# Nosemosis in Honeybees: A Review Guide on Biology and Diagnostic Methods
## 2 _Nosema_ Life Cycle and Spread  
Microporidia _N. apis_ and _N. ceranae_ are pathogens of the epithelium of the honey bee midgut. The parasite develops in all the castes of the bee colony (workers, drones, and queens). In the external environment, these microsporidia exist in the form of spores, which must be eaten (with food or water) by the bee in order for the infection to start [11]. The mature spore starts to germinate in the midgut lumen by extruding its polar filament. The spore requires a change in pH to activate the process of polar filament ejection. The polar filament penetrates the cell lumen, and the sporoplasm (the inside of the spore) flows through it into the host cell [12]. In the host cell, the sporoplasm transforms into a meront and starts to multiply (by division) into sporonts. After proliferation, sporonts transform into sporoblasts, which have a thick cellular wall. Sporoblasts transform into mature spores that pass into the lumen of the host gut [13]. Spores of _N. apis_ and _N. ceranae_ can pass from one epithelial cell to another and still reproduce, which contributes to their high pathogenicity [14; 15]. At the end of the process, the spores are passed into the rectum and exit in the feces (which are the main source of _Nosema_ infection) [11; 13]. Since the spores destroy the gut lumen, the bee cannot ingest or digest food. Therefore, there are considerable amounts of (undigested) sugar in the feces (which is attractive to other bees), and bees kick and eat it (the oral-fecal route of infection). Combs contaminated with feces are the basic source of infection [16]. _Nosema_ spores also spread through honey, pollen, bee bread, syrup, and other routes, thus trophallaxis (oral-oral food exchange between insects) is a direct way of sharing the spores between bees (Figure 1) [17; 18]. It causes dissemination of the spores through all the castes of the colony (including the queen) [19]. A study by [PERSON] and [PERSON] [20] shows that drones can be'superspreaders' of _N. ceranae_ spores through trophallaxis. The complete life cycle of _N. apis_ can last about 5 days [11]. In cage trials, the life cycle of _N. ceranae_ lasted 3 days [15]. Some studies show that when a 100% infectious dose (114 \(\times\) 10\({}^{3}\) spores per bee) is used, the youngest (newly emerged) bees are the most susceptible to the infection [21], while other studies suggest that, with the same infectious dose, 5-day-old bees are the most susceptible [22]. It has been proven that _N. ceranae_ can infect basal cells in the midgut while _N. apis_ cannot [15]. Epithelial cells in the midgut exfoliate constantly, so the basal cells proliferate and replace them; thus, when basal cells are infected, the intestinal wall is damaged irreversibly, which points to the higher pathogenicity of _N. ceranae_ in comparison with _N. apis_. Other in vivo studies have also  
Figure 1: Routes of _Nosema_ spp. infection and spread. confirmed the higher pathogenicity of _N. ceranae_. For instance, [PERSON] and [PERSON] [23] showed that the average infectious dose of _N. ceranae_ spores was over 4.5 times lower than of that of _N. apis_, and it possessed an LD50 of about 85 spores. However, in the United States, the results were very different. [PERSON] et al. [22] showed that the infectivity of _N. apis_ was higher than _N. ceranae_ (for bees of the same age and the same infectious dose). The contradictory results may be due to the different climatic conditions from which the bees originated or even a genetic component; for instance, Danish bees are resistant to _Nosema_ infections (in a long-lasting breeding program) and the pathogenicity of this microsporidium is minor there [24]. It may also be due to a black queen cell virus infection, which is known to aggravate the course of nosemosis significantly and which is not checked for in most _Nosema_ studies [25]. Both _Nosema_ species exhibit different temperature tolerances. _N. ceranae_ is very sensitive to freezing, while _N.