Yellow-head virus of Penaeus monodon is an RNA virus.
Yellow-head virus (YHV) causes acute infections resulting in very high mortality. First reports suggested that the viral core consisted of DNA and that the virus was a granulosis-type baculovirus. However, attempts at DNA extraction with high concentrations of purified virus gave only traces of DNA. Although selected recombinant clones derived from these pooled DNA traces did not hybridize with host shrimp DNA, they also failed to react with YHV-infected tissue by in situ DNA hybridization. Furthermore, negatively stained virions of YHV viewed by TEM were atypical for baculoviruses. Therefore, renewed attempts were made to extract RNA from purified YHV preparations. Haemolymph was collected aseptically by syringe from 200 artificially YHV-infected, live shrimp in terminal stages of the disease. Purified virions were prepared by a programme of centrifugation culminating in 22% to 45% Urofragin gradient ultracentrifugation. A band at the 30-37% interval of the gradient gave the cleanest preparation with the highest quantity of virions. By TEM these were enveloped, measured 150-170 × 40-50 nm and were surrounded by a fringe of knob-like projections approximately 11 nm in length. Nucleic acid was extracted using guanidium thiocyanate and purified by CsCl gradient ultracentrifugation. High-molecular-weight nucleic acid was obtained which was degraded by RNase-A but not by DNase I. Based on morphology of negatively stained virions by TEM and on RNA content, YHV resembles rhabdoviruses or coronaviruses, rather than baculoviruses. The findings necessitate cDNA preparation in the process to develop a nucleic-acid probe for YHV detection by the in situ or dot blot hybridization techniques.