Low viral loads and lymphoid organ spheroids are associated with yellow head virus (YHV) tolerance in whiteleg shrimp Penaeus vannamei

Abstract

Yellow head virus (YHV) is an invertebrate nidovirus that has caused mass mortality in penaeid shrimp since 1990. Several YHV types are known, but only the original type (YHV-type 1 or YHV-1) is highly virulent. Most studies have focused on acute YHV-1 infections and there is limited work on YHV-1 survivors. We compared moribund and surviving (14%) whiteleg shrimp Penaeus (Litopenaeus) vannamei from an experimental challenge with YHV-1. Although grossly normal, all survivors were positive for YHV-1 by specific, reverse transcriptase polymerase chain reaction (RT-PCR) assays, histological analysis or transmission electron microscopy (TEM), indicating that they were not resistant but tolerant to YHV-1. On the other hand, real-time PCR analysis revealed that mean YHV-1 copies/ng total RNA for survivors (2.8x10(4) +/- 6.9x10(4)) were approximately 40 times lower (P<0.05) than those in moribund shrimp (1.2x10(6) +/- 6.7x10(5)copies/ng total RNA). This was confirmed by strong positive immunohistochemical and in situ hybridization (ISH) reactions for YHV-1 in lymphoid organ tubules (LOT) of moribund shrimp and weak positive reaction only in lymphoid organ spheroids (LOS) of survivors. TEM revealed morphologically complete YHV virions in both groups. Furthermore, immuno-TEM and Western blot analysis revealed that YHV-1 structural proteins gp116 and p20 were present at comparable reactive levels in each group. Thus, YHV-1 tolerance was not associated with absence of gp116 as previously reported for palaemonid shrimp. Instead, it was associated with the presence of YHV-positive LOS and a relatively low viral load.

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“…These shrimp become infected, but the level of viral propagation is reduced when compared to unselected shrimp 34 . A similar result has been reported for P. vannamei survivors from a challenge with yellow head virus 35 . Again, the mechanisms underlying the low level of viral propagation are currently unknown, but it is possible that Se may serve as an important cofactor required by the animal to respond optimally to infection.…”

Insight into the lymphoid organ of penaeid prawns: A review

“…In contrast, using the same method (TEM), the presence of virions was illustrated within the spheroids but not in normal tubules [31,33]. Similarly, viral proteins were only found within the spheroid cells dsRNA-Togaviridae lymphoid organ vacuolization virus (LOVV) P. vannamei [33] by immunohistochemistry [36] and viral nucleic acid by in situ hybridization [53]. Other investigations found that ISH positive signal was detected both in normal lymphoid tubules and LOS cells [26,54] The variability in localisation of viral components within the lymphoid organ may be a sign of viral infection stages.…”

“…The spheroid formation might be related to the tolerance of penaeid prawn to viral infection. In the case of a survivor or in chronic infection, the spheroids were formed when animals controlled the infectious agents or at least reacted to them [36,37].…”

“…Viral genomes have been detected within the lymphoid organ by biomolecular techniques such as reverse transcriptase-polymerase chain reaction (RT-PCR) [26,36,48] or real time quantitative RT-PCR [27]. However these methods cannot distinguish whether the nucleic acid within the normal lymphoid tubule or spheroid cell, since whole the LO was used without any separation between stromal matrix cells and spheroid cells.…”

“…Moreover, in penaeid prawns infected with yellow head virus complex, monoclonal antibody (MAb) for immunohistochemistry (IHC) and digoxigenin (DIG)-labeled DNA probe of ISH showed a positive reaction only in the lymphoid tubules of moribund prawns (acute phase) and only in the LOS of the surviving animals (chronic phase) [36,48,56]. A similar result was observed in healthy P. monodon infected with Australian strain of yellow head virus, gill associated virus (GAV, previously named lymphoid organ virus, LOV) in which by TEM, the localization of this virus was only detected in the spheroid cells [45].…”