Extension Veterinarian
School of Veterinary Medicine
University of California, Davis
Veterinary Medical Teaching and Research Center
Tulare, CA 93274

Mastitis in dairy cows is classified in several ways. Sometimes it is classified as contagious and environmental depending on the suspected site of origin and method of transmission. Other times it is called acute or chronic depending on the duration length of infections and perhaps the appearance onset of clinical signs. Another common method is to refer to clinical or subclinical mastitis. In this scheme, clinical mastitis is obvious to the dairyman as the milk or udder is abnormal and perhaps systemic signs of disease are present. In contrast, subclinical mastitis is infections, which occur without obvious clinical signs such as abnormal milk, udder swelling or tenderness, or systemic signs such as fever, depression and drop in milk production. In both cases, milk production is lost and the dairymen probably should take some action.

The diagnosis of clinical mastitis is straightforward. Something is obviously wrong and it is easy to see. With subclinical mastitis, the problem is detection. Once detected, it is useful to estimate the production loss associated with this invisible form of mastitis. The solution is some type of somatic cell counting. When the somatic cell are counted, subclinical mastitis is visualized and the milk production loss can be estimated. Thus somatic cell counting (SCC) is a very useful measure to increase the awareness of subclinical mastitis and its effect on production and milk quality.

The types of infection that can be routinely monitored with SCC are most often the contagious types such as Staphylococcus aureus, Streptococcus agalactiae and the environmental mastitis pathogens, the environmental Streptococcus spp. A large percentage of the time, these infections occur as subclinical infections. Where Mycoplasma spp. are present, they can also be monitored with SCC. Indeed, some of the more infrequent mastitis pathogens such as Pseudomonas, Nocardia, and Prototheca can also cause increased SCC. Other environmentals (E, coli, Klebsiella) are difficult to monitor as the duration of elevated SCC is relatively short and hard to detect under most SCC monitoring systems. No problem, of course, as these most often cause clinical mastitis cases.

Somatic cell counts are useful both in individual cows and on a herd basis. On a herd basis, bulk tank somatic cell counts (BTSCC) from the entire milking herd can be used to monitor for the mastitis status of the herd. Often BTSCC are determined several times a month by the milk processor that purchases the milk. By observing the trends in the BTSCC, an early detection of increased BTSCC as an indicator of mastitis within the herd can easily be made. While there is still discussion regarding the satisfactory level of BTSCC to routinely accept, BTSCC below 200,000 to 250,000 cells/ml are commonly regarded as acceptable. On the other hand, many herd consistently keep their BTSCC below 200,000 cells/ml. At levels below 150,000 cells/ml., a herd is at or near the level of infection where perhaps it is no longer economically justifiable to attempt further decreases in BTSCC. However, some herds take great pride in the quality of their milk and have yearly BTSCC average below 100,000 cells/ml. One of our teaching herds with about 200 cows has maintained an average of about 90,000 cells/ml for several years.

It should be kept in mind that the BTSCC may be purposely manipulated to some extent to give the appearance of low mastitis prevalence and high quality milk. This manipulation becomes apparent where both regulatory (State of California) and management (Dairy Herd Improvement Association) BTSCC are available. The regulatory counts are often higher than the management. This occurs as the producers can select cows not to test for management purposes (generally known high SCC cows), however, all cows regardless of their SCC status that are in the milking herd contribute in proportion to their SCC and daily milk production to the regulatory BTSCC. While surely not intended, the BTSCC can on occasions be elevated when poor milking practices occur. When quarters are not forestripped, mild cases of mastitis can be overlooked. These mild cases can contribute lots of SCC’s to the BTSCC causing it to be elevated.

In herds with mastitis problems, the dairymen and the veterinarian often want to culture some milk from cows to determine the cause of the infections. SCC’s can be used to select the cows for culture. To gain confidence that the cows are truly infected, selection can be made on the basis of several consecutive SCC rather than a single count. The threshhold SCC can be set liberally high to insure that all selected cows have a high probability of being infected. Perhaps, 400,000 to 500,000 cells/ml could be selected as the cut-point. Depending on the desires of the dairymen, chronic or newly infected cows can be selected. Usually composite, 4-quarter sample results are available under most testing schemes. To be more precise, a CMT can be used to select individual quarters that are infected. Recently treated cows should be avoided unless treatment refractory cows are part of the mastitis history.

In some cases, it might be the desire of the dairymen to find all the infected cows for culture. This is a much more difficult situation and under field conditions is probably not possible. Under these conditions, a relatively low threshhold for selection should be used to increase the chances of selecting all the infected cows. Maybe 200,000 cells/ml should be used. However, even at this low threshhold, some very recently infected cows may be missed. Repeated selection and culture may improve this selection process.

Removing cows with chronic or repeated infections will reduced the BTSCC and decrease the chances of spread of contagious mastitis from infected cows to susceptible, non-infected cows. There is predictably a much smaller effect on environmental pathogens. Selection can be made in a similar manner as for milk culture. Use a series of SCC’s to select the chronic infections and use a high threshhold. Often, removal of these infected cows will have little effect on herd production as they are usually low producers. Other factors such as stage of lactation, reproductive status, milk culture results and response to previous therapy should also be considered before making the final culling decision.

Likewise, cows can be selected based on SCC to be milked last (smaller herds) or to be placed in a segregation pen (larger herds). The idea is to control spread of infection from infected cows to other cows during milking. A low threshhold should be used in this case to insure maximum selection pressure. Herd size and the infectious agent may be important factors when considering this action.

Consideration should be given to early dryoff of chronically infected cows (many elevated SCC’s). These cows can be removed from the herd and dryoff as early as 100 days before their anticipated calving date. Drycow antibiotic treatment gives the dairyman the best opportunity to treat resistant infections. Care must be taken to prevent over-conditioning with early dry as this may lead to other difficulties at calving time (fatty livers, DA’s, RP’s, metritis, milk fever).

Our research has shown that total herd drycow treatment is the best economical approach even in low BTSCC herds. In our study, treatment of all cows in a herd with BTSCC consistently below 250,000 cells/ml resulted in no change in the in the BTSCC, however, there was an average increased of over 400 pounds of milk during the first 120 days of the next lactation. This herd has not drycow treated for several years prior to our study.

As a general rule, lactating cows should not be selected for treatment based solely on SCC. The reason is basically economic. Most bacteria that cause elevated SCC are resistant to antibiotic therapy, particularly Staphylococcus aureus. Therefore, cost of treatment, discard milk and labor are not recovered as many of the treated cows do not recover. The sole exception would be in herds highly infected with Streptococcus agalactiae. With this bacteria, the cure is always high so that the cost of treatment can be recovered by increased milk production following treatment.

The dairyman always wants to know if the treatment for mastitis was successful. After treatment, the SCC will remain elevated for a period of time. If the infection is not resolved, the SCC will remain elevated. If a clinical cure only results, the SCC may decline but will at some point in the future increase again. When a bacteriological cure occurs and the infectious agent is eliminated, the SCC will slowly decrease. This may take several weeks or a month. Checking treated cows with a CMT with a few days after treatment will produce a very depressed dairyman! Dairymen should be advised of the anticipated results prior to treatment so that no false expectation will be developed.

The level of infection within a herd is based on the number of infections (prevalence) and the duration of these infections AND on the number of new infections entering the infected group (the new infection rate). By preventing new infections, the pool of infected cows will slowly decreased by spontaneous cure, treatment and culling.

To indirectly monitor the new infection rate, a breakpoint between infected and non-infected status must be established. Cows below the breakpoint are considered non-infected; cows above the breakpoint are infected. The new infection rate is the number of cows moving during a given time period from below the breakpoint to above the breakpoint divided by the number below the breakpoint at the beginning of the period (cows at risk).

The new infection rate is very useful in monitoring the effect of any changes on the BTSCC. Successful changes will tend to lower the NIR and in time lower the BTSCC.

When used based on trends or a series of SCC’s, SCC can be very useful to the dairy producer and their veterinarians in decision making regarding subclinical mastitis. However, SCC’s are not absolute and must be used with caution and good judgement. They are only part of a total mastitis control program. In my opinion they are of most value as they allow the dairymen to be able to see the invisible, subclinical infections.