Monday, June 20, 2016

White faeces disease in shrimp

Author: Poh Yong Thong
Published in January/February 2016 AQUA Culture Asia Pacific Magazine

Despite its presence since 2010, little is known of this disease now impacting shrimp farming in Asia.

Ecological disturbances in particular by global warming have resulted in the rise of surface water temperatures. In addition, the eutrophication of many coastal waters is due to increased shrimp farming intensity spurred by more than 2 years of attractive shrimp prices. These two conditions contributed to the onset of many new viral, bacterial and fungal shrimp diseases. A silent disease which has an impact in shrimp farms in most parts of Asia is the white faeces disease or WFD. Present since 2010 in Thailand, Dr Chalor Limsuwan, Kasetsart University attributed this new pathological entity to unusually high temperatures of more than 32˚C and high stocking densities which brought about increased levels of organic matter in the pond (Limsuwan, 2010). Around the same time, shrimp farms in Peninsular Malaysia, particularly in Sitiawan, in the state of Perak were infected with this disease. The spread of the disease died down for a while in 2012 but resurfaced since the end of 2014,
first in West Sumbawa but spread to East Java, Jogjakarta and Lampung in Indonesia, and also in Thailand and Malaysia.

A comparison of the gut of WFD infected shrimp (right) with the gut of a normal
shrimp. Picture courtesy of Iwan Sutanto, chairman of Shrimp Club Indonesia.

What is WFD ?

WFD becomes apparent when the digestive system of the shrimp malfunctions and the faeces turns from the normal brownish colour to pale white. The hepatopancreas becomes whitish and soft. The white faeces appear to be more buoyant than normal faeces and float on the water surface, appearing like faecal strings as shown in the photo below. Limsuwan (2010) said that in addition to the white faeces, infected shrimp show a loose exoskeleton and are also infested by epibiotic protozoa that cause a dark discolouration of the gills. Shrimp infected by WFD exhibit marked reduction in feed intake and a severe infection of WFD may result in up to 60% mortality.

What do we know ?

Here I discuss pre 2014 and post 2014 findings presented at seminars and in the literature on WFD in Asia. In 2010, Limsuwan reported the presence of Vibrio parahaemolyticus, V. fluvialis,V. alginolyticus and V. mimicus in the faecal analysis of WFD shrimp. Then, during the latter part of 2010, Limsuwan et al.(2010) further found V. vulnificus, V. fluvialis, V. parahaemolyticus, V. alginolyticus, V. mimicus, V. cholerae (non01) and Photobacterium damselae (V. damselae) in the haemolymph and intestines of WFD shrimp. In addition, in Vietnam, Ha et al. (2010) reported that the causative agent of WFD was the microsporidian
Enterocytozoon hepatopenaei or EHP.

In June 2014, the team of Visanu Boonyawiwat, Kasetsart University and Timothy W. Flegel, Mahidol University, Thailand (Sriurairatana, et al. 2014) revealed that the microvilli of WFD
shrimp peeled away from HP tubule epithelial cells and then aggregated in the tubule lumen – hence they coined the term ATM (aggregated transformed microvilli). The vermiform-like
bodies showed no cellular structure and were unlikely to be cellular microbiota. The cause of ATM is currently unknown, but the loss of microvilli and subsequent cell lysis indicate that their formation is a pathological process.

The authors further suggested that the disease in the prevalence of ATM has been coincidental with the increase in early mortality syndrome (EMS) or acute hepatopancreatic necrosis (AHPND) outbreaks. The causative agent of AHPND is V. parahaemolyticus which produces the toxin (which is regulated by quorum sensing or QS) that in turn causes sloughing of HP tubule epithelial cells. Thus, we may ask whether the same EMS toxin at low dosages is responsible for the formation of ATM or a mild form of WFD.

In summary, WFD could be due to an infestation of Vibrio bacteria, the dreadful microsporidia EHP or the physical sloughing off of hepatopancreatic tubules due to possibly a bacterial toxin. The exact causative agent to date is still unknown.

Possible causes

As presented in a workshop organised by the Shrimp Club Indonesia in Surabaya in October, 2015, based on studies by industry in Indonesia, Dr Arief Taslihan, Centre for Development of Brackish Water Aquaculture (BBPBAP), Jepara, Anwar Hasan, Biomin Indonesia and Sidrotun Naim S, Surya University, said that the triggers of WFD are usually:

• An algae crash with rising total ammonia nitrogen (TAN)
• Cyanophyta being replaced by dinoflagellata and ciliata/
protozoa
• High organic load (>100 ppm)
• High total Vibrio count in water > 1 x 102 CFU/mL
• Pond water with low transparency of <20 cm
• High alkalinity >200 ppm or <80 ppm,
• Prolonged low dissolved oxygen at <3.0 ppm

In its June 1, 2015 bulletin, feed producer PT Matahari Sakti reported that the total organic matter (TOM) in Indonesian sea water has increased to >50 ppm and this favours the growth of pathogenic bacteria which cause diseases. It stated that the environment is overloaded and cannot self-purify itself resulting in the imbalance in the ecosystem.

Prevention is better than cure

As until today, the exact cause of shrimp WFD is still unknown, the best that shrimp farmers can do to combat this disease is prevention. This means farmers must be on the alert for the
possible onset of WFD, and they must be more observant. The first recommendation issued by Thai experts was for farmers to decrease the stocking density during the hot season. This will result in a decrease in bottom organic matter and a reduction in the Vibrio loading in the pond bottom. Some have mitigated the disease by using probiotics containing Bacillus subtilis that block the growth of Vibrio bacteria.

To prevent WFD, start with thorough pond preparation, complete sterilisation of water, limit use of organic inputs to prevent excessively fertile water and pond bottom, stringent control of feed and careful monitoring of Vibrio and water quality. It is important to detect the disease as early as possible before more of the shrimp lose appetite. Constant observation
and surveillance in particular during night time is necessary. The discovery of partial white strands of faeces is cause for alarm.

Feed management

In feeding shrimp, we use sinking pellets which sink to the pond bottom. It takes time for a skilled worker to master the dispensation of the exact amount of feed to shrimp. Too little
feed will affect growth and health but too much feed will result in high organic load, ammonia, hydrogen sulphide and pathogenic bacteria in the pond. Uneaten feed and excessive excreta produced from excessive feeding will result in high organic matter in the pond, producing not only harmful gases such as ammonia and hydrogen sulphide which deteriorate the water quality, but also supplying food for the proliferation of pathogenic bacteria and harmful algae.

It is thus imperative that the shrimp farmer is able to dispense an optimal amount of feed to the shrimp pond by meticulous monitoring of feed trays. The feed trays are ingenious tools invented by the Taiwanese in the 1980s and are very useful in fine tuning the daily dispensing of feed in response to the daily changes in weather and water quality which affect the appetite of the shrimp.

Treatment of WFD

If the pond is infected by WFD, the following treatment can be attempted. This treatment protocol was developed and has been shown to be effective in some ponds.

• Reduce immediately the feed amount or even stop feeding for the whole day
• Run as many aerators as possible. Add additional aerators if available
• Blend 80 g fresh garlic/kg feed in a blender, mix with 150 ml of freshwater and top-dress the feed for 1 day’s feeding
• In lieu of garlic, add Potassium permanganate or KMNO4 to the feed at 5-10 ppm, for the daily feed ration
• Add a reputable probiotic at 3 times the normal rate to the pond
• The next day, add the same probiotics such as Bacillus to the feed and feed for a day
• Repeat the alternative top-dressing of garlic and reputable probiotics for 5 days.

Conclusion

Shrimp farmers must be aware that shrimp farming management and technology are constantly changing. What used to work previously may no longer be applicable due to the change in weather conditions and the eutrophication of the coastal ecosystem. Today, shrimp farmers who cling to past farming husbandry will find it hard to succeed. A critical aspect to this adaptation is feed management. Due to the deteriorating coastal waters containing a much higher organic load, the carrying capacity of the pond will be reduced. Excess uneaten feed will produce ammonia and hydrogen sulphide which act as feed for
pathogenic bacteria. Shrimp farmers who learn and adapt will stand a better chance of success.

The ultimate aim in shrimp health management is to reduce or dwarf the growth of pathogens by reducing excessive nutrient loading to the ecosystem through stringent feed management while optimising the quality of the environment as well as safeguarding the health status of the stock via genetics and nutrition.

Thank you for visiting our website

0 comments:

Post a Comment