Conventional histopathology, with routine hematoxylin and eosin stains (H&E), remains one of the most useful diagnostic tools currently available to diagnose diseases of penaeid shrimp. Molecular methods such as PCR-based assays and nucleic acid sequencing are also extremely useful due to unmatched sensitivity and specificity. However, molecular methods have limitations because they can only detect specific pathogens, depending on the chosen set(s) of PCR primers.
Alternatively, analysis of tissue sections by H&E histopathology allows a comprehensive health assessment of the shrimp. Many of the most important diseases of shrimp cause lesions so distinctive that they can be considered their pathological signature and a presumptive - and in some cases definitive - diagnosis can be reached in most instances.
Currently, there are four important diseases in the Central and South American region that affect the digestive gland, or hepatopancreas, and cause relatively similar clinical signs. These diseases include Septic hepatopancreatic necrosis (SHPN), Necrotizing hepatopancreatitis disease (NHP), Acute hepatopancreatic necrosis (AHPND) and Hepatopancreas microsporidiosis caused by Enterocytozoon hepatopenaei (HPM). These diseases may result in one or more of the following clinical signs: hepatopancreas atrophy and discoloration, brown to black streaks or spots observable with the naked eye or by wet mount examination, chronic mortalities, growth retardation, high size variation, flaccid bodies and soft carapace. These clinical signs are not specific and prone to erroneous presumptive diagnosis. A sample received at our laboratory early in 2024 was submitted because, based on clinical signs, it was thought to be affected by HPM. However, histological analysis (confirmed by PCR) demonstrated that it was actually NHP disease. Each one of these diseases causes distinct tissue lesions, which have been described in specialized publications. The objective of this presentation is to highlight and compare the distinctive morphological pathology (published and unpublished) characteristic of these diseases. This knowledge will enable us to accurately determine the cause of the disease, prevent misdiagnosis, and take appropriate measures to mitigate its effects.
All diagnostic methods have limitations. Histopathology is limited by its sensitivity. Very low levels of infection, or very early stages of a disease may be missed because the lesions upon which a diagnosis is based may not yet be present. For this reason, the best samples for histopathology analysis are those where the shrimp shows abnormalities, either physical or behavioral. Samples of asymptomatic shrimp usually do not yield useful information in terms of a diagnosis.