RESUMO
High mortality during the first part of the laying period was observed in Norwegian White Leghorns during the period 1988-1992. A longitudinal field study with repeated measurement of cumulative mortality was undertaken in the period from January 1994 to January 1996 to investigate (1) the mortality and susceptibility to Marek's disease (MD) in the Norwegian strain (NB41) compared to two imported layers, (2) the effect of MD on the total cumulative mortality in the period from 16 to 32 weeks of age in White Leghorn flocks, and (3) the effect of MD as judged by repeated measurement of cumulative mortality in the same period. All five layer hatcheries and 67% of the pullet-rearing farms in Norway participated in the field study. The egg-production farms were sampled by convenience. Recordings for the whole period were completed for 169 flocks in 101 farms. The statistical analyses were performed using both a general fixed-effects linear model and a mixed model with repeated measurements, with total flock-level cumulative mortality and flock-level cumulative mortality in four-week intervals as outcome variables, respectively. The overall cumulative flock-level incidence of MD was 12% (24% and 8% in NB41 and Lohmann White, respectively). MD was not recorded in any of the Shaver White flocks. A significant difference (p < 0.001) was found in (1) total cumulative mortality: 8.2% in the NB41 and 5.0% in the imported layers, and (2) 'interval-specific' cumulative mortality: 0.36% in the NB41 and 0.15% in the imported birds. A strong relationship was also demonstrated between MD and repeated measurements of 'interval-specific' cumulative mortality (p < 0.001) but not when cumulative mortality was used as an overall measure for the whole laying period (p = 0.11). The results from the repeated-measures analysis also indicated a stronger effect of MD on flock-level 'interval-specific' cumulative mortality in the NB41 than in the imported hens. The different cumulative mortality and susceptibility to MD observed in the NB41, compared to the imported hens, shows that the farmers will be able to reduce their losses by replacing the NB41 strain with one of the imported strains.