Chalkbrood Disease Caused by Ascosphaera apis in Honey Bees (Apis mellifera)—Morphological and Histological Changes in Infected Larvae

Chalkbrood is a mycological brood disease of the Western honey bee (<i>Apis mellifera</i>), caused by the fungus <i>Ascosphaera apis</i>. The aim of this study was the investigation of the pathology of artificially reared <i>Apis mellifera</i> larvae, experimentally infected with <i>A. apis</i> spores (1.0 × 10³ spores/larva). Non-infected larvae served as control. Five living larvae and every dead larva were collected daily (day 1-7 p.i.). All larvae were macroscopically measured, photographed, formalin-fixed, and histologically processed (hematoxylin-eosin stain, Grocott silvering). Histological sections were digitized, and the size of the larvae was measured (mouth-after length, area) and statistically analyzed. Twenty-six larvae from the collected larvae (<i>n</i> = 64; 23 dead, 3 alive) showed histological signs of infection from 3 d p.i. onwards. The dead larvae showed macroscopically white/brown deposits, indistinct segmentation, and a lack of body elongation. Infected larvae were significantly smaller than the controls on days 3 p.i. (<i>p</i> < 0.05), 4 p.i. (<i>p</i> < 0.001), and 6 p.i. (<i>p</i> < 0.05). The early time of death, the low number of transitional stages, and the strong penetration of the larval carcass with fungal mycelium indicate a rapid and fulminant infection process, which is probably relevant for spreading the disease within the colony.