Incidence Rate

The mastitis incidence rate

The incidence rate measures the number of new mastitis cases recorded over a period of time, usually in the form of number of cases per 100 cows over a 12-month period. It is useful to use in comparison with other farms and the national average, but as a comparative analysis technique it has several drawbacks. The most obvious is that some farms are likely to only record clinical mastitis cases, as subclinical cases will only be apparent through high Somatic Cell Count results. Some farmers may not include those mastitis cases which self-cure; there is also the potential for mild clinical cases to self-cure before having been identified.

The average UK mastitis incidence rate has dropped significantly from 150 cases per 100 cows per year in the 1960s to 30+ cases per 100 cows in the last ten years. Some evidence has suggested that there has been a recent slight increase in the incidence rate, but the reasons for this are complex and may be due to increases in milk yield and cow numbers in the average herd, and also on the emphasis on identifying and treating subclinical cases of mastitis that previously would not have been recognised.

It is important to recognise that, for recording purposes, one quarter infected once is counted as a single case, and if a cow has two infected quarters, that is in effect two cases of mastitis. If a quarter subsequently clears up and then mastitis recurs seven or more days later, this is then treated as a further case of mastitis.

Some farms also record the number of mastitis intra-mammary tubes used, but this information must be treated with caution, as it does not necessarily reflect the number of mastitis cases treated. Various factors influence the number of tubes used to treat mastitis cases:

  • The decision of individual stock persons on how many tubes are required, and how soon they are administered.
  • Veterinary advice may differ from vet to vet and practice to practice, depending on personal preferences of both vet and farmer and knowledge of which pathogens are present on the farm.
  • A variety of factors can affect the efficacy of and response to the drugs used, not least the types used, how well they are administered, and how acute the illness in the cow is.