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

Summary

Chalkbrood is a serious fungal disease that kills honeybee larvae, caused by the fungus Ascosphaera apis. This study examined exactly how the infection progresses by infecting larvae in controlled laboratory conditions and observing them over time under a microscope. The researchers found that once infected, larvae die very quickly within 3-7 days as the fungus spreads throughout their bodies, and the fungus continues to grow even after the larvae are dead, eventually forming visible spores on the dead larvae that spread the disease to other bees in the colony.

Background

Chalkbrood is a mycological brood disease of the Western honey bee (Apis mellifera) caused by the fungus Ascosphaera apis. While clinically recognized, the detailed pathogenesis of this disease has not been thoroughly investigated. Understanding the infection process is crucial for improving disease management in honeybee colonies.

Objective

To describe in detail the pathogenesis of chalkbrood using macroscopic and histological examination of artificially reared larvae experimentally infected with Ascosphaera apis spores under controlled conditions. The study aimed to compare infected larvae with non-infected control larvae to characterize morphological and histological changes during infection.

Results

Histological signs of infection were observed in 26 of 64 infected larvae collected, first appearing on day 3 p.i. Of these, 23 were dead and 3 alive. Dead infected larvae showed macroscopic white/brown deposits, indistinct segmentation, and lack of body elongation. Infected larvae were significantly smaller than controls on days 3, 4, and 6 p.i. (p<0.05 to p<0.001). A highly significant association existed between larval death and A. apis infection (p<0.0001).

Conclusion

A. apis infection follows a rapid and fulminant course from spore germination to larval death, with larvae dying as early as 3 days post-infection. The low number of transitional stages and intense mycelial infiltration indicate a severe infection process. The continued fungal growth and fruiting body formation in dead larvae is essential for horizontal transmission within the colony, important for understanding field infections in honeybee colonies.
  • Published in:Veterinary Sciences,
  • Study Type:Experimental Study,
  • Source: PMID: 39330794, DOI: 10.3390/vetsci11090415
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