Vairamorpha: A New Name for a Familiar Threat

Thursday 3 November 2022

Nosema disease refers to an obligate intracellular eukaryotic parasite that affects adult honey bees. It is caused by the pathogens Vairimorpha apis and Vairimorpha cerenae, previously known as Nosema apis and Nosema ceranae. In this week’s blog, we will be discussing how to accurately identify a clinical infection of Nosema in your honey bee colony.

Vairimorpha: A New Name for a Familiar Threat

Varimorpha spores are often present in honey bees at subclinical levels but when the population of spores exceeds a certain amount, the infected colony will begin to show disease signs. The disease disrupts the bees digestive system and they may succumb to premature death. Reduced worker longevity impacts foraging and subsequent honey yield.  Also, the inhibition of pollen digestion impacts the development of the hypopharyngeal gland in worker bees such that brood rearing declines. These are only a few symptoms of infection. Unfortunately, there are no observable symptoms of a clinical Variamorpha infection that can provide a clear diagnosis.

The only way to diagnose Variamorpha infection is by observing a sample of suspected bee’s gut content under a microscope. Vairimorpha spores live in the midgut of adult honey bees. Adult bees inadvertently ingest spores through contact with their environment or other honey bees. Once ingested, the spores infect the epithelial cells of the honey bee midgut and begin to germinate rapidly until there are so many that the cell ruptures, releasing spores into the digestive system. As such, accurate identification of a Vairimorpha infection requires observing, via microscopy, the midgut content of worker honey bees. 

Vairimorpha spores observed under a compound microscope at 400x magnification. See examples of egg-shaped spores circled in blue. (ATTTA©2022). 

To do this, a sample of bees must be collected, crushed, and observed under a microscope. Older foraging bees are best because they will have higher spore loads than younger bees. Collect a sample of 30 foraging worker bees, found near the bottom board or amongst honey frames, and kill them by freezing. Then, mix the sample of bees with 30 mL of water, to have 1 mL of water per bee, and crush the bees to create a slurry. This can be done with a mortar and pestle or in a sealed plastic bag with a rolling pin. Vairimorpha spores will be within this slurry and can be observed under a microscope.

A standard haemocytometer, or cell counting chamber, will make it possible to quantify the number of spores per bee in the sample. A haemocytometer has two wells per chamber which each contain a known volume of liquid (100nl). Using an eyedropper, obtain a sample of the bee-water slurry and drop it such that the liquid will fill the wells. A haemocytometer also has gridlines of known area. To quantify the number of spores in the slurry, count the number of spores in five of the twenty-five grid squares using 400x magnification. Standard procedure is to count the four corners and middle square, in each of the two hemocytometer wells (Cantwell 1970; Williams et al. 2011). Then, calculations must be done to find the number of spores per bee in the sample. First, add the total number of spores counted in each of the wells together and divide by two to find the average number of spores per well. Multiply this number by 5 to get the number of spores per well.  Multiply the average number of spores per well by 10 000 to provide the average number of spores per ml which equates to number of spores per bee. Presently, the economic treatment threshold is one million spores per bee (Williams et al. 2011). In this situation, an integrated pest management approach would suggest that chemical treatment be done to address the high levels of spores in the colony. Presently, Fumagilin-B is the only registered treatment of Vairimorpha in Canada and should be applied according to label instructions.

Vairimorpha infestations often accumulate over the winter and are highest in the spring. If overwintered colonies are weak and sickly in the spring, be prepared to consider the possibility of a Vairimorpha infection. ATTTA is equipped and happy to test your samples. Atlantic beekeepers can reach out using the contact information below for more information or to arrange testing!


Cantwell, G. E. 1970. Standard methods for counting Nosema spores. American Bee Journal 110: 52-54.

Williams, G., Shutler, R., Little, D., Burgher-Maclellan, C., & Rogers, M. 2011. The microsporidian Nosema ceranae, the antibiotic Fumagilin-B®, and Western honey bee (Apis mellifera) colony strength. Apidologie 42: 15–22.

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