Prevention and Solution for White Feces Disease (WFD) in Litopenaeus vannamei Culture
by Dr. Farshad Shishehchian, Ph.D., Terrestrial and Aquatic Ecology, Group CEO & Founder, Blue Aqua International Group
White Feces Diseases (WFD) is one of the most serious problems in shrimp culture and is currently causing lower productivity in L. vannamei shrimp farms. WFD becomes apparent when the digestive system in shrimp malfunctions and its feces turns from normal (brownish colour) to a pale white colour. White feces appear to be more buoyant than normal feces and float on the water surface. The shrimp hepatopancreas also becomes whitish and soft. Farmers have observed that as soon as they see white feces, shrimp eat significantly less. Early disease indications appear in both feed trays and on the water surface, where abundant fecal strings-white to somewhat yellow feces are observed (Fig.1).
Diseased shrimps tend to be darker in color and exhibit loose shells. White feces disease often occurs one to two months after stocking and is manifested as reduced feed consumption and low absorption of feed nutrients. WFD has caused significant economic losses to shrimp farmers, due to high FCR, slow growth and size variation of shrimp at harvest.
Fig.1. Water surface and pond edge of a L. vannamei culture affected by white feces disease (Limsuwan, 2010)
Signs and symptoms of WFD in shrimp:
-Dark discolouration of the gills (Fig. 2)
-Hepatopancreas and gut become white and pale in colour (Fig. 3)
-Floating white fecal strings (Fig.1)
-Loose shell (Fig. 4)
Fig. 2. A shrimp affected by white feces disease, showing darkened gills, with epibiotic protozoa (Limsuwan, 2011)
Fig. 3. (A) White coloured gut with discoloured hepatopancreas of WFS-affected shrimp and
(B) Normal appearance of gut and hepatopancreas of healthy shrimp (Kumara & Hettiarachchi, 2017)
Fig. 4. Loose shell in L. vannamei, left: normal muscle, right: gap between exoskeleton and muscle (Kannan, 2014)
Main causes of WFD (the exact cause is still unclear)
1. Pathogenic factors
1.1. Bacteria: Vibrio spp. (Fig. 5)
1. 2. Parasite: Gregarines protozoa (Fig. 6)
Fig. 5. Vibrio spp. on TCBS agar (Khan et al., 2007)
Fig. 6. Gregarine found in the intestines of white feces disease-affected shrimp (Limsuwan, 2010)
1. Accumulation of sludge
Feces, uneaten feed and dead phytoplankton are the main sources of sludge. It takes at least 120 days to obtain shrimp of marketable size, a long period for the build-up of organic matter. Sludge provides an oversupply of phytoplankton nutrition. Following the rapid bloom of phytoplankton, pH fluctuation will cause phytoplankton die off and settle to the pond bottom which will be creating an anoxic condition and resulting in increasing of toxic substrates. Proper placement of the aerators are essential to prevent sludge buildup, where there is insufficient oxygen for aerobic bacteria to decompose organic matter in the pond. This will cause the anaerobic bacteria to take over in the decomposition of organic matter — giving off byproducts such as hydrogen sulfide that are harmful to the shrimp culture.
2. Over-feeding and poor feed quality
WFD can also be related to poor feed management and feed quality. Instead of determining how much to feed based on the leftover feed on the feeding trays, farmers should set the maximum amount of feed according to the culture density. In addition, low feed quality results in low digestibility and low nutrients absorption. The undigested feed contributes to decline of water quality.
3. Deteriorating water quality
The water quality parameters must be monitored during the culture period and with regular maintenance. Peak mortality rates are seen in extremely low oxygen (< 3.0 mg/L) and at low alkalinity (<80 ppm) levels. Following low DO and alkalinity, the nitrification cycle will be affected, causing the accumulation of ammonia which caused stress to shrimp. Low alkalinity is at risk of drastic fluctuation of pH. Poor water quality caused stress in shrimp and are more susceptible to disease outbreaks.
Laboratory Diagnosis procedures:
- Wet the mount of gut to detect the presence of gregarine (Fig. 6)
- Bacteriology culture on TCBS to detect the presence of Vibrio sp. (Fig. 5)
- Histopathology of the gut to check the presence of gregarine (Fig. 7)
Fig. 7. Histological section showing gregarines in the anterior midgut cecum of pond-reared shrimp juvenile. (H & E, 200X). (Lavilla-Pitogo et al., 2000)
Preventive measures and treatment
Prevention is important to reduce the risk of serious aquatic animal diseases. Production shortages resulting from shrimp mortality, slow growth, and high FCR occur and affect the productivity of shrimp farms greatly. Eliminating the WFD triggers is the best prevention. Early detection and diagnosis are crucial factors to well-timed and prompt control.
- The pond should be well prepared before stocking:
Removal of sludge: all the sludge should be removed. Farmers may neglect to properly remove all the sludge out of the pond bottom especially from its center. In the traditional pond preparation, the simplest method is to let the pond dry normally under the sun for several days. In doing so, the pond bottom will be treated by sun and sludge will be reduced and the lower layer of sludge will be exposed to the air. However, in case of rain or cloudy weather, shrimp ponds cannot be dried out. In traditional pond preparation, sludge accumulation in dried soil cannot be removed totally.
To eliminate sludge accumulation, use PondGro as a tool for bioremediation.
- After shrimp harvest, dry off the pond bottom for several days (7–14 days)
- Fill up with water (40 –70 cm)
- Apply PondGro, 10 kg/hectare and molasses 100 liters/hectare
- Turn on the paddle wheels for mixing and distribute PondGro and molasses over pond bottom.
- Leave the aeration on during the treatment period for 7–10 days
- Fill up water to proper level and continue for water preparation
PondGro is specific blend of strong activity and viable aerobic and facultative anaerobic bacteria. It enhances biological degradation of sludge accumulation in pond bottom. Improves soil pond bottom quality during pond preparation and reduces toxic substrates such as ammonia and hydrogen sulfide build up from soil pond bottom.
2. Decrease stocking densities during the hot season.
3. Applying biosecurity to prevent diseases. A reliable source of stocks, proper detection and diagnostic methods to screen diseases.
4. Sufficient aeration is required to keep the oxygen level in water (3.5–4.0 ppm) before dawn so that even during the night, the organic matter can be decomposed by bacteria more efficiently ensuring that there will be enough water current to sweep all sediment towards to center of the pond.
5. Monitoring of shrimp health — effective management of the shrimp health requires consideration of delicate balance between the host, pathogen and environment. Disease and production problems vary during different phases of shrimp culture. Most often pathogens are present in association with the environment and shrimps are apparently healthy and show normal growth.
6. Application of probiotic will reduce the chances of diseases. Application of appropriate probiotics containing Bacillus subtilis that deter the growth of pathogenic Vibrio spp. bacteria. Also, the use of specific nutrients to promote the development of selected prebiotics in gut.
- Apply 250–500 g BactoGro per hectare in every 3 days.
- Apply 250 g SoilGro per hectare every 3 days.
Improving pond bottom and water quality by the application of probiotics capable of degrading wastes and be useful in improving water quality and pond bottom.
8. Feed management:
Caution against over feeding: Besides using the feeding tray to determine the amount of necessary feed, farmers should also set a maximum feed limit at each culture period depending on the assumed survival rate.
8.1 Use of functional feed additives with non-specific immune system booster ability, with high level of essential amino acids, fatty acids and low anti-nutritional factors which improves shrimp health and survival.
-Use SigPak*Aqua. This is a highly bioavailable functional ingredient formulated to improve feed quality.
Mix SigPak*Aqua with 5–10% of feed, coated by binder.
SigPak*Aqua has a high digestibility and high absorption. High levels of essential amino acid, essential fatty acid, phospholipid and cholesterol to enhance immune response. It is also acts as a natural pathogenic bacteria inhibito to overcome diseases and infection.
8.2 Use natural antibiotic feed additive and growth promoter to prevent and control incidences pathogenic bacteria
-Use AlphaGuard*LPlus as a natural product combined with medium and short chain fatty acid and plant extract (eucalyptus, oregano and thyme). It has a synergistic effect on inhibition of pathogenic bacteria in shrimp gut.
-Dosage of AlphaGuard*LPlus is 3–5 ml per 1 kg of pellet feed.
-Mix AlphaGuard*LPlus with binder
-Combine the mixture thoroughly to coat feed
-Air dry 30 minutes before feeding
AlphaGuard*LPlus Is an effective substitute of antibiotic and growth promoters. It improve growth rate, prevent and control incidences of diseases, and strengthens the immune system. Farmers must cut feeding during an extreme weather change, for example, during the heavy rain for several days where salinity changes more than 3 ppt and temperature below 22 oC in one day period.
9. Maintaining the proper concentration of phytoplankton by controlling the organic matter, removal of dead phytoplankton from the surface after phytoplankton crash, maintain the stability of the plankton to balance C, N and P ratio.
To control the watercolor (phytoplankton) in a proper range:
Daily monitoring of water transparency is very important. Farmers should not leave phytoplankton to grow until water color darkens (dark green, transparency less than 20 cm). When water color darkens and transparency gets close to 30 cm, the growth of phytoplankton should decrease before starting to die off by using FytoShade until get result of 40–50 cm transparency.
In the case of phytoplankton crash: remove died off phytoplankton from the water surface (Fig.13) before they sink to pond bottom and convert to large amount of ammonia.
-Apply 250–500 g BactoGro per hectare every 3 days for ammonia to decompose died off phytoplankton in suspension and to prevent bloom of Vibrio spp.
-Apply 250 g SoilGro per hectare every 3 days for nitrate to compete with phytoplankton and degrade settled died phytoplankton on pond bottom
-In the case of high level of ammonia (more than 3 ppm) and nitrite (depending on salinity 5–10 ppm), apply 1kg of NitroGro and 3–5 kg AlkaSet per hectare at 7.00–8.00 pm every 3 days until concentration of ammonia and nitrite go down (should measure ammonia and nitrite every 2 days). If ammonia rate is more than 5 ppm, apply 3–5 liter of AmmoTrap per hectare once a week.
If the pond water is very clear: apply 2–6 packs of FytoShade per hectare to reduce sun light penetration in water (stress factor) until 40–50 cm trasparency is resulted.
Establish a new phytoplankton community by applying 3–5 kg FytoGro per hectare and 3–5 kg AlkaSet per hectare at 7.00–9.00 am in every 3 days until phytoplankton recovers growth (transparency of 40 cm, water color become green or brownish green)
Fig 8. Dead phytoplankton on surface of the pond should be removed
- All the aerators should be turned on full power to speed up decomposition of wastes
- Stop feeding for one day
- Resume feeding gradually
- To control pathogen and sanitization in shrimp gut, give a chance to get rid of the pathogens and other diseases.
-If the cause of WFD is presence of gregarine protozoa, apply 3–5 g ParaGo (top dressing with pellet feed) continuously for 3–5 days.
-If the cause of WFD is presence of Vibrio spp. bacteria, apply 3–5 ml of AlpaGuard* LPlus to reduce Vibrio spp. in shrimp gut, and mix SigPak*Aqua with 5–10% of feed coated by binder to recover the destroyed gut microvilli and reduce risk of WFD
5. Application of appropriate probiotics containing Bacillus spp. block the growth of pathogenic Vibrio spp. bacteria. Also, these products are specific nutrients to promote the development of selected prebiotics in gut.
-Apply 250–500 g BactoGro per hectare every 3 days to colonize in water column and control the bacteria colonization and bloom of Vibrio spp. in water.
-Apply 250 g SoilGro per hectare every 3 days to colonize in pond bottom and degrade the waste, eliminating the toxic substance build up and reduce the stress factors in ponds.
-If the farmer finds co-infection of shrimps by gregarines and Vibrio together, it is recommended to apply both ParaGo, AlpaGuard* L Plus and SigPak*Aqua together.
Management of WFD based on different stages of disease:
The best decision to treat the WFD spread in the infected ponds is directly related to the time of diagnosis of symptoms (Fig. 13). During the culture period, depending on which stage of infection we are dealing with and possible symptoms, the farmers can solve the problem and recover the shrimps by the timely management.
In the 1st stage of disease (first week), if approximately 10% of shrimps are infected and the rest are in good health, it is the best time to treat the disease and prevent it before spreading to healthy shrimps.
In 2nd stage, it is still possible to control the disease and recover the shrimps that are infected with WFD.
In 3rd stage, with increasing time, the number of infected shrimps will increase to 50%; in this case, there is still possibility to recover some of the shrimps.
However, in the 4th stage, chances to recover shrimps will be extremely difficult and one cannot solve the problem.
Fig. 9. Stages of treatment of WFD infected ponds.
In the case that many shrimp in the pond are infected with WFD, it is usually very difficult to recover. Farmers should start early harvesting. Prevention is extremely important to ensure success.