Within-colony transmission of Microsporidian and Trypanosomatid parasites in honey bee and bumble bee colonies

Parasites are commonly cited as one of the causes of population declines for both managed and wild bees. Epidemiological models sometimes assume that increasing the proportion of infected individuals in a group should increase transmission. However, social insects exhibit behaviors and traits which can dampen the link between pathogen pressure and disease spread. Understanding patterns of parasite transmission within colonies of social bees has important implications for how to control diseases within those colonies, and potentially the broader pollinator community. We used bumble bees (Bombus impatiens Cresson) and western honey bees (Apis mellifera L.) infected with the gut parasites Crithidia bombi (Lipa & Triggiani) and Nosema ceranae (Fries et al.), respectively, to understand how the initial proportion of infected individuals impacts within-colony spread and intensity of infection of the parasites. In bumble bees, we found that higher initial parasite prevalence increased both the final prevalence and intensity of infection of C. bombi. In honey bees, higher initial prevalence increased the intensity of infection in individual bees, but not the final prevalence of N. ceranae. Measures that reduce the probability of workers bringing parasites back to the nest may have implications for how to control transmission and/or severity of infection and disease outbreaks, which could also have important consequences for controlling disease spread back into the broader bee community.

寄生虫（parasite）常被认为是规模化养殖蜂群与野生蜂群种群数量下降的诱因之一。流行病学模型（epidemiological model）有时会假设，蜂群内感染个体比例的提升会加剧寄生虫传播。但社会性昆虫（social insect）所具备的行为与性状，可削弱病原体压力与疾病传播之间的关联。探明社会性蜂群内部的寄生虫传播模式，对管控该蜂群内的疾病，乃至更广范围的传粉昆虫群落（pollinator community）均具有重要意义。本研究分别选取感染肠道寄生虫（gut parasite）熊蜂短膜虫（Crithidia bombi, Lipa & Triggiani）的北美大黄蜂（Bombus impatiens Cresson），以及感染肠道寄生虫东方蜜蜂微孢子虫（Nosema ceranae, Fries et al.）的西方蜜蜂（Apis mellifera L.）作为实验材料，探究感染个体初始比例如何影响寄生虫在蜂群内部的传播与感染强度。针对北美大黄蜂的实验结果显示，初始感染率越高，熊蜂短膜虫的最终感染率与单只蜜蜂的感染强度均有所提升。而针对西方蜜蜂的实验则发现，初始感染率的提升仅会提高单只蜜蜂的感染强度，并不会改变东方蜜蜂微孢子虫的最终感染率。通过采取措施降低工蜂将寄生虫带回蜂巢的概率，或可实现对寄生虫传播、感染严重程度及疾病暴发的管控，这对于遏制疾病扩散至更广范围的蜂群群落亦具有重要价值。