Prevalence and infection intensity of Nosema in honey bee (Apis mellifera L.) colonies in Virginia

Nosema ceranae is a recently described pathogen of Apis mellifera and Apis cerana. Relatively little is known about the distribution or prevalence of N. ceranae in the United States. To determine the prevalence and potential impact of this new pathogen on honey bee colonies in Virginia, over 300 hives were sampled across the state. The samples were analyzed microscopically for Nosema spores and for the presence of the pathogen using real-time PCR. Our studies indicate that N. ceranae is the dominant species in Virginia with an estimated 69.3% of hives infected. Nosema apis infections were only observed at very low levels (2.7%), and occurred only as co-infections with N. ceranae. Traditional diagnoses based on spore counts alone do not provide an accurate indication of colony infections. We found that 51.1% of colonies that did not have spores present in the sample were infected with N. ceranae when analyzed by real-time PCR. In hives that tested positive for N. ceranae, average CT values were used to diagnose a hive as having a low, moderate, or a heavy infection intensity. Most infected colonies had low-level infections (73%), but 11% of colonies had high levels of infection and 16% had moderate level infections. The prevalence and mean levels of infection were similar in different regions of the state.

Graphical abstractFigures depict results from a real-time PCR assay for the detection of Nosema apis and Nosema ceranae in honey bees. The figure on the left shows an amplification plot and how the data were used to diagnose hives as having a heavy, moderate, or low infection intensity. The figure on the right shows N. ceranae infection intensities in relation to colony strength.Figure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights► Nosema ceranae is present in approximately 70% of hives in Virginia. ► Spore counts are not reliable for the diagnosis of low level N. ceranae infections. ► Nosema apis is rare in Virginia (2.7%) and only occurs as a low level co-infection. ► No significant correlation was found between mean infection level and colony strength. ► No significant correlation was found between mean infection level and geographic area.