Bacterial infections of farmed shrimp are quite common and likely the major cause of mortality in farmed shrimp. Vibriosis is one of the most severe bacterial diseases in shrimp farming caused by Vibrio spp. These bacteria from part of the natural microbiota of wild and cultured shrimp and the marine environments.  Vibrio infections have become a major constraint of the production and trade in shrimp aquaculture. They are responsible for several diseases and mortalities of up to 100% and cause crop loses globally.

Vibrio usually associated with multiple etiological agents. Vibrio has been reported in penaeid shrimp culture systems implicating at least 14 species and they are Vibrio harveyi, V. splendidus, V. parahaemolyticus,V. alginolyticus, V. anguillarum, V. vulnificus, V. campbelli, V. fischeri, V.damsella, V. pelagicus, V. orientalis, V. ordalii, V. mediterrani, V. logei etc. However some Vibrio species have been identified as primary pathogen,  including V. parahaemolyticus and V. harveyi have been described as the main pathogenic species in shrimp. 

Clinical signs of Vibriosis

• • Redness of body
  • Antennae cut
  • Atrophy of hepatopancreas
  • White faeces
  • Delayed hemolymph
  • Luminescence in dark
  • Anorexia
  • Lethargy
  • Melanization
  • White patches in the abdominal muscle
  • Expanded chromatophores
  • Luminescence and necrotic areas in the uropods
  • Mortality

Shrimp diseases associated with Vibriosis

Vibrio can remain in the environment without causing disease, but can very easily switch from opportunistic and commensal to pathogenic when conditions change. Thus, its ability to cause disease or increase in virulence, is a complex process affected by many variables, including host, vibrio species, developmental stage, physiological conditions, environmental stress, and infection method.

They are several diseases associated with Vibriosis.

• • V.harveyi - Luminous disease
  • V.Parahaemolyticus – EMS/AHPND
  • Vibro sp. - White Faecal Disease (WFD)

Diagnosis

Diagnosis of vibrio infection is based on clinical signs and the histological demonstration of rod-shaped Vibrio bacteria in lesions, nodules or haemolymph. Excised organs and haemolymph may be streaked on a Vibrio-selective (TCBS) or general marine agar plate. Vibrio isolates may be identified by a number of methods, including: Gram stain, motility, an oxidase test, mode of glucose utilisation, growth in the presence of NaCl, nitrate reduction and luminescence.  

Vibrio threshold in shrimp farms:

There are many  vibrios maximum threshold standards in farms. Most of them are for Vibrio spp. in pond water. These are the common maximum thresholds used by shrimp farms:

• Total Vibrio Count (TVC): varies from 10³CFU/mL to 10⁴ CFU/mL. Some farmers adopt the TVC of 10² CFU/mL as the maximum threshold. Total Vibrio Count (TVC) in shrimp gut is 10⁵ CFU/g
• Vibrio colonies:10² CFU/mL for green colonies and 10³ CFU/mL for yellow colonies.
• Vibrio percentage: 5% to 10% TVC of total plate count, and 10% Vibrio green colonies of TVC.

How to prevent Vibrio problems?

Vibrios are difficult to eradicate because they adopt well to different environment conditions and can adopt state when facing adverse conditions. However, Vibriosis is controlled by rigorous water management and sanitation to prevent the entry of vibrios in the culture water and to reduce stress on the shrimps. Pond Management and robust gut health are important strategies to control vibrio, together with frequent sampling to monitor their levels in the shrimp gut and pond ecosystem.

Good site selection, pond design and pond preparation are also important. Maintain adequate water quality with low bacterial biomass, A stable phytoplankton bloom and a proper feeding program, Sterilize or filter recirculated water, Routinely monitor shrimp and pond for early diagnosis of a problem, Avoid temperature extremes or rapid variation in temperature, handling, overcrowding, and other stressors, Infections opportunistic and probably result from poor husbandry; may be secondary to other disease processes. An increase in daily water exchanges and a reduction in pond biomass by partial harvesting are recommended to reduce mortalities caused by vibriosis.

The combination of gut acidifiers, prebiotics, use of natural antimicrobials and Probiotics, Bacteriophage applications, immunostimulants and non-antibiotic substances has superior specificity against vibriosis and Luminescent Bacteria (LB) coupled with Best Aquaculture Practices (BAP) , which makes it an effective management tool for the control of vibriosis and bacterial toxicity in aquaculture.

Treatment

Use V Phages Growout immediately when various symptoms of Vibriosis as Luminescence in water, infections in shrimp and or low feed intake etc., correlated by lab test. 

EHP or Enterocytozoon Hepatopenaei is an yeast-like fungus belonging to a group called “microsporidia”, which are obligate intracellular parasites. It was first reported as an unnamed microsporidian from growth retarded black tiger shrimp Penaeus monodon from Thailand in 2004. This fungus infects the hepatopancreas of the shrimps, so it’s called as Hepatopancreatic microsporidiosis (HPM). It also has smaller spores (approximately 1 μm in length) and is currently known to infect both P. monodon and P. vannamei.

CAUSES

• Hepatopancreatic microsporidiosis (HPM) is an infectious shrimp disease caused by enteric fungi, Enterocytozoon hepatopenaei (EHP).
• EHP can be transmitted directly from shrimp to shrimp by cannibalism and cohabitation.
• Shrimps become infected by ingesting spores from the water, from sediment, from eating EHP-infected live feeds (polychaetes, molluscs, frozen artemia biomass etc).

SYMPTOMS

• The target organ of EHP is hepatopancreas and affects its digestive and absorptive functioning resulting in poor growth and immunity.
• EHP infected shrimp may have a thin cuticle, white muscle as a stress response, black spots on their eyestalks, in their muscle tissue and along the hind gut.
• It is associated with severe growth retardation in P.vannamei exhibits high size variability.
• Causes chronic mortality in severe cases.
• Associated with White Fecal Disease.

DIAGNOSTIC MEASURES

• Infection can be checked by microscopic examination ( at x100 oil immersion) of the hp and the gut of the shrimp.
• Infection can also be confirmed by molecular testing of the hp by PCR.

PREVENTION

Regular health assessments are recommended. If there is a large difference in the size of PL, if PL feeding activity is less than expected, the number of lipids in the hp drops, growth and moulting slows, then these are also good indications of infection. The number of swollen tubules may provide an indication of how advanced infections are.

Some important insights on practical solutions for managing EHP is described below:

HATCHERY

• The best approach for maturation and hatchery facilities to avoid EHP is to never use live animals (e.g., live polychaetes, clams, oysters, etc.) as feeds for broodstock. Use only SPF live polychaetes, SPF squid and other fresh feeds like Mussel (subjected to -20 degrees freezing for more than 48hrs) in maturation systems.
• Feed breeders with Redi-Mate in maturation system, a biosecure diet that could replace up to 50% of live polychaetes in commercial trials & 60% fresh feeds (without live polychaetes). 
• Adopt ultrafiltration system in the hatchery system with 0.1micron size or lower to exclude spores.
• Recommended to remodel maturation system by expanding maturation system using imported PPL and accept lower spawn size without the use of live feeds.

 NURSERY

• Stressors can provoke infection of EHP in ponds. Nursed PL can make animals robust and grow faster due to compensatory growth.
• Nursery cuts down culture period and reduce animals exposure to pathogens & stressors
• Adopt water filtration system in shrimp nurseries that could exclude spores.
• Use PL Raceway 40-9, a high quality nursery feeds proven to achieve FCR 0.8 at density 1-12pcs/Liter

GROWOUT

• The spores of EHP have thick walls and are not easy to inactivate. Even high levels of chlorine alone are not effective. Infected ponds should be treated with hot limes. Burnt lime CaMgO and Hydrated (Ca(OH)2) can increase pH and knock down spores at 6mt/ha. Need to moisten the pond prior to application. It needs water to react and activate to increase pH to 14. 
• Stock EHP PCR-negative post larvae 
• Treat all incoming water with 40ppm of 65% active chlorine or 15ppm KMnO4
• Ponds with sandy loam soil and high sedimentation should be PE lined. High suspended solids makes shrimps susceptible to EHP spores through oral ingestion. 
• Manage water quality with the use of effective probiotics. Shrimps infected with EHP and challenge with Vibrio (mixed infection) after exhibit white feces.