The incidence of various infectious and non-infectious diseases as causes of death in lambs up to 8 months of age was studied in relation to breed, inbreeding and other factors. The data from an upland grazing flock covered 6 years and involved the Scottish Blackface, Cheviot and Welsh Mountain breeds and the crosses among them.
Infectious and non-infectious diseases were directly responsible for almost a quarter of the mortality and their presence contributed to many more deaths. The organisms mainly responsible were Escherichia coli, Pasteurella spp. and Fusobacterium necrophorum.
The incidence of infectious diseases in general and enteritis in particular varied significantly between the breeds and was lower in the cross-breds than in the purebreds. Variation between the breeds in the incidence of non-infectious diseases was statistically significant only in respect of digestive upsets for which there was a similar ranking of breeds to that for infectious disease.
Inbreeding of the lamb (25–59%) significantly increased the incidence of infectious diseases, especially respiratory diseases, but not of non-infectious diseases. There was little difference between singles and twins in susceptibility to disease. The lower the birth weight of the lamb the greater the risk, particularly of non-infectious disease.