A longitudinal field study of mortality and Marek's disease in Norwegian and imported White Leghorns

High mortality during the first part of the laying period was observed in N orwegian White Leghorns during the period 1988-1992. A longitudinal field s tudy with repeated measurement of cumulative mortality was undertaken in th e 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 cumulati ve mortality in the period from 16 to 32 weeks of age in White Leghorn floc ks, and (3) the effect of MD as judged by repeated measurement of cumulativ e mortality in the same period. All five layer hatcheries and 67% of the pu llet-rearing farms in Norway participated in the field study. The egg produ ction farms were sampled by convenience. Recordings for the whole period we re completed for 169 flocks in 101 farms. The statistical analyses were per formed using both a general fixed-effects linear model and a mixed model wi th repeated measurements, with total flock-level cumulative mortality and f lock-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 N B41 and Lohmann White, respectively). MD was not recorded in any of the Sha ver White flocks. A significant difference (p < 0.001) was found in(l) tota l cumulative mortality: 8.2% in the NB41 and 5.0% in the imported layers, a nd (2) 'interval-specific' cumulative mortality: 0.36% in the NB41 and 0.15 % in the imported birds. A strong relationship was also demonstrated betwee n 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-measure s analysis also indicated a stronger effect of MD on flock-level 'interval- specific' cumulative mortality in the NB41 than in the imported hens. The d ifferent cumulative mortality and susceptibility to MD observed in the NB41 , compared to the imported hens, shows that the farmers will be able to red uce their losses by replacing the NB41 strain with one of the imported stra ins. (C) 1999 Elsevier Science B.V. All rights reserved.