The Effects of <italic>Nosema ceranae</italic> on honey bee health

Abstract

Honey bees are arguably the world's most iconic pollinator. The presence of honey bees in our landscapes has long invoked images of vitality, diligence, and cooperation. Unfortunately, the current state of honey bee health and productivity paints a rather different picture for this beneficial insect. The survival of honey bees, as well as the livelihoods of those who benefit from their labor (e.g., beekeepers, growers, and ranchers) is under threat from the culmination of a number of detractors to bee health. Exposure to pesticides, poor forage, mite parasites, and pathogens has resulted in heavy annual death of honey bee colonies in the U.S., Europe, and other parts of the world. Among the suspects thought to contribute to bee decline, the fungal pathogen, <italic>Nosema ceranae</italic>, is found at high prevalence in both healthy and declining/dead colonies. Since <italic>N. ceranae</italic> is thought to be a novel pathogen of the European honey bee, <italic>Apis mellifera</italic>, much remains unknown about its pathology at the individual and colony levels, as well as how infection may interact to form synergisms with other factors thought to be responsible for declining honey bee populations. Here, I review the breadth of research conducted on <italic>Nosema</italic> infection of honey bees in general. I give particular attention to observations on the detection of infection, cytopathology, viability and infectivity of spores, and incidence and geographic distribution. I rely on what is known about a similar and long-recognized pathogen of <italic>A. mellifera, N. apis</italic>, for context on the host-parasite relationship of <italic>Nosema</italic> infection of honey bees. As one part of my dissertation research, I applied background information about the effects of infection on the different castes to show how this may disrupt the social structure of a honey bee colony and contribute to its decline. The level of a honey bee colony or even a whole bee are complicated systems to study intracellular microbes such as <italic>Nosema</italic> that threaten honey bee health and productivity. Continuous cell lines have been lacking to further our understanding of the pathogenicity of <italic>Nosema</italic> infection in a simplified environment removed from the confounding effects of other factors such as poor nutrition or pesticides. For the second part of my dissertation research, I addressed this deficiency in honey bee research by developing an <italic>in vitro</italic> system derived from host cells that could be utilized for observing infection in a simplified environment. Whenever possible, I provide suggestions for future research that could broaden our understanding of this pathogen and ultimately improve honey bee health.