Secondary Contamination is the Main Source for Spread of Nosema bombycis Resulting in Outbreak of Pebrine Disease in Bombyx mori L

In nature, the population of Nosema bombycis (Microsporidia) causing pebrine disease is small and their development is extremely slow and only few ultimately producing spores. Pebrine infected silkworm, Bombyx mori larvae collected from sericulture field were alive till 3rd generation though the concentration of N.bombycis spore was very high (2.4 - 3.0 x 108 spores. mL-1). All larvae were died during 4th generation with extremely high concentration of pebrine spores (3.0 - 4.0 x 109 spores. mL-1) and mostly contain long polar tube (LT). Alternately, all larvae were died immediately (at 3rd stage of 1st generation) when it was artificially inoculated with same concentration of N.bombycis spores harvested from field (2.4 - 3.0 x 108 spores. mL-1) though concentration of spores harvest was very less (3.0- 4.0 x 106 spores. mL-1) and mostly contain short polar tube (ST). Artificially pebrine infected male moth when mated with healthy female moth took six generations to develop pebrine disease and all larvae were died at the 2nd stage with very less spore harvest (3.0 - 10.0 x 105 spores. mL -1). Survival percentage was increased in all generations (~92.0% at 4th generation) when silkworm rearing was conducted under new integrated disease management system.