White Feces Disease (WFD) is one of the most threatening diseases in Asian countries (e.g., Thailand, Vietnam, China, India, Indonesia) since 2009 until date leading to significant losses for shrimp farmers due to high FCR, slow growth and size variation at harvest. Generally, the white fecal strings floating on the pond surface indicates the WFD outbreak, and it occurs in black tiger shrimp (Penaeus monodon) and Pacific white shrimp (Penaeus vannamei). Although the specific disease-causing agent has not been identified, several studies have shown that WFD can be stimulated by opportunistic Vibrio species and Enterocytozoon hepatopenaei (EHP) an association with high WFD prevalence. However, the identification of causative pathogens and the replication of WFD infection in-vivo are still under development. This is one of the major drawbacks in consideration of prevention strategies for the outbreaks as well as in developing curative products to promote the early recovery of shrimp after WFD infection.
The laboratory induction method for WFD has been developed at ShrimpVet Laboratory, Vietnam, and the mode of disease infection was studied. The pathogenic strains of Vibrio spp. from WFD affected shrimp samples from shrimp farms in Vietnam were isolated by plate culture method, and the replication of the selected bacteria and the confirmation of disease occurrence via bioassay challenges on SPF (specific pathogen free) shrimp. An EHP challenge group, by cohabitation method, and a combined challenge with EHP (cohabitation) and WFD (per os) were set up and studied in parallel. A negative control group was also established to identify the significance in shrimp feeding behaviour and appearance of clinical signs in WFD challenged groups. During the post-challenge, the observed symptoms of challenged shrimp included loose shell, white/yellow hepatopancreas and gut tract filled with white feces strands, similar to the gross signs from the field specimens.
The wet mount method was used to check the occurrence of aggregated transform microvilli (ATM) in shrimp gut and hepatopancreas (HP) tissues, as formation of ATM is a precursor sign of disease. However, this method solely cannot confirm the disease infection. Hence, the histological assay on the shrimp HP and gut was performed to examine the severity levels of WFD infection and to deliver the case definition for WFD. Most importantly, the re-isolation of pathogens achieved positive results by recovering the exact strain of bacteria challenged to the animals, which can induce the symptoms and effect of WFD under in-vivo challenged condition, in this study.
Among three challenge groups, the combined challenge was the most effective way at inducing WFD under laboratory condition. The direct course of white feces could be stimulated under the influence of Vibrio spp. However, the underlying conditions of EHP infection leads to a persistent occurrence of WFD in whiteleg shrimp. To minimize the impact of WFD, two comprehensive management strategies were proposed from this study. Firstly, it is important to eliminate EHP out of the shrimp ponds or shrimp stock, and secondarily, the healthy stock should be protected against bacterial infections by maintaining the optimal health and minimal stress from the environment.