How to Control the Increased Bulk Milk Somatic Cell Count in Dairy Cows

Abstract

Subclinical mastitis in dairy cattle covers a much larger population than cows with clinical cases. To remedy this type of mastitis, it is necessary to pay attention to the number of somatic cells in bulk milk, the history of control measures against mastitis (dry cow therapy, post-milking teat dipping, increasing the level of immunity of livestock and the herd, reducing stress, and increasing mammary health), and necessary actions are divided into two groups, fast and gradual. In the rapid group, ensuring the pre-milking striping during milking, throwing away the flake or clots containing milk, ensuring the effectiveness of the treatment of clinical cases and reducing recurrent cases from the treated ones, culling or isolating some cows from the herd (cases with very high or chronic SCC, high days in milk, reduced milk production, lack of pregnancy, history of recurrent complication, old age, etc.) are included. In the group of gradual measures, ten mastitis control and prevention measures would be implemented. It is better to use intramammary antibiotic therapy only to eradicate Streptococcus agalactiae, and treat the rest of the infected quarters at the time of drying the cow. Because BMSCC is dynamic, the efficacy of actions needs to be monitored.

1. Introduction

Bovine mastitis is considered to be the most expensive infectious disease in the dairy industry [1]. Therefore, control (prevention and timely and effective treatment) of mastitis plays an important role in economic efficiency in the dairy industry. Techniques for detecting and fighting bovine subclinical mastitis are very diverse, but finding methods that are both new and practical is still ongoing.

Herd bulk milk testing, composite milk evaluation, and obtaining information from milking machine sensors that identify subclinical mastitis in modern herds are some of the methods that monitor the quantity and quality of milk, or the effectiveness of the measures being implemented to fight mastitis in the herd.

The purpose of this research is to investigate and introduce new methods and more importantly the common methods of combating this type of mastitis.

2. Definition of mastitis

Mastitis is an infectious disease that is diagnosed based on observation of an inflammatory response to an intramammary infection (IMI) [2]. Mastitis is characterized by a range of physical and chemical changes in the milk and pathologic changes in the udders [3]. When clinical signs caused by inflammation of glands or ducts or breast parenchyma appear in the milk (subacute or Grade 1) or udder (acute or Grade 2) or the cow itself (per acute or Grade 3), mastitis is called clinical type [2]. But, if there are no clinical signs of this inflammation, and the inflammation can only be detected directly (such as SCC), or indirectly by looking for changes caused by the inflammation (e.g., California Mastitis Test, CMT, CMT or testing electrical conductivity), it is called subclinical mastitis [4].

3. Mastitis is a multifactorial disease

Three categories of factors work together to cause mastitis, which include environmental and management conditions, the virulence of the pathogen, and cow (especially the udders) conditions (immune system, metabolic status, and antioxidant) [5, 6]. Some of these factors are considered primary factors and some as risk, predisposing, or virulence factors. Physical factors such as teat end lesions (e.g., frostbite or improperly functioning milking machine), chemical factors such as non-standard teat dip disinfectants, and microbial factors play a role in causing mastitis. However, the role of bacteria as mastitis pathogens is the most prominent among microbes [2, 5].

4. Definition of subclinical mastitis

To differentiate types of mastitis, the clinical signs caused by mastitis in milk (such as pus, wateriness, and bleeding), udder (pain, swelling, warmth, serum leakage, edema, swelling, and redness), and systemic signs in cows (grounding, dehydration, fever, decreased activity of the rumen, and increased heart rate and breathing) have been used [7]. However, if no inflammatory signs are visible, but an increase in SCC is seen, the quarter is considered to have subclinical mastitis [4]. A geometric mean bulk milk somatic cell count (BMSCC) lower than 400,000 cells/mL has complied with milk export [8]. Also, financial incentives were provided to milk producers to achieve and maintain very low BMSCC (<200,000 cells/mL) [9]. Today, the number of somatic cell counts (SCC) in bulk milk of a herd and SCC of the composition of milk of more than 200,000 (Cells/mL), and individual SCC of each quarter of more than 100,000 (Cells/mL), are an indicator of subclinical mastitis. An SCC scoring system that divides the SCC of composite milk into 10 categories from 0 to 9, known as the somatic cell score (SCS) (originally called the linear score), is becomings more widely used (Table 1). The number of somatic cells often does not have a normal distribution, but the score of somatic cells has a normal distribution and makes statistical calculations and comparisons easy. Each one-unit increase (or decrease) in SCS is associated with a doubling (or halving) of the SCC. The goal of monitoring linear score is to be less than 4 in 85% of the cows and 90% of first lactations. In total, herd cows should be less than 3.3 [10, 11].

Somatic cell count SCC (×10^3)	Somatic cell Score (SCS)(Linear score)
25	1
50	2
100	3
200	4
400	5
800	6
1600	7
3200	8
6800	9

Table 1.

Calculating somatic cell score (previously called the linear score) from the somatic cell count.

Adopted from Refs. [10, 11].

5. Consequences of subclinical mastitis

5.1 The outcomes in dairy products

In cases of subclinical mastitis, there is a decrease in herd milk production per head of a cow (Table 2), the amount of cheese produced from such milk, the milk is not being curdled in the yogurt production process, and the precipitation of milk during heating are some of the issues faced farmers or milk technology engineers in dairy factories. Increasing the microbial count in milk is a consequence of subclinical mastitis, which, in addition to increasing lipase, plasmin, and other harmful enzymes, reduces the growth of useful yogurt-producing bacteria during any other milk fermentation process [10]. The increase of bacteria prevents not only the production of yogurt but also reduces the stability of milk and reduces its quality.

Bulk tank milk somatic cell count (cells/mL)	Infected quarters in heard (%)	Production loss (%)
200,000	5	0
500,000	16	6
1,000,000	32	18
1,500,000	48	29

Table 2.

The estimated prevalence of infection and losses in milk production associated with bulk milk SCC.

5.2 The outcomes in involved cows

A decrease in conception rate, an increase in loss of embryo and fetus, and an increase in cases of anestrus and ovarian cyst are other morbidities that frequently occur in herds involved in subclinical mastitis [12].

6. Bacteriology of subclinical mastitis

Based on the location of the reservoir of bacteria, these microorganisms have been divided into two groups, infectious and environmental. In the contagious group, the reservoir is the cow (udders, teats, skin of the udder or mouth, and oral lymph nodes) and that can be spread by milking machines and the hands of milkers. In the environmental group, the reservoir is the cow husbandry, such as bedding and the environment under the cow’s feet. Although the distinction between contagious and environmental pathogens is still a useful learning tool, the distinction is not always clear; for example, Streptococcus uberis, and Streptococcus dysgalactiae display both “contagious” and “environmental” properties [13]. Common contagious organisms include Staphylococcus aureus, Streptococcus agalactiae, Streptococcus dysgalactiae, and Mycoplasma spp. Environmental organisms include Enterobacter aero-genes, Escherichia coli, Klebsiella pneumoniae, Proteus spp., Pseudomonas spp., Serratia spp., T. pyogenes, Corynebacterium bovis and other gram-negatives, environmental streptococci, yeast or fungi, and Proto-theca, zofeia. Some authors put coagulase-negative staphylococci (CoNS) in the teat skin opportunistic group [14, 15, 16].

Of about 140 to 150 species of microorganisms that have been identified [17], the main etiological agents include Staphylococcus aureus (coagulase-positive bacteria), CoNS, Streptococcus agalactiae, environmental streptococci, Escherichia coli and other coliforms, and Corynebacterium bovis [18, 19]. On the other hand, mastitis-causing bacteria are also divided into two major and minor groups. Most of the mentioned bacteria are of the major type. Only coagulase-negative staphylococci (S. hyicus, S. chromogenes, S. xylosus, S. sciuri, S. warneri, S. simulans, and S. epidermidis) and Corynebacterium bovis are included in the minor group. The minor bacteria are generally opportunistic and rarely cause clinical mastitis, but they increase the number of somatic cells to more than acceptable levels and may prevent some mastitis caused by other bacteria. Coagulase-negative staphylococci have a protective effect against colonization of the teat duct and teat skin by S. aureus and other major pathogens, except E. coli and the environmental streptococci. But, Corynebacterium bovis prevents clinical mastitis caused by the major bacteria [13, 15]. Among the above-mentioned pathogens, Mycoplasma bois is usually not isolated in milk culture by usual methods, but it increases the BMSCC greatly [20].

7. Common subclinical mastitis detection methods with a view to the future

After receiving a warning from milk processing factories that the number of milk somatic cells is high or observing traces of this problem in the herd (such as a decrement in milk production), various laboratory techniques are used at the farm or laboratory level to confirm the presence of this problem [21].

California Mastitis Test (CMT) [22], the electric conductivity of an individual quarter [23] or ultrasonography of mammary glands, precision technologies with in-line sensors; [9, 24] as an on-farm test, and SCC of a quarter, composite four milk or bulk milk cell count, N-acetyl-β-d-glucosaminidase (NAGase) test of composite or quarter samples, the concentrations of L-lactate, glucose, lactose, and acute phase proteins (serum amyloid A and haptoglobin) are of current test to identify subclinical mastitis in cows. Among the various tests, the somatic cell count is considered the golden test. The CMT, as an indirect indicator of the level of SCC is practical and usable as an on-farm or cow-side test. If the CMT is used to minimize the false negative rate and achieve the highest level of sensitivity, then the test should be read as negative (CMT = negative) or positive (CMT = trace, 1, 2, or 3). If the CMT is used to minimize the false-positive rate and achieve the highest level of specificity for culling decisions, then the test should be read as negative (CMT = negative or trace) or positive (CMT = 1, 2, or 3) [22, 23]. It should be noted that some of these tests indicate more damage to the mammary epithelium than to inflammatory cells. Therefore, their combination with inflammation monitoring tests such as SCC will have excellent results [25].

7.1 Portacell; portable somatic cell measurement device

In some new on-farm devices such as Portacell (made by Portacheck, USA), the amount of color created in the chemical reactions between the diesterase enzyme and the corresponding substrate is the basis for counting the number of somatic cells. In a farm test study to determine the efficacy of PortaSCC, 68 milk samples from bulk tanks were collected. Somatic cells of samples were counted with a light microscope and then with a PortaSCC on-farm set (Portacheck company, Moorestown, New Jersey, USA). Then, milk samples were cultured, and counted cells with two different methods were compared in each isolated bacteria to elucidate the effect of the kind of bacteria on diesterase enzyme-dependent PortaSCC activity. As a result, the Pearson correlation test showed a direct correlation between two different tests (r = + 0.828, P < 0.01). Sensitivity, specificity, and observed overall accuracy were 73.3, 94.4, and 0.785 respectively, when microscopic somatic cells cut-off was considered to be 2.5 × 10⁵/ml. However, the sensitivity and specificity of PortaSCC were 81.6 and 84.2%, respectively when the Rock curve determined 1.6 × 10⁵ cells in Porta SCC as a cut-off point, and microscopic SCC was considered as a golden test. McNamara test (P < 0.01) and kappa coefficient (0.537) showed that PortaSCC could be a substitute for microscopic somatic cell count. Pearson correlation test showed that there was a direct correlation between TBC and SCC counted with two different methods (r = 0.506, P < 0.01). Fisher exact test showed that PortaSCC counted cells were significantly more than microscopic counted cells in Bacillus cereus infected samples. Bacillus cereus had a significant effect on the activities of the Porta SCC set (P < 0.05). It is concluded that Porta SCC is a reliable and valuable screening test to identify subclinical mastitis [26].

7.2 Echo morphometric findings via ultrasonography

Ultrasonography has been used to diagnose or confirm the diagnosis of subclinical mastitis [27]. The epithelium diameter of alveoli, teat, and udder cistern would be measured in this method. Zanjirani and Mohammadsadegh [28] to determine the efficacy of ultrasonography in identifying subclinical mastitis, 30 Holstein cows in the Garmsar area, which were often in the third and fourth parity, were selected based on the CMT grade ≥ 1, and positive milk culture results, and considered as the experimental group. Then, the diameter of the teat, the diameter of the cistern, and the diameter of the alveoli were measured with a 5 MHz ultrasound probe. The healthy udder and contralateral to the same Cartier after confirmation of health by the CMT and negative culture results were considered as a control group and examined with the ultrasound probe. Results of that study showed that only alveolar epithelium diameter significantly increased in cases of subclinical mastitis (Figure 1). In this case, the best cut-off was estimated at 8 mm in the epithelial diameter of the alveoli. The sensitivity, specificity, and predictive value of all cases was 66.7%, and the predictive value of the negative cases was 100%. However, the kappa disagreement coefficient was 0.33 (P < 0.05). In examining the relationship between alveolar epithelium diameter and SCC, SCS, the level of lymphocyte, neutrophil, and the degree of hyperkeratosis, the correlation was only significant with SCS. (P = 0.049, r = 0.36) [28].

Figure 1.

Ultrasound findings in the mammary gland of cows. Part 1; subclinical mastitis; A = teat cistern, B = udder cistern, and C= alveoli. Part 2; a normal cow; D = teat cistern, E = udder cistern, and F = alveoli [28].

7.3 Micro RNA

Diverse miRNAs may play an important role not only in the diagnosis but also in the treatment of mastitis in cows. MiRNAs are posttranscriptional regulators that bind to complementary sequences on target mRNAs, usually resulting in translational repression in mammals [29, 30, 31]. It is estimated that each miRNA regulates on average 200 target genes through an interaction between the seed sequence and the complementary target sites [32].

Li et al. [33] attempted to identify and characterize novel and differentially expressed microRNAs in peripheral blood from healthy and mastitis Holstein cattle by deep sequencing, and found that the patterns of miRNAs expression differed significantly between the peripheral blood from healthy and mastitis Holstein cattle, which provide important information on mastitis in miRNAs expression.

Saenz-de-Juano et al. [34] tried to determine extracellular vesicle properties and miRNA cargo variability in bovine milk from healthy cows and cows undergoing subclinical mastitis and showed that the miRNA profile and particle size characteristics remained constant throughout consecutive days, suggesting that miRNAs packed in EVs are physiological state-specific. In addition, infected quarters were solely affected while adjacent healthy quarters remained unaffected. Finally, the cow-individual miRNA changes pointed toward infection-specific alterations (Table 3). In addition to microRNA 21, other microRNAs that are related to the increase of somatic cells are: bta-miR-223-3p, bta-miR-142-5p, bta-miR-146b-5p, bta-miR-2890, bta-miR-2284ab, bta-miR-22-3p. Meanwhile, the presence of some microRNAs has been associated with a decrease in the number of somatic cells and may be effective in resistance to mastitis. Such hairs are: bta-miR-19b-3p, bta-miR-29c-3p, bta-miR-374b-5p, bta-miR-339a-5p, bta-miR-141-3p, etc. [34].

High SCC vs. Low SCC
miRNA	Log2 FC	FDR	High SCC
bta-miR-223-3p	9.5	6.1E-11	↑
bta-miR-142-5p	7.4	3.4E-09	↑
bta-miR-146b-5p	2.9	3.1E-07	↑
bta-miR-2890	2.4	1.5E-04	↑
bta-miR-2284ab	1.4	3.3E-02	↑
bta-miR-22-3p	0.7	1.8E-02	↑
bta-miR-21-5p	0.7	7.0E-03	↑
bta-miR-27b-3p	−0.4	1.8E-02	↓
bta-miR-181a-5p	−0.4	3.3E-02	↓
bta-miR-10,174-3p	−0.4	1.2E-02	↓
bta-miR-29a-3p	−0.6	1.2E-02	↓
bta-miR-29b-3p	−0.8	4.3E-02	↓
bta-miR-2285bf	−0.9	2.8E-02	↓
bta-miR-141-3p	−1.0	2.8E-02	↓
bta-miR-339a-5p	−1.1	3.0E-03	↓
bta-miR-374b-5p	−1.1	2.4E-02	↓
bta-miR-29c-3p	−1.2	2.0E-02	↓
bta-miR-19b-3p	−1.5	3.3E-02	↓

Table 3.

List of differential miRNAs in milk EVs from High SCC versus Low SCC quarters. FC: Fold change; FDR: False discovery rate.

7.4 Bulk tank analysis

Using a sample of milk from the whole milk collected in the herd, which is called a bulk tank milk sample, facilitates the analysis of findings at the herd level. The proper method of sample preparation and the factors affecting its changes by different is to be discussed [35]. Sometimes, instead of one sample from the whole herd, several samples are prepared, each of which represents a specific group of cows in the herd, and it is called string sampling [36, 37]. Somatic cell count and the total number of bacteria in tank milk are usually checked to assess the state of the herd.

Bulk tank cultures may detect high numbers of specific contagious organisms (S. agalactiae) and indicate a herd problem that requires individual cow cultures. Usually, it is not enough to culture the tank milk once [38]. Mycoplasma spp. and S. aureus may be shed intermittently in milk by a small percentage of the herd and may be detectable only after repeated cultures. Except for cases of S. agalactiae and possibly mastitis due to other Streptococcus spp., standard plate count (SPC) are general indicator of milking and management problems [35].

An increase in the total number of milk bacteria can occur both in cases of mastitis and in cases of contamination of milk. The total number of milk bacteria is placed in three groups; standard plate count (SPC > 5000–10,000/mL), pre-incubation count (PIC>40,000/mL), and pasteurized laboratory count (PLC > 180–200/mL) [35, 36, 37]. Normally, the PIC level is less than four times the SPC, and cases that are more than four times usually increase the level of mastitis, especially subclinical, although with a lower probability, that it can be caused by milk contamination. If mastitis cases increase PIC, the number of SCCs will be more than 200,000. If the PIC was less than four times the SPC, but the SCC was more than 200,000, the presence of mycoplasma SPP is suspected (Table 4) [36]. The number of somatic cells is more than 200,000/mL in conventional herds and 150,000/mL in organic herds, indicating an increase in cases of subclinical mastitis and the loss of large amounts of milk due to this crisis. A cell counts greater than 400,000 usually indicates a condition that requires intervention and correction, but not everywhere.

SPC > 5000
PIC<4 × SPC (due to mastitis)		PIC>4 × SPC(Low hygiene in bedding, udder, and machine)
Bulk tank SCC		Bulk tank SCC
<200 × 10^3	>200 × 10^3	200 × 10^3 >	>200 × 10^3
High environmental and contagious mastitis pathogen	High LPC and high environmental pathogen	High environmental and contagious mastitis pathogen	High coliforms and LPC
Urgent control and prevention of mastitis	Prevention of environmental pathogens and evaluation of the milking process	Milk quality should be checked	The entire milking path and the milking machine must be checked and cleaned

Table 4.

A schematic diagram for identifying current and potential milk quality and mastitis problems in a herd.

Adopted from Ref. [36].

The advantages and disadvantages of the polymerase chain reaction (PCR) method in identifying clinical mastitis bacteria or herd tank milk have been explained by different researchers [39]. Despite the high cost of this method and not separating live bacteria from dead bacteria (it is important in the pathogenesis of bacteria), there is a lot of hope in its development, especially in identifying the causative agents of mastitis. However, the need to prepare a specific primer for each potential bacterium is one of its disadvantages, especially in the identification of bulk tank milk bacteria.

Analyzing a bulk tank milk sample is not enough to make a decision and determine the necessary action, and it is more logical to judge based on several samplings and preparation of the geometric mean, in herds with seasonal changes, the use of rolling SCC [35, 36, 37]. If the milk collected in the herd is stored in several refrigerators, as is done in large herds, it is recommended to use the milk from the refrigerators instead of the milk from the bulk milk tank of the herd, which is called string sampling. It is easier to search and find the group of animals involved in this method. In using bulk milk to monitor herd SCC, it should be remembered that subclinical mastitis is not the only cause of SCC increment. In addition to subclinical mastitis, not separating quarter’s milk with mastitis, mixing quarter’s milk being treated, and mixing quarter’s milk which is apparently treated but bacteriologically involved is the common cause of increased bulk milk SCC.

7.5 Composite milk analysis

One of the useful indicators in the evaluation of subclinical mastitis is the determination of composite milk somatic cell count. To prepare it, they mix the milk of four quarters from each cow. Estimating the geometric mean of the composite milk cell count is a good alternative to the bulk tank. A number greater than 200,000 for each cow indicates subclinical mastitis [4, 40]. It seems the degree of agreement between the number of somatic cells in tank milk and composite milk makes a difference according to the type of pathogen in cows’ udders and the herd, the median, average, and especially the standard deviation of the number of bulk tank somatic cells, the number of cows that spread large amounts of milk and somatic cells in the herd milk (the author).

As an abstract at the end of the research subclinical mastitis detection methods, it should be emphasized that the most reliable laboratory detection method is somatic cell counting and the most reliable on-farm or cow-side detection method is the case of CMT.

8. Decision-making to combating

In the absence of regular herd testing and precision technologies with in-line sensors, a general recommendation is to take a milk sample from the herd bulk tank or string bulk tank milk every month, and in cases where an unacceptable increase is seen that requires treatment, a composite milk sample or weighted cell count should be prepared from all the cows in the herd. In many developed dairy countries, there is a possibility to use the last herd test data or request an urgent herd test and obtain data at a cow level. Finally, as a field measure, the entire herd can be tested using CMT or other ancillary tests for the detection of inflammation. Some practitioners find it sufficient to use a composite milk sampling medium. In this way, you can find cows that need attention. Before determining the method of combating subclinical mastitis, common measures are introduced.

8.1 Short-term or long-term methods

The strategies aimed at the reduction of the number of somatic cells in cow or herd level milk can be divided into two groups: Short-term and long-term efforts.

Prompt and compliant treatment of clinical cases, shortening or prolonging the dry period, treatment of clinical cases that respond positively to the CMT test in 2 to 4 weeks after treatment [41, 42], pre-milking stripping and throwing away contaminated milk, and treating it if necessary are among the measures that reduce the herd level of somatic cell count very quickly and in a short time.

The rate of response to the treatment of subclinical cases caused by many bacteria, except for Strep agalactia during lactation, is very low and sometimes disappointing [29]. Treatment of involved quarters during lactation is recommended only in cases of Streptococcus agalactia in the form of BLITZ or mini-BLITZ, and the response to the treatment is relatively fast [43]. In the case of other bacteria, it is either not implemented at all or after the implementation of other measures [29].

Preventive measures include pre- and post-milking teat disinfection, blanket or selective therapy at the dry period, the use of teat sealants after placing dry cow ointment at the beginning of the dry period, disinfection of the milking machine, reduction of bacterial load in the herd, udder, and teats, increasing the level of immunity, and decrement the stress level of the cow in the herd [44, 45] are placed in the group of long-term measures.

8.2 Herd level or individual strategies

The fight against subclinical mastitis is carried out at two levels: 1- the herd level aiming to decrease bulk milk SSC, and 2- the individual level.

8.2.1 Heard level strategies

In addition to the standard measures to prevent and control mastitis at the herd level, efforts to increase the health of cows and udders during the dry and lactation periods, improve the immune system, improve the food supply, and many other such measures are carried out at the herd level that plays an important role in the control of subclinical mastitis. Determining the priority of necessary actions is somewhat difficult and is discussed by Shukken et al. [46].

8.2.1.1 More effective treatment of clinical cases

An incidence rate of 15 new cases per 100 cows per lactation is achievable in good herds and whenever this number is exceeded further investigation is warranted [7]. To achieve this goal, faster identification of new cases and more efficient therapy is needed.

8.2.1.2 Identification and disposal of involved milk

Pre-milking striping of all quarters and discarding the involved milk (stinky, diluted, warmer, or purulent) from apparently normal involved (red, painful, swollen, and sweaty) quarters can rapidly decrease bulk milk and somatic cell count. Persistence of clot for at least four pre-milkings, and the use of CMT solution is necessary to confirm milk contamination in mild cases of clinical mastitis [7, 22].

8.2.1.3 More complete treatment of clinical cases

In many milk-producing herds, the treatment is stopped after the signs of mastitis disappear, while if a bacteriological sample is prepared from the milk, active bacteria are still present in many cases. Such a situation increases not only the cases of return or recurrence of mastitis after treatment but also during the period of incomplete treatment and the presence of active bacteria after treatment, the number of somatic cells of quarters will be high. To reduce this situation, ideally, after the signs of the disease disappear and before stopping the treatment, a bacteriological sample should be taken from the milk. Since such a protocol is very expensive, and there is a good correlation between CMT and IMI [42, 47], it should be recommended that 3 to 4 weeks after stopping the treatment of mastitis, all the involved quarters should be tested with a CMT solution, and if they were involved to subclinical mastitis, intramammary treatment should be performed in one or two next milkings according to the CMT results. Such additional treatment is very useful, but as it is assumed, these quarters will increase the number of somatic cells in the bulk milk tank of the herd within 3 to 4 weeks of the mentioned gap. For this reason, it should be recommended that when mastitis signs disappear after a period of treatment, an additional intramammary treatment be given in the next milking (8 to 12 hours later).

8.2.1.4 Isolation or segregation strategy

In many herds especially in non-developed countries, more than 50% of cows that have an initial clinical case of mastitis will have a repeat clinical case in the same lactation. A 30% repeat case rate is considered an acceptable level 38]. It has been shown that 6–8% of herd cows are the cause of 40–50% of all clinical cases in the herd [48]. Therefore, if they can be found as quickly as possible and separated or removed from other cows in the herd, not only in the rest of the herd will there be a drastic reduction in cases of contagious mastitis but also the number of somatic cells in the herd will decrease rapidly.

8.2.1.5 Controlling the length of the dry period

An increase in the length of the dry period causes the development of intramammary infection and subclinical and clinical mastitis caused by coagulase-negative Staphylococci, Strep uberis, and mycoplasmas. Reducing the length of the dry period will be associated with the reduction of intramammary infection caused by Escherichia coli and Staphylococcus aureus [24].

8.2.2 Individual level strategies

To draw a plan to combat subclinical mastitis and determine the priorities of the fight before any action, factors such as composite milk of cows, DIM, the amount of milk production in the current lactation peak, the amount of milk production in the previous lactation, pregnancy status, and age of the cow should be estimated, and placed in an Excel sheet (Figure 2). Then, based on the number of somatic cells, they are sorted from maximum to minimum. In many livestock evaluations, many cows are found to be unacceptably past their delivery and initiation of lactation (DIM >180 d), are not pregnant, and have been given a small amount of milk in the current and previous lactations. After finding the above cows (usually many of them have already received a culling suggestion), they should be culled or segregated from other cows and their milking place should be separated from the milking of the herd so that their milk is separated from the others. This will quickly and significantly reduce the number of somatic cells in the herd.

Figure 2.

Somatic cell number analysis sorted from high to low along with other data (pay attention to the red rows).

In some cows, it can be seen that they are given a small amount of milk, they are pregnant, but the time has not yet come to dry them; however, it may be better to dry them depending on the type of bacteria and herd policy,

8.2.2.1 Treatment strategy

It is generally considered not advisable to treat subclinical mastitis during lactation [19, 34]. However, it is important to consider the causative organism (Streptococcus agalactiae is easily treated), the age of the cow (better in younger), days in milk (before the peak of lactation is good), the number of intramammary infections for the cow (lower is better), history of the quarter in the previous lactation, history of the resent clinical or subclinical mastitis, pregnancy state (nonpregnant may be culled), milk production levels in previous and present lactation (low milk producers may be culled), and the udder health status of the herd.

8.2.2.2 Should subclinical mastitis caused by all bacteria be fought in the milking period?

It should not be forgotten that all bacteria cause both clinical and subclinical mastitis, but subclinical mastitis caused by minor bacteria is usually not treated during the lactation period due to their supportive role. Minor bacteria such as Corynebacterium bovis and CoNS prevent the occurrence of primary bacterial mastitis with a slight increase in the number of somatic cells [5].

All major or minor bacteria, common or uncommon, gram-negative or gram-positive, and environmental, contagious or opportunistic cause subclinical mastitis, but only some of them are easily treated. Coliform infections caused by Escherichia coli and Klebsiella pneumoniae are usually short-lived. Although the number of somatic cells caused by E. coli is very high, their subclinical cases are not considered important [49]. Environmental streptococci such as S. uberis and S. dysgalactiae show poor response when treated during lactation, and after possible treatment, the probability of infection returning is high. Therefore, their subclinical cases are not considered suitable for treatment [50]. Subclinical mastitis caused by Staphylococcus aureus causes a relatively persistent infection in the udder and increases the number of somatic cells to a moderate extent, and due to poor response to treatment during the lactation period; treated only in young cows, when of shorter duration of infection and lower SCC, days in milk less than 100, in non-chronic and repetitive and high producing cows [51, 52]. In cases caused by Mycoplasma bovis, the number of somatic cells increases drastically, but the total number of bacteria in the bulk milk tank does not change. Its subclinical cases are not treated, and an attempt is made to cull or segregate affected cows [50].

Briefly, milk from CMT-positive quarters is aseptically sampled and submitted to the laboratory. Intramammary treatment is limited to quarters infected with gram-positive pathogens (streptococci and, in some herds, first- or second-lactation cows with new S. aureus infections). Milk from cows with no growth or other pathogens is managed by other means, including no intervention, segregation, dry-off, or “killing” of the quarter [9].

8.2.2.3 When the herd’s somatic cells should not be reduced

1. Somatic cells in the range of 300,000 to 400,000 indicate the activity of environmental bacteria in the herd. For this reason, in such a situation, although reducing somatic cells to 200,000 will increase milk production and achieve other benefits of SCC decrement, it will increase clinical cases caused by environmental bacteria [53, 54]. Therefore, in such a situation, it should be reduced only when it is ensured that the level of bacteria is low, the bedding is dry, the level of immunity of herd cows is high, and the level of stress in the herd is low. The total antioxidant capacity can be estimated from the imine level in cattle.

2. Never select a quarter or a cow that shows an increase in SCC in a single sampling for intramammary treatment to reduce SCC. Many increases in SCC may be temporary and resolve on their own [55].

3. Never choose a quarter or a cow that has shown an increase in SCC in more than five sampling times for intramammary treatment to reduce SCC. Such cases are considered chronic mastitis, which does not respond well to treatment, especially during the lactating period, and it is better to cull them. If it is difficult to cull them, such cows should be segregated and milked separately, so that, their milk does not mix with the milk of the rest of the herd. In cases where such cows remain in the herd, they may recover by being treated at drying and receiving dry cow (DC) ointment [55].

4. Corynebacterium bovis and CoNS should never be selected for intramammary treatment to reduce SCC unless the efficacy of post-milking teat dip and dry period therapy are acceptable. After ensuring the effectiveness of the post-milking teat dip and dry period therapy, intramammary treatment can be used to accelerate the recovery of the quarters from the above bacteria [45].

8.2.2.4 Classification of cows with subclinical mastitis before treatment

Schocken 2008 suggests two types of increase in somatic cells. In one type, the number of cows whose cells are more than the herd targets is less than 2% [46]. In such herds, chronic cases (they have been infected in more than five consecutive samplings are more important than cases of sudden and temporary increase. For this reason, bacterial culture and identification, culling or isolation, early drying off, and treatment during the dry season are more important than treatment during lactation.

In the herds where more than 2% of the cows in the herd are responsible for increasing the number of somatic cells from the predetermined goals in the herd, the heifers are encountered as the same as cows, and the affected cows are divided into three groups. If the rate of a new infection is more than 8%, the following efforts should be checked: Heifer health, milking procedures, purchased cows, days in milk (with increasing the cell count this indicator increases), and the seasonality of mastitis. In cases where the chronic infection is more than 5%, attention should be paid to the pattern of increasing the number of somatic cells. The patterns of SCC increment may be as follows: High- low- high or sudden increase (spikes), or chronic-high [34]. Segregation and isolation, culling, cessation of lactation, and rarely intramammary antibiotic treatment are considered finally. In cases where more than 15% of fresh cows are infected, the following matters are checked: Their health, treatment during the dry period, seasonality of mastitis, and food ration.

8.2.2.5 When should be subclinical mastitis treated?

There are several situations in which lactational therapy of subclinical mastitis may be indicated:

1. When the main cause of subclinical mastitis in a herd is Streptococcus agalactiae, all CMT-positive quarters in the herds in BLITZ strategy, or quarters with SCC > 5 × 1000⁵ in mini-BLITZ strategy would be treated with intramammary antibiotic therapy [43].

2. In the situation where cow husbandry has received the warning of high SCC (usually with bulk milk tank more than 400,000/mL) and does not have much time to reduce SCC, and S. agalactiae in the herd is not isolated, and so, BLITZ therapy cannot be performed, all CMT-positive quarters from which environmental streptococci or exceptionally S. aureus has been isolated in milk culture, and the cows are in their first or second lactation, and the days in milk (DIM) is before the milk peak, they are treated with an intramammary antibiotic for at least three times (usually once every 24 hours). To ensure the result, it is recommended to repeat the treatment up to eight times in the case of S. aureus. Cure rates of cows with subclinical mastitis caused by S. aureus during lactation are much lower. Reported cure rates following intramammary therapy are between 15 and 60% [51, 52].

3. When the cow suffering from subclinical mastitis with any bacterial agent following this type of mastitis, involved in other important reproductive diseases such as anestrus, ovarian cysts, or repeat breeding, the involved quarters should be treated.

4. In cases of positive CMT test in 2 to 4 weeks after stopping the treatment of the mastitis case due to the remedy and mitigate of clinical signs, the quarters should be treated.

5. CoNS are the main bacteria in the intramammary infection of heifers and are the common cause of clinical mastitis in heifers after parturition, so it may be necessary to diagnose and treat heifers before parturition. In this context, intramammary treatment with cloxacillin (200 mg) and cephapirin sodium (200 mg) has been useful. Intramammary or parenteral tilmycosin may also be helpful [3].

8.2.2.6 Antibiotic of choice

Lactational therapy of subclinical S. aureus mastitis using intramuscular penicillin along with intramammary amoxicillin infusion, compared with the intramammary infusion alone, and increased the cure rate to 40% (Almost twice as much as the other group) [29, 56]. In staphylococci sensitive to penicillin, the best antibiotic is penicillin, but due to the possibility of mixing in the author’s experience, it is usually ceftiofur sodium (injection solution in the form of 0.125 mg/100 ml intramammary or intramammary ointment is used. In herds with a high prevalence of S. agalactiae mastitis, BLITZ therapy followed by good sanitation procedures can be used for eradication of the pathogen, increased milk production, and reduced penalties for high SCCs. Erythromycin (30 mg), cloxacillin (500 mg), and ampicillin (250 mg) can be used for this purpose [29]. Extracts of various traditional medicinal plants have been used and effective in the treatment of mastitis, especially the subclinical type [57, 58, 59]. Some probiotics, such as Bacillus subtilis, have been effective in the treatment of subclinical mastitis, similar to antibiotics [60].

8.2.2.7 Non-steroidal anti-inflammatory drugs (NSAD)

The response of the immune system to the inflammatory stimuli of microbes [61], physical stimuli such as frostbite, or chemicals such as non-standard teat dip causes an increase in the number of somatic cells [62]. Therefore, in all cases of this type of mastitis, the use of non-steroidal anti-inflammatories will be effective. However, in such cases, anti-inflammatory drugs are usually not used unless the treatment strategy chosen by the veterinarian is to use these drugs, especially in cases where antibiotics are not prescribed. Meloxicam, ketoprofen, caprofen, and to some extent, flunixin meglumine have been recommended in such cases. The use of dipyrone and phenylbutazone has not been successful. Carprofen and meloxicam are cyclo-oxygenase-2 (COX-2) selective, single-dose, long-acting NSAIDs to treat bovine mastitis [63]. One-time use of long-acting drugs such as meloxicam and caprofen seems to be enough [63]. Systemic administration of these drugs is common. Intramammary NSAD is not available, and if necessary, intra-mammary ointments containing steroid anti-inflammatories such as hydrocortisone, prednisolone, and dexamethasone are used. The use of these medicinal products in pregnant cattle is not allowed because it causes abortion in cows pregnant for more than 4 months [64].

8.2.2.8 Warnings

It is also important to ensure that standard mastitis control procedures, such as post-milking teat disinfection and dry cow therapy, have been implemented. Treatment of subclinical mastitis cases in herds where the principles of control and prevention are not observed is associated with treatment failure or disease recurrence.

It is very important to pay attention to the bedding. Never use wet organic materials bagasse, or molasses, under the cow’s feet after intramammary treatment. Fungal infections after intramammary treatment are common. It is recommended to change the bedding, keep it dry, and burn the surface of the cow’s place before intramammary treatment, especially BLITZ therapy.

9. Conclusion

The somatic cell count of tank milk or string samples should be prepared every month. If there is a need to make some efforts and control the number of somatic cells (usually in the number of somatic cells more than 300,000/mL), a composite milk sample or weighted sampling is prepared and the information is placed in an Excel file and sorted from top to bottom. Cows with a cell count higher than 200,000/mL, and in most cases, more than 400,000/mL may need intervention, which includes treatment or other measures (separation of quarter milk with infected pre-milking, segregation of some cows, or cessation of lactation in some quarters).

The pregnancy status, chronicity and the level of high intensity of somatic cell count, days in milk, age and parity, and milk production level (at the time of sampling and the peak of lactation) should also be determined and the treatment method (culling, segregation, etc.) should be implemented based on all of the mentioned factors. Meanwhile, intramammary antibiotic therapy should be the last measure. Before intramammary treatment, it is necessary to pay attention to several health measures (Collecting manure, drying and disinfecting the bedding, not using the bedding containing organic substances such as bagasse, using effective post-milking teat dipping, and effective treatment during cessation of lactation).