A major outbreak of tuberculosis occurred in cattle on a farm in Dorset between 1970 and 1976. Six hundred and twenty-six cattle were slaughtered either because they reacted to the tuberculin test or had been exposed to infection. No source of infection was found until 1974 when badgers infected with Mycobacterium bovis were first discovered.
An analysis of the tuberculin test records of this herd and the six surrounding herds indicated that tuberculosis had been a sporadic problem since the early 1960's. Two peaks of infection occurred in the most severely affected herd in 1970 and 1974 when 29·8% and 27·3% of animals, respectively, reacted to the tuberculin test. These figures are exceptionally high. During the last 20 years there have been two periods when all the herds in the area had synchronous outbreaks consistent with a common source.
Analysis indicated that cattle were at greatest risk in April and May and suggest that there was re-exposure to infection at this time each year. In addition the cattle were apparently exposed to M. bovis, at sufficiently high levels for transmission to occur, for only a relatively short period of time.