Honey bees face a wide range of threats from visible pests to microscopic pathogens, all of which can impact their health and survival. While some diseases like Varroa destructor or Nosema spp. are well known among beekeepers, others remain less recognizable but are still important to understand. One disease is caused by the protozoan parasite Malpighamoeba mellificae, which infests the bee’s excretory system.

Understanding Malpighamoeba mellificae 

This amoeba disease of honey bees was first discovered over a century ago in Germany ¹. Malphighamoeba mellificae is a protozoan parasite that infects an essential organ in honey bees known as the Malpighian tubules which is part of the digestive system. It is transmitted through the fecal-oral route ². Usually, infection occurs when the feces of infected bees get on the comb, which then gets ingested ³.  

The life cycle of Malphighamoeba mellificae begins when honey bees ingest the cysts. Once inside the bee, the cysts transform into trophozoites, which are the active feeding stage of the amoeba ¹. These trophozoites invade and damage the Malpighian tubules. Eventually, they form new cysts that pass through the honey bee’s hind gut and are excreted.

The Malpighian tubules, as part of the excretory system in the bee’s abdomen, play a vital role in osmoregulation.  Osmoregulation is a cellular process which maintains fluid and salt balance within the bee’s body ¹. The tubules also remove nitrogenous waste from the hemolymph by passing it into the hindgut for excretion ¹. Additionally, the Malpighian tubules help filter out foreign substances that could cause harm to the bees, like insecticides, fungicides or antibiotics ⁴. When these tubules are damaged, their inability to function properly can significantly compromise the honey bee’s overall health. 

Figure 1: Malpighamoeba mellificae disease a) showing amoebic cysts in fecal samples from western honey bee. b) Cysts observed in Malpighian tubules under a light microscope. c) Normal appearance of Malpighian tubules in a non-infected honey bee and d) Discolored Malpighian tubules of honey bees who were infected with Malpighamoeba mellificae (image: Iredale et al, 2023 ²). 

When infected by Malpighamoeba mellificae, the Malpighian tubules become swollen and filled with cysts, which impairs their function and weakens the bee ⁴. This enlargement may result from thinning of the epithelial layer, caused by the accumulation of cysts or by trophozoites feeding on the tissue ⁴. At the colony level, this disease may lead to population decline, reduced lifespan, and signs of dysentery ³. It is often found alongside Nosema spp. infections ¹.

Diagnosing Malpighamoeba mellificae requires a compound light microscope and follows a similar method to screening for Nosema. About ten bees should be collected from the front of a hive and placed into a sealable bag with 10 mL of distilled water ³. The bees are then crushed to release internal contents, and a drop of the resulting liquid is placed on a microscope slide for examination of the 8-10 μm in diameter cysts at 400x magnification ³. However, this technique can also have its limitations. The amoeba can remain in its trophozoite stage for extended periods and only form cysts when its environmental conditions are poor ¹. While cysts are easy to identify due to their consistent shape, the trophozoites are much harder to recognize, especially in mild infections ¹.

Figure 2: Distribution of M. mellificae showering where disease is present (yellow), suspected presence (teal) and no presence/unreported (purple) (image: Gurland et al, 2024 ³). 

This disease has been reported in various regions where European honey bees are kept, including Canada ³. The disease may be seasonal, with higher prevalence in winter and early spring ³. Despite its long history, Malpighamoeba mellificae remains understudied, and no specific treatments are available. Malphigamoeba mellificae has not been proven to be a problem for beekeepers in our region. But we do need to understand the presence of the pathogen and the implications for honey bee health in the Maritimes.

 Written by Kaitlyn Newton, ATTTA Seasonal Apiculturist 

Connecting with ATTTA Specialists

If you’d like to connect with ATTTA specialists or learn more about our program, you can:

visit our website at https://www.perennia.ca/portfolio-items/honey-bees/

Email attta@perennia.ca

References:

1. Schäfer, M.O., Horenk, J. and Wylezich, C., 2022. Molecular detection of Malpighamoeba mellificae in honey bees. Veterinary Sciences, 9(3), p.148.
2. Iredale, M.E., Viadanna, P.H., Subramaniam, K., Tardif, E., Bonning, B.C. and Ellis, J.D., 2023. Report of amoebic disease in a colony of Western honey bees (Apis mellifera). Veterinary Pathology, 60(5), pp.709-713. 
3. Gurland, H.N., Fulton, J.C., Iredale, M.E. and Vu, A.T., 2024. Monitoring for Amoebic Disease (Malpighamoeba mellificae) in Honey Bee Colonies: ENY2112/IN1431, 10/2024. EDIS, 2024(5).
4. Rossi, M., Ott, S.R. and Niven, J.E., 2020. Malpighamoeba infection compromises fluid secretion and P-glycoprotein detoxification in Malpighian tubules. Scientific Reports, 10(1), p.15953.