Determination of infectious dose

The infectious dose (the dose that infects half of inoculated bees (ID50) or 100 % (ID100) can be calculated relatively precisely when experimental bees are individually inoculated as described in section and incubated in hoarding cages. The proportion of infected bees given different doses is then examined after 7-10 days. Usually, inoculums are prepared from a stock suspension of spores that is subsequently diluted (section The final quantity of spores given to bees is therefore theoretical (= calculated). Under some circumstances and when the protocol is adapted to the goal of the study, the technique of group feeding can be used and gives reliable results (Pettis et al., 2012; Tanner et al. (2012)). When the bees are mass fed, the quantity of spores given to each bee is an average count calculated from the overall quantity distributed to a given number of bees, but it should be noted, that group feeding is likely to yield an uneven distribution of spore dose among bees (section Similar to pesticide studies, where the lethal dose to 50 % (LD50) or 100 % (LD100) of tested insects are used to describe toxicity, the infectious dose can be described as the dose that produces infection in 50 % (ID50) or 100 % of exposed insects (ID100). For N. apis, ID50 has been determined to roughly 100 spores per bee (Fries, 1988), with 10,000 spores producing infection in all exposed honey bees (Czekonska, 2007; Fries, 1988). Recently, similar experiments were performed that compared N. apis and N. ceranae. Results revealed a slightly lower ID50 for N. ceranae (Forsgren and Fries, 2010). It should be noted that little is known about possible variations in infectivity between parasite isolates and/or different levels of susceptibility between host strains and, thus, infectious dose experiments need to be repeated using different bees and different spore sources.