Wednesday, February 8, 2017

Vibrio in shrimp gut: challenge and control strategy

After Litopenaeus vannamei was introduced in Asia, world shrimp production increased from 2 million tonnes (45% L. vannamei) in 2003 to almost 4 million tonnes (71% L. vannamei)
in 2011. Unfortunately, disease occurrences has followed the growth of shrimp production, particularly since 2008. Infectious myonecrosis disease (IMNV), white spot syndrome virus
(WSSV), white faeces and early mortality syndrome/acute hepatopancreatic necrosis disease (EMS/AHPND) have each hit shrimp production in Asia.

Viral diseases remain a big challenge. However, now the industry must be more cautious towards non-viral diseases, particularly bacterial disease caused by Vibrio spp. Vibrio has
been well-known as a major bacterial pathogen in shrimp since the 1990s, but it was more common to post larvae stage in the hatchery. However, there have been some reports of Vibriosis during the grow-out phase. Now, diseases  related to Vibrio affect shrimp production.
Vibrio and diseases
Vibrio plays a role in primary and secondary infections. As primary infection, Vibrio has become an even greater cause of concern after Tran et al. (2013) reported Vibrio parahaemolyticus as causative agent of AHPND. Interestingly a researcher in Vietnam found virulence genes of V. parahaemolyticus in Vibrio harveyi. In recent years, the concern on virulent gene (in plasmid) transference not only among V. parahaemolyticus strains but also to different Vibrio species has grown.

On secondary infection, Phuoc et al. (2009) showed that there was signifiant increase of Vibrio campbellii in shrimp haemolymph in co-infection with WSSV; increasing to 105 CFU/mL, from V. campbellii single infection of only 102 CFU/ mL. There was no difference in infected cells by WSSV with or without V. campbellii co-infection implying that co-infection with V. campbellii was not affected by WSSV virulence. However, co-infection of WSSV and V. campbellii or V. harveyi caused higher and faster mortality during normal WSSV infection.
Moreover, Manilal et al. (2010) reported that non-pathogenic Vibrio alginolyticus became virulent after a WSSV outbreak in the pond. In some cases, V. alginolyticus has a known probiotic role to compete with V. harveyi.
Monitoring Vibrio in shrimp
Based on the role of Vibrio in diseases, farmers should monitor the Vibrio level in pond water as well as in shrimp. Monitoring of Vibrio in pond water is already quite common. Many farms have Vibrio monitoring in pond water as a standard operating procedure (SOP), but not many farms today monitor Vibrio in shrimp body. As a further precaution, it is recommended that farmers collect samples to monitor Vibrio level in shrimp tissues such as haemolymph, hepatopancreas and gut.

The case of white faeces disease (WFD) in Chanthaburi, reported by Somboon et al. (2012) showed us how important it is to know Vibrio levels in shrimp tissues (haemolymph and the
gut). The report showed different total composition of Vibrio in haemolymph and gut in shrimp collected from normal pond and pond with an outbreak of WFD. Vibrio is normal flra in the shrimp pond ecosystem. It is present in water and also in shrimp. Controlling Vibrio at a normal level is important to keep shrimp in a desirable condition. Letting Vibrio growth exceed the normal range will lead to a higher susceptibility to diseases.
Additives to control Vibrio in shrimp gut
In the past, many farms applied antibiotics to control Vibrio, particularly during the hatchery phase. This led to many cases where shrimp exports were rejected due to banned antibiotics being detected or allowed antibiotics exceeding permitted levels.

There are specifi cases where antibiotics in correct doses in shrimp farming is allowed, although application should always be done under the supervision of a veterinarian. The veterinarian will design an application program that runs over the course of several days to treat sick animals. Importantly, antibiotics are not meant for everyday use for normal and healthy shrimp. Improper use of antibiotics can lead to several problems related to antibiotic resistance.

Antibiotic resistance of bacteria impacts the choice of therapeutic drugs humans and animals can use. Antibiotic residues in food-producing animals pose health concerns in humans. Not to be forgotten is the issue of ‘superbugs’–bacteria that are resistant to multiple kinds of antibiotics.

In order to control infectious bacterial diseases in shrimp aquaculture, application of natural, antibacterial additives accompanied with various strategies, such as improvement of both pond water quality and the host health, are recommended.

Organic acids could be one option to control Vibrio in shrimp gut. In an in vitro test, Adams and Boopathy (2013) reported that a low dose of formic acid has good inhibition effect on fie
Vibrio strains.The relative toxicity of the organic acids coincides with the relative lipophilicity. These growth patterns of fie strains of Vibrio were in agreement with pH change in the media. The pH value in the completely inhibited treatment was ≤ 5. The pH value of the control remained neutral to slightly alkaline. It is more likely that V. harveyi and other Vibrio species growth is inhibited when the media pH is below or at 5. When the media pH is above 5, V. harveyi is able to adapt to the acidic environment and survive. However, we cannot kill 100% Vibrio in the ecosystem.

Organic acids, phytochemical, permeabilizing complex
BIOMIN has developed a mix of organic acids blend, a phytogenic compound and a Permeabilizing Complex, known as Biotronic® Top3. Organic acids are blends of formic acid, acetic acid and propionic acid. The three acids have a strong effect on gram-negative bacteria such as Vibrio. The phytochemical, cinnamaldehyde, plays a role in inhibiting pathogenic bacteria division (antibacterial action), and keep the bacteria under quorum. Meanwhile, Biomin Permeabilizing Complex makes bacterial membrane permeable allowing acids, phytochemical and other antimicrobial substances to infitrate easily. With this the dose of acids can be reduced, limiting the negative effect on gut pH.

Several fild trials were conducted as in vivo test of Biotronic® Top3 to gauge the effect on shrimp growth and capability to control Vibrio levels in the gut of Penaeus monodon and
L. vannamei. For P. monodon, we conducted the fild test on 2 farms. There were 3 selected ponds in each farm as treatment, and 3 ponds each as control. The treatment feed on this fild trial was Biotronic® Top3 at inclusion level of 0.8 kg per tonne of feed. In L. vannamei, the trial was conducted in one farm where 2 selected ponds were used for the treatment diet and 2 ponds as control.

Figure 1 showed profie of Total Vibrio Count (TBC) in shrimp gut until days of culture (DOC 60). Treatment ponds showed similar results. TBC in treatment ponds (Farm A, B and C) declined to levels lower than in the control pond. It was in 104CFU/mL range which is a normal condition, particularly for P. monodon.
Harvest data from three farms is shown in table 3. Results showed that shrimp fed with treatment feed containing Biotronic® Top3 performed better than the control in terms of survival rate, weight, daily growth and biomass. Shrimp survival rate in treatment ponds in all three farms improved in comparison to the control. Importantly, Biotronic® Top3 (containing organic acids blend, phytochemical and Biomin’s Permeabilizing Complex) had no negative effect on shrimp growth. The average of daily growth of shrimp in treatment ponds was higher than 0.2 g/day.
It is important to monitor and control the Vibrio level in shrimp gut. Additives can be used to control Vibrio level. Organic acids (e.g. formic acids) can work to control many strains of Vibrio. However, pH reduction will be an issue as when we apply high dose of acids, shrimp growth is affected. A mix of acids blend, phytochemical and Biomin Permeabilizing Complex in Biotronic® Top3 at a very low dose has been proven in a fild test (in vivo) with no negative effect on growth.

By : Anwar Hasan

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