Transovarial transmission of two microsporidia in the silkworm, Bombyx mori, and disease occurrence in the progeny population

Peroral inoculation of Nosema bombycis and Nosema sp. M11 spores to 5th instar silkworm larvae was the most suitable method for obtaining infected adults for transovarial transmission studies. Mating tests with infected male and uninfected female adults indicated that N. bombycis and Nosema sp. M11 were not venereally transmitted. However, N. bombycis was transmitted transovarially to the progeny eggs at a frequency of 100%, whereas Nosema sp. M11 was transmitted at only 1.2 ± 0.41%. In larval progeny of the F 1 generation that were infected transovarially with N. bombycis, an acute lethal infection developed and most larvae died by the end of the 3rd stadium. Transovarial transmission of Nosema sp. M11, however, produced a chronic lethal infection and larval death did not occur until the end of the 5th stadium, when mortality increased up to 1.0% before eclosion.