Strategies for using on-farm somatic cell counters

Cow udders

Introduction

The concentration of somatic cells (SCC) in milk is directly correlated to the infection status of the udder. SCC has become a widely used measure of the state of the udder health. The accepted view, in general, is that individual cow SCC over 200,000 cells/ml is an indication that infection is present. Mastitis is of significant economical concern for the dairy farmer and causes reduced animal welfare.

A newly introduced optical cell counter, the DeLaval Cell Counter (DCC) is portable and battery operated. Measuring SCC with the DCC is easy, since manual preparation of the sample is eliminated. The appropriate reagent and sample volume is handled by the special cassette. The analysis result is given within a minute. The relevant question regarding the DCC is how to most effectively use the data for mastitis and milk quality control strategies. This text considers user strategies of the DCC.

Bulk tank somatic cell count monitoring strategy

DeLaval Cell Counter (DCC)

Tracking the raw bulk tank SCC (BTSCC) with the DCC and determining if the trend is changed or unchanged can help in managing and dealing with udder health problems. Apart from the animal welfare point of view, there is the financial aspect. Every dairy farmer should aim to keep the BTSCC low enough to gain maximum reward.

Newly freshened cows with infections, late lactation cows, or newly purchased cows can cause BTSCC to change. On a small farm – 30 to 60 cows – one or two cows producing high SCC can have a major impact on the BTSCC. It is important to know the herd status and detect a deteriorating trend as early as possible, so the cow(s) responsible for the elevated BTSCC can be identified and the milk separated from the shipping milk. Table 1 shows the effect on the BTSCC, when separating a high SCC cow from the rest of the herd.

Table 1.

Effect on BTSCC when removing a high somatic cell count cow from milking. All cows are assumed to produce an equal amount of milk

  Before removal of high SCC cow   After removal of high SCC cow  
Cow Cow SCC % of BTSCC Cow SCC % of BTSCC
1 2,000,000 54 - -
2 500,000 13 500,000 29
3 400,000 11 400,000 23
4 200,000 5 200,000 12
5 200,000 5 200,000 12
6 50,000 1 50,000 3
7 150,000 4 150,000 9
8 100,000 3 100,000 6
9 75,000 2 75,000 4
10 50,000 1 50,000 3
BTSCC 372,500   191,667  

The DCC works as an excellent complement to the weekly or monthly BTSCC reports from the dairy that are available in some countries. In other countries, this information may not be provided on a frequent basis. A good idea is to incorporate checking of the BTSCC - daily or every second day - in the farm routines. The values are easy to plot and are often easier to interpret visually than simply looking at numbers in a table. Establishing a trend line from the raw information provides a good overall view of the herd status (Figure 1).

Figure 1

Figure 1.

Plot of bulk tank somatic cell counts (BTSCC) with a trend line showing the development of the herd SCC.

Another way to display and review such information involves establishing an average for the BTSCC (Figure 2). Above and below the average are dotted lines which point out where values differ more than 25% from the average SCC. These lines represent typical random variation limits. Anytime BTSCC values exceed the upper limit there is reason for attention since SCC values then are beyond standard biological variation. If it happens one time it suggests a short term aberration. However, if the values stay above the line for an extended period - a change has occurred. It is then of utmost importance to identify the responsible factor(s) causing the elevated SCC values.

 Figure 2

Figure 2.

Plot of bulk tank somatic cell counts (BTSCC) with average and random variation limits.

Review of research

BTSCC represents the milk from cows actually milked into the bulk tank, while the SCC herd mean may include cows whose milk is discarded (Schroeder, 1997). Incidence of infection is the single most important factor affecting the BTSCC. An elevation in BTSCC is related to an increased infection prevalence and decreased milk production, as showed in Table 2 (Eberhart et al., 1982). The BTSCC indicates the udder health status of the herd and should be used to monitor trends and bring the dairyman’s attention to problems (Harmon, 1998).

Table 2.

Estimated proportion of infected quarters and losses in milk production associated with elevated bulk tank somatic cell counts (BTSCC) (Eberhart et al., 1982)

BTSCC/ml % infected quarters in the herd % production loss*
200,000 6 0
500,000 16 7
1,000,000 32 18
1,500,000 48 29

*Relative to BTSCC of 200,000 cells/ml.

BTSCC also fluctuates with season; the summer months usually give higher SCC than winter and early spring (Smith et al., 2002). Another thing that may affect BTSCC is the constitution of the herd; SCC tends to increase with age and stage of lactation (Harmon, 1998). However, if quarters are uninfected, only a small change could be displayed (Sheldrake et al., 1983). Hence, the major influence of parity and lactation stage on SCC is linked to the intramammary infection (IMI) status.

Contagious mastitis causing organisms such as Staphylococcus aureus, Staphylococcus agalactiae and Mycoplasma spp. often result in subclinical mastitis, a high percentage of infected quarters and a BTSCC of 500,000 cells/ml or more. Occasionally, it is environmental streptococci that cause substantial problems in high SCC herds. BTSCC is the first indication of the quantity of subclinical mastitis in the herd. If a problem is suspected, the high SCC cows in the herd can be identified and properly treated. A general rule is that cows with a SCC of 200,000 cells/ml or more are very likely to be infected. The ambition should be to have 90% of the cows at 100,000 cells/ml or less (Schroeder, 1997).

Harmon (1998) asserts that interpretation of monthly BTSCC records are a useful tool in herds experiencing infections due to contagious pathogens, since those tend to be of long duration and thereby likely detected in the monthly BTSCC. However, when it comes to environmental pathogens, IMI are less prevalent and have a tendency to be of shorter duration – less than 30 days. Even though the herd is experiencing clinical mastitis problems derived from environmental pathogens, the BTSCC may be as low as 300,000 or even 200,000 cells/ml. This is because the relatively small number of environmental infections in the herd at a specific time does not have a major effect on the BTSCC. Therefore the udder health status may not be representatively reflected in a monthly BTSCC record received from the dairy (Harmon, 1998). To monitor the long-term changes as well as changes of short duration, it is important to check the BTSCC on a more frequent basis.

A veterinarian using the DCC in the daily practice says: “Farmers also can have the reassurance that progress is being made, or not, on a daily basis. It has been useful to have been able to compare our results to those of their milk buyer if there is any dispute. Daily testing of bulk samples in problem herds allows us to have an early warning system and so if the herd cell count is rising we can go in and take more radical action earlier than having to wait for the weekly result from the milk buyer which may have been taken on a day when the cell count was low” (Edmondson, 2004).

Screening of cows to be purchased

Rapidly expanding dairy farms are typically purchasing hundreds of heifers or cows from various locations. Generally they have little background information on these cattle. Smaller dairy farms may occasionally purchase a few animals from another farm. The new animals may be carrying infectious diseases including mastitis problems. It is beneficial to determine the infection status of purchased cattle before they are commingled with the existing herd.

Using the DCC to determine if SCC is normal or not before purchasing any cows is a good way of preventing udder problems from entering the herd. It is recommended for buyers to always do their own independent test, even if the seller provides information. The cattle sellers may also use the DCC to assure potential buyers that the animals they are selling are not troubled with mastitis.

Cows with abnormally high SCC should be considered suspicious and a milk sample should be cultured to determine the involved bacteria species. These animals should remain at the seller’s location or kept separated from the main herd until their infection status is determined. Purchased cows infected with S. aureus, S. agalactiae or Mycoplasma spp. are likely to lead to a serious problem once integrated into the main herd. These organisms are highly contagious and spread easily and rapidly and can infect many cows in the uninfected herd if precautions are not taken. A less extensive method than testing individual cows is testing BTSCC samples from the farms purchasing cows. A high count is a warning of prevalence of mastitis, while a low count indicates good udder health in the herd. Look over past BTSCC records to find out whether there has been any problem with elevated SCC.

Review of research

The milk producer should minimize the risk of infectious disease to be brought into the herd when purchasing animals. Buying heifers instead of cows will reduce the risk of transmitting mastitis problems from another herd. On the other hand a large number of heifers calving during a short period of time require facilities and labour which may easily be underestimated (Smith and Harner, 2004). Contagious mastitis caused by S. agalactiae, S. aureus and Mycoplasma spp. are not uncommon when introducing new cows into the existing herd. Other mastitis causing pathogens such as Arcanobacterium pyogenes and Prototheca spp. can also cause problems. Checking SCC is an effective way to identify and judge the degree of udder infection. However, it does not identify the infecting organism (González and Wilson, 2002).

González and Wilson (2002) discourage potential dairy cattle buyers from using one single BTSCC sample to conclude anything about the present mastitis status in the herd. Even though a positive test indicates the presence of udder infection in the herd, a negative test does not guarantee that the herd is free from infections. To increase the reliability it is recommended to collect at least three samples of BTSCC with three to four days interval or to alter¬natively collect milk samples for seven consecutive days. Animals that can not be checked before the purchase, e.g. dry cows, heifers and calves, should be tested as soon as they calve to avoid bringing contagious mastitis pathogens into the herd (González and Wilson, 2002).

Dry cow/fresh cow monitoring

Many cows develop problems around the dry period, including the time of calving or one to two weeks after calving (Figure 1). Three possibilities have to be considered regarding dry cow infections:

  • Cows may have gone dry with no infections, but sometime during the early lactation a new infection may have developed.
  • Cows may have had an infection at dry-off which was not cured and it flared up again after calving.
  • Cows may have been infection free at dry-off and throughout the dry period but became infected at calving or shortly thereafter.

 Figure 3

Figure 3.

Infection risk around dry-off and calving.

In terms of mastitis control, dairy farmers should monitor the SCC of their cows when they go dry and after calving. The DCC can be used for sampling cows close to dry-off and for sampling recently calved cows that are three to five days in milk. The post-calving samples are to be compared with the dry-off samples.

Dry cows

At dry off, every cow should be tested to determine the SCC - and if necessary the bacterial status of high SCC quarters. Using the DCC, SCC can be checked quickly and accurately at dry off and a decision on dry cow therapy can be made. With appropriate dry cow treatment, new infections at the beginning of the dry period may be held to a minimum.

With help of the DCC it is easier to practise selective dry therapy and eliminate unnecessary use of antibiotics. An US farmer, practising selective dry therapy says: “We have elected to sample dry cows with the DCC at final milking. We’ve identified those that have an SCC of 200,000 or less and elected not to dry-treat them – to save money”.

The DCC is also a good tool to catch up cows that at the last monthly Test day show a low SCC and would be dried off without dry therapy - but just before dry-off have an increased SCC. The individual quarter testing by the DCC will reveals the infected quarter(s), and the dry-off strategy will be changed as a consequence.

Review of research

The early dry period is a high risk period for new IMI. Schukken and co-workers (1993) found, in 68 cows from a low SCC herd, which were infused at dry off with an intamammary antibiotic on the left or the right side of the udder, that 6 out of 10 clinical cases were caused by new IMI in untreated quarters in early dry period i.e. within 10 days after dry-off. Another clinical case was observed 12 days after dry-off. However, only 10 of 136 untreated quarters (7%) encountered mastitis that might have been prevented by antibiotic infusion. This low percentage may have probably occurred because of a high risk for some cows and a low risk for others. The selection and testing of appropriate cows, i.e. cows exposed to high risk factors - e.g. previous clinical mastitis experience, high SCC, age and teat lesions - to infuse at dry-off is therefore beneficial.

Fresh cows

Fresh cows have a high probability to become infected by mastitis - even if dry cow therapy is used since the drugs are only effective for 14 to 21 days. Therefore, it is important to control udder status soon after parturition. Testing quarter or whole udder samples soon after calving could be done with the California mastitis test (CMT) but the result is not particularly accurate if SCC is below 500,000 cells/ml. Typically, in uninfected heifers and cows the initial SCC at calving may be 600,000 cells/ml or greater - but in uninfected animals SCC drops already three days post-calving. A practice to consider at day three after calving is to test the SCC of all quarters or a composite udder sample of all fresh cows, using the DCC. Cows with a SCC above 200,000 may be monitored more closely and evaluated for treatment. When detecting infections at this stage the cows are more easily treated and they have not yet been assigned to their milking string.

Review of research

Identifying and eliminating mastitis in early lactation may have significant economic benefits, e.g. preventing clinical mastitis in early lactation, decreasing the amount of discarded milk, and reducing BTSCC (Dingwell et al, 2004). Minimizing fresh cow mastitis is important to achieve peak milk yield and maximum lactation yield. Jones and Bailey (1999) reported that when clinical mastitis occurred before peak production, lactation milk yield was reduced by 0.51 kg/day in one study, by 11% in another study and 341 kg during the 60 days following onset of lactation. For subclinical mastitis cases, each doubling of SCC above 50,000 cells/ml has been shown to result in a loss of 0.4 and 0.6 kg of milk per day in first lactation and older cows, respectively (Hortet and Seegers, 1998).

Rajala-Schultz and co-workers (1999) studied the effect of clinical mastitis on milk yield in 24,276 dairy cows using monthly Test day milk yields. When mastitis occurred before peak milk, the daily losses during the first 14 days after the onset of mastitis varied from 1.1 to 2.5 kg. Milk yield in cows of all parities never reached the pre-mastitis levels for the rest of the lactation. When mastitis occurred between peak and 120 days after parturition, cows in all parities remained at a significantly lower level for the rest of lactation. Cows in first lactation seemed to be affected most severely by mastitis in this period. Many cows had mastitis so early that they did not have any monthly Test day taken before the onset of mastitis. 25% of the cows contracted mastitis within the first four days after parturition. The distribution of cases of mastitis during lactation is shown in Figure 2. (Rajala-Schultz et al., 1999).

 Figure 4

Figure 4.

The distribution of cases of mastitis during lactation (After Rajala-Schultz, et al., 1999)

Ruegg (2003) monitored the number of days that milk was discarded for 225 cases of mild and moderate mastitis. She found that cows that had high SCC preceding the clinical case required longer treatment and longer period of milk discard than cows that had low SCC before developing clinical mastitis. She points out the importance of early detection of mild and moderate cases of mastitis. Long periods of subclinical infections can allow some mastitis pathogens to invade secretory tissue – which makes it difficult for antibiotics to penetrate scar tissue and effectively destroy the bacteria.

Monitoring heifer’s mastitis

The SCC of heifers should be tested soon after calving. Heifers have not had a chance to develop immunities to diseases found in the milking herd. It is quite common for heifers to develop mastitis prior to calving, at time of calving or during the first two weeks after calving. If a heifer develops mastitis and loses a quarter, her value drops. There is also the risk of reduced lactation milk production, delaying the point when she will be profitable for the farmer.

Monitoring the SCC shortly after calving (day 3-5), provides an indication of udder status. Heifers with high SCC need sampling for bacteriological culture to identify the possible infecting organism and treatment or management options. Treatment success is greater, the earlier the infections are detected and dealt with.

Review of research

In prepartum heifers, many infections take place shortly prior to calving. Jones and Bailey (1999) reported that S. aureus, was found in 10% of 615 heifers in 10 Washington herds. As much as one-third of all heifers’ mastitis may be caused by S. aureus. It only takes a few months for a few S. aureus infected cows to infect a high percentage of the herd (Kirk and Mellenberger, 1990). Jones and Bailey report further on, that in many herds, much of the subclinical mastitis is caused by S. aureus and/or environmental Streptococci. Many IMI in pregnant heifers can persist for long periods of time, they are associated with elevated SCC, and may harm mammary development and affect milk production after calving. Studies have found that inflammatory responses in infected quarters of pregnant heifers may subsequently reduce milk production by as much as 18%. Most IMI became clinical. Over 60% of staphylococcal infections continued into second lactation. There were very few spontaneous recoveries without treatment (Jones and Bailey, 1999).

Using the DCC as a complement to monthly individual SCC testing

In monthly cell count recordings (Test day), the SCC in individual cow’s composite milk are determined. These data can be used to identify high SCC cows and to determine which cows are likely to be infected. At the time when these data return, the DCC can be used to confirm if high SCC animals are still high; the high SCC quarter may be identified by taking quarter samples. Thereafter a decision can be made on treating this quarter. The DCC testing is particularly beneficial in fresh cows, because many clinical cases of mastitis occur early in lactation and will not be picked up by monthly Test day. The DCC can be used to monitor cows between monthly Test days. Another good use of the DCC is to test cows that will be dried off. This information can be used with guidance from the herd veterinarian to develop selective dry treatment strategies.

Treatment evaluation

Work with your veterinarian to develop a prudent treatment program and keep records of all treatments and responses. For treatable mastitis, the most effective strategies include presumed identification of mastitis pathogens and the use of antibiotics for an appropriate duration for the expected pathogen. After treatment, the DCC will show if the cows responded to treatment or if they are still suffering a problem. By a minimum of 14 days after treatment, response to antibiotic therapy can be judged by the DCC. The SCC should be examined for a period of time to determine if the infection was really eliminated or just suppressed for a period and then re-emerged.

References

Dingwell, R., K. Leslie, L. Timms, Y. Schukken, J. Sargent, 2004. Evaluation of the Californian mastitis test to determine udder health status of early lactation dairy cows. Iowa state university animal industry report 2004.

Eberhart, R.J., L.J. Hutchinson and S.B. Spencer, 1982. Relationships of bulk tank somatic cell counts to indices of herd production. Journal of Food Protection, 45:1125-1128.

Edmondson, P., 2004. The practical applications of the DeLaval DCC tester. Unpublished. 4 pp.

González, R.N. and D.J. Wilson, 2002. Realistic milk culture programs for herd expansion. National Mastitis Council Annual Meeting Proceedings, pp 118-124.

Harmon, R.J., 1998. Somatic Cell Counts- Myths vs Reality. http://www.ces.uga.edu/Agriculture/asdsvm/Dairyscience/Scc.htm (2004-10-07).
Presented at 1998 Regional Meeting of National Mastitis Council, Bellevue, WA.

Hortet P. and H. Seegers, 1998. Calculated milk production losses associated with elevated somatic cell counts in dairy cows: review and critical discussion. Vet. Res. 29(6):497-510.

Jones, G.M. and T.L., Bailey, 1999. Mastitis Control in Heifers and First Lactation.
Virginia Polytechnic Institute and State University.
http://www.ext.vt.edu/pubs/dairy/404-281/404-281.html (2004-10-07).

Mellenberger, R, and J., Kirk, 1990. Mastitis control program for environmental Strep.-infected dairy cows. University of Wisconsin Department of Dairy Science Milk Quality Resources.
http://www.uwex.edu/milkquality/PDF/enironmental_strep.pdf (2004-10-17).

Rajala-Schultz, P.J., Y.T. Gröhn, C.E. McCulloch and C.L. Guard, 1999. Effects of clinical mastitis on milk yield in dairy cows. J. Dairy Sci. 82:1213-1220.

Ruegg, P.L., 2003. Key Udder Health Items: Treatment of Clinical Mastitis. University of Wisconsin Department of Dairy Science Milk Quality Resources. http://www.uwex.edu/milkquality/PDF/treatment_of_clinical_ mastitis.pdf (2004-11-15).

Schukken, Y.H., J. Vanvliet, D. Vandegeer, F.J. Grommers, 1993. A randomized blind trial on dry cow antibiotic infusion in a low somatic cell count herd. J. Dairy Sci. 76:2925-2930.

Sheldrake, R.F., R.J.T. Hoare and G.D. McGregor, 1983. Lactation stage, parity, and infection affecting somatic cells, electrical conductivity, and serum albumin in milk. Journal of Dairy Science, 66:542-547.

Schroeder, J.W., 1997. Mastitis Control Programs: Milk Quality Evaluation Tools for Dairy Farmers. North Dakota State University Extension Service. http://www.ext.nodak.edu/extpubs/ansci/dairy/as1131w.htm (2004-10-18).

Smith, J.W., A.M. Chapa, W.D. Gilson and L.O. Ely, 2002. Somatic Cell Count Benchmarks. Cooperative Extension Service, University of Georgia.
http://www.ces.uga.edu/pubs/PDF/b1194.pdf (2004-10-06).

Smith, J.F. and J. Harner, 2002. Planning a Dairy Expansion.
http://www.milkproduction.com/Planning a Dairy Expansion (2004-11-02).

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