The international shrimp farming industry depends in part on shrimp supplied by breeding centers where the Specific Pathogen Free (SPF) status of their stocks is continuously monitored. According to guidelines by the World Organization for Animal Health (WOAH), the recommended methods for surveillance are based on PCR, which targets a limited number of listed pathogens. On occasion, a smaller number of samples is also included for histological analysis. Here, we report the re-discovery of a Reolike virus (RLV) facilitated by the inclusion of samples for histological analysis as part of a surveillance program.
Lesions highly suggestive of RLV infection were discovered during a health assessment of SPF Penaeus vannamei by using conventional H&E histology (Fig. 1A). Follow-up work by transmission electron microscopy (TEM) demonstrated the presence of viral particles with morphology consistent with RLV (Fig. 1B). Samples from the same population of shrimp were also analyzed by PCR for diseases listed by the WOAH and none were detected. The shrimp (3.5 g avg. wt.) had exhibited normal behavior and were otherwise healthy.
RLV infections have been reported in crustaceans, including penaeid shrimp. However, the relevance of RLV infection has been historically difficult to determine due to the presence of other ailments in the same shrimp. According to our observations, the virus appears to be very contagious, but we have not yet observed evidence of high virulence in the short term. We do not know what the long-term effect of the infection might be for example, on growth, survival, fertility, or resilience against other diseases or to stress due to sub-optimal grow-out or environmental conditions.
Current work on virus sequencing will provide insight into the taxonomic position and will allow us to design molecular tests based on PCR. These tests will be helpful in preventing the spread of a virus whose effect on the shrimp farming industry is yet unknown. This discovery exemplifies the usefulness of histological analysis and serves as a cautionary note to avoid relying solely on molecular methods for disease surveillance programs.