3. Future perspectives
The Nosema spp. parasites in honey bees still remain largely enigmatic. The described field symptoms differ between the parasites (Fries et al., 2006), as do the seasonal prevalence (Higes et al., 2008a, b). These observations suggest that the main mode of transmission between bees could differ between N. apis and N. ceranae. Faecal deposition within bee hives is associated with N. apis infections, but this is not the case with N. ceranae (Fries et al., 2006). The main mode of parasite transmission for N. apis is believed to be a faecal-oral route through soiled comb (Bailey, 1953). There is a need to elucidate the main mode of transmission for N. ceranae to understand more of the epidemiology of this parasite. There is also a profound lack of data on differences in susceptibility of both parasites among different honey bee strains. Using standardized laboratory infectivity tests, it is probably possible to find differences in susceptibility to infection, both within and between strains of bees. With such information, breeding for resistance can be undertaken and genetic markers for resistance could possibly be located for genetic marker aided selection for disease resistance. As with differences in resistance in the host, it is likely that different isolates of the parasites differ in infectivity as well as in virulence. Although such differences have never been documented, along with differences in host susceptibility, they could complicate interpretations of experiments and possibly explain some contradictory results published on parasite virulence.
Ring tests among laboratories would be very useful for the scientific community but remain to be organized and funded.
The use of cell culture for studying the Nosema parasites is still in its infancy. With further developments, where continuous propagation of Nosema spp. in cell cultures becomes possible, new insights into infection biology may be gained. Also, there is a need to develop a reliable method for the long term storage of infective N. ceranae spores.
Lastly, a further complication when studying the Nosema parasites in honey bees is the associated virus infections. Infection with N. apis is associated with three unrelated viral infections; black queen cell virus (BQCV), bee virus Y (BVY) and filamentous virus (FV). A combination of N. apis and BQCV is distinctly more harmful than either infection alone, infection with BVY adds to N. apis virulence, whereas no such influence is seen with FV infections (Bailey et al., 1983). Interestingly, N. ceranae infections have been shown to be negatively correlated with deformed wing virus (DWV) (Costa et al., 2011).