of the virus to the insertion in the genome of its penaeid shrimp hosts (Tang and Lightner 2006). One publication reviewed the several different papers written about IHHNV (Vega-Heredia et al. 2012), but it is not the only one. The development of genetic lines of P. stylirostris resistant to IHHNV also has been reported (Tang et al. 2000). During the decades since its discovery and association to RDS, IHHNV became some sort of scapegoat to justify every problem associated with poor growth under pond conditions. Regarding this there is an excellent publication that reviews all the pathogens associated with “growth syndromes” (Valappil et al. 2021). The list includes ten potential pathogens of penaeid shrimp, including viruses such as Hepatopancreatic Parvo virus (HPV), Monodon baculovirus (MBV), Baculovirus penaei (BP), Yellow head virus Type 4 (YHV Type 4), in addition to various species of bacteria and microsporidians. Lightner (1985) reported “runts” associated with hemocytic enteritis, an observation that one of us (XR) has also noticed in histopathology sections. Consequently, when observing “runts” and growth problems in culture ponds, aquaculturists should think beyond the simple presence of IHHNV or a new emerging pathogen in which such association have been reported. During the decades that followed the Kalagayan et al. (1991) publication and when examining shrimp sampled from ponds using more sensitive techniques such as PCR and quantitative PCR (qPCR), slowly and little by little the evidence has emerged that the presence of IHHNVwas not affecting the levels of production in P. vannamei in Ecuador, the country that has become the largest shrimp producer in the world. It is possible to find IHHNV-positive shrimp without RDS and obtain good yields. This shows that it is possible to select and develop genetic lines of shrimp that can tolerate or resist this virus. Natural selection works one way or another. The previous fact does not deny that there are publications that associate the presence of IHHNV to negative effects on growth and production in P. monodon (Sellars et al. 2019). Nevertheless, the question for the investigators that find an association is the same one that Kalagayan et al. (1991) proposed more than 30 years ago: what is the role of genetics, the influence of the stock used for the experiments and bioassays, and its sensitivity and tolerance to this and other viruses? In relation to this and in this context, there are publications that support opposing views. Withyachumnarnkul et al. (2006) maintain that the presence of IHHNV in P. monodon does not seriously affect growth and fecundity in IHHNV-positive shrimp with light infections. Maybe these differences are caused by the different genetic lines used in the experiments, done in different countries and by different research groups. It is undeniable that IHHNV-resistant/tolerant populations exist and can be developed and selected. Maybe this approach is the path to follow in the future of the shrimp farming industry. This selection has probably already occurred in nature, as when selecting wild penaeids as the source of some broodstock, these were already tolerant to IHHNV, as their parents were the survivors of the effects of this virus. This possibility was discussed by Aranguren et al. (2022) regarding the survival of a population of SPF P. stylirostris used in their experiments, as there was a high survival rate to the viral challenge in their bioassays. These results were unthinkable during the years when IHHNVwas discovered and its effects on the survival of P. stylirostris described almost four decades ago. If this selection also occurs in cultured shrimp populations and there is evidence of no negative effects at commercial levels, why not then retire (delist) IHHNV from the list of Obligatory Notifiable Diseases by the World Organization of Animal Health? This proposal has a precedent as it was done with two viruses that caused negative effects to the industry in the beginning and after their discovery. Baculovirus penaei and Monodon baculovirus were both retired and withdrawn from the list in 2009 and were not included in the Manual for Diagnostic Tests of for Aquatic Diseases in the 2012 edition as obligatory notifiable pathogens (OIE 2019). It is evident that any virus, under the right conditions of stress and in shrimp stocks with limited genetic diversity, will cause problems in the physiology of shrimp or any cultured crustacean, leading to death. A recent publication reports how viral integration of IHHNV in P. monodon could be used, using RNA interference (RNAi) to help shrimp respond to the presence of the virus, as well as the detection of the areas of the genome that could be used to identify more resistant ( C O N T I N U E D O N P A G E 3 2 ) A shrimp farm in Ecuador. A shrimp nursery pond in Ecuador.
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