Samples of Specific Pathogen Free (SPF) penaeid shrimp Penaeus (Litopenaeus) vannamei (23.46 g avg. wt.) were received by our laboratory for conventional H&E histological analysis to determine the cause of unusual accumulations of a black substance in the sixth abdominal segment. This black material was clearly visible through the cuticle when the shrimp were alive and had a dorsolateral distribution (Fig 1A). The cuticle was not compromised; the material appeared to be embedded into the musculature. Once the shrimp was preserved with Davidson’s fixative and the musculature turned white due to protein coagulation, these black spots were not visible anymore. A total of 4 shrimp out of 5 submitted for analysis presented with this abnormality.
Histological analysis revealed severe necrosis of the posterior midgut caecum (Figure 1B). The lesion was heavily melanized, hence the black appearance of these accumulations. Smaller, satellite lesions were also observed in the vicinity. Heavy bacterial colonization was found interspersed with the necrotic remains of the caecum and between concentric layers of melanization that had replaced the organ. Aside from bacterial infection, no signs of any other pathogenic agent(s) were found. In addition to caecum necrosis, severe hemocytic enteritis (HE) was found in the midgut at the sixth, third and first abdominal segments. The severity of HE appeared to be higher at the sixth segment and lower at the first, suggesting a progression of the lesion from the posterior midgut towards the anterior. HE was also accompanied by bacterial colonization, possibly by the same type of bacteria found in the remnants of the posterior caecum. No lesions or abnormalities were found in other areas of the digestive system or the rest of the body.
Since HE has been linked to Translucent Post-larvae Disease (TPD), and in some instances to Acute Hepatopancreatic Necrosis Disease (AHPND), tissue samples of necrotic caecum were taken from Davidson’s fixed shrimp and tested by PCR with primers specific for virulence factors for AHPND and TPD. However, the presence of these pathogens could not be confirmed. Determination of the etiologic agent causing posterior midgut caecum necrosis and sepsis will be useful in screening candidate SPF stocks of P. vannamei.