Mastitis continues to be the most costly disease facing dairy producers today (10, 12). The major components of this economic loss include reduced production, milk withhold, premature culling, increased labour, costs of therapy, and reduced genetic improvement (3). Even though great technological advances have been made in dairy production systems, it remains that intramammary infection with bacterial organisms leads to substantial losses from both clinical and subclinical mastitis (1, 4, 10, 14).

A large number of organisms have been identified as causing bovine mastitis. These organisms can be classified as either major or minor pathogens. The major pathogens can be further subdivided into organisms causing contagious intramammary infection and agents from environmental sources (13). The most common contagious organisms are Streptococcus agalactia and Staphylococcus aureus. These organisms are spread from the infected quarters to susceptible quarters of other cows in the herd. Milkers’ hands can also act as a source of Staphylococcus aureus. Over the past 15 to 20 years, significant progress has been made in the control of contagious pathogens. Udder health programs, such as post-milking teat dipping and total dry cow therapy, have had a great impact in reducing bulk milk tank somatic cell counts (BTSCC) (7), while improving milk quality and milk production (15, 16). Segregation and culling of chronically infected cows have also aided in reducing herd prevalence of contagious infections.

On the other hand, many dairy herds have a high incidence of intramammary infection with environmental pathogens. The predominant organisms involved are E. coli, Klebsiella sp., Streptococcus uberis and Streptococcus dysgalactia (13). Other coliform agents and environmental Streptococci are also involved. Intramammary infection with these pathogens are frequent, but often of short duration. These infections usually result in clinical mastitis of widely varying severity. Udder health management programs to control environmental mastitis problems include cleanliness and housing issues, management around calving and pre-milking cow preparation (17).

Over the years, many studies have reported the predominant pathogens involved in subclinical intramammary infection, as well as in cases of clinical mastitis. There is a great deal of diversity in these reports. In addition, many herds have seen changes in pathogen type over time. For example, a 24 year observational study on a California dairy demonstrated a gradual switch from clinical mastitis caused by contagious organisms to cases caused by environmental organisms. Although there was a significant decrease in contagious pathogens, the rate of clinical mastitis during the time period changed only slightly (2). In a 1995 study, 65 Ontario dairy herds were used to collect data on clinical mastitis rates and the distribution of the pathogens present. In this study, 2840 complete cow-lactations were recorded. The overall lactational incidence of clinical mastitis, not associated with teat injury, was 19.8%. In other words, one cow out of every five had at least one case of mastitis during her lactation (14). These cases were caused by a wide variety of bacterial pathogens. A 1988 study found that herds with low BTSCCs (< 150,000) had a higher level of clinical mastitis than did herds with contagious mastitis problems (BTSCC > 700,000) (6).

The implementation of milk quality penalty and incentive programs throughout the dairy industry has increased the need for programs to monitor udder health status in dairy herds. Indirect measures of mammary gland inflammation have gained widespread use for this purpose. Dairy Herd Improvement Somatic Cell Counts (DHI SCC), as well as on-farm use of the California Mastitis Test (CMT), are extremely useful tools for monitoring udder health status. In addition, automated programs for the collection and analysis of SCC data can be extremely useful for problem-solving (13). However, SCC and CMT have inadequacies when used to definitively determine the presence of intramammary infection, and are not at all useful for identification of the organism involved (13, 19).

In summary, there is a real need for information about the bacteria causing mastitis. Different mastitis organisms require different treatment regiments and control strategies. The distribution of pathogens varies widely between herds, as well as within a herd, as a result of lactational and seasonal factors. Simply stated, bacteriological culture of milk is essential to determine the presence and type of pathogen involved in cases of intramammary infection. Issues associated with cost, shipping and handling samples, lag-time in reporting results, as well as difficulties in storing and using this data, have resulted in under-utilization of milk culture through diagnostic laboratories. In order to implement appropriate therapy decision-making protocols and to fine-tune overall herd udder health management programs, timely and accurate milk bacteriological test results would benefit producers and veterinarians. The HyMast Milk Bacteriological Test System represents an example of new technology, that can increase the availability of this information.

The HyMast test (HyMast®, Pharmacia & UpJohn Animal Health, Kalamazoo, Michigan) is a rapid bacteriological test system which has been promoted as a tool to assist veterinarians and producers in decision-making for clinical mastitis therapy. The test consists of a plastic vial with a screw-top cap, that has a paddle attached. The paddle is embedded with a different type of selective growth media on each side. Bacterial growth on one side of the paddle indicates the presence of gram-positive (i.e. Staphylococci, Streptococci) organisms, whereas growth on the other side indicates the presence of gram-negative (i.e. coliforms) organisms.

The test is easy to use. Milk from an aseptically collected sample (quarter or composite) is placed in the vial, and the paddle/cap is replaced. The vial is inverted and rotated two to three times so that the milk completely covers the culture media on both sides of the paddle. The milk is discarded (or saved in a vial for further diagnostic bacteriology, if appropriate), and the cap loosely replaced to allow air to enter the vial. The entire test vial is incubated up to 36 hours. The operator checks the paddles for bacterial growth at 8 to 12 hour intervals.

To date, the HyMast test has been evaluated for its ability to identify Staphylococci, Streptococci and coliforms. The sensitivity (ability to correctly identify bacteriologically positive samples) and specificity (ability to correctly identify bacteriologically negative samples) for three selective media has been determined (11). The HyMast was compared to the ‘gold standard’ of milk culture in a milk quality laboratory, using the guidelines set in the National Mastitis Council’s Laboratory and Field Handbook on Bovine Mastitis. The sensitivities and specificities of each selective medium and each mastitis causing organism, are shown in Table 1. The sensitivity of each medium for determining negative samples was high. The sensitivity of the HyMast test for identifying its target organisms was moderate at best (11).

The specificity for classifying organisms in the broad categories of gram-positive and gram-negative organisms was also examined. The sensitivity and specificity of the test for identifying gram-positive organisms was 0.80 and 0.76, respectively. The sensitivity and specificity for identifying gram-negative organisms was 0.60 and 0.98, respectively (11). These results suggested that further evaluation of the HyMast test in the field was indicated.

There are at least four potential applications for the HyMast system in mastitis control programs. These are:

• Clinical mastitis decision-making: The HyMast test can be used in a decision-making protocol to selectively target therapy towards gram-positive organisms.
• Udder health biosecurity: The HyMast test may be used as a preliminary bacteriological screening test prior to introducing purchased animals into the herd.
• Evaluating the success of dry cow management and therapy programs: In herds with concerns about the rate of new intramammary infections during the dry period, the HyMast teat may provide a useful system to evaluate the effectiveness of the dry cow management program. Similarly, concerns about the success of therapy during lactation can be evaluated using the HyMast test to culture samples.
• Assessment of cows with newly elevated SCC: The HyMast test may be used to determine the infection status of cows with newly elevated SCC on DHI records.

The approach to therapy of clinical mastitis cases on a particular farm is greatly influenced by a combination of several factors. Characteristics of the cow in question, such as stage of lactation, genetic merit, current reproductive status and presence of other problems, will come into consideration. Other points to consider include the costs and benefits of treatment, dollars lost due to milk withhold, the probability that the treatment will succeed, the chance of a relapse, the present cull value of the cow, the availability of replacement animals, and the risk of antibiotic residue violations (9). The specific pathogen present can greatly influence some of these factors and, in turn, the appropriate approach to treatment.

Staphylococcus aureus infections tend to be chronic in nature and are often subclinical. Studies have shown that treatment during lactation is questionable and cure rates are quite variable and often poor (9). Recent information suggests that the success of therapy may be improved by selection of candidates for therapy based on lactation number, days in milk, and somatic cell counts. On the other hand, Streptococcus agalactia has a very good cure rate during lactation and the dry period. For the environmental organisms, specifically the coliforms (gram-negative bacteria), there are no approved products for the treatment of pathogens. Most of these infections are clinical and of short duration. Studies have shown that antibiotics for mild or moderate cases are not indicated and do not impact on the course of infection or the outcome (9). Cases can be managed with supportive treatment. The environmental Streptococci (gram-positive bacteria) are known to be responsive to treatment and treatment is indicated, However, there is a variable cure rate. This group of organisms has the greatest range in antibiotic susceptibility.

Most dairy herds use one of two general approaches to mastitis therapy:

• the use of intramammary antibiotics, and
• frequent stripping of infected quarters (with the aid of oxytocin) without application of antimicrobials.

The decision as to which of these therapeutic regimens is most appropriate, for a given cow or herd, is complex. Although many factors are involved in determining the outcome of this choice of therapy, the bacterial cause of the infection is of paramount importance. Recently, studies have found no difference in clinical or bacteriological cure rates between environmental mastitis cases treated with antibiotics versus those treated with oxytocin and stripping (8). However, these studies also found a much higher relapse rate in the oxytocin groups, thereby reducing the long-term benefit of this protocol. Many of the relapses occurred in cases caused by environmental Streptococci (8). The use of antibiotic therapy in this group of bacteria may, therefore, be advantageous. Alternatively, in herds which have controlled contagious mastitis, one-third of the clinical cases yield no growth on bacteriological culture. A further one-third of cases are caused by coliforms, for which there is no approved intramammary treatment (2). In these situations, the conservative approach of oxytocin and stripping would appear to be indicated. The HyMast bacteriological test system, which can be used on-farm, will allow producers and their veterinarians to determine the bacterial agent associated with a case of clinical mastitis in a timely fashion (usually 24 hours). A decision-making protocol for clinical mastitis therapy using the HyMast test system is shown in Figure 1.

The HyMast test is convenient, easy to use and to read. The test results provide the opportunity for meaningful education and discussion of management changes. Other benefits include improved mastitis records and herd level incidence data. This information on the herd milk bacteriological profile can also be used to support the implementation of other mastitis control strategies for a given herd. Over the past five years, there has been an increase in the number of published studies pertaining to bacterial resistance to antibiotics in human medicine. Although attempts to link these changes in resistance to antimicrobial use in food-animal agriculture are not proven, the allegations should not be ignored. In today’s society, the public is demanding a more targeted use of medicines in agriculture. In view of these issues, the HyMast bacteriological test system seems attractive.

Rapid expansion of herd size is occurring throughout the dairy industry: with the addition of cows, herds are at a significant risk of introducing contagious bovine pathogens, such as Streptococcus agalactia and Staphylococcus aureus. Prior to purchase and entry into the herd, these cows should be evaluated for the presence of existing intramammary infections caused by contagious pathogens.

Milk cultures of individual cows prior to, or at the time of, purchase is the most effective method for preventing the introduction and spread of contagious pathogens in the herd (19). The HyMast test may have considerable potential as a screening method. However, this use of the HyMast test has not been formally evaluated.

Dry cow management programs represent a cornerstone of the mastitis control efforts on most dairy farms (18). However, with the many environmental and nutritional factors affecting dry cows, it is important to continually evaluate the effectiveness of dry cow management. DHI SCC information for fresh cows is not reported for cows less than 15 days in milk. Therefore, it is possible that some cows may not receive a SCC test until 45 days in milk, because of the test intervals. Use of CMT on fresh cows may be a cost-effective screening tool for intramammary infection. In combination with CMT, the HyMast test system applied to fresh cows may provide useful information on the infection status of individuals coming out of the dry period. This application of the HyMast test has not been formally evaluated.

Individual cow DHI SCC is widely used. Elevation of SCC above a threshold is commonly used as an indicator of a new infection. However, for this purpose, SCC is not particularly sensitive. Treatment of cows based on elevated SCC alone can be very costly (5). Bacteriological milk culture of cows with elevated SCC may provide valuable information. This information can then be used to make management decisions.

Our current research involves an in-depth evaluation of the HyMast culture system as an aid in decision-making for clinical mastitis therapy. This project is a collaborative effort between members of the Ontario Veterinary College, the Atlantic Veterinary College, Cornell University, Kemptville College, the Ontario Ministry of Agriculture Food and Rural Affairs, and Pharmacia & Upjohn.

The study is being conducted on 26 commercial diary farms in Ontario and New York State. Clinical cases are randomly assigned to one of three treatment groups (assigned by farm and by time):

• a no antibiotic treatment group which receives only oxytocin and stripping as therapy.
• a total treatment group in which all cases are treated with intramammary antibiotic. This group acts as the positive control.
• a HyMast decision group which bases therapy on culture results from the test according to the flow chart shown in Figure 1.

Cases of clinical mastitis with systemic signs, or resulting from teat injury, are not enrolled in the study. All infected quarters are re-sampled between days 7-14 and days 21-28 after finding a case, to establish bacteriological cure.

The field trial will evaluated the usefulness of the HyMast test, both bacteriologically and economically, in making therapeutic decisions in dairy cows with clinical mastitis. Further information will be obtained with regards to the diagnostic characteristics of the test. The usefulness of the HyMast test in cows with newly elevated cell counts on DHI records, will also be evaluated.

This paper has emphasized the importance of bacteriological information, not only in the treatment of clinical mastitis but also in control strategies. Selectively targeting antibiotic therapy towards gram-positive organisms, versus cases caused by gram-negative organisms or having no bacterial growth, will allow us to use antimicrobial therapy when appropriate. The HyMast bacteriological test system is a tool which will allow for making timely and informed decisions. It may fill an important niche in the diagnosis and management of clinical mastitis, as well as in other aspects of udder health management programs. Further research is needed to determine the appropriate methods of implementation, as well as the economic impact of this new technology.

1. Bartlett, P.C., J. Van Wilk, D.J. Wilson, C.D. Green, G.Y. Miller, G.A. Majewski, and L.E. Heider. 1991. Temporal patterns of lost milk production following clinical mastitis in a large Michigan Holstein herd. J. Dairy Sci. 74:1561.

2. Bennett, R.H. 1990. Clinical mastitis from environmental pathogens: Analysis of a large commercial dairy. Int. Sym. Bov. Mas., Indianapolis, Indiana, pp 181-185.

3. Craven, N. 1991. Antibiotic therapy and mastitis control, economics and future prospects. Mammites des Vaches Laitieres, Soc. Francaise de Boviatrie, Paris, pp 113-118.

4. DeGraves, F.J., and J. Fetrow. 1993. Economics of mastitis and mastitis control. Vet. Clinics of N. Amer.: Food Anim. Pract. 9(3):421.

5. Dohoo, I.R., and K.E. Leslie. 1991. Evaluation of changes in somatic cell counts as indicators of new intramammary infections. J. Prev. Vet. Med., 10:225.

6. Erskine, R.J., R.J. Eberhart, L.J. Hutchinson, S.B. Spencer, and M.A. Campbell. 1988. Incidence and types of clinical mastitis in dairy herds with high and low somatic cell counts. J. Am. Vet. Med. Assoc. 192:761.

7. Godkin, M.A. 1997. Milk quality update. Animal Health News Ceptor , Vol 5, No 2, pp4-5.

8. Guterbock, W.M., A.L. Van Eenennaam, F.J. Anderson, I.A. Gardner, J.S. Cullor, and C.A. Holmberg. 1993. Efficacy of intramammary antibiotic therapy for treatment of clinical mastitis caused by environmental pathogens. J. Dairy Sci. 76:3437.

9. Guterbock, W.M. 1995. Rational treatment of clinical mastitis. 2nd Western Large Herd Dairy Management Conference, Las Vegas, NV, pp50-59.

10. Hoblet, K.H., G.D. Schnitkey, D. Arbaugh, J.S. Hogan, and K.L. Smith. 1991. Economics of clinical mastitis. Proc. 30th Annual Meetings of the National Mastitis Council, pp24-29.

11. Leslie, K.E., D.A. Barnum, J.C. McTaggart, A. Bishiri, and R. Swackhammer. 1995. Evaluation of a field-based, selective-media bacteriological test system for diagnosis of intramammary infection. Proc. 3rd Int. Mast. Sem., Tel Aviv, Israel, (S2)66-(S2)67.

12. Miller, G.Y., P.C. Bartlett, S.E. Lance, J. Anderson, and L.E. Heider. 1993. Costs of clinical mastitis and mastitis prevention in dairy herds. J. Am. Vet. Med. Assoc. 202:1230.

13. Radostits, O.M., K.E. Leslie, and J. Fetrow. 1994. Herd Health: Food Animal Production Medicine. 2nd ed. Toronto: W.B. Saunders Company. pp.229-273.

14. Sargeant, J., H.M. Scott, K.E. Leslie, K.D. Lissemore, D.F. Kelton, and S.W. Martin. 1995. Clinical mastitis in Ontario dairy herds. Proc. Ontario Assoc. Bovine Pract., Guelph, Ontario.

15. Schukken, Y.H., K.E. Leslie, A.J. Weersink, and S.W. Martin. 1992. Ontario bulk milk somatic cell count reduction program. 1. Impact on somatic cell counts and milk quality. J. Dairy Sci. 75:3352.

16. Schukken, Y.H., K.E. Leslie, A.J. Weersink, and S.W. Martin. 1992. Ontario bulk milk somatic cell count reduction program. 2. Dynamics of bulk milk somatic cell counts. J. Dairy Sci. 75:3359.

17. Smith, K.L., and J.S. Hogan. 1997. Risk factors for environmental Streptococcal intramammary infections. Proc. Udder Health Management for Environmental Streptococci, Guelph, Ont., pp.42059.

18. Smith, K.L., D.A. Todhunter, and P.S. Schoenberger. 1985. Environmental pathogens and intramammary infection during the dry period. J. Dairy Sci. 68:402.

19. Wilson, D.J., and R.N. Gonzalez. 1997. Evaluation of milk culture, SCC and CMT for screening herd additions. Proc. 36th Annual Meeting of the National Mastitis Council, pp.127-131.