A survey of mortality in 51 caged laying flocks.

A survey of mortality was carried out in 51 caged laying flocks between point of lay and slaughter and lasted from March 1971 to June 1973. Flocks were composed of 26 white egg strains and 25 brown egg strains. Total flock population sampled was 281,000 laying fowl. Types of holding that were sampled or sub-sampled were representative of units on which 80% of the laying fowl are maintained in England and Wales. Sampling was performed at four Veterinary Investigation Centres of the Ministry of Agriculture, Fisheries and Food (M.A.F.F.) by obtaining a single day's mortality per fortnight and at a fifth Centre by collecting a specified day's mortality at weekly intervals. In 42 of the 49 flocks for which overall mortality figures were available, mortality did not exceed 16% (mean +/- SEM = 9.12 +/- 0.53) and the monthly mortality rate did not rise above 2.5%. The total number of specimens examined was 2,615. Primary and secondary diagnoses were recorded and of the former, kidney lesions (20%), disorders of the reproductive tract (15.1%), cannibalism and other injury (11.0%), Marek's disease (10.6%), staphylococcal infection (7.1%) and obesity, fatty liver and ruptured liver (7.7%) were the principal conditions encountered. Excluding those specimens for which the cause of death was unestablished, this group of conditions comprised 78.9% of all primary diagnoses. Significant variation in mortality during successive periods of lay was recorded in reproductive disorders, cannibalism, Marek's disease, staphylococcal infection and associated diagnoses of ruptured liver and obesity. No relation was found between flock size and mortality.