Abstract
Accompanying acute hepatopancreatic necrosis disease (AHPND) in cultivated Asian shrimp has been an increasing prevalence of vermiform, gregarine-like bodies within the shrimp hepatopancreas (HP) and midgut. In high quantity they result in white fecal strings and a phenomenon called white feces syndrome (WFS). Light microscopy (LM) of squash mounts and stained smears from fresh HP tissue revealed that the vermiform bodies are almost transparent with widths and diameters proportional to the HP tubule lumens in which they occur. Despite vermiform appearance, they show no cellular structure. At high magnification (LM with 40-100x objectives), they appear to consist of a thin, outer membrane enclosing a complex of thicker, inter-folded membranes. Transmission electron microscopy (TEM) revealed that the outer non-laminar membrane of the vermiform bodies bore no resemblance to a plasma membrane or to the outer layer of any known gregarine, other protozoan or metazoan. Sub-cellular organelles such as mitochondria, nuclei, endoplasmic reticulum and ribosomes were absent. The internal membranes had a tubular sub-structure and occasionally enclosed whole B-cells, sloughed from the HP tubule epithelium. These internal membranes were shown to arise from transformed microvilli that peeled away from HP tubule epithelial cells and then aggregated in the tubule lumen. Stripped of microvilli, the originating cells underwent lysis. By contrast, B-cells remained intact or were sloughed independently and whole from the tubule epithelium. When sometimes engulfed by the aggregated, transformed microvilli (ATM) they could be misinterpreted as cyst-like structures by light microscopy, contributing to gregarine-like appearance. The cause of ATM is currently unknown, but formation by loss of microvilli and subsequent cell lysis indicate that their formation is a pathological process. If sufficiently severe, they may retard shrimp growth and may predispose shrimp to opportunistic pathogens. Thus, the cause of ATM and their relationship (if any) to AHPND should be determined.
Highlights
The results presented in this manuscript describing transformation, sloughing and aggregation of hepatopancreatic microvilli into vermiform bodies superficially resembling gregarines was obtained over a period of 6 years in a piecemeal fashion as a series of initially independent, sideline observations made during the course of dedicated research projects on a variety of known shrimp pathogens ranging from viruses to bacteria, fungi and parasites
The results revealed that 96% (24/25) of the ponds exhibiting white feces syndrome (WFS) contained shrimp specimens that presented vermiform bodies resembling gregarines
There was a decrease in feed consumption and growth rates were reduced as revealed by average daily weight gain (ADG) for the whole crop operation of less than 0.1 g/day compared to 0.2 g/day in normal ponds
Summary
The results presented in this manuscript describing transformation, sloughing and aggregation of hepatopancreatic microvilli into vermiform bodies superficially resembling gregarines was obtained over a period of 6 years in a piecemeal fashion as a series of initially independent, sideline observations made during the course of dedicated research projects on a variety of known shrimp pathogens ranging from viruses to bacteria, fungi and parasites. AHPND Outbreaks began in cultivated shrimp Penaeus (Penaeus) monodon and Penaeus (Litopenaeus) vannamei China in 2009 and thereafter spread progressively to Vietnam (2010), Malaysia (2011) and Thailand (2012), the cause of the disease was not known at that time. It was not until 2011 that a case definition for AHPND was first described (referred to as acute hepatopancreatic necrosis syndrome or AHPNS at the time) by D.V. Lightner from the University of Arizona at a seminar organized by the Vietnamese Department of Animal Health in Hanoi in June 2011 (unpublished). The unique diagnostic characteristic of the disease is massive, medial sloughing of shrimp hepatopancreatic (HP) tubule epithelial cells as a result of a currently unknown toxin(s) that originates from the causative bacteria colonizing the shrimp stomach
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