Nosema Disease in Honey Bees

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Nosema is a serious disease of honey bees caused by a microsporidian. A microsporidian is a type of single-celled fungus that reproduces by spores. The nosema organisms live and reproduce in the honey bee midgut where they steal nutrients and prevent digestion.

The mature microsporidian has a spring-loaded lancet that injects spores into the epithelial cells lining the gut. Normally, epithelial cells release enzymes that digest the honey bee’s food. But after spores have been injected into an epithelial cell, they reproduce and grow into mature microsporidians that fill the cell and inhibit the formation of enzymes.

When the epithelial cells erupt to release their enzymes, they end up releasing mature microsporidians instead, each with its own spore-shooting lancet. With so many organisms interfering with her digestion, a honey bee worker will starve to death, even when she has plenty to eat.

Hungry Bees Cannot Thrive

A malnourished honey bee doesn’t live long. On average, a starving worker’s lifespan is shortened by 50-75%. In addition, the worker’s hypopharyngeal glands—which normally produce food for the young—do not develop properly. And since workers don’t live long, new workers are forced to forage before they are ready, which further reduces colony efficiency.

If heavily infested with nosema, a colony will soon spiral out of existence, often leaving a tiny cluster of bees, a queen, and more brood than the small number of workers can raise. Many researchers now believe that so-called Colony Collapse Disorder may have been caused by a proliferation of Nosema ceranae.

Two Types of Honey Bee Nosema

For many years, the only nosema in North American was Nosema apis. Symptoms usually appeared in late winter or early spring and were associated with “spring dwindle,” an old-fashioned term used to describe colonies that failed just before spring build up.

But in 2007, a new nosema was discovered in American honey bees. Nosema ceranae was originally a pathogen of the Asian honey bee, Apis cerana. Researchers speculate the fungus transferred into European honey bees at about the same time as varroa mites. But since we weren’t looking for it, the fungus went undetected until populations exploded a dozen years ago.

When a pathogen enters a new area, the first wave of disease is usually the worst because the most susceptible organisms quickly become infected. Later, as those that survived the first wave reproduce, you begin seeing some immunity, causing the prevalence of the disease to decrease. With nosema, the first wave coincided with CCD, but now the overall incidence seems less.

The Dysentery Connection

An important thing to understand about nosema is that it has nothing to do with dysentery. In spite of conventional wisdom, no one has ever found a scientific link between the two conditions.  A colony can have nosema or dysentery or both, but one does not cause the other. Historically, both Nosema apis and dysentery occurred in early spring during cold and damp weather, so people assumed they were related.

When Nosema ceranae came on the scene, beekeepers noticed that it did not produce dysentery. Since Nosema ceranae affects summer colonies when dysentery seldom occurs, the two diseases were unlikely to occur simultaneously. Further research showed that, in truth, neither species produces dysentery.

Nosema Symptoms and Treatment

Because dysentery and nosema are not related, you cannot conclude your colony in infected simply by the presence of bee droppings. In fact, the only way to diagnose nosema is by preparing a sample of bee abdomens and analyzing it under a microscope. The procedure is not difficult, so even a beginner can learn it. Alternatively, many university extension offices can analyze a sample for you.

Standard cell counts, however, can’t tell you which species is present. But for practical purposes, the species doesn’t much matter since no antibiotics are currently available for either one.

Nosema is an Opportunistic Disease

Honey bee nosema seems to be an opportunistic disease. In other words, at least some spores can be found in most bee hives. Even surprisingly high counts have been found in perfectly healthy and productive colonies, which makes us wonder what triggers a collapse.

Nosema acts like the common cold. Cold viruses are everywhere, yet most of us seldom come down with symptoms. Health care professionals have speculated that other conditions such as physical exhaustion, mental depression, lack of exercise, or poor diet make us more susceptible. The same may be true of a bee colony.

Nosema disease seems to be worse after pesticide exposure, in areas of poor forage, or in the presence of varroa mites. It makes sense. Pesticides and poor forage weaken the immune system, while poor forage and varroa mites deprive the bees of proper nutrition. Coupling any one of these with the nutrient-stealing nosema fungus will make the situation even worse and perhaps tip the colony over the edge.

How to Protect Your Colonies

Since colonies can thrive in the presence of nosema, we know bees have some natural immunity. The best thing we can do for our bees is capitalize on that immunity by providing good living conditions and minimizing other threats.

How to best manage a colony depends on your local climate. However, since nosema is a fungus, it is wise to keep a hive dry and remove any excess moisture. In addition, you should assure your bees have adequate forage and provide supplements when forage is scarce. Avoid exposure to pesticides, control varroa mites, and monitor your colonies for other conditions including brood diseases and robbing insects. In addition, the University of Guelph recommends that beekeepers replace their oldest brood frames on a regular basis. If you replace two of every ten frames each year, you can significantly lower the number of spores in a hive.

We no longer have a magic potion to control microsporidians, but healthy colonies can fend off most any ailment or predator. A healthy colony has an amazing ability to take care of itself, so if we provide the basics, the bees can usually handle the rest.

Have you tested a colony for nosema? If so, what were the results?