The Honey Bee Parasite Nosema ceranae: Transmissible via Food Exchange?
To test the hypothesis he set up a series of hoarding cages with varying degrees of separation between infected older bees and non-infected young bees. This way the older bees would feed the younger if able and potentially spread spores of N. ceranae to them. While the test cannot rule out any fecal transmission, (the spores are still present there, the fungus just doesn't give the bees the runs) it does cut down on exposure via that pathway. The three degrees of cage separation were as follows: A single screen between the cages would allow for the transfer of food but not allow intermingling, a dual screen would keep the older bees from even getting a chance to feed the younger ones, and a control of isolated young bees would, well, act as a control for natural exposure.
First, to ensure infection, the bees destined to be used as "Older bees" were taken from their hives and marked. They were then coated with a sucrose solution laced with N. ceranae spores. As they cleaned themselves they ingested the spores leading to infection. These bees were then allowed to incubate their new parasite for 12 days before being collected and used in the first of three trials. A second set of trials was conducted with the older bees not being artificially infected but picked to represent a natural spread in colony.
The "Younger bees" for the test were taken directly from combs using an aspirator quickly after emergence and isolated until test runs.
For the testing the cages were placed next to each other in the aforementioned set ups for 4 days. In all sets the older bees were fed a sucrose solution and in the case where young would not be reachable they too were fed the same type solution. However; in the young bees directly adjacent to the old bee cage, no food was provided on days 2-4. This forced them to receive food via feeding from the older bees. Mortality of the young bees during this time was no different than the rest of treatment times.
After the exposure was complete the bees were then allowed to incubate for 10 days before being dissected to detect infection. For the dissection each bee had its midgut extracted and opened under a microscope. Then spore presence was observed by looking at 25 field views for each sample. Due to the high colonization of infected bees only bees counted with >100's of spores were counted as infected.
And the infection rates showed very strong correlation with the single screen exposure. In fact, while there was not much notable difference between the dual screen and the control group, the single screen bees showed a 13 times higher rate of infected young bees.
The researcher does take special time to note that while precautions to prevent defecation spread were taken, and that no feces were observed on the separation screen, there is still potential that the fecal-oral pathway could still potentially be a factor in spread. But despite that the study gives the first experimental support to the hypothesis that N. ceranae is spread through oral-oral exchange.
Michael L. Smith (2012). The Honey Bee Parasite Nosema ceranae: Transmissible via Food Exchange? PLoS One DOI: 10.1371/journal.pone.0043319
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