White Feces Syndrome: Latest understanding and preventive measures

Firstly described around 2010, White Feces Syndrome (WFS) is an important disease for shrimp farming in Asian countries that can cause severe mortalities and economic losses. This disease is a complex syndrome whose occurrence remains unclear. Factor triggering WFS is yet unknown and this might not be the result of one single agent.
Description, etiology and preventive measures: Aquaneo’s experts give an update.

White Feces Syndrome: a syndrome that causes mortality and reduce performances

The White Feces Syndrome refers to the presence of floating white fecal strings in ponds. It can be observed in both farm-reared Black tiger shrimp (Penaeus monodon) and Pacific white shrimp (Litopenaeus vannamei). The syndrome causes several issues, including high mortality, growth retardation, size heterogeneity and elevated feed conversion ratio. Survival is generally reduced by 20-30% (Tamilarasu et al., 2020) and the cumulative mortalities can be as high as 50% in the summer periods (Hou et al., 2018). Early disease indications (floating fecal strings) appear in feed trays and at the water surface (figures 1a-1b), with a sudden reduction in feed consumption. It usually happens in grow out ponds around the second month after stocking.
The shrimp feces turns from normal (a brownish color) to a pale white color, leading to the name of this disease. White feces appear to be more buoyant than normal feces and float on the water surface. The shrimp hepatopancreas also becomes whitish, soft, and look empty due to the lack of feed. The midgut is distended and filled with white to yellow substance (figure 1c). Also, a loose exoskeleton and dark-colored gills can be observed.

Figure 1 – Source: Sriurairatana et al. (2014)

But these feces are not “true” feces. While normal ones are mostly composed of undigested feed, the white feces are composed of leftovers of tissues from hepatopancreas, gut mucus, and frequently contain a mixed bacterial component and massive number of spores from the microsporidian parasite Enterocytozoon hepatopenaei (EHP) (Pachumwat et al., 2021). Vibrio species have also been found in the feces of infected shrimps (Suguna, 2020). When the contents of the gut or fecal strings were examined with a light microscope, they consist of masses of vermiform bodies that look like protozoan gregarines (Sriurairatana et al., 2014). This explains why at some point, it was supposed that gregarines were the causative agents of WFS.
Sriurairatana et al. (2014) described the White Feces Syndrome as coming from a disorder in the microvilli from epithelial cells of the hepatopancreas. Transformed microvilli, peeled away, are collected in the tubule lumens and aggregated together as vermiform bodies. Stripped of microvilli, cells undergo lysis. The vermiform bodies are almost transparent with no cellular structure. They accumulate with tissue debris, mucus, EHP spores and eventually Vibrio bacteria at the hepatopancreas-midgut junction and in the midgut. These aggregated and transformed microvilli lead to the formation of white fecal strings, which are finally excreted (figure 2).

Figure 2 – Source: TECHNA

Histology performed by Sriurairatana et al. (2014) on a cross section of hepatopancreas tubule (figure 3a) show a modified, sloughed B-cell in the lumen with microvilli spread over its surface. Epithelial cells with normal microvilli and others with abnormally thin layers, or denuded of microvilli, with lysis process ongoing were also seen.

Figure 3 – Source: Sriurairatana et al (2014)

By transmission electron microscopy (figure 3b), hepatopancreas epithelial cells show normal and transformed microvilli and an early stage in the aggregation of transformed and sloughed microvilli surrounded by an enclosing membrane. After, aggregated transformed microvilli form gregarine-like entities.

Environment, microbiota and stressful conditions: key factors

White Feces Syndrome is a syndromic condition of complex etiology, similar to diarrhea in humans (Pachumwat et al., 2021), and its origin is still unclear. Recent study shows that a full intestinal ecosystem alteration, rather than a single pathogen, is associated with WFS (Huang et al., 2020). On the other hand, many studies show an important impact of the environment and stressful conditions on this syndrome. These adverse conditions can also facilitate the development of opportunistic pathogens and create an unbalanced digestive bacterial community. Furthermore, anti-nutritional factors, toxins such as mycotoxins contained in feeds might create damage to the hepatopancreas, thus promoting degraded hepatopancreas conditions that favor the occurrence of WFS.

Like humans, the shrimp digestive system hosts a large number of microorganisms dominated by bacteria, which constitute a complex microbial ecosystem called the microbiota. It has multiple functions and plays an important role in maintaining host health, nutritional absorption or immune response. The balance in the intestinal microbiota population is crucial and the loss of microbial diversity in the intestine can cause host disease.

Figure 4 – Source: Huang et al. (2020)

Recent studies (Hou et al., 2018 and Huang et al., 2020) reported a close relationship between dysbiosis of the intestinal microbiota and White Feces Syndrome. According to these authors, the intestinal bacterial communities of white feces affected shrimp are significantly less diverse and more heterogeneous than normal shrimp (represented respectively by Shannon index and Bray-Curtis dissimilarity on figure 4).

The profile of the intestinal bacterial community is significantly different between healthy and white feces affected shrimp. According to Huang et al. (2020), increased abundances of opportunistic pathogens (Vibrio, Candidatus Bacilloplasma, Aeromonas and Photobacterium species) and decreased abundance of the beneficial bacteria (Chitinibacter spp.) were the common features associated with white feces affected shrimp. Furthermore, this study demonstrated that the transplantation of intestinal microbiota from white feces infected donors to healthy shrimp leads to similar symptoms in about one third of cases. Hou et al. (2018) reported that the overgrowth of intestinal Candidatus Bacilloplasma and Phascolarctobacterium species and depression of Paracoccus and Lactococcus, known to be beneficial to the host, might result in the occurrence of WFS.

White Feces Syndrome is often associated with the microsporidian EHP whose spores were found in and around the transformed microvilli. As described by Huang et al (2020), the microsporidian was firstly proposed as causality of WFS while this hypothesis was not supported by subsequent study). Recently, Pachumwat et al. (2021) showed that EHP may be a component but not the only causative agent. According to Sathish Kumar et al. (2022), WFS is a shrimp diarrhea associated with severe EHP infection that may be caused by a combination with an unknown agent.

The severe loss of microvilli may predispose shrimp to other enteric pathogens. The study of Somboon et al. (2012) indicated that most of the shrimp that had white feces had significant higher amounts of Vibrio bacteria, including V. vulnificus, V. fluvialis, V. parahaemolyticus, and V.alginolyticus, in their hemolymph and intestine, than in control shrimp.

Preventive measures

Even though the exact causative agent or factor triggering White Feces Syndrome is unknown, some actions could be taken to avoid its outbreak or, at least, reduce severe impacts and losses of this syndrome. We can divide possible actions in two different categories, the shrimp and its environment.  

Shrimp

It has been proven that WFS firstly target the hepatopancreas cells of shrimp. Therefore, promoting a better condition of this organ could help to counter the WFS effects. This could be done by using a high quality, highly digestible feed. Quality ingredients and proper formulation will lead to facilitated digestion and less demand on the hepatopancreas which could be more resilient.

Choosing raw materials that contain low levels of toxins or anti-nutritional factors is also important; as these compounds will directly affect hepatopancreas. Indeed this organ is handling a lot of the toxins in the shrimp.

Furthermore, the feeding ratio should be adapted. High feeding rates are stressful for the animal and its digestive system. Lowering the amount of feed during risky periods is as important as the feed quality.

We’ve seen that White Feces Syndrome impact and losses are mostly caused by opportunistic pathogens such as EHP or Vibrio bacteria. Specific additives could be used to limit the proliferation of these pathogens in the digestive gut of shrimp. Immunity of the animals could also be increased by the use of specific ingredients, called immune stimulants. Thanks to its long time experience and knowledge in the field of additives; Aquaneo designed a product called WF Detox. It helps the shrimp immune system and has actions on the pathogens and overall microbial population of the digestive tract . WF Detox also incorporates plant extracts that prevent membrane degradation and help toxin excretion. These actions aim at an improved hepatopancreas status. WF Detox should be used as a preventive additive when conditions are in favor of WFS outbreak. It can also be used at higher doses when WFS is present in the ponds.

Environment

Besides shrimp, the environment also needs to be considered. Indeed, pond is a complex ecosystem that has a direct impact on the animal, but also on the surrounding pathogens. It is important to monitor parameters and adapt farming practices accordingly.

First, high temperature (Tamilarasu et al., 2020), but also fast temperature fluctuations induce stress to the animals that could help the White Feces Syndrome outbreak or increase its severity.

Other important parameters to monitor include: pH, alkalinity, ammonia, nitrite and oxygen levels. Similarly, maximum and/or minimum levels as well as strong fluctuation could favor the WFS outbreak or increase its severity. High densities, poor pond-bottom quality and high planktonic blooms are also negatively affecting environment quality and animals.

Feeding and feed quality also have an impact on the environment as the undigested feed will remain in the ecosystem, polluting it and ensuring conditions for the growth of opportunistic pathogens. As already mentioned above, feeding should be adapted and reduced when risk increases (values close to the limits and strong fluctuations).

Strict enforcement of biosecurity measures are also of importance to keep pathogens as low as possible in the environment. Opportunistic pathogens could have a very strong impact in the case of WFS outbreak.

Finally, it is important to adapt and anticipate. Indeed, prevention is more effective and less expensive than cure. Farm management should be adapted according to the conditions. For example, ammonia toxicity is different according to pH and temperature. Therefore, ammonia limit should be adapted according to these parameters. Furthermore, if the ammonia level is increasing, don’t wait until it reaches the limit to take actions. It is important to anticipate.

Feeding should also be adapted to the overall pond conditions and not only shrimp size and temperature. Feed choice or the use of additives such as WF Detox should be linked with the risk of White Feces Syndrome outbreak.

For more information, please do not hesitate to ask our experts at Aquaneo!

References

• Hou, D., Huang, Z., Zeng, S. et al. (2018) Intestinal bacterial signatures of white feces syndrome in shrimp. Appl Microbiol Biotechnol 102, 3701–3709. https://doi.org/10.1007/s00253-018-8855-2
• Huang, Z., Zeng, S., Xiong, J. et al. (2020) Microecological Koch’s postulates reveal that intestinal microbiota dysbiosis contributes to shrimp white feces syndrome. Microbiome 8, 32. https://doi.org/10.1186/s40168-020-00802-3
• Prachumwat A., Munkongwongsiri N., Eamsaard W., Lertsiri K., Flegel T.W., Stentiford G.D., Sritunyalucksana K. (2021) A potential prokaryotic and microsporidian pathobiome that may cause shrimp white feces syndrome. bioRxiv 2021.05.23.445355. https://doi.org/10.1101/2021.05.23.445355
• Sathish Kumar T., Makesh M., Alavandi S.V., Vijayan K.K. (2022) Clinical manifestations of white feces syndrome, and its association with Enterocytozoon hepatopenaei in Penaeus vannamei grow-out farms: A pathobiological investigation. Aquaculture 547. https://doi.org/10.1016/j.aquaculture.2021.737463
• Somboon M., Purivirojkul W., Limsuwan C (2012). Effect of Vibrio spp. in white feces infected shrimp in Chanthaburi, Thailand. Kasetsart University Fisheries Research Bulletin (Thailand). Vol. 36 N. 1 Page 7-15
• Sriurairatana S, Boonyawiwat V, Gangnonngiw W, Laosutthipong C, Hiranchan J, Flegel TW (2014) White Feces Syndrome of Shrimp Arises from Transformation, Sloughing and Aggregation of Hepatopancreatic Microvilli into Vermiform Bodies Superficially Resembling Gregarines. PLoS ONE 9(6): e99170. https://doi.org/10.1371/journal.pone.0099170  
• Suguna, T. 2020. Disease Diagnosis Prevention and Control of Diseases in L.vannamei. Int.J.Curr.Microbiol.App.Sci. 9(9): 764-776. doi:https://doi.org/10.20546/ijcmas.2020.909.096
• Tamilarasu A, Nethaji M, Bharathi S, Lloyd Chrispin C, Somu Sunder Lingam R. Review on the emerging white feces syndrome in shrimp industry. J Entomol Zool Stud 2020;8(5):680-684