Reestablishing Histopathology as an Essential Component of Health Assessment for Farmed Shrimp in the Era of Molecular Diagnostics

Infectious disease remains one of the major impediments to the growth and sustainability of shrimp farming worldwide. Despite extensive efforts, including surveillance and diagnostic testing, movement of infected broodstock and post-larvae across countries and continents has been a primary driver of disease introduction into new geographic locations. Often the etiology of a disease and diagnostic tools used to prevent its spread were developed only after the disease was detected somewhere new. From the 1970s to the mid-1990s, disease diagnosis in shrimp hatcheries and farms depended on clinical presentation, farm records, histopathology, and traditional microbial culture techniques for bacterial and fungal pathogens; followed by characterization of the isolated microbes. For viral pathogens, histopathology and electron microscopy were the primary tools for disease discovery and basic characterization since there was (and still is) no immortal cell line for crustaceans. In the late 1990s and early 2000s, molecular techniques supporting genomic characterization of pathogens and polymerase chain reaction (PCR)-based diagnostic tools started gaining momentum in the realm of pathogen discovery and disease diagnosis. The late 2000s saw rapid development of molecular diagnostic tools for a wide array of shrimp pathogens and today PCR-based assays have become the most widely relied upon methods of disease diagnosis and surveillance in the shrimp industry. Molecular diagnostics have a number of strengths, including high sensitivity, specificity, speed, and robustness, but like every modality, also have limitations. PCR-based diagnostics detect only those pathogens being targeted and fail to detect unknown pathogens or even newly emerged strains or genotypes of a known target pathogen. Beyond the presence or absence of a target pathogen, PCR-based diagnostics do not provide information about overall health status or disease state of an animal. Progressive over reliance on molecular diagnostics and concurrent neglect of histopathology is creating unforeseeable challenges for shrimp farming worldwide. In this commentary, we provide empirical evidence of how overreliance on PCR-based diagnostics can lead to erroneous diagnoses and failure to detect a new genotype of an existing World Organization for Animal Health (WOAH, Paris, France)-listed pathogen, and an emerging pathogen. These findings illustrate the risks of over reliance on a single methodological approach and emphasize the need to preserve the practice of pairing molecular diagnostics with conventional histopathology. Concurrent utilization of histopathology and molecular diagnostics are critical components of safeguarding the global shrimp farming industry by reducing detection failures of new or rapidly evolving pathogens, improving disease detection and identification, and improving animal health management practices.