Effects of Streptococcus dysgalactiae polyarthritis on lamb growth and mortality and risk factors for disease

This study describes an outbreak of infectious polyarthritis due to Streptococcus dysgalactiae in a New Zealand sheep flock, evaluates the effects on lamb growth rates and mortality and investigates risk factors for disease development. Data from 76 affected lambs and up to 2223 unaffected lambs were used. On average, affected lambs had a 54.4 g/day reduction in growth rate from birth to weaning; at an average weaning age of 82 days this amounted to a 4.5 kg reduction in weaning weight. The pre-weaning mortality of affected lambs was 10% higher than for unaffected lambs. The only factors that were significantly associated with development of the disease were time of birth and birth area (groups of paddocks with similar physical characteristics) whereas there was no association with lamb birthweight, gender, litter size or vigour score or with dam age, body condition score or maternal behaviour score.