Horizontal transmission pathways of terrestrial microsporidia: A quantitative comparison of three pathogens infecting different organs in Lymantria dispar L. (Lep.: Lymantriidae) larvae

Pathways for horizontal transmission of three microsporidian species, such as transmission via silk, feces, cadaver, exuviae and direct contact, were studied in Lymantria dispar larvae. The midgut infecting Endoreticulatus schubergi caused low larval mortality; 60% of infected L. dispar survived until the adult stage. An infected larva released on average 1.6 × 108 spores with feces until pupation, indicating that this is the main route for transmission. The more virulent fat body parasite, Vairimorpha disparis, caused the early death of infected L. dispar larvae. The high spore load of the cadaver (4.6 × 109 spores/cadaver), which was independent of the inoculation dosage, indicates that the release of spores from decomposing cadavers is the main transmission pathway. The third studied microsporidium, Nosema lymantriae, caused a systemic infection and killed the host mostly in the larval stage (>90%). Spores were released from living hosts with feces (2.7 × 108 spores in total), and later from decomposing cadavers (4.9 × 109 spores/cadaver on average). Thus, we assume that both pathways are important for this species. Susceptible larvae contracted an infection with N. lymantriae or E. schubergi when they came into contact with either infected larvae or their exuviae.