As one of the possible causes of honey bee losses, the gut pathogen Nosema ceranae has proved controversial. It seems accountable for colony losses in Spain, yet elsewhere it seems not to cause obvious problems despite being widespread. Two papers published recently in the Journal of Apicultural Research help to shed some light on the problem, with close studies of this organism and its relationship with its host.
Nosema ceranae, a microsporidian fungus, is a relatively new pathogen of honey bees, having only been discovered in 1994 on the Asian honey bee Apis cerana. It was found on the western honey bee Apis mellifera in 2005. It seems to have spread rapidly around the world and apparently displaced the similar species Nosema apis. This second parasite is a minor chronic infection of the western honey bee, known for more than one hundred years. However, no-one is absolutely certain of the history of these two species because their spores are indistinguishable under an ordinary light microscope. So far, few laboratories throughout the world are able to perform the costly molecular techniques necessary to reliably determine between the two Nosema species.
In a new paper, Dr Aneta Ptaszyńska and colleagues from the Maria Curie-Skłodowska University, Lublin, and the University of Life Sciences in Lublin, Poland have demonstrated that Scanning Electron Microscopy (SEM) provides a solution. Close examination of the cell walls of Nosema spores revealed that they differ between the two species, with Nosema ceranae having a characteristic sculpted cell wall. The authors also studied the damage caused by the parasite to the intestines of infected bees. They found that the midgut of affected bees is completely covered with Nosema. This spore-made layer may be the cause of bee malnutrition and higher mortality of foragers.
In the second paper, Dr Predrag Simeunovic and colleagues from the University of Belgrade, Serbia, carried out a three-year study on the laying capacity of queen bees as affected by age and infestation by Nosema ceranae. The results clearly demonstrated not only that as queen bees age so colony productivity declines, but that interactions with Nosema ceranae infestation also occur. Curiously, although queen bees decline in all other respects during their third year, their egg-laying capacity was found to increase. This may be an influence of Nosema infection which was greatest in the oldest queens. The authors suggest that the infected queens may significantly increase their rate of egg-laying to compensate for the losses of heavily infected workers due to Nosema.
IBRA Science Director Norman Carreck says: “These two new papers add to our knowledge of this problematic pathogen and how it adversely affects honey bees.” (IBRA News Release)