# 2.1.1. Colony level infection

The degree of Nosema spp. infection in a colony has most commonly been described through the average number of spores per bee in a pooled sample (Cantwell, 1970; OIE 2008). However, some studies suggest that the best way to determine the degree of infection is to estimate the proportion of infected bees in the colony (L’Arrivee, 1963; Doull, 1965; Higes et al., 2008a; Botías et al., 2012a). Nevertheless, there is a good correlation between the proportion of infected bees and the average number of spores in a pooled sample of bees (Fries et al., 1984), but not in all cases (Higes et al., 2008a). Evaluating the proportion of infected bees is much more laborious than to count the number of spores in a pooled sample, so pooled sampling will probably remain an important tool for quantifying infections in colonies. Because there is a wide variation in the numbers of spores found in individual bees in pooled samples, when the highest precision is needed, it may still be motivating to investigate the proportion of infected bees. The highest proportion of infected bees are found in foraging bees (Higes et al., 2008a, b; Botías et al., 2012a,b; Meana et al., 2010; Smart and Sheppard, 2011), as is the greatest infection (spores per bee) (El-Shemy and Pickard, 1989). Recent studies suggest the importance of determining the proportion of infected house bees to establish the viability and impact from infection on colonies (Botías et al., 2012a). Considerations regarding pooled sampling versus individual bee diagnosis are discussed in section 2.2.1.2.