Quantitative risk assessment of WSSV transmission through partial harvesting and transport practices for shrimp aquaculture in Mexico

This quantitative risk assessment provided an analytical framework to estimate white spot syndrome virus (WSSV) transmission risks in the following different scenarios: (1) partial harvest from rearing ponds and (2) post-harvest transportation, assuming that the introduction of contaminated water with viral particles into shrimp culture ponds is the main source of viral transmission risk. Probabilities of infecting shrimp with waterborne WSSV were obtained by approaching the functional form that best fits (likelihood ratio test) published data on the dose-response relationship for WSSV orally inoculated through water into shrimp. Expert opinion defined the ranges for the following uncertain factors: (1) the concentrations of WSSV in the water spilled from the vehicles transporting the infected shrimp, (2) the total volume of these spills, and (3) the dilution into culture ponds. Multiple scenarios were analysed, starting with a viral load (VL) of 1 × 102 m L−1 in the contaminated water spilled that reached the culture pond, whose probability of infection of an individual shrimp (Pi) was negligible (1.7 × 10−7). Increasing the VL to 1 × 104.5 m L−1 and 1 × 107 m L−1 yielded results into very low (Pi = 5.3 × 10−5) and high risk (Pi = 1.6 × 10−2) categories, respectively. Furthermore, different pond stocking density (SD) scenarios (20 and 30 post-larvae [PL]/m2) were evaluated, and the probability of infection of at least one out of the total number of shrimp exposed (PN) was derived; for the scenarios with a low VL (1 × 102 m L−1), the PN remained at a negligible risk level (PN, 2.4 × 10−7 to 1.8 × 10−6). For most of the scenarios with the moderate VL (1 × 104.5 m L−1), the PN scaled up to a low risk category (PN, 1.1 × 10−4 to 5.6 × 10−4), whereas for the scenarios with a high VL (1 × 107 m L−1), the risk levels were high (PN, 2.3 × 10−2 to 3.5 × 10−2) or very high (PN, 1.1 × 10−1 to 1.6 × 10−1) depending on the volume of contaminated water spilled in the culture pond (VCWSCP, 4 or 20 L). In the sensitivity analysis, for a SD of 30 PL/m2, it was shown that starting with a VL of 1 × 105 m L−1 and a VCWSCP of 12 L, the PN was moderate (1.05 × 10−3). This was the threshold for greater risks, given the increase in either the VCWSCP or VL. These findings supported recommendations to prevent WSSV spread through more controlled transportation and partial harvesting practices.