occurrence and reoccurrence of clinical mastitis

Occurrence and Reoccurrence of Clinical Mastitis

D. MORSE, M. A. DeLORENZO, C. J. WILCOX, R. P. NATZKE, and D. R. BRAY

ABSTRACT

Clinical mastitis records for 6.5 yr (July 1977 through November 1983) from a large north Florida dairy were analyzed. Observed frequencies of clinical mastitis were calculated in 7240 Holstein and Jersey lactations. Least squares analyses used Holstein and Jersey lactations >200 d and limited maximum parity to 5 (6732 lactations, 5738 episodes). Solutions for number of occurrences of clinical mastitis adjusted for month of parturition and breed effects were .47, .59, .94, 1.27, and 1.50 for parities 1 through 5. Solutions for number of occurrences across lactation in Holsteins was greater than for Jerseys (1.28 vs..63). Solutions for occurrence of first episode in Holsteins increased from .35 in parity 1 to .71 in parity 5. Oc- currence of episodes two through five increased as parity increased. Similar trends were found in Jerseys with the exception of parity 2. Correlation of residuals between specific episodes increased as episode increased. From episode 2 through 5 probability of another occurrence of clinical mastitis was >.75. Solutions for proportion of occurrences of clinical mastitis occurring during the first 35 d postcalving were .60, .36, .31, .30, and .28 for parities 1 through 5. Clinical mastitis in first parity cows occurred primarily during the first 35 d postcalving.

INTRODUCTION

Few researchers have reported clinical mastitis data. Jackson (4) indicated that 48.0%

Received October 10, 1986. Accepted July 1, 1987. 1 Florida Agricultural Experiment Station Journal

Series Number 7614.

Institute of Food and Agricultural Sciences Dairy Science Department

University of Florida Gainesville 32611

of lactations had at least one occurrence of clinical mastitis (CM). Morse (5) observed 43.3% of all lactations had at least one occurrence of CM, and as parity (PAR) increased, percent of lactations treated and retreated for CM increased. In PAR 1, 37.8% of observed lactations were treated with an average of 1.4 occurrences of CM per treated lactation. In PAR 4, 54.6% of lactations were treated with 2.9 occurrences per treated lactation. Other researchers (1, 2, 5, 7) found frequent occurrence of clinical signs within 1 mo postcalving.

Although dairy producers recognize CM and not subclinical mastitis (bacterial infection), the major emphasis of reported mastitis research has centered around analyses for bacterial infection. In many instances occurrence of CM may not be recorded, as it is infrequent when compared with occurrence of bacterial infection. A large number of animals is required over an extended period to obtain adequate records of CM because CM manifests in a small percentage of cattle at any time. Where herds have few animals (<100 cows per herd), num- erous cooperating herds are required to obtain data regarding CM. Management, a key factor related to CM, becomes confounded with herd.

In order to produce milk efficiently, variable costs associated with milk production need to be decreased. Utilization of records pertaining to health costs can reduce nonfeed variable costs. The objectives of this study were to calculate probabilities of occurrence and reoccurrence, to predict number of occurrences of CM within a lactation, and to analyze occurrence of CM during the first 35 d postcalving. Probability of reoccurrence could be used to calculate cow lactation value before chemotherapeutic treatment.

MATERIALS AND METHODS

Data

Clinical mastitis records for 6.5 yr from a large north Florida dairy were analyzed. An

1987 J Dairy Sci 70:2168-2175 2168

OCCURRENCE OF CLINICAL MASTITIS 2169

episode (EPI) was def ined as a single occurrence o f CM. Herd management practices are de- scribed elsewhere (5). Lactat ions > 2 0 0 d with known breed and PAR (7240 lactat ions, 6641 EPI) were used.

Lactat ions (n = 6732) with 5738 EPI were analyzed. Data recorded included breed (B) (Holstein, Jersey) , cow, PAR (1 through 5), m o n t h of calving (M, grouped in to six 2-mo intervals), to ta l number of episodes (TEPI) within a lacta t ion (0 through 7), and occurrence or absence o f each individual episode wi th in a lac ta t ion (NEPI). Occur rence o f CM during the first 35 d postcalving was also recorded. Because less than 1% of lactat ions had more than seven EPI, value for m a x i m u m TEPI and NEPI was t runcated at 7.

Statistical Analyses

Observed f requencies o f each NEPI were calculated f rom edi ted data and compared with corresponding least squares solutions. Two measures of observed f requency were calculated. F r e q u e n c y of at least n EPI (NEPI = n) wi thin a pari ty was calculated for Holsteins and Jerseys as number of animals with n EPI divided by number o f animals for each of four PAR groups. F requency of at least one more EPI wi thin P A R was calculated as n u m b e r o f animals with NEPI = n divided by number o f animals with NEPI = n--1. Average number o f clinical occurrences per lacta t ion was calculated as TEPI within B and P A R group divided by n u m b e r of lactat ions in the same B and PAR group. The average n u m b e r o f EPI per lactat ion with clinical signs was calculated as TEPI wi th in B and P A R group divided by the actual n u m b e r o f lactat ions with clinical signs. This was more descriptive than an average n u m b e r of EPI per lacta t ion as EPI were at- t r ibuted to the lactat ions in which they occurred.

Presence or absence of each NEPI and TEPI were mode led with f ixed effects B, PAR, M, random effect o f COW(B), and error by least squares. COW(B) was absorbed as a f ixed effect . Init ially, two fac tor in teract ions ( B ' P A R , B ' M , and PAR*M) were tested for significance. Third and higher order in teract ions were assumed negligible. Prel iminary analyses o f TEPI and specific episode within lacta t ion (NEPI = 1 through NEPI = 7 ) i n d i c a t e d second order interact ions of B*M and P A R * M were

no t significant (P>.70) and were deleted. In terac t ion o f B ' P A R and main effect M were significant (P< .0001) . The reduced mode l was Y = /_t + B + COW(B) + M + PAR + B ' P A R + error, where Y represented presence of each NEPI and TEPI a n d / / w a s the popula t ion mean. Cow variance was es t imated by Henderson 's m e t h o d 3 (3). This variance represented within cow covariance be tween observat ions of a par t icular NEPI across PAR. Residual correlat ions be tween NEPI pairs wi thin-cow lac ta t ion were c o m p u t e d .

Data were par t i t ioned into 10 sets corresponding to each B ' P A R category. Oc- currence o f each NEPI (NEPI = 1 through NEPI = 7) was mode l ed with variat ion a t t r ibuted to M and error (Y = g + M + error). As each cow had only one record, cow was not included in the model , and residual te rm represented error (e 2) and var ia t ion due to cow (02). Correlat ions o f residuals be tween NEPI within cow were computed .

To predict percent o f occurrences o f CM by d 35 o f lactat ion, occur rence of CM by d 35 o f lac ta t ion was mode led . Interact ions tested were B ' P A R , B ' M , and PAR*M. Analyses o f occurrence of CM by d 35 of lac ta t ion tes ted interact ions o f B ' P A R , B ' M , and PAR*M. When all three in te rac t ion terms were included, B ' P A R was no t significant (P>.25) and was r emoved f rom the mode l . Af t e r analysis with B*M and PAR*M, P A R * M was deleted (P> .10). Probabi l i ty for B*M was .0051. The reduced mode l was Y = bt + B + COW(B) + M + PAR + B*M + error where Y was occurrence of CM by d 35 of lacta t ion. COW(B) was absorbed as a f ixed effect . Third and higher order interact ions were assumed negligible.

RESULTS AND DISCUSSION

Observed Frequencies

Observed f requencies of CM for Holstein and Jersey lactat ions are in Table 1. Across all lactat ions and breeds, the f requency of occurrence of at least one EPI (NEPI = 1) was .42 with an average o f .92 TEPI per lacta t ion. Jackson (4) indica ted that 48.0% of all cows had at least one occur rence o f CM during 12 mo. In Holsteins, f r equency of at least NEPI = 1 increased as PAR increased. In P A R 1 and 2, approx imate ly 40% of animals had signs of CM. Half of third lac ta t ion animals had at least

Journal of Dairy Science Vol. 70, No. 10, 1987

2170 MORSE ET AL.

TABLE 1. Observed frequency of at least n episodes within a lactation and average number of episodes (NEPI) per lactation (X) by parity and breed. 1

Episode number (NEPI) Parity 1 2 3 4 5

Holstein 1 .39 .11 .03 .02 .57 2 .41 .19 .11 .07 .05 .94 3 .51 .25 .16 .11 .09 1.33 4+ .60 .36 .24 .17 .13 1.90 Overall .47 .21 .12 .08 .06 1.10

Jersey 1 .34 .03 .39 2 .20 .03 .25 3 .32 .11 .04 .02 .01 .52 4+ .41 .16 .08 .06 .03 .82 Overall .32 .08 .03 .02 .01 .48

Both breeds .42 .17 .09 .06 .04 .92

1 Standard deviations <.03. Empty cells had fewer than 15 observations.

NEPI = 1 and about half o f those animals had NEPI - 2. In PAR 4, 60% of lactat ions had at least NEPI -- 1. Average TEPI per Holstein lactat ion was .57, .94, 1.33, and 1.90 for the four PAR groups. A more descriptive term average TEPI per t reated Holstein lactat ion (expression o f average TEPI per lacta t ion with clinical signs) was calculated as 1.5, 2.3, 2.6, and 3.2 for PAR groups 1, 2, 3, and 4+. Nei ther average TEPI per lacta t ion nor average TEPI per t rea ted lactat ion was available f rom Jackson (4) as total number of lactat ions involved was no t ment ioned .

Observed f requency of CM in Jerseys was lower than Holsteins. A b o u t one-third o f PAR I Jerseys had at least NEPI = 1. Fo r PAR 2 the f requency of NEPI = 1 was .20. Th i r ty percent of Jerseys in PAR 3 received t rea tment , and in PAR 4+, 40% of animals had at least NEPI = 1. Average TEPI per Jersey lacta t ion was .39, .25, .52, and .82 for PAR groups 1, 2, 3, and 4+. Average TEPI per Jersey lacta t ion with clinical signs was 1.1, 1.2, 1.6, and 2.0. In bo th breeds, f requency of re t rea tment wi thin lacta t ion increased as P A R increased.

Work by Smi th et al. (8) repor ted average m o n t h l y percent of cows with clinical signs f rom January 1979 through February 1984 ranged f rom 13.2 to 22.0. Actual percent of cows with clinical signs during 12 m o (percent of lactat ions with clinical signs) and TEPI per

lacta t ion or per lacta t ion with clinical signs was no t available.

In Holsteins, as PAR increased f rom 1 to 4+, f requency of an animal with NEPI = 1 having NEPI = 2 increased f rom .28 to .59 (Table 2). Within a PAR, f requency of NEPI + I increased as NEPI increased. As an example, f requency o f NEPI + 1 in par i ty 4+ increased f rom .59 to .82 as NEPI increased f rom 1 to 6. Similar t rends were present in Jerseys; however , observed f requency values were smaller. Jackson (4) observed f requency of reoccurrence ranged f rom .64 to .76 as NEPI increased f rom 1 to 7.

Calculat ion o f f requency values for two or three more EPI can be obta ined by mul t ip lying toge ther observed f requency values in Table 2. Observed f requency of a third P A R Holstein wi th NEPI = 2 to have at least three more EPI equals (.64) (.72) (.75) = .35.

Least Squares Analyses

Independen t variables used to explain variat ion in TEP1 within lac ta t ion and occurrence o f each NEPI included B, COW(B), M, PAR, and B ' P A R . Probabil i t ies for independen t variables ranged f rom <.0001 to .0146. The R 2 values ranged f rom .49 to .61 with cow con- t r ibut ing at least 85% of mode l sum of squares. Repeatabi l i ty for TEPI (the number of EPI wi thin lactat ion) was .13 [a~/(a2c + a~)] .



TABLE 2. Observed frequency of at least one more episode within lactation, by episode number (NEPI), parity, and breedJ

Episode number (NEPI) Parity 1 2 3 4 5 6 7

Holstein 1 .28 .29 .54 .48 2 .46 .57 .62 ,69 3 .48 .64 .72 .75 4+ .59 .68 .70 .74- Overall .4-5 .58 .67 .72

Jersey 1 .10 2 .17 3 .34 .35 4+ .38 .49 .71 .60 Overall .24 .39 .61 .62

Both breeds .40 .55 .67 .71

.67 .74 .86

.68 .72 .64

.81 .82 .75

.71 .78 .75

1 Range of standard deviations was .02 to .09. Empty cells had fewer than 15 observations.

Repea tab i l i ty o f NEPI = 1 across lac ta t ions wi thin a cow was .20. We would expec t 20% of animals with NEPI = 1 in t he cur ren t lac ta t ion to have at least one EPI in the nex t lacta t ion. This suggests successful dry cow t r e a t m e n t and m a n a g e m e n t pract ices exis ted as the p robab i l i ty of a lac ta t ion having clinical signs was greater than the repeatabi l i ty of NEPI = 1 across all lac ta t ions wi th in a cow. Similar values for NEPI = 2, NEPI = 3, and NEPI = 4 were .14, .04, and .04. Natzke and Evere t t (6) calculated a repea tabi l i ty value for at least one occur rence o f CM in a lac ta t ion as .31. Managemen t pract ices in the i r 25 herds were ex t r eme ly variable. Our repeatabi l i ty value ( .20) was lower and based on up to five consecut ive lactat ions, whereas the .31 value of Natzke and Everet t (6) was ob ta ined f rom only two consecut ive lacta t ions .

Solut ions and ar i thmet ic means for TEPI per lac ta t ion in Hols te in and Jersey lacta t ions calculated by PAR are in Table 3. In Hols te ins , so lu t ions for TEPI increased wi th PAR. In Jerseys , wi th the e x c e p t i o n o f PAR 2, TEPI increased wi th PAR. The low value for PAR 2 was unexpla inable . For bo th , least square solu t ions and a r i thmet ic means for TEPI per lac ta t ion across b reeds increased wi th PAR. This indicates tha t he rd hea l th records could p red ic t CM prob lems in animals in later PAR.

Solu t ions for TEPI by M, adjusted for B, PAR, and B ' P A R , are in Table 4. Animals calving f rom July th rough December had fewer

EPI than those calving f rom January th rough June .

Solu t ions for NEPI by B and PAR, adjus ted for M are in Table 5. In Holsteins, fewer first calf heifers had a first EPI than did f i f th lacta- t ion animals (.35 v s . . 7 1 ) . Within PAR, Hol- s teins usually had m o r e EPI than did Jerseys.

F r e q u e n c y of occur rence of each NEPI wi th in lac ta t ion by PAR and B is in Table 1.

TABLE 3. Least squares solutions for number of episodes within lactation and least squares solutions and arithmetic mean number of episodes per lactation by parity.

Least squares solutions 1 Breed Arithmetic

Parity Holstein 2 Jersey s Average 4 mean

1 .44 .50 .47 .56 2 .85 .33 .59 .73 3 1.31 .59 .94 1.09 4 1.71 .83 1.27 1.57 s 5 2.12 .89 1.50 Overall 1.28 .63

Adjusted for month of parturition. 2 Standard errors .05 to .09. s Standard errors .07 to . 13. 4 Standard errors .04 to .08. s Lactations 4+.


2172 MORSE ET AL.

TABLE 4. Least squares solutions for number of episodes per lactation by month of calving. 1

Month of calving 2 Solutions 3

January, February 1.08 March, April 1.01 May, June 1.11 July, August ,94 September, October .75 November, December .84

Adjusted for breed and parity. 2 Month effects significant (P<.0001). s Standard errors .05 to .09.

Tab le 5 has c o r r e s p o n d i n g least square so lu t ions ad jus ted for M. Observed f requencies and so lu t ions fo l lowed similar t r ends . Fo r ins tance , values f rom Table 1 for Hols te ins NEPI = 1 in par i t ies 1, 2, and 3 were .39, .41, and .51. Co r r e spond ing least square so lu t ions were .35, .42, and .57. Ef fec t o f seasonal calving on observed freqiaencies did n o t yield large dif- ferences b e t w e e n so lu t ions and observed values. A l t h o u g h dairy p roduce r s m a y no t have t he t e c h n o l o g y to ca lcula te least square values for m a n a g e m e n t cr i ter ia , s imple ca lcu la t ions o f observed f r equenc ies can be useful .

Cor re la t ions of residuals are in Table 6. A f t e r e x p l a n a t i o n of va r ia t ion in occur rence of CM due to B, COW(B), PAR, M, and B ' P A R , co r re l a t ion o f NEPI wi th NEPI + 1 increased as EPI w i t h i n l ac ta t ion increased. Cor re l a t ion o f NEPI = 1 wi~h NEPI = 2 was .43; co r re l a t ion

b e t w e e n NEPI = 5 and NEPI = 6 was .82. Some c o m p a r a b l e observed values are l is ted in Table 2. Obse rved f requenc ies of at least one more EPI for NEPI = 1 t h r o u g h NEPI = 4 were .40, .55, .67, and .71. C o r r e s p o n d i n g so lu t ions f rom Tab le 6 were .43, .64, .77, and .82.

Cor re l a t ion o f residuals a f t e r accoun t ing for va r ia t ion due to M are in Tab le 7. These are p robab i l i t i e s o f repea t o c c u r r e n c e f rom NEPI = n to NEPI = n + 1 w i th in each B ' P A R category. In Hols te ins , p robab i l i t y of repea t occur rence was .43 and .52 in P A R 1 a f t e r NEPt = 1 and NEPI = 2. In P A R 2 t h r o u g h 5, p robab i l i t y of r epea t o c c u r r e n c e af te r NEPI = 1 ranged f rom .56 to .62. O t h e r p robab i l i t i e s were > .70 . Jerseys fo l lowed similar t r e n d s of increased p r o b a b i l i t y o f repea t o c c u r r e n c e as b o t h NEPI increased wi th in P A R and as P A R increased. Values for residual cor re la t ions were lower. P robab i l i t i e s of repeat occu r r ence wi th in l ac ta t ion ( > 6 0 % ) were far g rea te r t han the r epea t ab i l i t y o f NEPI = 1 across all l ac ta t ions (20%) w i t h i n cow. These resul ts indica te t h a t cows r e s p o n d e d p o o r l y to l ac t a t ion t he r apy (high r epea tab i l i t y wi th in l ac ta t ion) , whereas response to dry cow t h e r a p y or spon t aneous recovery was grea ter ( low repea tab i l i ty across l ac ta t ion w i t h i n cow).

Resul ts o b t a i n e d f rom these analyses in- d ica te t h a t m a i n t e n a n c e o f he rd hea l th records could p rov ide useful i n f o r m a t i o n when ap- p rop r i a t e ca lcu la t ions are m a d e and evaluated. On- fa rm c o m p u t e r use can has t en m u n d a n e ca lcu la t ions and enab le m a n a g e m e n t pe rsonne l to act on o b t a i n e d results ins tead of pe r fo rming calcuIat ions .

TABLE 5. Least squares solutions for occurrence of episodes by breed and parity,

Solutions for occurrence of episodes I 1 2 3 4 5

H 2 J H J H J H J H J Parity

1 .35 .38 .07 .04 .02 .02 .01 .02 .00 .02 2 .42 .21 .18 .05 .10 .02 .06 .02 .04 .02 3 .57 .33 .27 .13 .17 .06 .12 .O3 .09 .02 4 .65 .38 .39 .19 ,24 .11 .16 .07 .11 .04 5 .71 .48 .49 .22 31 .09 .21 .05 .17 .03

2 Standard errors .01 to .05 adjusted for month of calving.

2 H = Holstein; J = Jersey.



TABLE 6. Correlations of residuals between occurrence of specific episodes regardless of parity.a

Episode number (NEPI) NEPI 2 2 3 4 5 6 7

1 .43 ,26 ,19 .17 .15 .13 2 .64 .49 .41 .34 ,29 3 .77 .64 .52 .44 4 .82 .68 .58 5 .82 .71 6 ,87

1Model included breed, cow in breed, month of calving, parity, and breed*parity (P<.01).

2 Number of episode within lactation.

In analysis of CM by d 35 o f lacta t ion probabil i t ies for PAR and B*M were < .0001 and .0029. Variance c o m p o n e n t es t imates for o 5 and 02 were .0376 and .1640. Solut ions for occurrence of CM within 35 d postcalving for B*M are in Table 8. These solutions represented the p ropor t ion o f lactat ion EPI that occurred during the first 35 d postcalving. Tables 3 and 4 present solut ions for number of EPI wi thin a lactat ion. Adjus ted for PAR, occurrence of CM was more prevalent during the 1st m o postcalving in Holsteins, which calved during the warmer months , May through October . In

Jerseys, a higher percent o f CM occurred by d 35 of lacta t ion in animals calving in May and June.

Solut ions for PAR, adjusted for B and M, indicate percent of P A R EPI occurr ing by d 35 o f lac ta t ion decreased f rom .60 to .28 as P A R increased f rom 1 to 5 (Table 9). Also included are ar i thmet ic means (5). The solutions are larger than calculated ar i thmet ic means. In this case, presence o f seasonal calving reduced the expec ted prevalence o f CM within the first 35 d postcalving.

High prevalence o f CM postcalving agreed with values and trends found in the l i terature. Berger and Francis (1) repor ted bacterial and clinical findings for samples obta ined f rom 1945 to 1947 f rom cows with clinical quarters. Both bacterial and clinical severities were greatest during the 1st m o postcalving. 0 l iver et al. (7) repor ted 30% of all infected cows showed signs o f CM during the 1st mo of lactat ion. Clinical signs were evident in 10 to 15% of infected cows during the remaining m o n t h s o f lactat ion. Withers (9) repor ted a lmost one-third o f CM cases began by peak lacta t ion with the major i ty occurr ing during the first 5 d postcalving. In a s tudy of CM in first and second PAR of 24 herds (1672 cows) (2), 33% of first lactat ion animals had CM during the 1st mo o f lacta t ion. In second PAR, 28% of CM cases occur red by d 30 o f lacta t ion.

TABLE 7. Correlation of lactation residuals between NEPI ~ and NEPI+I, by breed and parityJ

Episode number (NEPI)

Parity 1 2 3 4 5 6

Holstein 1 .43 .52 .73 .69 .73 .87 2 .58 .72 .77 .82 .81 .86 3 .56 .76 .82 .85 .81 4 .58 .73 .81 .82 .89 195 5 .62 .74 .76 .88 .87 .90

Jersey 1 .26 .45 .45 . . . . . . 2 .37 .46 .70 17; 3 .51 .57 .81 . 86 . . . . . . 4 .54 .70 .80 .73 .81 5 .53 .66 .91 .84 .83 172

NEPI = Number of episode within lactation. 2 Model accounted for month of calving.


2174 MORSE ET AL.

TABLE 8. Least squares solutions for occurrence of clinical mastitis by d 35 of lactation by breed and month of calving ~

Breed solutions Month of calving Holstein 2 Jersey 3

January, February .16 .50 March, April .24 .40 May, June .31 .60 July, August .43 .38 September, October ,40 .35 November, December .23 .45

1 Model included breed, cow in breed, calving, parity, and breed*month of calving.

2 Standard errors .03 to .05. 3 Standard errors .07 to . 10.

month of

An inverse relat ionship was found be tween PAR and percent of EPI in the first 35 d of lacta t ion. As PAR increased, percent of EPI during the 1st mo of lac ta t ion decreased. Thus, it appears that immedia te pos tpar tum care to min imize CM would be mos t eff icient in first PAR animals. Al though no bacter iological analyses accompany our clinical findings, these results suppor t successfulness o f dry cow t r ea tmen t when compared with lactat ion therapy. Incidence of CM in first par i ty cows (which received no dry cow or precalving treatment ) was greater as a percent of lacta t ion EPI occurr ing within 1 m o postcalving when compared with CM in PAR 2+ animals.

CONCLUSIONS

Frequency of animals having recurrent CM increased as PAR and NEPI increased. Abi l i ty to predict recurrence o f CM, and thus disposal of milk, may play a key role in future dairy culling models . Incorpora t ing frequencies o f reoccurrence into sof tware may aid dairy farmers in ident i fying and possibly culling animals before an excessive milk is discarded. This decision depends on the probabi l i ty o f sub- sequent occurrences within lacta t ion. Con- sidering f requency o f reoccurrence , the animal 's stage of lactat ion, stage o f gestat ion, and accumula ted days of discarded mi lk during the current lacta t ion, an es t imate o f net income could be made prior to animal t r ea tment . If analysis resulted in a negat ive value, a dairy

TABLE 9. Least squares solutions and arithmetic means for occurrence of clinical mastitis by d 35 of lactation by parity. 1

Means Parity Solutions 2 ~3

1 .60 .50 2 .36 .28 3 .31 .24 4 .30 .214 5 .28

~Model included breed, cow in breed, month of calving, parity, and breed*month of calving.

Standard errors .02 to .03. 3.~ = Arithmetic means; standard deviations

<.02. 4 Parities 4+.

farmer may be more inclined to cull rather than t reat the animal in quest ion.

Establ ishment , main tenance , and ut i l izat ion of herd health records can assist in making economica l ly eff icient decisions pertaining to t rea tment , re t rea tment , and culling. This will reduce the number o f animals with recurrent clinical signs and min imize associated losses. This paper provides me thods that should be incorpora ted into farm management practices. Calculat ions o f reoccurrence should be made by individual dairies to represent best the effects of managmen t practices and occurrence o f CM. High CM immedia te ly postcalving may decrease peak milk p roduc t ion and thus alter total lactat ion product ion . The economic impact o f high CM postcalving combined with probabil i ty of repeat occurrences o f CM stresses the useful- ness of herd health data to reduce economical ly ineff icient repeat offenders . Future research needs to address clinical reoccurrence and the economic impact associated with high occurrence o f CM during the first 35 d of lacta- t ion.

REFERENCES

1 Berger, J., and J. Francis. 1951. Mastitis in practice 1. Epizootiology and bacteriology. Vet. Rec. 63:283.

2 Bunch, K. J., D. J. Heneghan, K. F. Hibbitt, and G. J. Rowlands. 1984. Genetic influences on clinical mastiffs and its relationship with milk yield, season and stage of lactation. Livest. Prod. Sci. 11:91.


OCCURRENCE OF CLINICAL MASTITIS 2 1 7 5

3 Henderson, C. R. 1953. Est imation of variance and covariance components . Biometrics 9: 226.

4 Jackson, E. R. 1971. Elimination of infection. Pages 25--35 in Control of bovine mastitis. F. H. Dodd and E. R. Jackson, ed. Natl. Inst. Res. Dairying, Reading, Engl.

5 Morse, D. 1986. Characterization and statistical analysis of clinical mast i t is occurrence. M. S. Thesis, Univ. Florida, Gainesville.

6 Natzke, R. P., and R. Everett. 1975. The elimina- t ion of mast i t is by culling. Page 303 in IDF semi-

nar on mast i t is control. 7 Oliver, J., F. H. Dodd, F. K. Neave, and J. M. Lee.

1956. Udder infections in the dry period. II. The effect of withdrawing secretion from the dry udder on the incidence o f infection. J. Dairy Res. 23: 194.

8 Smith, K. L., D. A. Todhunter , and P. S. Schoen- berger. 1985. Environmental mastitis: cause, prevalence, prevention. J. Dairy Sci. 68:1531.

9 Withers, F. W. 1957. Wastage and disease incidence in dairy herds. Vet. Rec. 69:446.


occurrence and reoccurrence of clinical mastitis

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