How can Nosema infections weaken honeybee colonies?

Nosema infections, caused by single-celled microsporidians such as N. apis and N. ceranae, weaken honeybee colonies primarily by invading the gut epithelium of the bees. This infection leads to nosema disease, which manifests as a serious form of dysentery and can cause varying degrees of paralysis in the host.123

Beyond the direct physiological impact on individual bees, these infections are implicated as stressors that may contribute to colony collapse disorder (CCD). The weakening of bees due to Nosema often occurs in combination with other pathogens, such as the deformed wing virus, or environmental stressors like pesticide exposure, which can suppress immune function and promote the proliferation of these parasites. Furthermore, researchers have noted that honeybees raised in environments with high levels of pesticide residue exhibit increased susceptibility to N. ceranae, suggesting that chemical stressors may exacerbate the severity of Nosema infections within a hive.1