Repeat after me: Nosema does not cause dysentery
Over the past year, Randy Oliver has published several informative articles in the American Bee Journal about the relationship between nosema disease and honey bee dysentery. If you haven’t read these pieces, you should. His message is simple: there is no evidence to support the theory that nosema disease causes dysentery.
I’m dismayed, however, that many beekeepers are still having trouble with this idea. People continue to insist their bees died of nosema based solely on the presence of feces on the frames or the fronts of their hives.
Correlation vs causation
I decided to take a different tack on this subject and start with the difference between causation and correlation. If you’ve never been able to understand the difference, don’t be alarmed. It’s never too late to learn.
To demonstrate the difference, many statistics books use the study done in New York City many years ago that showed that ice cream sales correlate with homicide rates. Every summer in the city, as ice cream sales increase, the homicide rate goes up. That is an interesting correlation, but does the sale of ice cream cause homicides? Or does more homicide whet our appetite for ice cream? Of course not.
Both of these events—ice cream sales and homicides—are affected by hot weather. People eat more ice cream in hot weather than in cold weather. Also, people tend to be more irritable and perhaps short-tempered in extreme heat. Some of those irritable, short-tempered folks are prone to staying out later in the evening, trying to stay cool, a situation that can spike the homicide rate.
Both the sales of ice cream and the increase in homicides were linked to a common cause, hot weather, but homicides did not cause an increase in ice cream sales or vice versa. In other words, the incidence of both events correlated: when one increased, the other increased. But that does not mean that one caused the other.
Nosema disease and dysentery
Likewise, nosema disease and honey bee dysentery often correlate. When one goes up, the other may go up, especially in a winter hive when bees are confined, but that does not mean that one causes the other.
Nosema: Nosema disease in honey bees is caused by a microsporidian, a tiny unicellular fungus that reproduces by spores. The fungus enters the digestive system of a bee through the mouth and travels to the midgut. There, a little spring-loaded lancet injects the bee’s epithelial cells with spores. Once injected into a cell, each spore reproduces by forming lots of copies of itself.
The spores interfere with the digestive enzymes that are normally produced in the epithelial cells. When the epithelial cells try to release digestive enzymes, they end up releasing nosema spores instead. With a lack of digestive enzymes, the bee weakens from malnutrition and may eventually starve to death.
Honey bee dysentery: Honey bee dysentery is a separate ailment. As you know, honey bee dysentery is not caused by a pathogen but by an excess accumulation of water in the honey bee gut. You can think of it as bee diarrhea. In cold weather when honey bees can’t get outside to defecate, they must hold their feces, sometimes for months. They are designed to do this, but if they consume too much liquid over time, they sometimes reach a point at which they can no longer “hold it” and they end up defecating inside the hive or on the front porch. This unfortunate mess is what we call honey bee dysentery.
Two conditions can occur together
Your bees can have nosema disease with or without dysentery and they can have dysentery with or without nosema. One does not cause the other and one is not dependent on the other. If you see dysentery on the frames inside your hive or running down the front of it, the only thing you can conclude is that your colony has dysentery. To quote Randy Oliver in his May 2019 article, “The only way for a beekeeper to diagnose nosema infection is by microscopy.” If you don’t have it tested, you don’t have a clue.
On the other hand, bee colonies can easily have both ailments at the same time. Dysentery can be quite harmful to colonies because of the sanitation problem it creates. Many diseases are easily transmitted by fecal material, so if bees are defecating in the hive, there is a much greater probability that pathogens will be transferred from bee to bee, including nosema. So if a bee has nosema, dysentery can increase the rate of bee-to-bee transmission. However, if a colony is free of nosema, no amount of dysentery will produce it.
Timing causes confusion
Nosema apis shows up primarily in late winter or early spring, just like dysentery, so it is easy to see why beekeepers began connecting the two. It was only when Nosema ceranae—often a summertime ailment—began showing up with no signs of dysentery, that beekeepers began resurrecting the question about the nosema-dysentery connection. In the years of research Randy did, he was never able to find a single study that proved that either nosema species was a cause of dysentery.
I agree that we have to stop parroting what others are saying, wildly spreading disinformation like spores. When someone tells you their bees died of nosema, don’t be afraid to question them: “Did you have them tested?” If they say, “No, I didn’t have to” take everything else they say with a grain of salt. If they didn’t have those colonies tested, their conclusion is nothing more than a wild guess.
I highly recommend reading Randy’s May 2019 article, “It’s ‘Common Knowledge’ that Nosema Causes Dysentery…But is it Actually True?” He presents lots of supporting evidence from the work of others.
Honey Bee Suite