Nosema spp. – Honeybee Parasites
Nosema apis and Nosema ceranae Cause Honeybee Disease
Nosema apis has long been known as an intestinal parasite of European honeybees. Nosema ceranae seems to be new and may be causing colony collapse disorder.
Nosema apis and Nosema ceranae, agents of nosemosis in honeybees, belong to the Microsporea--a class of parasites that infect mostly insects, but also other invertebrates including fish, birds, reptiles, amphibians, and mammals. Until recently, the Microsporea were grouped with single-celled parasites; however, current evidence suggests that Microsporea may be fungi.
Spores of Microsporea range in size from about three to six micrometers (a micrometer, or micron, is one one-thousandth of a millimeter), the size of many intestinal bacteria. Nosema apis spores are four to six micrometers long and two to four micrometers wide. They are marvelous organisms, perfectly designed for invading the intestinal cells of their hosts.
The spore has a tough resistant wall protecting it from conditions in the host and in the environment. Inside lies a coiled polar tubule (or polar filament)—think of a tightly coiled spring packed into an egg. When the time is right, the coil releases with such force that it penetrates first the spore wall and then the cell wall of the host. At the same time, the spore empties itself into the tubule and all the spore contents are deposited inside the host cell. The complete life cycle of N. apis is as follows:
- Inside intestinal cells of an infected bee, parasite nuclei divide repeatedly, producing large amoeba-like organisms with multiple nuclei.
- Parasite nuclei become walled off from each other as separate organisms, which continue to divide. Some mature into spores.
- Infected bees pass spores—sick bees often have diarrhea and defecate in the hive, greatly increasing the chances that other bees will be exposed.
- Other bees come in contact with the spores and ingest them. Conditions in the bees’ intestines trigger the explosive release of the polar tubule causing nosemosis.
Keepers of European honeybees, Apis mellifera, are familiar with N. apis and nosemosis. Infected workers, drones, and queen bees become weak and don’t live as long. Queens suffer damage to reproductive organs and bee populations can drop dramatically. Spring tends to be a high risk time until sick bees die off and new healthy brood emerges: when parasite levels are low, the colony appears normal. Nosema apis is a serious bee parasite but it can be managed—N. ceranae seems to be a different story.
In an eerie replay of the spread of the varroa mite, N. ceranae may have jumped from the Asiatic honeybee, Apis cerana, to the European honeybee. Early this century, the incidence of nosemosis in European hives rose dramatically, with enormous colony losses. Coincidentally, N. ceranae, previously thought to be confined to Eastern hives, was discovered in Europe. The sequence of events lead beekeepers and scientists to link the appearance of N. ceranae with “colony collapse disorder,” a massive die-off of honeybees. Nosema ceranae has now been found in North American honeybee colonies—and colony collapse is happening there too.
Is there a link between N. ceranae and colony collapse disorder? Scientists question whether:
- Nosema ceranae has recently spread from A. cerana bees, or whether it has been present, undetected, in A. mellifera for a long time.
- Nosema ceranae can cause colony collapse disorder by itself, or whether there’s a combination of factors. Varroa mites, a virus, pesticides, or some other factor may be adding to the bees’ stress.
- Nosema ceranae, or some other organism, has somehow become more dangerous.
With the collapse of honeybees worldwide, the search is on to find the answers.
Related articles:
Honeybee Crisis and Biodiversity
Sources:
Moray Beekeepers. “Nosema ceranae.”
“Nosema.” In: Fundamentals of Beekeeping.
Roberts, Larry S. and John Janovy Jr. Foundations of Parasitology 6th Ed. Boston: McGraw Hill, 2000.
