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Nicomedes Aparicio, Jesús Pérez Paniagua,Rafael Baselga, Iñaki Albizu.

The present paper comments the initial aspects of a program for the control of mastitis in small ruminants, though without addressing the specific actions of each pathogen. An explanation is provided of somatic cells counts (SCC), how they are evaluated, how and when samples should be collected, and how the animals should be followed-up on.

THE SOMATIC CELL COUNT (SCC) OF AN FARM

In situations of mammary gland infection, neutrophils migrate to the site of infection and thus increase the somatic cell count (SCC). However, the SCC is not composed only of neutrophils appearing in milk as a consequence of infection but also macrophages and desquamated epithelial cells — though this does not imply that the presence of mastitis is increasing on the farm.

The SCC can be determined individually in the animals included in a production control plan; in this case, we obtain an individual estimation that may substitute the California test. However, mastitis is a group problem, where for effective control it is absolutely necessary to know the mean SCC of the farm, recorded from a tank sample.

INTERPRETATION

An isolated SCC is not a very important piece of information, though it does give an idea. As an example, an SCC of 300,000 is suggestive of an excellent farm (from the mastitis perspective), while a figure of 1,500,000 is initially indicative of problems. In our opinion, a SCC of 750,000 indicates a farm with an acceptable level of subclinical mastitis.

The SCC is a dynamic parameter than can show important natural variations, for example due to incorporation of colostric milk, or milk of animals very close to drying and diminished milk production. These variations are in turn much greater in small herds, where the influence of a single animal upon the average value is important. Fluctuating values can also be indicative of minor accidents, e.g., stress can elevate the SCC. These minor variations are of little importance, however, provided we have identified their underlying cause and the variations do not constitute a change in tendency. The true value of the SCC is found in the fact that the following of its tendency provides an idea of what is happening on the farm, and of the efficacy of our actions.

A farm with a low SCC is not necessarily problem-free, for in many cases the milk from clinical mastitis is not destined to the tank. For this reason, in addition to using SCC as an indicator of subclinical processes, a follow-up of the clinical processes is required, recording them and establishing a diagnosis. For example, an ovine farm with an average SCC of 300,000 may have a serious problem of gangrenous mastitis due to Staphylococcus aureus.

Fig. 1. The moving mean SCC is a perfect indicator of the course of subclinical mastitis on a given farm.

FOLLOW-UP

Figure 1 provides a practical example of the course of SCC in a herd that may drop to under 300,000 (for example, as a result of withdrawal of animals of the lactation batch), only to increase again at the next control (for example, due to the introduction of new animals, or because of point-problems in some animals). Due to the variability of the count obtained every two weeks, the use of a moving geometric mean provides a closer to true value, and its graphic interpretation reflects the tendency of mastitis in the herd as a whole. The moving geometric mean constitutes a measure of the previous 6-12 months to which the reading of the last month is added while the oldest reading is removed.

A good control program should ensure that the SCC exhibits a decreasing tendency. This is of course much easier at the start, when we begin with very high values, and the simple elimination of some problem animals suffices to improve the count; however, a decreasing tendency proves very difficult when moderate values (for example 500,000) have been reached.

RELATION OF SCC TO PRODUCTION AND QUALITY

Table 1 shows the approximate relation between SCC and production and the California test. Clearly, mammary gland inflammation, no matter how mild, will have negative repercussions upon production. In addition, the presence of antibiotics, bacterial contamination, or the existence of a very high SCC negatively affects the quality of the milk, its stability when subjected to thermal sterilization procedures, cheese yield and the maturation and storage of cheese.

Table 1. Relation between cell count and bovine production (Bovis, 1992).

PHYSIOLOGICAL VARIATION

Many causes of SCC variation have been discussed in individual animals without infection (age, lactation month, etc.). However, these variations are only truly important in colostric milk or close to drying, when milk production is very low. The figure below reflects the variation observed in the course of ovine lactation (Fig. 2).

Fig. 2. The milk of an animal without mastitis shows an increased SCC both at the start and the end of lactation.

INTEREST OF A PROGRAM FOR THE CONTROL OF MASTITIS

The interest of establishing a control program is reflected by the following considerations. In goats and sheep, a simple control program including revision of the milking machinery, improved milking routines, teat cleansing, the elimination of chronically sick animals, modification of the milking order, treatment of drying and the use of autovaccines against staphylococcal mastitis and/or Mycoplasma-induced mastitis, can afford a production increment of 10-25%. Obviously, this production increase fully justifies the economic cost of the program. Moreover, the removal of animals is reduced, with a lesser expenditure attributable to antibiotics and veterinary services, increased production of milk without inhibitors, and a lesser risk of violent mastitis due to Streptococcus agalactiae or Mycoplasma spp.

HOW TO PRESENT THIS SERVICE TO THE FARMER

A program for the control of mastitis must be economically profitable for both the farmer and veterinarian. In our opinion, this service should entail 2-3 visits to the farm a year, where the veterinarian should correct the observed deficiencies and perform sampling for microbiological study. Of course, some farms will suffer point problems requiring a larger number of visits. The fact of not visiting a given farm does not imply neglect; the veterinarian should receive a copy of the SCC data to thus ensure that the course of the herd is favourable, or at least stable. In this context the farmer must play an active role, performing the California test, examining udders and recording any problems.

HOW TO START A PROGRAM FOR THE CONTROL OF MASTITIS

The way to use SCC as an indicator of mastitis has already been explained. The usual situation at the start of such a program is to find SCC of over 2,000,000 per ml. Our next step should be to establish the cause of the problem. In goats and sheep, our experience suggests that the milking machines very often present deficiencies (frequently as a result of the purchase of "economical" systems). Our recommendation for all farms starting the control program is to begin by revising the milking machinery (if any). The steps could be as follows:

• Study the available data: SCC, bacteriology, latest milking machine revisions.
• Visual inspection of the animal stabling area (available space, bedding cleanliness, etc.)
• Visual inspection of the milking machines (cleanliness, teat cup and rubber conditions, oil stains in the vacuum pump, absence of collectors, etc.)
• Inspection of the milking process (routine, animal behavior, over-milking, teat condition at withdrawal, etc.)
• Palpation of the udders (nodules, differences between udders, indurations, ecthyma, impetigo, etc.) and of the retromammary lymph nodes (unilateral or bilateral infarction).
• Collection of samples from the animals yielding positive California tests, and from those with infarction nodes. It is advisable to obtain samples from first-birthing animals and in the first stages of lactation, since these were presumably previously healthy — thereby contributing to identify the etiologic agent responsible for the new infections.

HOW TO PERFORM THE CALIFORNIA TEST

The technique is simple: Mix 2 ml of California reagent with 2 ml of milk. In animals with mastitis, the milk develops clots — unlike in healthy animals. The test is scored from - to ++++, according to the degree of milk clotting. The California test should be performed independently for each udder.

The California test is sometimes difficult to interpret. What is more important: the shift to purple or the change in viscosity? How is the reading related to clinical, subclinical and chronic mastitis?

HOW TO COLLECT A SAMPLE FOR MICROBIOLOGICAL DIAGNOSIS

Sampling first requires cleaning of the teat with cotton soaked in 70% alcohol. After discarding the first two streams of milk, 2-3 ml (more is not needed) should be collected in a sterile bottle. Blood extraction tubes are ideal for this purpose, since their narrow opening is less prone to contamination than a wide-lipped tube; moreover, such tubes are sterile, and all veterinarians have access to them. In no case should preservatives be added to the milk sample; moreover, samples from animals treated with antibiotics, or mixed milk from more than one udder or animal should not be submitted for analysis.

HOW TO ASSESS THE SEVERITY OF THE PROBLEM

On starting a mastitis control program, it is common for a large number of animals on the farm to yield positive results with the California test or present lesions. It is important to know this situation, using the mastitis control sheet (Fig. 3) to the effect, where in addition to the date and number of the animal the presence of lesions and/or California test positivity is to be recorded, along with any observed clinical processes. The farmer should record these data, which are essential in order to identify animals that repeatedly yield positive test results. These animals are chronic and should be gradually eliminated.

Fig. 3. Record the results of the palpations and California test readings, since they afford much information — particularly for identifying chronic animals. The figure shows how these data should be registered. This example shows an animal that should be eliminated due to repeated California test positivity and the presence of lesions.

CLINICAL, SUBCLINICAL AND CHRONIC MASTITIS

In order to determine the etiology of chronic mastitis, the farmer should collect and freeze samples before treatment is applied. On the other hand, the samples from animals with positive California test results will inform of the etiology of subclinical and chronic mastitis. In this context, subclinical and chronic mastitis are normally responsible for most of the economic losses, due to their very high incidence. When submitting samples for microbiological analysis, it should always be specified whether mastitis corresponds to clinical, chronic or subclinical processes.

MYCOPLASMA

Mycoplasma is present in latent form on most (if not all) farms, and takes advantage of stress in the herd (shearing, vaccinations, temperature changes, rain, etc.) to induce clinical alterations at any time in lactation. In any case, it should be pointed out that the presence of Mycoplasma in a milk sample is not necessarily indicative of agalactia. In fact, such situations are very often associated with subclinical mastitis.

In another study we recorded a substantial increase in the presence of Mycoplasma in California test-positive milk samples following a decrease in other pathogens (staphylococci and streptococci). In our opinion, this can be explained by the notion that mastitis control programs have effectively brought many pathogens under control — thus opening an ecological space and opportunity for Mycoplasma.

CORRECT ANTIBIOTIC USE

Mastitis is a group problem in which the treatment of individual animals and in lactation should be minimized. Treatment should be applied in drying, with a specific product for drying, and as part of the prophylaxis of the farm. The product required depends on the results of the antibiogram and on the etiology of the underlying process. In any case, it is widely accepted that no drying treatment is preferable to treatment under bad conditions. For correct drying it is necessary to clean the teat with alcohol and take great care not to inflict damage in inserting the cannula.

Problems arise in such treatment because of the way in which farmers dry their animals. Progressive drying where the animals are emptied several times is always dangerous for various reasons: because the animal is manipulated more often, the keratin plug sealing the teat sphincter and thus protecting the udder is eliminated, and the antibiotic used in drying is also eliminated.

Table 2: As a norm, we propose these drying recommendations.