Shrimp farming is constantly under attack by viral infections (White Spot and Taura virus) that are not treatable and result in significant economic losses. Worldwide 1.6 million tons of shrimps are farmed using 3.2 million tons of feed.  

1. Effect of beta-1,3/1,6-D-glucan on  survival of  L. vannamei infected with White Spot Virus.  White Spot Syndrome Virus (WSSV) is a devastating infection that has been known to kill 100% of the shrimps in a pond.  In the first half of 2000, a severe outbreak of WSSV reduced the total shrimp crop in Ecuador by 70%.  The immune system of shrimps consists of 3 different types of macrophages that are responsible for controlling viral and bacterial infections.  As in higher animals these macrophages can kill microorganisms directly and can synthesize substances toxic that will kill microorganisms.  Shrimp macrophages are similar to mammalian macrophages and should therefore be activated by the same substances that activate human macrophages.

To examine whether beta-1,3/1,6-D-glucan activates macrophages, the resistance to WSSV was examined under laboratory conditions.  Shrimps were acclimatized to laboratory conditions, placed individually into 1 liter of seawater and infected by feeding them with 50 mg/ animal of WSSV infected shrimp tissue. 

The results of this study show that beta-1,3/1,6-D-glucan increases resistance to WSSV by shrimps.  Survival was highest in the group receiving the diet containing the 100 g/ton beta-1,3/1,6-D-glucan (70.6%), followed by the diet  containing 50  gr/ton (45.5%), then 500 grams per ton (27.3%) and the control (23.1%) respectively (Figure 6).

 

 

Figure 6. Survival of L. vannamei shrimps infected with WSSV fed a control diet or diets containing different concentrations of beta-1,3/1,6-D-glucan

 

 

 

Of particular interest is the result that shows that high the concentration of beta-1,3/1,6-D-glucan does not increase survival.  This result is consistent with a receptor mediated effect. Receptor mediated reactions are subject to receptor down-regulation, that is when the active substance (ligand) is administered in quantities large enough to occupy all receptors on a cell, the number of receptors or the affinity of the receptor for the ligand decreases and the response decreases or is abolished.  Based on the results of this study as well as on theoretical considerations, it is critical that to be of benefit, beta-1,3/1,6-D-glucan must be administered in appropriate concentrations that can vary from species to species.  It is significant to remember that too low a concentration will not be effective and that too high a concentration will also not be effective.

2.  Effect of Beta-1,3/1,6-D-glucan on the growth and survival of penaeid shrimps

The White Spot Syndrome Baculovirus Complex (WSBV) of Shrimp has drastically reduced the shrimp crop around the world.  Following initial detection in northeast Asia in 1992-1993, WSBV is now widely spread throughout most of the shrimp growing regions of Asia, the Indo-Pacific region, North America and Latin America.  WSBV is one of four diseases known to cause high mortality at shrimp farms, and disease outbreaks are a primary concern because they have the potential to wipe out an entire crop of shrimp.  While it is possible to apply antibiotics to fight bacterial infections, there is no effective means to combat viral infections in shrimps.

Since beta-1,3/1,6-D-glucan was effective in increasing the survival of shrimps infected with WSSV in the laboratory, it became important to examine whether beta glucan would be effective under field conditions.  A trial was conducted in which shrimps (L. vannamei) in growout ponds infected with WSSV were fed a diet containing 100 g of beta-1,3/1,6-D-glucan per ton of feed.  Shrimps were grown in ponds of 4-12 ha in size, at a stocking density of 15-30 animals/m². Twenty two ponds were fed a regular commercial diet, 2 ponds were fed a diet containing beta-1,3/1,6-D-glucan for 50% of the growth cycle, 1 pond for 80% of the growth cycle and 1 pond 90% of the days in the cycle.  Shrimp were fed daily according to farm protocols.  At harvest, total pond production (lbs) was recorded at the processing plant and production (lbs/ha) estimated.

 

Table 8.

Comparative data obtained in a field trial conducted in commercial ponds

 

 

Pond production ranged from an average of 2,695 lbs/ha in ponds that  fed control diet to a record harvest of 4,030 lbs/ha in a pond fed a diet containing 100 grams/ton beta-1,3/1,6-D-glucan for 80% of the number of days in the cycle (Figure 7).  The average production for the 22 ponds that were fed the control diet was 2,695±691 lbs/ha, whereas in the four ponds fed the diet containing beta-1,3/1,6-D-glucan the production was 3,582±482 lbs/ha, or 32% increase over the regular feed. 

 

Figure 7.  Shrimp production (pounds/ha) in ponds fed the diet containing beta-1,3/1,6-D-glucan  during 0 (n=22), 50 (n=2), 80 (n=1) and 90% (n=1) of the days in the growout cycle.

 

 

 

Shrimp are entirely dependent on non-specific immune mechanisms to resist infections, since they are not equipped with cells analogous to antibody producing lymphocytes in fish and warm-blooded animals. It would therefore be beneficial to add to shrimp feed substances that would optimize their immune system to better resist and control infectious agents.  Beta-1,3/1,6-D-glucan is an ideal additive since it is non-toxic, its mode of action is understood and it is available at a cost that is economically attractive.

 

Table 9.

Theoretical cost-benefit of the beta glucan supplemented diet

 

	Regular Diet	Beta-1,3/1,6-D-glucan Diet
Number of Ponds	1	1	1	1	1	1
Has/pond	1.0	1.0	1.0	1.0	1.0	151.0
Days	120	120	120	120	120	120
PL/m2	20	20	20	20	20	20
% Survival	30	40	50	60	70	80
Shrimps/m2 harvested	6.0	8.0	10.0	12.0	14.0	16.0
Average weight (g)	15	15	15	15	15	15
Lbs/ha	1982.4	2643.2	3304.0	3964.8	4625.5	5286.3
FCR	1.8	1.8	1.8	1.8	1.8	1.8
Feed Consumed	3568.3	4757.7	5947.1	7136.6	8326.0	9515.4
Cost Control Diet at $.198/lb	706.5	942.0	1177.5	1413.0	1648.5	1884.1
Cost B-G Diet at $0.22/lb	813.6	1084.8	1355.9	1627.1	1898.3	2169.5
Pond Yield ($) at 2.20/lb	4361.2	5815.0	7268.7	8722.5	10176.2	11630.0
Pond Yield Increase (lbs)		660.8	1321.6	1982.4	2643.2	3304.0
Increased Revenue($) at 2.20/lb		1453.7	2907.5	4361.2	5815.0	7268.7
Difference (revenue $/ha)		369.0	1551.5	2734.1	3916.7	5099.2
Difference (%)		8.5	35.6	62.7	89.8	116.9

 

Economic significance.  In semi-intensive and intensive shrimp farming, feed cost alone can account for two-thirds of the variable costs. The results of this trial show that beta-1,3/1,6-D-glucan added to the feed can improve survival by at least 10%, without depressing growth, feed efficiency, feed intake, and protein utilization. Table 9 shows the theoretical financial benefit of supplementing the diet with beta-1,3/1,6-D-glucan, as well as improvement in feed usage and savings to production costs at the farm. 

Assuming 10% increase in survival and a shrimp value of US $2.20/ lb ($4.84 per kg), the table above makes it clear that a survival increase of 10% adds significantly to revenues. A 30% increase in survival (as in shrimps fed the diet containing beta-1,3/1,6-D-glucan for 80 days) increases revenues by over $4000.00 per 1 hectare pond.

 

3.  Effect of beta-1,3/1,6-D-glucan Addition to Feed on Shrimp Survival In TSV infected Ponds.  To evaluate effect of supplementing shrimp feed with beta-1,3/1,6-D-glucan on survival and growth of the white shrimp, Litopenaeus vannamei in commercial ponds, six 6-acre ponds were randomly assigned to two treatments with three ponds for each treatment. Ponds l, 2, 4 were fed the regular diet and ponds 3, 5 and 7 were fed the same regular diet supplemented with beta-1,3/1,6-D-glucan at 100 grams/ton.

Shrimp growth was recorded weekly by samplings using an 8-foot cast net. Each week the shrimps were classified and weighed and the weighted average determined. Feeding was done three times a day and the feeding rate in each pond varied based on water quality, sampling results, feed tray data and other variables. Feeding was done by broadcasting along in 12 trays in each pond. The test period lasted for a limited period of the growth cycle, accounting for about 50% of the total feed to the test ponds.

The survival in ponds fed the diet containing beta-1,3/1,6-D-glucan ranged from 51% to 66% with an average of 57%. In comparison, survival in ponds fed the control diet ranged from 44% to 51 % with an average of 45%. The results show that survival is on average 25% higher in ponds fed the diet containing beta-1,3/1,6-D-glucan compared to the survival in ponds fed control feed (Figure 8). It must also be noted that during the growout cycle all six ponds were infected with TSV and suffered mortalities.

 

 

 

Figure 8.  Comparison of survival (%) among ponds fed with a normal diet and a diet containing beta glucan at a dose of 100 grams/ton.

 

 

 

 

 

 

 

 
The weekly growth rate for the shrimp fed the diet containing beta-1,3/1,6-D-glucan was higher than those fed the control diet (Figure 9). The average weekly growth in ponds fed the control diet ranged from 1.00 to 1.40 g/week with an average of 1.19 g/week, whereas in ponds fed the beta-1,3/1,6-D-glucan diet growth ranged from 1.26 to 1.90 g/week with an average of 1.53 g/week, an average increase of 28.6%.

 

 

  

 

 

Figure 9.  Weekly growth among ponds fed control (¾) and beta-1,3/1,6-D-glucan diet (¾).

 

 

 

 

 

 

 

Table 10 shows that the average Feed Conversion Ratio (F.C.R.) for the ponds fed the control diet was 2.28 to 2.89 with an average of 2.65 whereas the FCR in the ponds fed the diet containing beta-1,3/1,6-D-glucan ranged from 1.78 to 2.80 with an average of 2.14, 20% lower. The high than normal FCR's in these ponds is a result of the TSV infection during the late stage of the cycle. 

 

Table10.

Comparative data obtained in a field trial conducted in commercial ponds

 

Pond#	Feed	Age at Harvest (weeks)	Total Stocked	Density Per acre	Avg. Weight (g)	Overall Growth (g/week)	Final Survival (%)	Biomass (Pounds)	Feed (lbs/day)	Cumm. Feed (pounds)	F.C.R.
1	Control	22 5/7	500,000	83,333	19.9	0.97	44.00	9626	168	26670	2.77
2	Control	19 1/7	400,000	66,667	22.8	1.32	42.00	8221	140	18716	2.28
4	Control	25 2/7	400,000	66,667	23.3	1.05	51.00	10497	172	30371	2.80
	Average		43333.3	72222.2	22.0	1.1	45.7	9448.0	159.7	25252.3	2.6
3	b-glucan	22 2/7	400,000	66,667	23.5	1.24	66.00	13785	163	36084	1.35
5	b-glucan	25 1/7	400,000	66,667	22.0	0.99	55.00	10763	171	30109	2.80
7	b-glucan	18 5/7	400,000	66,667	22.8	1.49	51.00	10210	138	18139	1.75
	Average		400,000	66,6667	22.8	1.2	57.3	11559.3	157.4	24544	2.1

The increase in survival (45.7 vs 57.3) resulted in a 22 5 increase in overall yield (Fig.  10) 

 

 

 

 

 

Figure 10.  Comparison of shrimp yield (lb/acre) between ponds fed a control or a beta-1,3/1,6-D-glucan supplemented diet.

 

 

 

 

Economic significance.  The results presented show that the addition of beta-1,3/1,6-D-glucan to shrimp feed increases survival and may result in better feed conversion without depressing growth, feed intake, and protein utilization.  Table 11 shows that the addition of adding beta-1,3/1,6-D-glucan to the diet improves in feed usage and savings to production costs at the farm the financial benefit.

 

Table 11.

Cost-benefit of using beta-1,3/1,6-D-glucan in the diet, assuming a 10% increase in survivability and assuming a shrimp value of US $3.0 per pound ($6.50 per Kg)

 

	Control Diet	Beta-1,3/1,6-D-glucan Diet
Number of ponds Has/pond Days PL/m2 % Survival Shrimp/m2 harvested Average weight (g) Pounds/Ha FCR Feed consumed Cost of control feed at $0.198/lb Cost of B-G Feed at $0.28/lb Difference between diets ($)/ha Pond yield at $3.0/lb Pond yield increase (pounds) Increase revenue value at $3.00/lb Difference (revenue $/ha) Difference (%)	1 1.9 150 17 45 7.7 22.8 3787.8 1.7 6382.9 1247.8 1436.9   11121.1	1                   1                    1                    1                  1 1.8                1.8                 1.8                 1.8                1.8 150               150                150                150               150 17                 17                  17                  17                 17 50                 55                  60                  65                 70 8.5                9.4                 18.2               11.1               11.9 22.8             22.8                22.8               22.8               22.8 41118.9       4530.8            4942.7           5354.6           5766.5 1.7               1.7                  1.7                 1.7                 1.7 7082.2         7702.4            8402.6           9182.9           9803.1 1386.4         1525.1           1663.7            1882.4           1941.0 1596.5         1756.2           1915.8            2075.5           2235.1 218.1           231.1              252.1             273.1             294.1 12356.8       13592.5          14828.2         16063.9        17299.6 411.9           823.8              1235.7           1647.6          2959.5 906.2           1812.3            2718.5            3624.7         4538.8 -698.3          56.2                802.7             1549.2          2295.7 -6.2              0.5                  7.2                 13.9              28.6

 

 It is evident from the results presented that the addition of beta-1,3/1,6-D-glucan to shrimp diet increases survival under commercial growing conditions.  The underlying theoretical reasons for this effect of beta-1,3/1,6-D-glucan are the increased ability of the immune system of shrimps to combat infections.   Since the immune system of shrimps is comprised only of macrophages it is reasonable to expect that any substance that is capable of activate macrophages and/or increase their number would increase the ability of the shrimp to kill infectious agents and thus keep the animal healthier.