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The main portal of entry for shrimp-related pathogens
T
he antennal gland of Penaeid shrimp is a much more complex organ than previously thought. An intrabladder inoculation study showed the high susceptibility of the nephropore to both pathogens, white spot syndrome virus (WSSV) and Vibrio campbellii. The authors concluded that during conditions such as sudden salinity drop like during heavy monsoon rains, aggression, establishment of social dominance and feed intake combined with a high virus load in the surrounding water, the antennal gland has to be considered a major portal of pathogen entry. Furthermore, a new morphology was proposed. Additional findings show that pathogens may indeed enter through this organ naturally, infecting shrimp and that the diverticles, connected with the antennal gland are involved in molting. These insights into the molting process and pathogen entry open doors in fundamental biology and the potential development of disease control measures This summary is extracted from an article titled: The shrimp nephrocomplex serves as a major portal of pathogen entry and is involved in the molting process by GaĂŤtan De Gryse, Thuong Van Khuong, Benedicte Descamps, Wim Van Den Broeck, Christian Vanhove, Pieter Cornillie, Patrick Sorgeloos, Peter Bossier and Hans Nauwynck. www.pnas. org/cgi/doi/10.1073/pnas.2013518117
One of the major bottlenecks in the marine shrimp aquaculture industry is the occurrence of diseases which may cause serious financial losses to farmers. Although many studies have been conducted to identify pathogens and to implement treatments, there is still a lack of information on one very important aspect - pathogen entry in the host. The main portal of entry for shrimp-related pathogens remains vague; without clear knowledge and understanding on the entry portal, diseases may be difficult to treat and control. In shrimp, the cuticle constitutes a strong pathogen barrier; regions which lack cuticular lining can be the Achilles heel where pathogens may be able to penetrate into the shrimp body. The shrimp's excretory organ, the antennal gland, which does not have a cuticle-lined lumen, may be an entry portal to pathogens.
sections (10Âľm) of the cephalothorax. Besides the histological 3D reconstruction with AMIRA software 6.0, a different imaging technique was used to confirm the correctness of the histology-based 3D reconstruction: serial micromagnetic resonance imaging (ÂľMRI). Test animals used for the infectivity studies were specific pathogen free (SPF) penaeid shrimp, Penaeus vannamei with a mean body weight of 2.8g. Infectivity studies were carried out to compare white spot syndrome virus (WSSV) and Vibrio campbellii infections in shrimp subjected to intrabladder, intramuscular or peroral inoculations, and results were interpreted against those obtained from control treatments. A study to investigate natural WSSV infections via the antennal gland was also carried out.
The antennal gland
The study fully described the anatomy of the shrimp's excretory organ - the hitherto little known antennal gland (Figure 1) which was identified as a major portal candidate. Dissection of the antennal peduncle revealed a white, bean-shaped structure which was bilaterally located at the transition from the peduncle to the cephalothorax. This bean-shaped structure was identified as the compact glandular compartment (CGC) of the antennal gland. The CGC was found to be connected to a urinary bladder, which, depending on the size of the shrimp, could either be visualised by the naked eye or by bright-field microscopy. The antennal gland was found to be a threepart organ: a coelomosac responsible for the filtration of the haemolymph; an efferent labyrinth which alters the filtrate (together they constitute the CGC); and a terminal ductus consisting of a bladder, several diverticles, and a small ductus leading to the nephropore, which seals the antennal gland from the outside world. The involvement of the antennal gland during molting was also established in this study. Micromagnetic imaging not only confirmed the morphology in vivo, but also revealed the filling of the caudal extensions to be linked to the molting stage of the shrimp. The contents of the caudal extensions of the antennal gland in perimolt were, on average 3.25 times greater when compared to intermolt.
This study was thus planned to investigate the role of the antennal gland as a portal of entry of pathogens. The antennal gland is involved in haemolymph filtration and osmoregulation; these functions, together with the fact that certain infectious diseases such as white spot syndrome emerge upon a salinity drop during heavy rainfall, further strengthened the authors' belief that the antennal gland could act as a major portal of pathogen entry.
Materials and Methods
The antennal gland and surrounding organs were studied using a three-dimensional (3D) reconstruction by superposition of serial hematoxylin and eosin-stained
January/February 2021 AQUA Culture Asia Pacific
SEM image of the nephropore at the antennal base of P. vannamei shrimp. Black arrowhead is pointing to the rostrum; 1, rostral valve; 2, caudal valve; white arrowheads, fissure.
