Experimental infection of infectious myonecrosis virus (IMNV) in the Pacific white shrimp Litopenaeus vannamei (Boone, 1931)

Abstract

Bioassays are essential in detecting resistant animals based on standardized challenge tests. A controlled bioassay system for Infectious myonecrosis virus (IMNV) was developed through two methods of infection, by ingestion and injection, in Litopenaeus vannamei. For ingestion bioassays, shrimp (2–3.9 g) were exposed to four feeding infection treatments (feeding with infected tissue-FIT without stress; FITS-salinity stress; FITES-temperature and salinity stresses and FITA-alkalinity stress). No signs of disease were observed and 30 % of the animals were infected, with FITS treatment exhibited the higher proportion of infected animals (p ≤ 0.05). In the injection treatments, a stock inoculum was quantified (viral load of 9.82 × 105 IMNV copies µg−1 of total RNA) and a tenfold serial dilution was tested at 28 °C and, from 10−5 to 10−9 at 26 °C. Over 60 % of infected animals were obtained for dilutions between 10−5 and 10−7 at 28 °C, while at 26 °C; these values ranged from 30 to 50 %, with a clear effect of low water temperature in reducing the proportion of infected shrimp. In general, higher survival probabilities of infected shrimp were achieved 5 days after exposure, followed by a quick decrease at the tenth day post-inoculation. Median infectious dose (ID50) was determined between 2.44 × 10−8 and 1.19 × 10−7 at 28 °C and between 1.76 × 10−6 and 1.28 × 10−5 at 26 °C for 15 days post-infection, respectively. Injection treatments based on known infectious dose were the most reliable method of estimating viral infectivity for IMNV selection. Moreover, FITS treatment was identified as an environmental risk factor, contributing to increase in IMNV infection prevalence.