IHHN disease of Penaeus stylirostris: effects of shrimp size on disease expression

Abstract. Populations of Penaeus stylirostris, free of infectious hypodermal and haematopoietic necrosis virus (IHHNV), were divided into five size groups (PL₂ [2‐day‐oldpostlarvae, approximately 5.0 mg], 3.7 g, 14.5 g, 21.3 g and 36.9 gaverage initial weight), and experimentally exposed to IHHNV per or. Statistical comparisons (histologically‐derived) of infection severity ratings from six weekly sample periods indicated the relative abundance of pathognomonic Cowdry type A intranuclear inclusion bodies (and, hence, the severity of the disease) differed between sizes. The results of comparisons showed a tendency for larger animals to have a significantly lower prevalence of inclusion bodies and associated histopathology than smaller animals. The results also indicated that the smaller (average weight) the group was, the shorter was the period of time from initial exposure to the first histological detection of the disease. Thus, the larger (in average weight) a population of P. stylirostris is upon initial exposure to IHHNV, the less acute will be disease manifestation within a given time and the longer will be the period before the disease is first histologically detectable. A predictive equation that defines the epizootiological relationship of IHHN disease within an intensively cultured population of P. stylirostris is also presented.