Honey bees Apis mellifera) infected with the parasitic mite, Varroa destructor, or the microsporidia, Nosema ceranae, have changes in the chemical profile of their skin and in their brains, finds research in BioMed Central's open access journal BMC Ecology. Despite this, parasitized bees were not expelled from the hive, which, the authors say, supports the hypothesis that stressed bees leave the hive altruistically to prevent the spread of infection.
This study from INRA (French National Institute for Agricultural Research) investigated the effect of parasitization on honey bees living in hives at Avignon. Individual bees were infected with either the ectoparasite Varroa, which lives on the bees, or endoparasite Nosema, which invades their bodies, and reintroduced to the hive. After a few days the effect of infection on bees and their behavior was monitored.
Parasitization caused changes in the levels of active genes in the brains of infected bees. Varroa altered the activity of 455 genes, including genes involved in GABA and serotonin signaling, while Nosema affected 57. Twenty genes were common between the two infections and several of the up-regulated genes are involved in oxidative stress, neural function and foraging behavior. Parasitized bees also tended to have a higher viral infection as well, adding to their disease burden, -- even if they did not have physical symptoms.
Hydrocarbons on the cuticle of bees provide a 'family' scent allowing bees from the same hive to recognize each other. The levels of these chemicals was altered by infection with either the endo- or ecto-parasite nevertheless infected bees were treated as normal by other bees -- social interactions including antennal contact, grooming, feeding, and vibration, continued -- and they were not expelled from the hive.
Dr Cynthia McDonnell who led this study commented, "Parasitized bees tend to leave the colony earlier to perform foraging activity, which could lead to a significant depopulation of the colony. However, very few studies have analyzed the impact of parasites on bee phenotypes, e.g. brain and behavior. We found that parasitized bees were not attacked by their nestmates suggesting that they leave the hive voluntarily, perhaps in response to the changes in gene expression in their brains. This social removal and the underlying mechanism might be a general and conserved response to parasitism, given that it was observed with extremely different types of parasites"
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