What is EMS?

What is EMS?

For many years, the aquaculture community considered Vibrio spp. to simply be natural biota in both wild and farmed shrimp. However, it has become one of the primary disease-causing pathogens in shrimp aquaculture.

The diseases caused by Vibrio are referred to as “vibriosis.” They have become widespread due to shrimp farming intensification. A recently emerging disease, commonly known as Early Mortality Syndrome (EMS) or Acute Hepatopancreatic Necrosis Disease (AHPND), causes mass mortality in farmed Penaeid shrimp. The disease was first reported in Southern China in 2010 and subsequently spread throughout Southeast Asia, making its way through Vietnam, Thailand, and Malaysia, and Latin American countries such as Ecuador. Reports indicate that a specific strain of the bacterium species, Vibrio parahaemolyticus, is the primary causative agent of the EMS/AHPND. Recently, however, the aquaculture community has discovered other Vibrio strains that are potentially capable of causing the disease.

Therefore, approaches to controlling the presence and activity of vibriosis, in general, provide the best likelihood of preventing EMS/AHPND outbreaks.

Identifying EMS in Shrimp Ponds

EMS is capable of infecting even healthy shrimp under a variety of conditions. The disease affects all stages of shrimp with the most significant economic losses focused on grow-out conditions. Mass mortality, up to 100%, typically occurs in post larvae (PL) within the first 35 days of stocking. The target species are both the vannamei and black tiger shrimp. Vannamei are the most susceptible to the EMS. The causative agent in the bacterium is a specific plasmid, producing PirA and PirB toxins, which causes acute necrosis in the shrimp’s hepatopancreas, resulting in clinical symptoms such as:

• Pale, atrophied hepatopancreas
• Empty stomach and midgut
• Slow growth
• Soft and loose exoskeleton
• Erratic swimming, or spiraling and dropping to the bottom of the pond

Causes of EMS

A number of factors are attributed to causing EMS:

• Inferior quality of post larvae stock
• Bad stocking practices, including stocking PL too young and at too high a density
• Characteristics of farms that do not have reservoirs for treating water prior to filling grow out ponds
• Poor pond and water preparations
• Poor farming management practices such as antibiotic use, insecticide use, or uncleaned equipment
• Poor water quality maintenance
• Stress from fluctuating water temperatures and high salinity

Dominance of fast growing Vibrio spp. in pond water which can out-compete slower-growing, beneficial bacteria. Also, the practice of totally disinfecting ponds and water and stocking immediately after can contribute to the epidemic spread of EMS. This is because water needs time to develop a new community of beneficial bacteria to out-compete the fast-growing Vibrio.