Since 2002, the majority of Thai shrimp farmers have had problems with severe growth retardation when rearing black tiger shrimp, Penaeus monodon. Normally after four months of uneventful culture, they expect to obtain an average animal weight of 25-35 g. Instead, they find that over 50% of the shrimp average only 5-10 g.

Survival in affected ponds is usually quite normal at 60-80%, although some farmers report somewhat lower percentages. In severe cases, up to 80% of the shrimp are small. For these ponds, the average growth is less than 0.12 g/day, in contrast to the 0.2 g/day or higher growth normally expected for P. monodon over four months of culture.

Monodon Slow Growth Syndrome
In spite of the large number of small shrimp, some shrimp in these ponds grow normally. This differential growth rate results in a very high coefficient of variation (CV) that ranges 30-80%. CV is a statistical measure of variability that is calculated by dividing the standard deviation by the mean.

The small shrimp are mostly active, but in many cases their body color is darker than normal. They can also have unusually prominent blue and yellow stripes on the swimming and walking legs.

The malady reflected in these small shrimp is now known as Monodon Slow Growth Syndrome (MSGS). It can be likened to Runt Deformity Syndrome (RDS), which is caused by Infectious Hypodermal and Hematopoietic Necrosis Virus (IHHNV) in P. vannamei. Very high CV are also observed in this condition.

Switch to P. vannamei
Obviously, MSGS is disastrous for Thai shrimp farmers. Many say it is almost worse than going through a White Spot Syndrome or Yellow Head Virus outbreak, since they have to carry on feeding the shrimp up to six months just to reach a break-even point. That is a long time to work for no profit.

Such bad experiences have caused many producers to turn to white Pacific shrimp, P. vannamei, since farmers have found that good specific pathogen-free (SPF) P. vannamei
stocks grow at a more predictable rate. Unfortunately, the rush for P. vannamei seedstock resulted in the trading of nonSPF stocks, and some unwary farmers had the unfortunate experience of jumping from the frying pan into the fire when they were hit with RDS instead of MSGS.

New Infectious Agent Suspected
The authors are attempting to find the cause of MSGS, and suspect a new infectious agent may be the cause. The reason for this speculation is that a survey of 32 ponds exhibiting MSGS using light microscopy, transmission electron microscopy (TEM), bacterial culture, and polymerase chain reaction (PCR) failed to reveal any known pathogen or pathogen combination, although many single or multiple infections by viruses, bacteria, and protozoa were found.

These included Monodon Baculovirus, Hepatopancreatic Parvovirus, and IHHNV. In addition, a number of unknown, purported viral particles ranging 17-45 nm in diameter were found in the lymphoid organs of some specimens. Although symptoms of HPV, which reportedly causes growth retardation in P. monodon, were found in some ponds, HPV was not present in all the problem ponds. The number of sampled shrimp with bacterial infections was relatively small.

The protozoans included a low prevalence of external Zoothamnium below 1% and a higher (10-15%) prevalence of a very small intracellular protozoan eventually identified by TEM as a new microsporidian in hepatopancreatic cells. The microsporidian is new for P. monodon in Thailand. Its prevalence was quite substantial in the MSGS ponds.

Again, none of the agents was ubiquitous in the problem ponds. In addition, a correlation analysis revealed that the severity of infection for these agents, either individually or collectively, tended to be equivalent in large and small shrimp in the same pond. About 10% of the MSGS ponds were negative for the known pathogens tested. Further, the known agents were all previously reported in Thailand, while the MSGS problem surfaced rather quickly all………..