TPD in shrimp

TPD “translucent post-larva disease” in whiteleg shrimp has become a “hot” phrase in the current period. Having only appeared in Vietnam for a short time since the end of 2023, TPD has been recorded in many shrimp farming provinces in the Mekong Delta such as Bac Lieu, Ca Mau, Tra Vinh, … with a rapid spread rate and causing huge economic losses.

Causes and Symptoms of TPD in shrimp

Clinical signs of TPD are recorded as pale or colorless hepatopancreas and digestive tract, empty digestive tract, causing the body of the diseased shrimp to become transparent and opaque, the shrimp gradually weaken, and swimming speed decreases. Recent studies have shown that the cause is Vibrio Parahaemolyticus (Vp- JS20200428004-2) – a species of bacteria belonging to the genus Vibrio but has the highest mortality rate in this group of bacteria ever.

The mortality rate of whiteleg shrimp postlarvae infected with TPD at a concentration of 1.83×107 CFU/mL was the highest among all infection groups and 100% mortality was observed at 28 hours post-infection. 100% mortality in the 1.83×106 CFU/mL group occurred at 40 hours post-infection. Finally, 75% mortality in the 1.83×104 CFU/mL group (the lowest bacterial concentration used in the infection trials in this study) occurred at 48 hours post-infection.

In contrast, there was no mortality in the negative control group during the test period. In the challenge test, the 50% lethal concentration (LD50) after 24 hours was 9.84×105 CFU/mL, and the time to 50% mortality in the 56-hour challenge tests at doses of 1.83×107; 1.83×106, 1.83×105 and 1.83×104 CFU/mL was 15.7 hours; 24.6 hours; 28.9 hours and 38.6 hours, respectively. That is, the bacterial concentration reached 1.83×10^6 CFU/ml in 24 hours, causing mass shrimp deaths at a very fast rate.

Figure 1: Clinical signs of shrimp infected with TPD

Postlarval opacity disease mainly affects shrimp from 2 to 15 days old, the incidence can be up to 60% on the second day after the first observation of abnormal individuals, and even up to 90–100% in severe cases on the third day.

Figure 2: Dead shrimp infected with TPD

Histological analysis to understand how TPD attacks

1. Histopathological examination of naturally infected shrimp showed that necrosis, as well as sloughing of epithelial cells, occurred in the hepatopancreatic tubules and midgut.
2. In the early stage of infection, there was mild necrosis of epithelial cells in the hepatopancreatic tubules (Figure a, b), but severe necrosis and sloughing of epithelial cells in the midgut (Figure d). Necrosis and sloughing of epithelial cells became very severe, and typical signs of bacterial invasion both in the hepatopancreatic tubules and midgut lumen could be observed in the acute and severe stages of infection (Figure c, d, f).
3. In the early stage, necrosis and sloughing of epithelial cells also appeared in the gastrointestinal tract, midgut.
4. As the bacterial infection became more severe, severe necrosis and sloughing of epithelial cells were observed in both the hepatopancreatic tubules and the gastrointestinal tract in the midgut during the middle stage of infection.
5. Bacterial infection-induced melanization was evident in hepatopancreatic epithelial cells, and similar melanization symptoms were also observed in hepatopancreatic epithelial cells of naturally infected shrimp. Clear signs of bacterial invasion also occurred in both the HP lumen and the digestive tract at the severe stage of infection.

Figure 3: Histology of TPD-infected postlarvae

Impact of TPD

TPD in shrimp can cause high mortality in shrimp populations, leading to reduced productivity and profits for farmers. The disease can also cause shrimp to become weakened, reducing their growth ability and ability to resist other diseases. Importantly, there is currently no effective treatment or cure for TPD.

Conclusion

Although there is no effective medicine or treatment, understanding TPD in shrimp, from its causes, symptoms to its impact on the shrimp farming industry, is essential to prevent and treat TPD in shrimp.

You can refer to products from Feed, Breeding, Biological Products and Water Recirculation Models of GrowMax Group to apply to the farming process to help prevent and treat TPD effectively.

 

Source from Shrimp Farmer Magazine