Prevention and treatment of White Feces Disease (WFD) in Shrimp
By Dr Farshad Shishehchian Ph.D., Terrestrial and Aquatic Ecology
Group President & CEO/Founder, Blue Aqua Group of Companies
White feces disease (WFD) is one of the most serious problems in shrimp culture. WFD on L. vannamei shrimp farming is currently causing lower productivity. WFD becomes apparentwhen the digestive system of shrimp malfunctions and feces turns from normal (brownish color) to pale white color. White feces appear to be more buoyant than normal feces and float on the water surface. Shrimp hepatopancreas becomes whitish and soft. Farmers have observed that as soon as they see white feces, shrimp eat less. Early disease indications appear in both feed trays and at water surface, where abundant floating fecal strings white to some-what yellow feces are observed (Fig.1) and sometimes could also be found on feeding trays. Diseased shrimp tend to be darker in color and after some time their bodies will lose firmness and become soft and limp, and eventually will die. White feces disease often occurs one - two months after stocking and is manifested as reduced feed intake and feed absorption in the shrimp’s gut. WFD has caused significant economic losses to shrimp farmers, because of high FCR, slow growth, and variable sizes of shrimp at harvest.
Fig.1. Water surface and pond edge of a Litopenaeus vannamei culture affected by white feces disease (Limsuwan, 2010)
Signs and symptoms of WFD in shrimp:
- Dark discoloration of the gills (Fig. 2)
- Hepatopancreas and gut become white and pale in color (Fig. 3)
- Floating white feces strings (Fig.1)
- Slow growth
- Infected shrimps show loose shell (Fig. 4)
Fig. 2. A shrimp affected by white feces disease, showing darkened gills (Limsuwn, 2011)
Fig. 3. Comparison of healthy shrimp (left) to WFD infected shrimp (right) (Limsuwan, 2011)
Fig. 4. Shell loss effect of WFD, left: anormal shrimp, right: loose shell (Limsuwan, 2010)
Main causes of WFD (the exact cause is still unclear)
1. Pathogenic factors
1.1. Bacteria: Vibrio spp. within hepatopancreas and midgut (Figs. 5)
1.2. Gregarins protozoa in hepatopancreas and midgut (Fig. 6)
Fig. 5. TCBS agar plate shows Vibrio spp.in feces (Limsuwan, 2010)
Fig. 6. Gregarine parasite in squash mount of intestine (Limsuwan, 2010)
2.1. Accumulation of sludge
Accumulation of sludge in intensive shrimp ponds impacts its production. Sludge is derived from feces, uneaten feed and died off phytoplankton. Sludge deposits, are suspected to be the responsible for most of the biological oxygen demand (BOD), mineralization of nutrition from organic matter, and formation of toxic metabolites. Sludge provides over-supply of phytoplankton nutrition. Phytoplankton in ponds which grows fast and dies off fast cannot control its optimum concentration. Following the fast growth of phytoplankton, pH fluctuates fast as well. Next, the phytoplankton die off and sink to the pond bottom and cover soil surface causing anoxic condition and toxic substrate will come up.
2.2. Dirty pond bottom and feeding area
To reach good shrimp production, it is important to pay attention to cleanliness of the pond bottom and feeding area. It takes at least 120 days to obtain market size shrimp, thus if the ponds bottom is not managed well during that period, large amounts of organic matter may build up all. If the waste is not all swept to the center of the pond, then the area of the pond bottom that is clean will be smaller and shrimp will not have enough space to live in clean area. Many aerators are required to provide sufficient oxygen. If the placement of the aerators is wrong, the sludge builds up until there is insufficient oxygen for aerobic bacteria to decompose organic matter. Subsequently anaerobic bacteria will take over to decompose organic matter. These bacteria give off byproducts such as ammonia, nitrite, hydrogen sulfide that are harmful to shrimp.
2.3. Over feeding and poor feed...
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