The effects of microsporidian (Thelohania) infection on the growth and histopathological
changes in intensive pond-reared Pacific white shrimp (Litopenaeus vannamei) were studied. Postlarvae
10 (PL10) were stocked at 125 PL/m2 in three earthen ponds of approximately 5 rai (8,000 m2) in an
area with a salinity of 30-35 ppt. Shrimp were fed with commercial pelleted feed throughout the 120
days of the culture period. Shrimp were first observed to be infected with microsporidian parasites
between 20-25 days after stocking. The highest prevalence of infection between 25-28% was found at
day 60 in all growout ponds. Subsequently, the prevalence of infection decreased until the shrimp were
harvested at day 120 when only 3-5% of shrimp remained diseased from the three rearing ponds. Shrimp
infected in both hepatopancreas and abdominal muscle had the lowest weight, which was significantly
different (P