Early Mortality Syndrome (EMS) in Shrimp
As most knowledgeable aquaculturists know all too well, disease is a natural component of aquaculture. Moreover, the absence of disease is unnatural and there are no agricultural practices that do not at the very least suffer on occasion from the impact of disease. While it should be the goal of all science-based aquaculture to prevent diseases to the maximum extent possible, the unfortunate truth is that this ideal is not achievable nor perhaps even ultimately desirable.
The last major disease outbreak (others are occurring as I write this) was named EMS after its noted impact. PLs were dying within the first 30 days of stocking in large numbers with a characteristic pathology. Recent reports suggest that a virus, (SHIV), may also be implicated to some extent in the overall impact.
Few involved in the shrimp farming industry are not familiar with this EMS (also known as AHPNS or AHPND). The disease has been well characterized and the early damage to the shrimp is a result of the bacterial strains involved producing a highly hepatotoxic material.
A strain of a common marine bacterium, Vibrio parahaemolyticus is the cause of EMS (Early Mortality Syndrome). Early on researchers thought that perhaps the ability to produce disease was related to the presence of a virus that infects the bacterium (called a bacteriophage). Subsequent research failed to corroborate this and we now know that extrachromosomal circular pieces of DNA (called plasmids) carry the genes for expression of this potent bivalent protein toxin.
Recently, it has been determined that this is not actually a new pathogen and that this toxin was present many years before the recent global impact.
Management is primarily through exclusion. The evidence is strong that poor biosecurity practices contribute to the consistent presence of the toxin-encoding genes. The disease process is complex and multiple vibrio species have been found to carry these genes. Accumulated organic matter which leads to quorum sensing is thought to be a major driver of toxin expression. Elimination of accumulated organic matter via the use of sumps and microbial products that lower vibrio loads along with the use of SPF nucleus breeding center (NBC) PLs has been successful in dramatically reducing the impact.
It is important to note that the use of SPF PLs from true NBCs is perhaps the most critical element of control. For the most part, as with all too many things in shrimp farming, you are far more likely to be sold PLs from someone who claims to have these and validates the claim through PCR, a method that is not suitable for this purpose without considering other factors. This ensures that EMS will persist in one form or another. The toxin can weaken animals and can be present in populations that are PCR "negative".