Experimental vertical transmission of covert mortality nodavirus in Exopalaemon carinicauda

Viral covert mortality disease (VCMD) has caused serious losses to shrimp aquaculture in China in recent years and the ridgetail white prawn Exopalaemon carinicauda has been suspected to be one important factor in perpetuating the high prevalence of covert mortality nodavirus (CMNV) infections due to its perennial presence in shrimp farming ponds and water from natural habitats. Experiments were carried out to determine the possibility of vertical transmission of CMNV in E. carinicauda in this study. CMNV infection in gonads, fertilized eggs and larvae was investigated by using the methods of reverse transcription nested PCR (nRT-PCR), in situ hybrization (ISH) and transmission electron microscopy (TEM). The ovarian tissue and testis tissue of artificially infected parental E. carinicauda were proved to be CMNV-positive by nRT-PCR. Fertilized eggs were also found to be CMNV-positive by nRT-PCR whether the fertilized eggs originated from the CMNV-positive female broodstock mated with the CMNV-negative male broodstock, or they originated from the CMNV-negative female broodstock mated with the CMNV-positive males. The results of ISH indicated that the positive signals were evident in the oocytes within ovarian tissue and nauplii. By TEM analysis, CMNV virions were observed in oogonia, oocytes, spermatocytes, fertilized eggs and nauplii. The presence of CMNV in fertilized eggs and larvae indicates that CMNV can transmit vertically via sperm and oocytes in E. carinicauda, which highlights the high probability of vertical transmission of CMNV in the main species of cultured shrimp and prawns.