Mastitis is a multifactorial disease, closely related to the production system and environment in which the cows are kept. Mastitis risk factors or disease determinants can be classified into three groups: host, pathogen and environmental determinants.
Genetic resistance
Genetic resistance to mastitis has been well researched. There is a body of work on the genetics of SCC and subclinical and clinical mastitis (Poso and Mantysaari, 1996). This work has established a favourable genetic correlation between low SCC and mastitis incidence at the cow level. Others have reported on the significance of certain BoLA alleles in resistance to Staphylococcus aureus-infection (Aarestrup et al., 1995). A particular concern is that factors increasing resistance to one udder pathogen might predispose to mastitis caused by other pathogens.
In the past, efforts to introduce mastitis resistance traits into dairy cow breeding schemes have also been hampered by the negative genetic correlation with increased milk yield. It has often been considered uneconomic to attempt to improve both mastitis resistance and milk yield simultaneously, particularly since the heritability of milk yield is markedly higher than that of mastitis resistance (Strandberg and Shook, 1989).
Recent introduction of bull proofs for SCC by the Animal Data Centre in the UK allows farmers to choose bulls for AI that will produce daughters with lower SCC profiles (McGuirk, 1998).
SCC
The correlation between SCC in the milk and the immune response of the udder to infection is not clear. There is evidence to suggest that minor pathogens increase milk cell counts and can help to protect the udder against mastitis. For example, it has been shown that Corynebacterium bovis-infection protects the udder against infection by major pathogens (Kurek, 1980).
Several field studies have also concluded that low BTMSCC herds have a higher incidence of environmental mastitis compared to herds with high BTMSCC (Schukken et al. 1990; Green et al. 1996; Waage et al. 1998). There is also experimental evidence to suggest that moderate to high individual cow milk SCC can provide protection against experimental infection by environmental mastitis pathogens (Matthews and Harmon, 1989).
Cow and udder conformation
Udder and foot conformation have been shown to be important risk factors for mastitis (Seykora and McDaniel, 1985; Grindal and Hillerton, 1991). Most conformation related traits have high heritability, and are generally recognised by farmers and herdsmen as major selection criteria for breeding. There is an emphasis on appropriate breeding in organic standards, and hence breed may be an important factor influencing both mastitis resistance and susceptibility characteristics.
Milk yield
There is substantial evidence to suggest that high yields are linked to high mastitis levels (Grohn et al., 1990; Chassagne et al.,1998; Rajala and Grohn, 1998).
Nutritional status
Various nutritional factors may lower a cow’s disease resistance. The phagocytic activity of macrophages in milk is significantly inhibited by ketone bodies. Several studies have shown that the outcome of experimental E. coli-infection is related to the in vitro ability of polymorphonuclear cells to react to chemo-attractants (Lohuis et al., 1990). Energy, protein and mineral/trace element deficiencies may also affect disease resistance and SCC levels. Baars & Opdam (1998) found a reduction in CMSCC in heifers when the animals received a bolus of trace elements and vitamins. Selenium supplementation has been linked to improved immunity to mastitis (Conrad and Smith, 1986; Erskine et al. 1987; Weiss et al,, 1990).
Disease surveys of organic dairy herds both in the UK and in Denmark and Norway suggest that ketosis occurs at the same or a lower level than data from conventional farms indicate (Vaarst, 1995; Weller et al., 1996; Ebbesvik, 1993).
Organic nutritional management of dairy cows may increase the risk of mastitis by causing unrecognised mineral deficiencies in the absence of routine supplementation. On the other hand, organic feeding restrictions are less likely to lead to high yields and ketosis, decreasing mastitis risk.
Age of host
Both mastitis incidence and SCC levels are higher in older cows (Emanuelson and Peersson, 1984; Bendixen and Astrabad, 1989; Harmon, 1994). Organic livestock production standards do not directly discourage culling of livestock for age or parity, but positive animal health and welfare aim at longevity, and the culling of healthy young animals would probably be considered as contradictory to positive welfare management. There is very little information on the longevity of organic dairy cows. A UK survey of 16 established organic farms found an average parity of 3.1 on these farms (Hovi and Roderick, 1999).
Stage of lactation
Most mastitis surveys show that 2/3 of all clinical mastitis cases occur in early lactation (Faye and Fayet, 1986; Jones and Ward, 1989). Mastitis research carried out on Danish organic farms did not find any increase in dry period mastitis (Vaarst, 1995), whereas a UK survey found that 50% of the surveyed farms had relatively high levels of dry period mastitis in comparison with conventional farms that had virtually no dry period mastitis (Hovi and Roderick, 1999).
Organic standards prevent the use of routine antibiotic DCT on dairy cows (UKROFS, 2000). It is possible that British dairy farmers, who have been accustomed to use antibiotic DCT on all cows at drying-off, find it more difficult to dry cows off without this prophylactic support than their Danish counterparts who have never used DCT on a routine basis.
Other diseases
It is recognised that other diseases, particularly ketosis, milk fever, lameness and post-puerperal endometritis/metritis, are closely associated with mastitis incidence (Oltenacu et al., 1988; Grohn et al., 1989; Peeler et al., 1994).
Information from surveys of disease incidence or prevalence on organic dairy farms from the UK and other European countries suggests that disease levels are either similar or, particularly in the case of ketosis, lower than on conventional dairy farms (Ebbesvik, 1993, Von Weber, 1993; Vaarst, 1995; Weller et al., 1996).
Vaccination status of host
E. coli is the only mastitis pathogen that has been commonly vaccinated against in the US. Vaccines based on R-mutant coliforms have been shown to lower the severity of clinical symptoms but appear to have no effect on the prevalence of infection (Clark and Roekel, 1994; Hogan et al., 1995). An E. coli vaccine has recently been introduced to the UK market. Routine use of such a vaccine, particularly since there is some doubt as to its efficacy in preventing infection, would not be acceptable in an organic herd. It could, however, be used as a part of a disease reduction strategy alongside other management and husbandry improvements in herds where E. coli has been identified as a major problem.
Mastitis pathogens
Whilst over 100 different micro-organisms have been identified as causative agents of mastitis, only a few species of staphylococci, streptococci and Gram-negative organisms are of economic or epidemiological significance. The importance of the various mastitis pathogens has also markedly changed throughout the past 50 years as a result of different control and husbandry methods used. Major mastitis pathogens are classified as being either environmental or contagious.
The routine use of antibiotics and improved understanding of the complex aetiology of mastitis have meant that the targeting of control, and even eradication, of some mastitis pathogens, has became more efficient. Increased emphasis on somatic cell count reduction and targeting certain contagious micro-organisms (i.e. Streptococcus agalactiae) may have changed the relative importance of the principal mastitis pathogens in the national herd. Low SCC herds may be more susceptible to environmental mastitis caused by Escherichia coli, which are becoming more important, whilst Str. agalactiae is rapidly disappearing (Jones, 1998). This phenomenon is coupled with apparent changes in the virulence of some pathogens (Str. uberis) and with the emergence of previously non-pathogenic or minor pathogens (coagulase-negative staphylococci) as mastitis causing pathogens (Myllys et al., 1994; Watt, 1997).
There is also increasing evidence that bacteria that until recently have been considered non-pathogenic or opportunistic udder pathogens are becoming more common as primary mastitis pathogens. These bacteria include Corynebacterium bovis and coagulase negative staphylococci.
Whilst mastitis levels in organic herds have been studied in various European countries, including the UK (Hovi and Roderick, 1999; Weller et al., 1996), very little information exists on the occurrence of different mastitis pathogens in organic dairy herds. Hovi and Roderick (1999) found no difference in the prevalence of different pathogens in a limited sample of organic and conventional farms. It has been suggested, however, that udder infections with contagious pathogens, particularly those that show fewer clinical signs, would become more important in organic herds where blanket antibioitc dry cow therapy is not used (Baars and Barkema, unpublished).
Contagious mastitis pathogens
The contagious pathogens usually have a mechanism to adhere to the epithelial cells of the udder or to become intracellular, in order to protect themselves from the intramammry defense mechanisms. Staphylococcus aureus, Streptococcus agalactiae and Streptococcus dysgalactiae belong to this group of pathogens. Actinomyces pyogenes is an opportunistic, contagious mastitis pathogen, usually spread by flies. Mastitis caused by these microbes is often chronic and causes elevated SCC levels. It is possible to eradicate contagious mastitis pathogens from a herd by aggressive antimicrobial therapy and/or culling and biosecurity (Pyorala, 1995). Antibiotic dry cow therapy has been seen as a major factor in diminishing the significance of these pathogens in British dairy herds (Jones, 1998).
Environmental mastitis pathogens
Environmental mastitis bacteria include a large number of both Gram-positive and Gram-negative species. Str. uberis, Str, equinus, Enterococcus faecalis and Enterococcus faecium of the Gram-positive species and Escherichia coli, Klebsiella spp., Enterobacter spp., Serratia spp. and Pseudomonas spp. of the Gram-negative are the most common environmental pathogens of the bovine udder. Str. uberis and E. coli, however represent by far the largest proportion of the identified intramammary infections caused by environmental pathogens in the UK (Jones, 1998).
The significance of Str. uberis and E. coli has grown in the past 15 years as Str. agalactiae and S. aureus have been controlled successfully in many herds. It has also been suggested by various surveys that these pathogens, particularly E. coli, have become more significant in herds with low somatic cell counts (Schukken et al. 1990; Green et al. 1996; Waage et al. 1998; Peeler et al., 1999). It is estimated that currently approximately 20% of mastitis cases in the UK are caused by Str. uberis, and a similar proportion by E. coli (Jones, 1998).
Whilst it has been recognised that teat injuries, wet bedding and contamination with faecal material are important risk factors for E.coli mastitis, it has also been suggested that improvements in hygiene are not reducing E. coli mastitis incidence, as cows are becoming more susceptible to the disease (Green and Bradley, 1998). One of the reasons for the increased susceptibility is likely to be the increased milk flow capacity in cows, leading to milk leaking (Schukken et al., 1990)
Opportunistic udder pathogens
Opinions on the significance of Corynebacterium bovis as an udder pathogen vary greatly in the literature. Some workers suggest that, whilst intramammary infections with C. bovis cause increased SCC in affected quarters, the presence of this minor pathogen provides protection against major pathogens (Lam et al., 1997; Schukken et al., 1990). Others, however, find no protective effect (Hogan et al., 1988).
In many countries with intensive dairy production, coagulase negative staphylococci (CNS) have been identified as emerging mastitis pathogens, suggesting that increasing numbers of bacteria considered non-pathogenic, until recently, are capable of causing clinical itramammary infections (Myllys, 1995). Generally, it is accepted that the mastitis caused by these organisms is mild or subclinical. It has been suggested that the susceptibility of dairy cows to mastitis caused by CNS is a reflection of lowered resistance in the cow’s udder (Myllys, 1995).
Increased opportunity for growth of pathogenic udder microbes in the cow’s environment
Organic standards recommend the use of straw or other appropriate bedding material and require that all animals have access to dry lying areas. Loose housing is also a requirement. Whilst a requirement for drying lying areas is likely to decrease the risk of mastitis, loose housing on straw yards is likely to increase the risk of environmental mastitis.
Introduction of new pathogens into the herd
The most likely risk factor for introducing new mastitis pathogens into a dairy herd is a new cow or heifer that carries an infection. A closed herd policy is the best safeguard against this risk and organic standards recommend limiting the number of animals brought in annually to 10% of the herd.
Culling policies
The presence of chronically infected cows in a dairy herd is a well-recognised risk factor for mastitis. Due to poor cure rates with antibiotic treatment and due to their contagious nature, S. aureus infections are seen as particularly dangerous.
Mastitis treatment practices
Mastitis treatment practices can affect the transmission of pathogens within the herd. If the main aim of the treatment is not to eliminate the pathogen as quickly as possible, the duration of infection increases, increasing the risk of transmission.
A UK survey of organic dairy farms revealed no difference in mastitis levels between the farms that used primarily antibiotics and those using primarily alternative therapy, mainly homeopathy (Hovi and Roderick, 1999).
The fact that antibiotic DCT is not used as a method of treatment for chronic cases of mastitis or for cows that have an udder infection at drying-off is likely to increase the duration and prevalence of udder infections in the herd. This may be reflected in higher subclinical mastitis levels and a higher dry period incidence in organic herds (Hovi and Reoderick, 1999).
Milking hygiene
The main aim of milking hygiene is to prevent the spread of contagious mastitis from one cow to another and the introduction of environmental or contagious bacteria inside the teat canal during milking. The most effective way of avoiding these risks is to milk infected cows separately or last and to keep udders, teat and the milking machine clean. None of the UK organic dairy herds surveyed by Hovi and Roderick (1999) practised separation of infected animals at milking.
Disinfecting of teat after or before milking
Teat disinfecting or teat dipping after milking has a major effect on the microbes growing on the teat. The dipping practice was first introduced to prevent the spread of contagious pathogens during milking. Disinfecting of teats before milking has also been recommended as a way of preventing the spread of contagious pathogens during milking (Bramley and Dodd, 1984). Whilst these methods have been very successful in combating contagious mastitis, it has been suggested that, as well as killing off the pathogenic microbes, the disinfectants also destroy other microbial flora that function as "healthy" competition against colonisation of the teat, particularly by environmental mastitis-causing organisms (Green et al., 1996).
The organic standards discourage the use of chemicals and encourage natural resistance to disease. However, most organic dairy farmers surveyed by Hovi and Roderick (1999) used post-milking teat dipping, at least during the housing period, and some even used pre-milking teat disinfection.
Teat injuries
Teat injuries are also likely to lead to improved survival of pathogens on the teat. The main causes of teat injuries tend to be lameness and inappropriate housing systems (Oltenacu et al., 1988). Existing information from organic dairy herds in the UK and elsewhere suggests that lameness is not a specific problem for organic dairy herds (Weller and Cooper, 1996).
Udder cleanliness
Udder cleanliness is an important factor in the general resistance to mastitis. Dirt on udders and teats increases infection pressure, damages skin and prevents beneficial, commensal flora from establishing. As organic standards require provision of dry bedding areas, udder cleanliness is enhanced. In a UK survey of organic dairy farms, housing conditions were scored above average in most herds (Hovi and Roderick, 1999).
Milking machine and milking technique
Milking machine faults and poor milking techniques are probably among the main environmental risk factors for mastitis, alongside housing hygiene. Unstable or excessive vacuum, faulty pulsation, liner slippage for various reasons and teat cup hygiene contribute to mastitis risk by either damaging the patency of the teat canal or by causing pathogenic organisms to enter the teat canal during milking (Manninen, 1995). The organic production standards do not have any special implications for milking techniques or milking machine maintenance. In a UK survey of organic dairy farms, all farms had their milking machines tested twice per year or more often (Hovi and Roderick, 1999)
Husbandry practices
The teat canal remains open for up to 45 minutes after milking. A recommended husbandry practice is to prevent the cows from lying down until the teat canal has closed, to prevent bacterial penetration. In a UK survey of organic dairy herds, only 6 out of 16 organic farms prevented cows from lying down directly after milking (Hovi and Roderick, 1999).
Stockmanship
Stockmanship and other characteristics of the herdsman have been considered important enough by some workers to warrant a separate category among disease determinants (Schwabe, 1984). There is, however, very little published information on mastitis and stockmanship. Organic standards offer little guidance on stockmanship. The human-animal relationship is mentioned in the old UKROFS standards, specifying that animals be housed in conditions allowing them regular sight, smell and sound of human activity, but this has been removed from the current standard (UKROFS, 2000).
The conventional approach to mastitis control and prevention
For the past 25 years, the theory and practice of conventional mastitis control in the UK have been based on the Five Point Plan, developed at the National Institute for Research in Dairying in conjunction with the Central Veterinary Laboratory, in the 1960s.
The Five Point Plan For Control of Mastitis in Dairy Herds
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The Five Point Plan is unsuitable for organic dairy farms, due to its emphasis on antibiotic DCT for every cow at drying-off as well as on the elimination of existing infections by antimicrobial therapy and culling. Strict control of somatic cell counts by antimicrobial prophylaxis and aggressive culling does not fit with the organic principles of positive health care, reduction of chemical inputs and animal welfare. Furthermore, the Five Point Plan offers very little guidance on disease prevention strategies recommended by organic standards or on mastitis treatment without antimicrobials.
The organic approach to mastitis control and prevention
The organic livestock production standards emphasise positive health care and prevention of diseases, particularly by improving or supporting an animal’s own defence mechanisms (EU Regulation 1804/1999, UKROFS, 2000). Good husbandry, breeding of resistant animals and optimisation of production levels are seen as the cornerstones of mastitis prevention. The main issues related to mastitis and mastitis control covered by the EU and UKROFS standards are listed below:
| Aspect of mastitis control | Organic standards |
| Patency of teat duct | +Breeding for resistance and improved housing encouraged |
| Intramammary resistance | +Breeding for resistance encouraged +Production stress controlled by restricted feeding practices -Routine vaccination not encouraged -Routine selenium supplementation not encouraged |
| Elimination of existing infections | -Use of dry cow therapy restricted -Number of antibiotic treatments in any year restricted |
| Prevention of new infections | +Closed herd policy encouraged +Improved housing and low stocking density encouraged -Use of dry cow therapy restricted |
A survey of 16 established and 7 conventional farms and their approaches to mastitis control was carried out in 1997-1998 in the south of England and Wales (Hovi and Roderick, 1999). The producers surveyed considered that the mastitis situation on their farms was generally good and in most cases better than the recorded mastitis incidence suggested. It was evident that mastitis treatment records kept on all farms were under-utilised, and that bacteriological identification of mastitis pathogens was not commonly used as a monitoring mechanism.
Most surveyed farmers considered low herd SCCs as a risk factor for mastitis and felt that it was impossible to maintain low SCCs without using antibiotic DCT and culling healthy, young cows.
On organic survey farms, mastitis control measures related to the milking machine, milking hygiene, housing type or hygiene, husbandry and detection of mastitis did not differ from those observed on conventional farms.
Animal health and conversion plans required by organic standards were not evident on any of the survey farms. The majority of the organic farms (14/16) used homeopathic nosodes as a part of their mastitis control strategy. Whilst the use of fly control in the absence of acceptable lactating cow fly repellents was perceived as being difficult to apply on some farms, others had developed successful strategies that were acceptable under organic standards. None of the survey farms supplemented dry cows routinely with minerals or trace elements.