Introduction
Mastitis is one of the three most significant health problems of the UK dairy herds, together with lameness and fertility problems. The effect of mastitis on milk quality has recently gained added importance with the introduction of an EU-wide upper legal limit for somatic cell counts in milk destined for liquid market at 400 000 cells/ml. Public interest in the welfare of production animals and the recognition of mastitis as a major source of pain for affected cows give added focus to mastitis concerns.
Organic dairy farmers have identified mastitis as a major concern in the UK, mainly due to non-use of antibiotic dry cow therapy and the need to maintain low somatic cell counts in the milk. Animal welfare aspects of controlling and treating mastitis are also important on organic farms, where the maintenance of high welfare standards is important.
Mastitis – causative factors
Our understanding of mastitis has developed in several stages over the past 100 years. An association between mastitis and pathogenic micro-organisms was established in 1887. Most major pathogens were identified by the 1940s. When antimicrobial therapy became available for production animals in 1945 it proved effective in the control of some, but not all, mastitis pathogens. This prompted further research into potential husbandry related causes of mastitis. In the 1960s, the multi factorial aetiology of bovine mastitis was commonly recognized.
Today, mastitis is considered to be a multi factorial disease, closely related to the production system and environment that the cows are kept in. Mastitis risk factors or disease determinants can be classified into three groups: host, pathogen and environmental determinants.
Mastitis and somatic cell counts (SCC)
Somatic cell counts (SCC) have long been used as a way of measuring milk quality. Most dairy companies base their milk pricing policy, among other things, on SCC values of the milk. The SCC levels in the national dairy herd in the UK have declined steadily since the 1970s and are now well below 200 000 cells/ml, both in bulk tank milk and in average individual cow milk in milk-recorded herds. The maximum legal limit for saleable milk is 400 000 cells/ml.
The somatic cells consist mainly of immune cells that enter the milk compartment of the udder. Only a minority of these cells are dead cells from the udder tissue. There are always small quantities of immune cells in the cow’s milk, and their function is to protect the udder against infection by bacteria. The older the animal gets, the more somatic cells it tends to have in its milk. Similarly, the SCC levels are higher immediately after calving and towards the end of each lactation.
When bacteria do enter the udder, the number of immune cells increases rapidly, as the immune system attempts to overcome the infection. Once the infection has been cleared, the SCC levels gradually drop to normal. This can sometimes take weeks, however. In cases of chronic infection, where the bacteria persist in the udder, the SCC levels can remain high throughout the lactation.
High SCC levels in the milk cause deterioration of the milk quality. It has been shown that levels above 500 000 cells/ml decrease cheese yield and affect yoghurt making. The shelf life of milk is also affected, but at a higher level of SCC.
Consistently high SCC levels in a herd are usually a sign of high levels of subclinical mastitis. Most cases of subclinical mastitis are caused by contagious mastitis bacteria (S. aureus or Str. agalactiae), even though Str. uberis is increasingly considered to cause chronic mastitis as well.
Masititis in the UK
In the UK, no national survey data on clinical mastitis has been produced since 1986. Annual incidence in England and Wales was reported at 54.6 cases per 100 cows in 1980 and 41.2 in 1982. A recent survey of DAISY (Dairy Information System) recorded herds in England and Wales reported the following parameters for clinical mastitis: 39.9 cases per 100 cows; 25.9% of cows affected and a recurrence rate of 18.3%. Another survey of English and Welsh dairy herds participating in an udder health programme found an average clinical incidence rate of 34 cases per 100 cows. Whilst these surveys offer no data on the prevalence of different mastitis pathogens, sporadic evidence from small field surveys and summaries of VI-centre isolations of mastitis pathogens suggest that E. coli has become the most important mastitis pathogen in the UK, with Str. uberis as the second most important pathogen. This anecdotal evidence also suggests that staphylococcal mastitis has significantly declined in significance in the past 30 years. It is, however, suggested that there are great regional variations in the prevalence of different mastitis pathogens, and that S. aureus still remains an important mastitis pathogen in the north of England and Scotland.
During the past 30 years, there have been even more significant changes in the levels of somatic cell counts of the milk produced in the UK. Bulk tank somatic cell counts have dropped from an estimated 750,000 cells/ml in 1968 to a present estimate of 180,000 in 1996. Individual cow milk cell counts (CMSCC) in the milk recorded herds have similarly declined from an average of 573,000 cells/ml in 1971 to 168,000 in 1996. A similar decline in both bulk tank and individual cow somatic cell counts has been observed in other European countries during the same period.
Whilst mastitis control programmes have had a major impact on the improvement of udder health in the British national dairy herd, it has been suggested that introduction of milk quota systems within the EU, quality payments based on bulk milk somatic cell counts and, most recently, legislation setting maximum somatic cell count levels for saleable milk within the EU have been the major encouraging and motivating factors in bringing about the changes described above. It has also been pointed out that the incidence of clinical mastitis has not declined at the same rate as the somatic cell counts in the past 15-20 years, but has rather remained at an endemic level in the national herd.
Mastitis in organic dairy herds
Mastitis incidence and patterns were surveyed in 16 organic and 7 conventionally managed dairy herds in the south of England and Wales in 1997-1998. Clinical mastitis incidence in survey herds is presented in the table below. Whilst overall mastitis incidence was significantly lower (P<0.001) in O herds than in C herds, the incidence rates during the dry period were significantly higher in O herds than in C herds (P<0.001). There was a wide variation in incidence rates amongst both O and C herds. The lower incidence in O herds was related to a very low incidence in one large herd.
| Table 1. Mastitis incidence (cow cases/100 cow years) | ||
|
O herds |
C herds |
|
| Overall |
36.4 |
48.9 |
| Lactation |
37.6 |
54.5 |
| Dry period |
28.9 |
9.2 |
Average individual cow SCC levels were significantly higher in O herds (135,000 cells/ml) than in C herds (84,000 cells/ml; P<0.001), resulting in high subclinical mastitis levels in O herds (individual cow SCC> 200,000 cells/ml in 34% of all measurements).
Another UK survey of dairy farms converting to organic milk production found average levels of 45.8 cases of clinical mastitis/100 cows on 11 farms at the end of the conversion period, with mean annual somatic cell counts of 299,000.
A German study of 268 organic dairy herds identified mastitis as the most important health problem. Whilst incidence rates for mastitis were similar to those on conventional farms, the culling rates for mastitis were higher than on conventional farms studied health and disease control on 14 organic dairy herds in Denmark, and found similar levels of mastitis incidence to comparable conventional herds. A Dutch study has identified somatic cell count control as a critical area in mastitis control under organic production standards. In the study, S. aureus mastitis is seen as the main mastitis problem on organic dairy farms, and the difficulty in controlling it is attributed to poor diagnosis and non-use of antibiotic DCT