Somatic Cell Counts: Interpretation for Individual Cows

Table of Contents

  1. Introduction
  2. Factors Affecting Somatic Cell Counts
  3. Interpretation of Individual Cow SCC
  4. Uses of SCC on Individual Cows for Management Decisions

Introduction

The "Somatic Cell Count Service" of the Ontario Dairy Herd Improvement Corporation (OHDIC) has been available for nearly two years. Approximately 40% of all DHI herds and 11% of ROP herds are presently utilizing the service. Bulk tank somatic cell counts (SCC) have become a widely used measure of milk quality, such as is given in the monthly statement of the Dairy Inspection Branch. Every producer should understand the factors which affect SCC and how to use the SCC program as a herd management tool.

Bulk tank SCC or the herd averages of individual cow SCC results are reliable indicators of the state of udder health in the herd. In general, bulk tank counts or herd averages of under 200,000 indicate excellent udder health and counts of over 500,000 indicate a definite problem with subclinical mastitis. The objective of this Factsheet is to comment on the interpretation of SCC for individual cows.

Factors Affecting Somatic Cell Counts

The factors affecting SCC are as follows:

  1. Mastitis (udder infection).
  2. Teat or udder injury.
  3. Number of quarters with mastitis.
  4. Age of Cow.
  5. Stage of lactation.
  6. Season.
  7. Stress.
  8. Day to day variation.
  9. Technical factors.
  10. Management factors.

Table 1, a sample report from the ODHIC Somatic Cell Count Program, has been compiled from actual individual cow SCC results from Ontario herds. These individuals were selected to demonstrate the various factors affecting SCC and to show what interpretations were made and how they were used.


Table 1. Sample monthly somatic cell count report sent to Ontario Dairy Herd Improvement Corporation producers. (See text for interpretation of results.)

Ontario Dairy Herd Improvement Corporation

Somatic Cell Count Report

Name: Mr. I.M.A. Dairyman
R.R.# 33
Anywhere, Ontario

Veterinarian: Dr. John Doe
Anywhere Vet Service
Box 100
Anywhere, Ontario

Herd Number Count Number Assoc. Number Batch Number
9999 99 9999 99

Test 1

D/M/Y: 25/09/81

Cow Inv. No. Barn Name or Number Cell Count Index
001 Annie
44,000
4
002 Patty
6,000
1
003 Helen
59,000
6
004 Mamie
2,834,000
283
005 Dawn
294,000
29
006 Toni
464,000
46
007 Belle
30,000
3
008 Beauty
144,000
14
009 Flashie
92,000
9
010 Mary
34,000
3
011 Toots
82,000
8
012 Babs
Sèche
--
013 Victoria
2,834,000
283
014 Sue
1,628,000
163
015 Rosa
1,828,000
183
016 Joan
Sèche
--

Test 2

D/M/Y: 19/08/81

Cow Inv. No. Barn Name or Number Cell Count Index
001 Annie
41,000
4
002 Patty
11,000
1
003 Helen
693,000
69
004 Mamie
2,608,000
261
005 Dawn
256,000
26
006 Toni
793,000
79
007 Belle
41,000
4
008 Beauty
178,000
18
009 Flashie
Sèche
--
010 Mary
31,000
3
011 Toots
313,000
31
012 Babs
1,627,000
163
013 Victoria
2,608,000
261
014 Sue
3,843,000
380
015 Rosa
1,742,000
174
016 Joan
1,019,000
102

Test 3

D/M/Y: 15/07/81

Cow Inv. No. Barn Name or Number Cell Count Index
001 Annie
16,000
2
002 Patty
21,000
2
003 Helen
Sèche
--
004 Mamie
3,500,000
350
005 Dawn
163,000
16
006 Toni
34,000
3
007 Belle
28,000
3
008 Beauty
130,000
13
009 Flashie
Sèche
--
010 Mary
529,000
53
011 Toots
32,000
3
012 Babs
1,317,000
132
013 Victoria
3,500,000
350
014 Sue
712,000
71
015 Rosa
1,516,000
152
016 Joan
1,614,000
161

Test 4

D/M/Y: 10/06/81

Cow Inv. No. Barn Name or Number Cell Count Index
001 Annie
10,000
1
002 Patty
Sèche
--
003 Helen
Sèche
--
004 Mamie
Sèche
--
005 Dawn
165,000
17
006 Toni
23,000
2
007 Belle
50,000
5
008 Beauty
289,000
29
009 Flashie
279,000
28
010 Mary
Sèche
--
011 Toots
53,000
5
012 Babs
630,000
63
013 Victoria
2,509,000
251
014 Sue
1,360,000
136
015 Rosa
Sèche
--
016 Joan
790,000
79

Test 5

D/M/Y: 08/05/81

Cow Inv. No. Barn Name or Number Cell Count Index
001 Annie
20,000
2
002 Patty
Sèche
--
003 Helen
72,000
7
004 Mamie
Sèche
--
005 Dawn
329,000
33
006 Toni
31,000
3
007 Belle
2,611,000
261
008 Beauty
4,520,000
452
009 Flashie
127,000
13
010 Mary
Sèche
--
011 Toots
47,000
5
012 Babs
391,000
39
013 Victoria
1,299,000
130
014 Sue
382,000
38
015 Rosa
Sèche
--
016 Joan
1,542,000
154

Test 6

D/M/Y: 08/04/81

Cow Inv. No. Barn Name or Number Cell Count Index
001 Annie
14,000
1
002 Patty
74,000
7
003 Helen
64,000
6
004 Mamie
143,000
14
005 Dawn
279,000
28
006 Toni
Sèche
--
007 Belle
48,000
5
008 Beauty
123,000
12
009 Flashie
149,000
15
010 Mary
101,000
10
011 Toots
Sèche
--
012 Babs
52,000
5
013 Victoria
Sèche
--
014 Sue
Sèche
--
015 Rosa
1,814,000
181
016 Joan
848,000
85

 

Cow Inv. No. Barn Name or Number Lactation Average Lifetime Average
Cell Count Index Cell Count Index
001 Annie
25,896
3
25,896
3
002 Patty
12,646
1
18,865
2
003 Helen
596,230
60
224,451
22
004 Mamie
2,908,360
291
721,382
72
005 Dawn
222,883
22
178,929
18
006 Toni
528,212
53
374,463
35
007 Belle
768,139
77
768,139
77
008 Beauty
1,029,803
103
189,902
19
009 Flashie
92,000
9
128,996
13
010 Mary
97,280
10
71,058
7
011 Toots
105,804
11
99,634
10
012 Babs
--
--
535,115
54
013 Victoria
2,489,169
249
1,563,911
156
014 Sue
1,692,000
107
438,000
44
015 Rosa
1,667,000
167
1,782,099
178
016 Joan
753,911
75
328,191
33

 

Herd Summary

Test (D/M/Y) Cell Count Mastitis Index % of herd over index 50
25/09/81 914,000 91 38%
19/08/81 408,017 41 13%
15/07/81 413,212 41 25%
10/06/81 399,247 40 33%
08/05/81 322,987 32 40%
08/04/81 319,233 32 14%
  1. Mastitis. The most important factor affecting the SCC of an individual cow is the infection status of her udder. General agreement rests on the values of less than 100,000 cells/mL for uninfected cows and greater than 300,000 for cows infected with significant pathogens such as Staph. aureus or Strep. agalactia. Cows with SCC between these values may have recovered from an infection, sustained an injury or be infected with a less important organism such as C. bovis.
    Examples are shown in Table 1. The first five cows listed in Table 1 represent potentially different states of udder health as reflected by the somatic cell count. Subject 001 had low monthly and lactation average somatic cell counts and was uninfected throughout the first lactation. Subject 002 had low monthly, lifetime, and lactation average somatic cell counts and was uninfected throughout the first lactation and after the second calving. Subject 003 had a low somatic cell count for the first lactation and was not dry treated. She calved with subclinical mastitis in one-quarter, which was found on culture to have Streptococcus agalactiae infection, was treated successfully, and returned to a low somatic cell count. Subject 004 was not dry treated, calved with clinical mastitis in two-quarters, was found on culture to have Staphlococcus aureus infection, did not respond to treatment, and continued to have an elevated somatic cell count. Subject 005 had a moderately elevated somatic cell count throughout lactation because of a persistent teat canal infection with C. bovis.
  2. Teat or Udder Injury. Somatic cells consist primarily of leukocytes (white blood cells) that are present in the udder in response to infection and to repair damaged tissue. Somatic cells also include epithelial cells which make up the internal lining of the mammary gland tissue and are normally replaced during the events of lactation. When the udder or teat is severely injured there are large increases in SCC. Some elevation in counts in these cases is in response to the increased prevalence of mastitis with injury. Subject 006 is an example cow with a stable injury to her udder. SCC returned to normal without antibiotic treatment.
  3. Number of Quarters with Mastitis. The dilution of high cell count milk from infected quarters with low cell count milk from uninfected quarters can be an important consideration in the interpretation of individual cow SCC. Subject 003 has one-quarter infected combined with high milk production, while subject 004 has two-quarters infected and lower milk production, therefore higher somatic cell counts.
  4. Age. Higher SCC have been found in the milk of older cows. This is primarily due to an increased prevalence of mastitis in older cows. It may also be the result of a greater cellular response to infection or of a greater amount of permanent udder damage after infection in older cows. For example, consider two cows from the same herd infected with Strep. ag. and treated in the same month. Cow 007, a first lactation heifer, has a high SCC and a good response after treatment of the infection. Cow 008, a fourth lactation cow with a very high SCC, has a slower reduction in somatic cell count after treatment.
  5. Stage of Lactation. Somatic cell counts are elevated immediately after calving and remain elevated for up to two weeks. lnterpreting increases in SCC early in lactation as evidence of mastitis must be done with caution. The counts of cows late in lactation are higher than the average throughout lactation, but this is due to an increased prevalence of subclinical infections in late lactation. When cows are not infected, there is no change in SCC due to stage of lactation or daily milk yield. Some cows will exhibit an increase in cell count at the end of lactation without having mastitis, but it only occurs immediately before drying off or after milk production has dropped below 4 kg/day. For example, cow 009 was sampled three days before going dry after a long lactation. Cow 010 was sampled seven days after calving. She had blood-tinged milk and considerable udder edema. There is no significance in the moderate increases in the SCCs of these cows.
  6. Season. After the first full year of the ODHIC Somatic Cell Count Program, the results reflect seasonal variations similar to those found in SCC programs in the USA. Counts are lowest during the winter and highest during the summer months of July and August. The reasons for these seasonal variations are, as yet, unknown and only speculated to be the effects of housing and temperature changes on infection status.
  7. Stress. Changes such as isolation of an individual, mixing groups of cows or being chased by a dog have been shown to increase SCC in the absence of mastitis. However, it has been reported that there was no increase in SCC associated with cows being in heat.
  8. Day to Day Variation. There can be considerable differences in SCC from individual cows from test day to test day and even if samples are taken on successive days. It has been suggested that this is a normal physiological variation. However, other suggestions are that these periodic large increases in cell counts are due to stress or injury infections that were eliminated before being detected. Regardless of the reasons for the variations it is advantageous to study at least the last five counts and the lactation average in making an interpretation with respect to an individual cow. Some veterinarians request their clients to construct at chart that will show a cow's SCC results for the entire lactation before making important management decisions. Cow 011, a third lactation cow, has had a consistently low SCC and no problems with udder health. However, on one test day her SCC was moderately increased. Culture and sensitivity tests were negative for growth of mastitis-causing organisms so the elevated SCC was attributed to normal variation. No treatment was given and normal counts resumed.
  9. Technical Factors. The methods of transportation, storage and electronic counting of the milk sample all can have an influence on the resultant values. Different labs in Ontario use slightly different testing machines and may find differing values on the same milk sample, especially when the counts are very low. However, these minor differences are relatively unimportant provided there is consistency in the handling, and processing of samples.
  10. Management Factors. Mastitis control procedures such as teat dipping, dry cow treatment, milking machine maintenance and the use of single service paper towels have all been useful in reducing SCC. These are secondary effects manifested through the elimination of existing cases of mastitis and the prevention of new udder infections.

Interpretation of Individual Cow SCC

The aim of the SCC program for individual cows is to detect cows with subclinical mastitis and to do so with a reasonable degree of accuracy, even when only one-quarter is infected. No matter what threshold value is used to classify a cow as positive or negative for mastitis, there will always be some classified incorrectly. It is important to remember that SCC indicates more about the general state of health of the udder than simply whether or not mastitis is present.

Cows without mastitis should have SCC of less than 100,000. Cows infected with unimportant organisms have counts between 100,000 and 300,000 cells/mL. Cows with SCC of greater than 300,000 are most likely infected. However, most authorities have agreed that an appropriate threshold value to divide uninfected cows from infected cows would be approximately 200,000 cells/mL. It appears that as long as stage of lactation, age, season and series of counts have been considered, the level of 200,000 cells/mL produce a reasonable success rate in classification.

Uses of SCC on Individual Cows for Management Decisions

There are several management decisions that can be affected by the interpretation of SCC on individual cows. All of the previously discussed factors that affect SCC should be considered in these decisions. If these decisions are made wisely, considerable improvements in herd mastitis control and profitable milk production will be realized. Your veterinarian can help with these deliberations as part of the herd health program. The ODHIC SCC Report, with your authorization, can be sent directly to your veterinarian.

Milk Culture and Sensitivity Testing

Individual cow SCC provide a basis on which to decide if cows should be milk sampled and cultured for the presence of mastitis-causing organisms. It is recommended that cows with high SCC (greater than 500,000 cells/mL) be considered for milk culture and sensitivity testing. If the elevated counts persist for two or more tests, even in the absence of clinical mastitis, and the cow is towards the beginning of lactation the results of culture and sensitivity testing may be very useful. However, it is only beneficial if the culture reveals information which will lead to the successful recovery of the cow, a reduced risk of transmission of infection or improved production. These benefits must be weighed against the costs of milk sampling, shipment to the lab, charges for the tests, drug costs and discarded milk. SCC is helpful in selecting cows for culture, which in turn is the basis for further decisions.

Treatment During Lactation

A control program that includes selection of lactating cows for treatment solely on the basis of high SCC would have several drawbacks. Depending on the level of cell count used to decide treatment was indicated, many uninfected cows might be treated. There would be many infections treated with antibiotics to which the organism is resistant. The dosage and duration of treatment may be inadequate. Strep. ag. causes a significant subclinical mastitis with high SCC and loss of production. This organism is sensitive to penicillin and it is recommended that these cases be treated during early lactation. Recent research conducted in Pennsylvania has investigated the cost-benefit relationship of treating mastitis during lactation. This work has shown that there are very few cases of subclinical mastitis for which treatment would increase the profitability of the cow after 120 days into lactation. This took into account all of the costs involved, all of the possible gains and the probability of a cure. In summary, high SCC in conjunction with culture and sensitivity testing can be useful in identifying cows infected with Strep. ag. in order that they may be treated during early lactation. However, the economics of this approach for other types of mastitis and for cows in later stages of lactation should be carefully deliberated.

Drying Cows Off Early

There is no doubt that the best method of eliminating infections of the udder is treatment with an appropriate long-acting antibiotic at the beginning of the dry period. Most authorities suggest that for current management and environmental conditions in Ontario, a total cow dry treatment program is most effective. However, if selective dry treatment is practiced the decision as to which cows should be dry treated can be based on high SCC with considerable success. If total dry treatment is used there are situations in which cows with persistently high SCC and relatively low production would benefit from being dried-off early and dry-treated. There is evidence to suggest that a repeat dry treatment three weeks after the first therapy could increase success rate. The decision to dry-off early and/or use multiple dry treatments can be effectively based on SCC. Teat dipping for 10 days after last milking and for 10 days prior to calving will reduce the rate of new infections.

Culling

The most important decision that can be affected by SCC is culling. Cows that have persistently high cell counts from lactation to lactation, even though dry treatment and teat dipping have been used, should be considered for culling. This decision can be strengthened with culture results that are positive for organisms such as Staph. aureus or Mycoplasma spp. Even without such culture results, a high SCC from lactation to lactation should be considered right along with other information such as the cow's production records, butterfat test, type classification and broodcow potential in making a decision to cull. Culling on the basis of SCC eliminates both an unprofitable producer and a potential source of new infections.

Milking Routine

In some management situations it may be possible to alter milking and hygiene procedures for high SCC cows in order to minimize the spread of organisms. For example, cows with subclinical mastitis detected by a persistent high SCC could be milked last or the milking machines could be sanitized after milking these cows.

Examples of Management Decisions

Milk sampling for culture and sensitivity testing is indicated for many of the cows listed in Table 1: 003, 004, 007, 008, 012, 013, 014, 015, and 016.

For subjects 003, 007 and 008, the isolation of Strep. agalactiae early in lactation suggests that treatment will result in successful recovery and will be economically advantageous. In subject 004, the isolation of Staph. aureus suggests that treatment during lactation is unlikely to be successful. Subject 013 had severe clinical mastitis followed by a persistently high somatic cell count; culture of the milk revealed Corynebacterium pyogenes and hence a guarded prognosis for recovery. Culling should be considered. A high somatic cell count in subject 012 indicates subclinical mastitis late in lactation. Proper dry treatment is the best approach. Subject 014 was treated on high basis of a somatic cell count and a slight reduction of the count resulted. The cow relapsed into cubclinical mastitis, and no significant change was noted in the count. Subject 015 had a persistent high somatic cell count from lactation to lactation even though good herd mastitis control procedures were being practiced. This cow is a definite candidate for culling. Subject 016 could be a candidate for early drying off or repeat dry treatment.


For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca
Author: K.E. Leslie - Veterinarian/Ontario Veterinary College
Creation Date: 03/84
Last Reviewed: 07/12