The Electrical Conductivity of Milk for the Detection of Subclinical Mastitis in Cows: Comparison of Various Methods of Handling Conductivity Data with the Use of Cell Counts and Bacteriological Examination

The Electrical Conductivity of Milk for the Detection of Subclinical Mastitis in Cows: Comparison of Various Methods of Handling Conductivity Data with the Use of Cell Counts and Bacteriological Examination

Br. uet. }. (1978 ), 134, 308 THE ELECTRICAL CONDUCTIVITY OF MILK FOR THE DETECTION OF SUBCLINICAL MASTITIS IN COWS: COMPARISON OF VARIOUS METHODS OF...

2MB Sizes 0 Downloads 15 Views

Br. uet. }. (1978 ), 134, 308

THE ELECTRICAL CONDUCTIVITY OF MILK FOR THE DETECTION OF SUBCLINICAL MASTITIS IN COWS: COMPARISON OF VARIOUS METHODS OF HANDLING CONDUCTIVITY DATA WITH THE USE OF CELL COUNTS AND BACTERIOLOGICAL EXAMINATION By M. PEAK ER

Agricultural Research Council Institute of Animal Physiology, Babraham, Cambridge

SUMMARY

From samples of foremilk taken from all four glands of cows on a single occasion, the efficacy of diagnosis of subclinical mastitis using a new application of conductivity measurements was compared with that using cell counts. For these trials boundaries previously set for conductivity tests were used, and similar boundaries for classification were established for cell counts. As in previous papers the cows were divided into three categories as a result of bacteriological examination. It is concluded that for classifying cows from the analysis of quarter foremilk samples taken on a single occasion, cell counts and conductivity measurements are of comparable effectiveness, although it is pointed out that the rapidity and ease of making conductivity measurements offer distinct advantages .

INTRODUCTION

In previous papers from this laboratory, the accuracy of detecting subclinical mastitis in cows by means of milk conductivity measurements has been assessed . It was shown that the success rate is very high when the conductivity of the foremilk from all four glands is determined every day and a calculation is performed to assess the degree of parallel variation between the four glands from day to day (Linzell & Peaker, 1971 , 1972; Linzell, Peaker & Rowell, 1974). The efficacy of conductivity measurements on the milk of all four glands, based on absolute conductivity or a calculation of differential conductivity (see below) made on a single day, has also been assessed . A high degree of success was apparent in classifying un infected cows and those with a gland or glands infected with markedly pathogenic organisms; these methods were less successful in identifying those cows habouring organisms which are often regarded as being less pathogenic (Linzell et al., 1974 ; Linzell & Peaker, 1975). In these studies the true status of the cows was determined by bacteriological examination.

DETECTION OF SUBCLINICAL MASTITIS

309

Largely because the correlation between the conductivity of a milk sample a nd the cell count is by no means exact, a number of workers recommended tha t conductivity should be abandoned as a diagnostic aid (see for example, Postle & Blobel, 1965; Little, Hebert & Forbes, 1968 ). However, we pointed out that such compariso ns a re of doubtful significance since one variable measures the degree of damage to the mammary epithelium while the other reflects the disease-comba ting resp o nse o f the cow to infection of the udder (Linzell & Peaker, 1975). Thus it seems unlikely tha t the correlation between cell count and conductivity would be very close. Nevertheless, it is clearly of interest to compare methods based on cell count and conductivi ty b ecause either method could be applied at a single examination of cows in a herd . It is clearly less expensive in terms of equipment, time and labour to make conductivi ty measurements than it is to determine cell counts. The question examined in this paper, therefore, is whether similar degrees of accuracy can be achieved using conductivity and cell counts. In addition to comparing differential and abso lute conductivity methods with cell counts, the accuracy of another way of evalua ting conductivity data has been assessed.

MATERIALS AND METHOD S

Procedures The main part of this paper is concerned with two trials, nine mo nth s apa rt, in Jersey cows of herd B. Foremilk samples were taken from each quarter befo re afternoon milking; conductivity measurements (using a bench appara tus) , cell count determinations (using a Coulter counter) and bacteriological tests were made exactly as described previously (Linzell et al. , 1974). On the basis of the bacteriological tests, the cows were divided into three categories as in previous studies: (i) uninJected (no organisms cultured from any of the glands), (ii) mildly-infected (infection of o ne or more glands with non-haemolytic, coagulase-negative staphylococci a nd/or Corynebacterium spp.), (iii) severely-infected (untreated clinical cases or infectio n with streptococci and/or haemolytic, coagulase-positive staphylococci). It should be no ted that the classification is on the type and not on the number of bacteria, so that some of the severely-infected cows, for example, had relatively few bacteria a nd therefore were presumably in the early stages of infection. Cows which had been treated with antibiotics within the preceding four weeks were excluded. Al so excluded were three cows in the second trial which had been in lactation for more than 305 days since more data are required to assess the value of conductivity measurements in very la te lacta tion (Linzell & Peaker, 1975). On another occasion in herd B and in a Friesian herd (herd 0), samples were taken for comparison of conductivity and cell count methods; bacteriological tests were not performed . Evaluation rif data Conductivity. Conductivity is expressed as the molarity of a sodium chl oride solutio n of the same electrical conductivity as the milk sample. The data were treated as previously for absolute conductivity and differential conductivity; absolute conductivity is the highest value from any of the four glands, and differential conductivity is the same

310

BRITISH VETERINARY JOURNAL, 134, 4

figure divided by the lowest value of the four \Malcolm, King & Campbell, 1942; Davis, 1947). A modification of differential conductivity was proposed by Quayle & Greatrix ( 1973). Electrodes were arranged so that milk from the four glands formed the four arms of a conductivity bridge and a signal proportional to any out-of-balance was obtained. This out-oj-balance conductivity is proportional to the product of the conductivities of two quarters divided by the product of the conductivities of the other two, the lowest product being the denominator. In a healthy cow one would expect this quotient to be close to I· O. Linzell et al. (1974 ) tested the principle of this method by manual calculation and found it to be less sensitive than other methods because occasions arose when, for example, in a cow infected in two glands, one high value was included in the numerator and the other in the denominator, or when a severelyaffected and an uninfected gland were balanced against two moderately-affected quarters . Therefore we suggested that for this method to be of value it would be essential to switch the quarters to different positions in the bridge circuit during the measurement. In that trial we used the quotient of the product of the fore-quarters divided by the product of the hind-quarters or vice versa (called out-oj-balance one-way in the present paper). In a mastitis detector manufactured by Monitor & Control Systems Ltd. of Birkenhead two positions are included - the two fore-quarters v. the two hindquarters, and the two left quarters v. the two right quarters . Diagnosis is then based on the out-of-balance signal from both positions. The effectiveness of using this procedure (out-oj-balance two-way ) has been calculated; in addition the effect of adding the third possible position (left fore x right hind v. right fore x left hind) (out -oj-balance three-way) has been assessed . In other words a cow was classified as infected if the outof-balance value exceeded a certain figure (see below) in either of the two positions for the two-way test or of the three positions for the three-way test. Cell counts. Absolute cell count is the highest value from any of the four glands, and differential cell count is the same figure divided by the lowest of the four values.

B oundarie sfor classification The boundaries used to distinguish between un infected and infected cows (without classifying the degree of infection) on the basis of absolute conductivity, differential conductivity and out-of-balance conductivity were the ones set previously by Linzell et al. (1974) (Table 0. A similar approach was used to set the boundaries for cell count collected on two occasions during the first trial in herd B, i.e. the highest figure for uninfected animals and the lowest for severely-infected animals were used . For absolute cell count there was no overlap between these two values and so two categories were established - uninfected and infected animals (Table 0. For differential cell count, there was not only considerable overlap but the high quotients obtained in uninfected animals with very low absolute cell counts seemed to preclude the application of this method of analysis. However, when an arbitrary lower limit of 0 ·3 x 10 6 cells/ml was set for the application of the method and only the highest diflcrential value for uninfected animals was used, the boundaries for the two categories (uninfected and infected) could be established; these are shown in Table I. With the inclusion of a figure of absolute cell count this is really a modified differential method .

311

DETECTION OF SUBCLINICAL MASTITIS TABLEI

BOUNDA RI ES USED TO PREDICT THE BACTERIOLOGICAL STATUS OF COWS FROM QUARTER FOREMILK CO DUCTlVITY AND CELL COUNT DETERMINATIONS

Ciassification

Conductivity methods

Cell count methods

Uninfected

Uncertain

Infected

Differential Absolute (mM) Out-or-balance

< 1·159 <53 < 1· 186

53-56·5

> 1·159 >56·5 > 1·186

Diflerential Absolute (cells x 106/ m l)

< 1·48" < 1·02

> J.48 > 1·02

• and cows with absolute cell count of < 0·3 x 106 cells/ml.

It should be reiterated that these tests apply to samples of foremilk taken from all four glands, that they do not apply in the case of the absolute values to milk mixed from all four glands, and that they may have to be modified as more data accumulate from larger numbers of animals. RESULTS

Comparison of methods Although the boundaries between classes were set during daily monitoring of cond uctivity and periodic determination of cell counts in the first trial, the data used {'or the present tests were not part of this procedure. Therefore because both trials in herd B were independent of the data used to establish the boundaries, the results have been combined and are shown in Table II. In identifying the 15 uninfected cows, all methods were equally and completely accurate ; in other words there were no false positive indications. For the 21 mildly-infected cows, out-of-balance conductivity correctly identified 10 to 13 as infected depending on the number of combinations of positions used in the calculation, while differential and absolute conductivity identified nine and eight respectively. The highest success rate for these cows was obtained with differential cell count ( 14 identified) while absolute cell count detected 11. For the 15 severely-infected cows, a similar result was apparent for all methods apart from out-of-balance one-way and two-way, with 13 identified by differential cell count and 14 by differential conductivity, absolute conductivity, out-of-balance threeway conductivity and absolute cell count. Overall, the success rate in classifying the total of 51 cows was highest for out-ofbalance three-way conductivity and differential cell count, with 42 (82%) correctly identified. Absolute cell count correctly classified 40, out-of-balance two-way 39, differential conductivity 38 and absolute conductivity 37; the lowest success rate was achieved by out-of-balance one-way with 33 (65%) correct. The advantage of using two-way and three-way out-of-balance conductivity over the one-way method is evident from these results, especially for severely-infected cows.

00

"0

TABLE II EFFECT IV ENESS OF USING CONDUCTIVITY OR CELL COUNT MEASUREMENTS ON QUARTER FOREMILK SAMPLES ON A SINGLE OCCASION TO PREDICT INFECTION

No. oj cows classified Cell count methods

Conductivity methods

:1en

Out-oj-balance Bactenological status

Cl

!1C

::r: < ['r1

Classification

Differential

Absolute

I-way

2-way

.i-way

Differential

Absolute

Correct Uncertain Incorrect

15

15

15

15

15

15

15

['r1

0

0

0

0

0

0

0

>!1C

Correct Uncertain Incorrect

9

8 5 8

10

12

13

14

II

II

9

8

14 I 0

8

12

14

37 6 8

33

39

42

42

40

18

12

9

9

11

--l

Uninfected (15 )"

M ildly- inlected (21)

0

12

Severely-infected ( 15) Correct Uncertain Incorrect

14

Overall (51)

38

Correct Uncertain Incorrect

13

~ Z

-<

'-<

0

13

10

C

14

>-

Z

.r

-

2

3

!1C

( )O

• Number in each group in parentheses. The boundaries between classes and the bacteriological status are described in Table I and in the text. The results are the combined data from two trials in herd B.

.~

~

DETECTION OF SUBCLiNICAL MASTITIS

313

Thus the percentage of these cows correctly diagnosed increased from 53% for the onew~y test to 80 and 93% using the two-way and three-way tests respectively.

Similarity of reports based on conductivity and cell count methods Another way of assessing whether conductivity and cell counts are similar in their efficacy in classifying cows is to compare the number of identical interpretations based on each method in a group of cows, without reference to the presence of bacteria. For this analysis data from 28 Jersey cows of herd Band 17 Friesians of herd 0 were used. Both differential and absolute conductivities and differential and absolute cell counts were combined in interpreting the data, using the boundaries shown in Table I. For conductivity a cow was regarded as 'infected' if she was classified as infected either by differential or absolute conductivity; as 'uncertain' if she was not classified as infected but as uncertain by absolute conductivity; and as 'uninfected' if she was not in the other two categories. For cell counts, a similar procedure was used except that in this case there was no uncertain category. Of the 45 pairs of interpretations, 41 (91%) were identical and four were not. This overall similarity again indicates the comparable effectiveness of conductivity and cell count methods.

DISCUSSION

The present results indicate that of the various methods that have been described for handling conductivity data for the detection of subclinical mastitis from quarter foremilk samples taken on a single occasion, the out-of-balance three-way method was the most effective in the trials, and this, together with the two-way out-of-balance method has clear advantages over the other methods (absolute conductivity, differential conductivity and out-of-balance one-way conductivity). The success rate in classifying cows, using boundaries between categories previously established, of this most successful of the conductivity methods assessed was 100% for uninfected cows, 93% for cows with highly pathogenic organisms in the mammary gland(s) (the severelyinfected group) and 63% for those infected with less pathogenic organisms (the mildlyinfected group); the overall success rate in the trials was 82%. However, it should be made clear that the results obtained with all modern evaluations of conductivity tests are based on relatively small numbers of, mainly Jersey, cows, and that more extensive field trials are required. It is also clear from the present results that the lack of a good correlation between the electrical conductivity of a milk sample and its cell (leucocyte) content (correlation coefficient of approximately O· 7 for the data obtained in the present experiments) does not indicate that conductivity measurements are inferior to cell counts in the detection of subclinical mastitis in individual cows. Thus it appears from this small trial that cell counts and the best of the methods of handling conductivity data would be virtually equal in their effectiveness with sampling on a single occasion. However, the rapidity and ease of making conductivity measurements offers distinct advantages over cell count determinations. A further advantage is that the accuracy of detecting all types of infected cows, even at the earliest stages of infection, increases with daily sampling

314

BRITISH VETERINARY JOURNAL, 134, 4

and, Instead of differential, absolute or out-of-balance conductivity, a relatively sophisticated calculation to test the degree of parallel variation between the four glands can be applied to the data; this yields the highest success rate of these methods, and offers the possibility of automation such that conductivity could be determined on the milk from all the glands of every cow at every milking (Linzell et al., 1974). REFERENCES

DAVIS,]. C . (1947). Dairy Inds 12, 35. LINlELL,]. 1. & PEAKER, M . (1970. Vet. Rec. 89,393 . LINlELL,]. 1. & PEAKER, M. (1972 ). Br. vet.). 128,284. LINlELL,]. 1. & PEAKER, M . (1975 ). Br. vet.}. 131,447. LINlELL,J. L., PEAKER, M. & ROWELL,]. C. (1974).]. agric. Sci. , Camb. 83,309. LITILE, T. W . A., HEBERT, C. N. & FORBES, D. (1968). Vet. Rec. 82,431. . MALCOLM,]. F. , KING, C. W. & CAMPBELL, M. M. (1942 ). Proc. Soc. agric. Bact. (Abstracts), 1942, 30. POSTLE, D. S. & BLOBEL, H. (1965). Am.]. vet. Res. 26,90. QUAYLE,]. C. & CREATRIX, C. R . (1973 ). British Patent 1, 314, 32B.

(Accepted for publication 22 September 1977)