The Challenge of Shrimp Diseases

Aquaculture is one of the fastest growing food-production sectors but the economic impact that parasites and bacterial, fungal and viral diseases have on the industry is highly significant for the many countries that rely heavily on this industry. Research into the diseases that affect penaeid shrimps that are grown in aquaculture systems is therefore vital, writes Bob Carling for TheFishSite.



Two diseases that are currently being actively in Shrimp farming are:

1. The bacterial infection, Acute Hepatopancreatic Necrosis Disease (AHPND) – also called Early Mortality Syndrome (EMS)



2. The fungal infection, hepatopancreatic microsporidiosis caused by Enterocytozoon hepatopenaei (EHP).






AHPND

Acute Hepatopancreatic Necrosis Disease (AHPND) is a problem of the main countries that farm shrimps – China, Thailand, Vietnam and Malaysia. AHPND can occur in the first 30 days after stocking shrimp into ‘grow-out’ ponds, which is why AHPND is commonly, but incorrectly, called early mortality syndrome (EMS).

The disease is caused by a bacterium that colonizes the shrimp gastrointestinal tract and produces a toxin that causes tissue destruction and dysfunction of the the hepatopancreas, the shrimp digestive organ. The culprit is the bacterium Vibrio parahaemolyticus (Vp), a common enough bacterium in brackish saltwater which, when ingested, can cause gastrointestinal illness in humans.

The species of shrimp affected are:
  • giant tiger shrimp (Penaeus monodon)
  • whiteleg (or Pacific white) shrimp (Litopenaeus vannamei, formerly Penaeus vannamei)
  • Chinese white shrimp (Penaeus chinensis)
  • Mortality of infected shrimp stock can exceed 70%.
Prevention, control and mitigation measures include:
  • Improved biosecurity at the farm, zone, national, and regional levels
  • Zonal management of production units
  • Disease risk assessments, and
  • Development and implementation of aquatic veterinary health plans.
Unless they are certified by independent third parties as being free from AHPND, the use of live feeds for shrimp broodstock should be discouraged – or, where live feeds are used, these should be sterilised or frozen to reduce the likely transfer of AHPND.

Stocking density can have an effect, as does the use of probiotics. The use of Biofloc technology – enhancing water quality by balancing carbon and nitrogen in the system – has also been reported as helping in reducing the impact of AHPND 

EHP

In comparison to AHPND, very little is known about the effects on aquaculture systems by the fungal infection hepatopancreatic microsporidiosis (HPM) caused by Enterocytozoon hepatopenaei (EHP).
Hepatopancreatic microsporidiosis was first detected in P. monodon and then the infection is also reported as being an increasing problem in India, exacerbated by flooding 

What is known so far is that EHP infects only the tubule epithelial cells of the hepatopancreas of the shrimp. The spores are very small (1.1 × 0.60–0.07μm) and have a polar filament with 4–5 coils.

Although previously EHP was only reported as being found in the shrimp hepatopancreas tubule epithelial cells.

In 2010, EHP was reported as being associated with ‘white faeces syndrome’ (WFS), but further experiments have however failed to show the association with WFS, although transmission has been demonstrated. EHP spores are extremely hardy and EHP can be transmitted horizontally between shrimp. As a result EHP infection can spread progressively and is believed to intensify with successive shrimp crops over time.

Prevention, control and mitigation

Detection and screening is done by using nested polymerase chain reaction (PCR) and loop-mediated isothermal amplification (LAMP) tests. 

Histological analysis for the spores is possible, but is difficult unless performed by expert histopathologists. Live feeds should be avoided and/or freezing/sterilization should be carried out.

Research is currently being prioritised on:
  • A better understanding the lifecycle of EHP
  • The EHP transmission pathway
  • Development of real-time PCR testing
  • Easier histopathological identification of spores
  • Development and registration of surface disinfectants to control EHP
  • Development and registration of drug treatment for the treatment of live shrimp infected with EHP
  • Development of cell lines to culture microsporidia
Shrimp farmers have tried a variety of measures to try and eradicate EHP from hatcheries, nurseries and grow-out farms but with little success. There has been some experimentation with alternative treatments, including coccidiostats used to treat poultry like Monensin, but no research has yet been conducted to ensure the product is safe to use in shrimp production systems.



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