Vertical transmission of Infectious myonecrosis virus in Litopenaeus vannamei

•IMNV vertical transmission in SPF and non-SPF L. vannamei broodstocks was investigated.•Positive Real-time PCR detection of IMNV in eggs and ovaries demonstrated vertical transmission.•Maternal transmission was the most common transmission.•Low sperm viability was determined in IMNV-infected males.

In the farmed whiteleg shrimp (Litopenaeus vannamei), Infectious myonecrosis virus (IMNV) accounts for 70% of the cumulative mortality during the growout cycle. Although the horizontal transmission of IMNV has been demonstrated, vertical transmission remains unknown, which may be a crucial factor to the prevention of IMNV from one generation to the next. Thus, the aim of the present study was to determine the possibility of the vertical transmission of IMNV by experimental infection in SPF and non-SPF L. vannamei broodstocks using an intramuscular procedure. Evaluations involved the use of molecular methods (Real-time PCR and conventional PCR) for spermatophores, ovaries and eggs tissues. The effect of IMNV infection on sperm viability was also evaluated. Conventional PCR detected IMNV in spermatophores, mature ovaries and muscle tissue from experimentally infected specific pathogen-free (SPF) broodstock. Real-time PCR confirmed IMNV replication in all samples (spermatophores, mature ovaries, muscle tissue and eggs — fertilized or not) from naturally and experimentally infected non-SPF broodstock. No significant difference in viral load of the reproductive organs/tissues was found between naturally or experimentally infected shrimp (both sexes) (p ≥ 0.05). However, a significant difference was found regarding mean viral load in the muscle tissue of all shrimps (males and females) in comparison to other tissues, with dissimilar viral load values for naturally and experimentally infected muscle tissue for both sexes (p ≤ 0.05). With regard to spawning, all infected females spawned and no significant difference was found in the viral load of eggs produced by naturally and experimentally infected females. The eggs were classified as two morphological types (type B and A2) based on morphology and post-spawning development. Sperm viability of experimentally infected males was not determined due to the low cell density, whereas mean viability among naturally infected males was 82%. In the control group, mean sperm viability was 91%. The results suggest that IMNV can be transmitted vertically in L. vannamei. However, nauplii did not hatch out, which prevented a better understanding of the mechanisms by which eggs become infected following spawning (egg surface or by transovarial transmission). Nonetheless, the 100% positive samples from ovaries and eggs and the low percentage of viable cells from naturally infected males suggest that females are the primary source of the vertical transmission of IMNV to eggs.Statement of relevanceThis study reports the first scientific evidence of IMNV vertical transmission in L. vannamei. The study suggests the use of IMNV-free broodstock as an effective prevention measure in order to minimize propagation in L. vannamei farming operations.