In the 1970’s in mid-west US, most cows were kept in tie stall barns where physical contact between cows and the spread of M bovis were very limited. The death rate of young calves was reduced with the move to individual hutches and no physical contact, and pasteurising milk. In the 1980’s if mastitis didn’t respond to antibiotics it was assumed the cow had M bovis and she was culled. Sick cows were isolated quickly so spread was slow. Average herd size in Wisconsin in the 1990’s was 55.
In modern US dairy farms with larger herds and free stall barns, dry lots or grazing, cows have much more contact so disease spreads through the herd far more quickly. Current mastitis cultures indicate nearly 10% of herds in the Western States have M bovis. In the 1900’s when Sue was in the US looking at well fed herds with poor profitability, M bovis was sometimes the cause. Even where there was no clinical disease, cows used energy for immunity when M bovis was active, so feed efficiency was worse. Cows use 300-400 grams of glucose per day in response to inflammatory or infectious challenge. This disease costs farmers money even if there are no clinical cases. Infected herds can initially appear OK but if cows come under stress there can suddenly be a lot of clinical cases.
In NZ we already have free range cows, large herds and more cow movement, so the spread will be faster if the disease is not contained. Having a closed herd and pasteurising milk for calves can help reduce the spread.