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Preventive Veterinary Medicine, 10 ( 1990 ) 1-14 1 Elsevier Science Publishers B.V., Amsterdam

The association between serological evidence of exposure to Campylobacterfetus and productivity

in dairy cattle

Saeed Akhtar 1, Hans P. Riemann ~, Mark C. Thurmond 2, Charles E. Franti I and T h o m a s B. F a r v e r ~

m Department of Epidemiology and Preventive Medicine, School of Veterinary Medicine, University of California, Davis, CA 95616 (U.S.A.)

2Department of Medicine, Veterinary Medicine Teaching and Research Center, School of Veterinary Medicine, University of California, 18830 Road 112, Tulare, CA 93274 (U.S.A.)

(Accepted for publication 8 August 1990)

ABSTRACT

Akhtar, S., Riemann, H.P., Thurmond, M.C., Franti, C.E. and Farver, T.B., 1990. The association between serological evidence of exposure to Campylobacterfetus and productivity in dairy cattle. Prey. Vet. Med., 10: 1-14.

We investigated the relationship between serological evidence of exposure to Campylobacterfetus (CF) and various indices of productivity in a cross-sectional study of 178 dairy cows from three California Dairy Herd Improvement Association (DHIA) herds. Blood samples were collected from the lactating cows during December, 1986. Enzyme-linked immunosorbent assay (ELISA) was used to determine antibody titers against CF, Haemophilus somnus (HS) and Leptospira hardjo (LH). The data on demographic and productivity parameters were obtained from DHIA individual-cow records of December, 1986. The data obtained pertained only to the current lactation of the cows. The variables CFETUS, HSOMNUS and LEPTO represent the serological status of a cow either as negative or positive against CF, HS and LH antigens, respectively. CFETUS and HSOMNUS represent the serological evidence of natural exposure to corresponding bacterial agents. However, LEPTO was assumed to be the level ofvaccinal titer against LH since all the cows studied had been vaccinated routinely against this infectious agent during September, 1986. Five indices of productivity (recent calving interval, calving to conception interval, number of services per conception, test-day's milk production and percent mature equivalent milk production) were used. HSOMNUS, LEPTO and other covariates suggested by previous studies were included in modeling the relationships of interest. Stepwise multiple linear regression analyses were carried out to study the adjusted relationship of CFETUS with each measure of productivity.

Multivariate analyses revealed that the adjusted relationships of CFETUS with recent calving interval, calving to conception interval, number of services per conception and test- day's milk production were non-significant. However, after adjusting for possible covariates, CFETUS-positive cows had an average of 7.43% less mature-equivalent milk production than CFETUS-negative cows. Among the covariates, LEPTO had a strong and independent relationship with recent calving interval and with percent mature-equivalent milk production (having unstandardized partial regression coefficients of - 0.77 and 4.53, respectively).

We conclude that the presence of antibodies against CF was not associated with reproductive efficiency of cows. The association of CF seropositivity with one index of milk production is the first

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2 S. AKHTAR ET AL.

indication that latent CF infection may be associated with subclinical mastitis (perhaps through a hypersensitivity reaction). However, this aspect needs further investigation.

INTRODUCTION

Bovine venereal campylobacteriosis (BVC) is caused by the micro-aero- philic, Gram-negative bacterium Campylobacter fetus (CF). This organism has two antigenically-indistinguishable subspecies: C. fetus ssp. fetus and C. fetus ssp. venerealis. (Hoffer, 1981 ). In females the disease is character- ized clinically by mild endometritis and abortion at 4-7 months of gestation, as a result of localized infection. Early embryonic deaths due to CF infection are common and often go unnoticed, thereby prolonging the calving-to-conception interval, increasing the number of services per conception, and increasing the risk of premature culling for the cow owing to her poor production performance relative to her herdmates (Martin et al., 1982; Roberts, 1986). Transmission of CF to a susceptible female through coitus or artificial insemination with infected semen can lead to a clinical and/or an asympto- matic carrier state (Ware, 1980 ). Systemic immunization has been employed to protect females against CF infection but such immunization is not considered adequate as the sole measure of control in infected herds (Wilkie and Winter, 1971 ).

There are few reports regarding the significance of prevalent serologic titers against CF with respect to reproductive efficiency in dairy cattle. Further- more, epidemiological data concerning the possibility of adverse effects on milk production by non clinically-apparent CF infection are also sparse.

Logan et al. (1982) and Neill et al. (1982) isolated aerotolerant Campy- lobacter from two outbreaks of atypical mastitis in the same dairy herd. In both of these reports, the organisms isolated were biochemically and culturally similar to strains of C. fetus ssp. fetus and C. fetus ssp. venerealis re- covered from abortions in cattle by Ellis et al. ( 1977 ).

Maddux and Williams ( 1975 ) reported that milk production in a 150-cow dairy herd (rolling herd average 4536-6638 kg and constant during the pe- riod of 1963-1969) had declined to 1540 kg in 1970-1971 because of CF infection. However, they did not elucidate the underlying biological mechanism through which CF infection mediated its effects in causing such a great reduction in milk yield.

Campylobacterfetus infection in dairy cattle abnormally prolongs the days open and calving interval (Roberts, 1986). The demonstrated inverse relationship of these two parameters of reproductive efficiency with milk production (Speicher and Meadows, 1967; Louca and Legates, 1968 ) also provided the basis for the present study concerning the association of serological evidence of exposure to CF and milk production.

EXPOSURE TO C. FETUS AND PRODUCTIVITY IN DAIRY CATTLE 3

The specific objectives of this study were: (1) to study the association between serological evidence of natural exposure to CF and reproductive efficiency; (2) to study the association between serological evidence of natural exposure to CF and milk production performance. A cross-sectional epide- miologic study design was used to test specific hypotheses of association between serological evidence of exposure to CF and different measures of productivity in dairy cows. Multivariate analyses were used for appropriate control of nuisance parameters identified in the literature.

MATERIALS AND METHODS

This research was a part of the subsampling component of the California National Animal Health Monitoring System. Three herds were selected from two counties of California. These herds were included as a convenience sample based on the willingness of the farmers to cooperate throughout the duration of the project. Herd codes were, 1, 3 and 4. Herds 1 and 3 were located in Sacramento county and Herd 4 in Tulare County. All three herds were en- rolled in the California Dairy Herd Improvement Association (DHIA). The study population was all lactating cows in the three dairy herds during December 1986.

Blood sample collection and analysis

Blood samples were collected from 535 lactating cows from the three herds during December 1986 by the senior author with the assistance of fellow grad- uate students. After sampling, blood samples were transferred to the laboratory. The blood was allowed to clot at refrigeration temperature overnight and sera then were separated and frozen at - 2 0 °C until analyzed. Enzyme- linked immunosorbent assay (ELISA) (Behymer et al., 1985 ) was used to analyze the serum samples against antigens of CF 1, Haemophilus somnus 2 (HS) and Leptospira hardjo 3 (LH). The ELISA results for each serum sample was interpreted either as negative or positive against the corresponding antigen, based on the method of de Savigny and Voller (1980).

Data collection, storage and validation

The source of the production, reproduction and demographic data was DHIA individual-cow records (ICR) of lactating cows for December 1986. Individual-cow records for Herds 1 and 3 were kept and updated at the DHIA

~Virion Inc., Morris Town, NJ. 2Department of Veterinary Microbiology, University of California, Davis, CA. 3The United States Department of Agriculture.

4 s. AKHTAR ET AL.

center at Provo, UT; these were obtained as image print on computer mag- netic tape. A computer program was written in Fortran to transfer the required data into a data file on a computer (DEC/PDP-11 ) in the Depart- ment of Epidemiology and Preventive Medicine. Individual-cow records for Herd 4 were obtained from the DHIA center at Tulare, CA and the required data were entered manually into the computer data file.

Serological data (indexed by cow number within herd) were stored on microcomputer disk using a database manager (Rbase 5000). At the end of complete data entry, all data in the computer file were checked against data sheets from the serological laboratory. Similarly, the computer file of production and reproduction data on Herd 4 was also compared manually with orig- inal ICR. Serological data from the microcomputer disk were transferred to computer data file. The two data files containing product ion and reproduction data and serology data for December 1986 were linked together.

Definition of variables

The variables used in the various statistical analyses are summarized in Table l and described here. The disease variables CFETUS, HSOMNUS and LEPTO represent the serological status based on ELISA titers. CFETUS and HSOMNUS are the evidence of natural exposures to corresponding bacterial agents whereas LEPTO was assumed to represent the level of vaccine titers, as all the cows in the study were vaccinated during September 1986 as a rou- tine preventive measure against LH. Percent of herd mature equivalent was the relative value of a cow's mature equivalent production expressed as a per- centage of mature equivalent herd average for milk.

Most variables in Table 1 are self-explanatory and pertain to the current lactation.

Statistical analysis

Complete records on all the variables included were available only for 178 cows. Statistical analyses were performed using the Biomedical Computer Programs (BMDP) (Dixon et al., 1985 ). All statistical tests were carried out at a = 0 . 0 5 unless otherwise stated. Descriptive statistics on each of the response and explanatory variables were computed using BMDP 2D. The descriptive statistics on some of the productivity variables for the three herds also were computed. The matrix of zero-order correlations was printed to ex- plore the collinearity among the variables and to select from among the indices of productivity as the outcome variables in regression models.

EXPOSURE TO C. FETUSAND PRODUCTIVITY IN DAIRY CATTLE 5

TABLEI

Description of variables used in the study of the relationship between serological evidence of exposure to Campylobacter fe tus and productivity in dairy cattle from three dairy herds in California, Decem- ber 1986

Demographic variables F A R M Farm identification ( 1, 3 or 4) B R E E D Breed of cow (Holstein or Guernsey) C L A C N Current lactation number A G E F C Age at first calving (months) A G E L C Age at last calving (months)

Production variables D I M I L K Days in milk since current calving T D M I L K t Test-day's milk production (lb) P E R M E l Percent herd mature equivalent

Reproduction variables C T F S I Calving to first service interval (days) C T C E P I 1 Calving to conception interval (days) (pregnant cows only ) N O S P C ~ Number of services per conception (pregnant cows only) R E C C I J Recent calving interval (months) P R E D D Previous days dry

Disease variables C F E T U S I H S O M N U S ' L E P T O l L O S O C C

Serological status against Campylobac ter fe tus Serological status against Haemophi lus sornnus Serological status against Leptospira hardjo Logarithm of somatic cell count on test-day milk sample

t See text for definition

Multiple regression analysis

Stepwise multiple linear regression (BMDP 2R) was used to study the adjusted relationship of CFETUS with each measure of productivity. Nominal and binary variables (Table 1 ) were specified as dummy variables in the regression models. The dummy variables HSOMNUS, FARM3, FARM4 and BREED were forced and maintained in all regression models because infection, farm sites and breed are established confounders in the dairy literature (Laben et al., 1982; Corbeil et al., 1985). FARM1 was kept as reference cat- egory for comparison among the dummy variables for various levels of FARM, in the regression model.

RESULTS

The final data set consisted of 535 records, including 178 records with no missing entries for any of the variables (Table 2 ). NOSPCand CTCEPI (per-

6 S. AKHTAR ET AL.

TABLE 2

Summary statistics for production, reproduction and disease variables for all available records (535 cows) and 178 complete records from three herds in California, December 1986

Variable ~ All available records Complete records

Mean Standard n Mean Standard deviation deviation

AGEFC 29.55 8.58 414 27.75 5.51 AGELC 50.22 22.63 526 59.52 22.53 CLACN 2.62 1.63 533 3.41 1.59 DIMILK 209.43 111.95 534 211.32 91.06 TDMILK 53.30 19.14 510 55.67 20.46 PERME 101.27 15.61 469 101.11 14.81 CTFS1 72.28 28.05 463 78.10 29.12 CTCEP1 123.76 64.97 230 121.17 58.84 NOSPC 2.15 1.42 227 1.92 1.02 RECC1 13.65 3.44 246 13.57 3.35 PREDD 65.93 35.20 343 69.77 28.17 LOSOCC 5.18 0.52 508 5.23 0.48

pp2 p p

CFETUS 0.47 535 0.44 HSOMNUS 0.32 535 0.42 LEPTO 0.41 535 0.59

~See text and Table l for definition. 2point prevalence of ELISA-positive cows.

taining to pregnant cows) had 57% missing values. RECCI and PREDD had 54 and 36% missing values, respectively. AGELC and CLACN were highly correlated (r=0 .95) and therefore only CLACN was included in all subse- quent analyses because of its larger sample size.

The descriptive statistics on all the variables included in the study, both for complete and incomplete records, also are presented in Table 2. Descriptive statistics on some of the productivity variables for the three herds and their herd sizes are presented in Table 3. Herds 1 and 3 were exclusively Holstein herds whereas Herd 4 had cows of both Holstein and Guernsey breeds. Sum- mary statistics on production and reproduction variables for CF seropositive and CF seronegative cows are presented in Table 4. The proportions of HSOMNUS- and LEPTO-positive cows among CF seropositive cows were 0.30 and 0.8 l, respectively, whereas the corresponding proportions among CF seronegative cows were 0.51 and 0.45, respectively.

Tables 5-9 show adjusted relationships of CFETUS with the most recent calving interval, calving to conception interval, number of services per conception, test-day's milk production and percent mature equivalent milk production, respectively. For simplicity, our discussion of the results of the


TABLE 3

Summary statistics by herd on some productivity variables of three California Dairy Herd Improve- ment Association herds, December 1986

Variable ~ Herd 1 Herd 3 Herd 4

M e a n S D 2 Mean SD Mean SD

TDMILK 57.1 19.0 49.9 17.9 54.2 19.8 (26.0-114.0) (12.0-97.0) (15.0-106.0)

PERME 100.6 13.9 104.0 14.9 99.3 16.9 (62.0-139.0) (63.0-135.0) (47.0-168.0)

RECCI 12.5 2.0 10.3 1.2 14.3 3.8 (10.0-20.0) (9.0-11.0) (9.0-30.0)

NOSPC 2.1 1.5 2.1 1.4 2.3 1.4 (1.0-8.0) (1.0-8.0) (1.0-6.0)

CTCEP/ 106.0 55.1 104.7 39.4 146.1 80.0 ( 47.0-262.0 ) (44.0-193.0 ) (40.0-318.0 )

CLACN 2.6 1.5 2.6 1.5 2.8 1.8 (1.0-7.0) (1.0-7.0) (1.0-11.0)

Herd size 113 198 224

tSee text for definition. 2SD = standard deviation.

TABLE4

Summary statistics on production, reproduction and disease variables for Campylobacterfetus seronegative (n = 107 ) and seropositive ( n = 71 ) cows from three dairy herds in California, December 1986

Variable I Serological status against Campylobacterfetus

Positive Negative

Mean S D 2 Mean SD

D I M I L K 212.21 83.62 210.73 96.06 TDMILK 58.76 19.58 53.62 20.87 PERME I 01.85 14.81 100.63 14.87 CLACN 3.13 1.03 3.64 1.85 AGEFC 29.03 7.33 26.90 3.67 AGELC 54.60 19.94 63.44 26.3 l CTCEPI 100.69 47.70 133.54 61.86 NOSPC I. l0 1.05 2.00 1.79 PREDD 61.83 25.75 75.04 28.58 RECCI 12.42 18.04 14.32 18.29 LOSOCC 5.14 0.41 5.28 O. 51

ISee text for definition. 2SD = standard deviation.

8 S. AKHTAR ET AL.

TABLE 5

Resul ts o f the s tepwise mul t ip le regression analysis o f the effects o f some disease and demograph ic variables on recent calving interval (RECCI) f rom three dairy herds in California, December 1986 ( n = t 7 8 )

Independen t b2 SE(b)3 Beta 4 Sig(t)5 variable t

CFETUS - 0 . 8 5 0.60 - 0 . 1 3 0.155 HSOMNUS - 0 . 2 7 0.42 - 0 . 0 4 0.524 LEPTO - 0 . 7 7 0.43 - 0 . 1 1 0.072 FARM3 - 4.63 1.86 - 0.15 0.014 FARM4 - 0.30 0.61 - 0.04 0.620 BREED - 0.23 0.67 - 0.02 0.733 CLACN - 3.43 0.42 - 1.62 0.000 AGELC - 0 . 2 8 0.03 1.88 0.000 Cons tan t 9.68 Mult iple R 2 0.441

~See Table 1 for def ini t ion. 2Uns tandard ized partial coefficient. 3Standard error o f b. 4Standardized b coefficient. 5p value associa ted with partial t s tat is t ics o f b.

T A B L E 6

Resul ts o f the s tepwise mul t ip le regression analysis o f the effects o f some disease and demograph ic variables on calving to concept ion interval ( C T C E P I ) f rom three dai ry herds in California, Decem- ber 1986 ( n = 7 7 )

Independen t b2 S E (b ) ~ Beta 4 Sig(t)5 variable I

CFETUS - 8.59 16.02 - 0.07 0.593 HSOMNUS 5.75 10.21 0.05 0.576 FARM4 19.17 16.56 0.16 0.251 BREED - 11.56 14.78 - 0 . 0 7 0.437 DIMILK 0.59 0.07 0.72 0.000 PERME - 0.70 0.40 - 0.15 0.083 Cons tan t 42.92 Mult iple R 2 0.551

~See Table 1 for defini t ion. 2Uns tandard ized partial coefficient. 3Standard error o f b. 4Standardized b coefficient. 5p value associa ted with partial t-statistics o f b. Note: FARM3 could not be forced in the mode l owing to its zero tolerance.


TABLE7

Results o f the stepwise multiple regression analysis o f the effects o f some disease and demographic variables on number o f services per concept ion (NOSPC) from three dairy herds in California, De- cember 1986 (n = 77 )

Independent b2 SE(b)3 Beta 4 Sig(t)5 variable ~

CFETUS 0.16 0.31 0.08 0.600 HSOMNUS - 0.20 0.21 - 0.10 0.343 FARM4 0.30 0.32 0.14 0.351 BREED - 0.53 0.28 - 0.18 0.065 DIMILK 0.01 0.00 0.59 0.001 CLACN 0.14 0.07 0.18 0.061 CTFSI - 0.01 0.00 - 0.30 0.002 Constant 0.49 Multiple R 2 0.462

ISee Table 1 for definition. 2Unstandardized partial coefficient. 3Standard error o f b. 4Standardized b coefficient. 5P value associated with partial t-statistics o f b. Note: FARM3 could not be forced in the model owing to its zero tolerance.

TABLE 8

Results o f the stepwise multiple regression analysis o f test-day's milk product ion (TDM1LK) on disease and some demographic variables f rom three dairy herds in California (n = 178)

Independent b2 SE (b)3 Beta 4 Sig(t) ~ variable j

CFETUS 0.83 2.65 0.02 0.753 HSOMNUS 0.46 1.92 0.01 0.809 LOSOCC - 5 . 1 5 1.97 - 0 . 1 2 0.010 FARM3 - 16.85 8.53 - 0.09 0.050 FARM4 - 3.83 2.79 - 0.09 0.172 BREED - 15.09 2.98 0.23 0.000 DIM1LK - 0.17 0.01 - 0.74 0.000 CLACN 1.83 0.61 0.11 0.026 Constant 101.70 Multiple R 2 0.689

ISee Table 1 for definit ion. 2Unstandardized partial coefficient. 3Standard error o f b. 4Standardized b coefficient. 5p value associated with partial t-statistics o f b.

10 s. AKHTAR ET AL.

TABLE 9

Results of the stepwise multiple regression analysis of percent mature equivalent milk production (PERME) on disease and demographic variables from three dairy herds in California (n = 178 )

Independent b2 SE(b)3 Beta 4 Sig(t)s variables t

CFETUS - 7.43 3.27 - 0.25 0.024 HSOMNUS - 0 . 1 5 2.30 - 0.01 0.948 FARM3 - 1.18 10.17 -0 .01 0.908 FARM4 - 8.26 3.33 -0 .27 0.014 BREED 2.87 3.61 0.06 0.429 DIMILK 0.04 0.01 0.24 0.001 CLACN - 1.55 0.69 - 0.16 0.037 LEPTO 4.53 2.34 0.15 0.054 Constant 101.05 Multiple R 2 0.1487

~See Table 1 for definition. 2Unstandardized partial coefficient. 3Standard error of b. *Standardized partial coefficient. 5p value associated with partial t-statistics of b.

regression analysis centers around the unstandardized partial regression coefficients.

CFETUS status of a cow had no significant association with any of the three indices of reproductive efficiency considered in this study (P> 0.1 ). How- ever, being a CFETUS-positive cow was associated with a slight increase in the number of services per conception.

The adjusted relationship of CFETUS with TDMILK was also non-significant ( P = 0.75 ). However, CFETUS was significantly associated with PERME in a multivariate model (P=0 .02) and this relationship is in the expected direction. Among the covariates, FARM4, DIMILK, CLACN, and LEPTO explained a substantial proportion of the variance in PERME. LEPTO had a strong positive and independent relationship with PERME and this con- founding variable needs further examination. FARM4 being significant rep- resents a significant difference in PERME responses for dairies 4 and 1.

DISCUSSION

The convenience nature of the sampling procedure limits generalization of the results to the possible target population of dairy herds in California. In- formation bias in the results of this study also may have resulted from the use of complete records (178 ) instead of including the 535 incomplete records. However, it was not possible to assess systematically the degree of bias in this sample of the study population. Power calculations (Kelsey et al., 1986 ) in-

EXPOSURE TO C FETUSAND PRODUCTIVITY IN DAIRY CATTLE 1 1

dicated that available sample sizes for NOSPC and CTCEPI models were suf- ficiently large to permit the detection of moderate levels of abnormality (associated with CF seropositivity) in these response variables.

The true sensitivity and specificity of ELISA used in this study were unknown for CF, HS and LH antigens but the relative values of these parameters of ELISA were deemed to be high based on results of this test for other bacterial antigens (Ruppanner et al., 1980; Behymer et al., 1985). (The latter studies have reported relative sensitivity and specificity as high as 99 and 97%, respectively, for the antigens used.) Nevertheless, some classification errors might have occurred in using ELISA to determine the exposure status of cows against CF, HS and LH antigens. However, based on the reasoning of Rosendal and Martin ( 1986 ) for serological tests of unknown sensitivity and specificity, we can assume that classification errors, if consistent for seropositive and seronegative cows, gave us conservative results concerning the classification of cows as seronegative or seropositive against CF, HS and LH antigens. Furthermore, we expect that such possible classification errors were consistent in our study because all serological tests were performed under uni- form laboratory conditions (test antigens, negative and positive control sera and laboratory technician were constant throughout the serological analysis).

Results from the multivariate analyses did not support the prior concep- tualized hypotheses of lower breeding efficiency of CFETUS-positive cows as compared with CFETUS-negative cows. It often has been claimed that CF infection as detected mainly by a serological test is associated with impaired fertility in dairy cattle (Frank et al., 1967 ). The present and a previous study (Donaldson et al., 1967) did not substantiate those claims. The difference in the results of our study and those reported by others might be a result of the crude assessment made in previous studies of differences in these reproductive parameters between two groups of cows (other studies did not account for the effects of many other variables which are related to reproductive performance).

Alternatively, a positive ELISA reaction might be reflective of one or more of the following possible situations: ( 1 ) high levels of specific immunoglob- ulins against CF may be due to constant antigenic stimulation in the cervico- vaginal area, from immobilized organisms which have been rendered aviru- lent by specific opsinins (Wilkie and Winter, 1971 ); (2) convalescent im- munity among cows which had regained a normal fertility level following infection (Hoffer, 1981 ); and/or (3) cross-reactivity with antigens of some coliform bacteria having antigenic homology with CF (Grohn and Genigeor- gis, 1985).

The association of CFETUS with TDMILK was non-significant in the multivariate model. This is contrary to the report of Logan et al. (1982), of re- duced milk production in dairy cattle owing to an outbreak of atypical mastitis caused by aerotolerant Campylobacter which were culturally and

12 s. AKHTAR ET AL.

biochemically similar to C. fetus ssp. fetus and C. fetus ssp. venerealis. This discrepancy between our results and those of Logan et al. (1982) might be due to the fact that the studies were dealing with two different clinical entities i.e. clinical and subclinical, from the same bacterial infection.

PERME is used to spot low-producing cows and investigate the reasons for low production (Quesenberry and Murrill, 1983). CF seropositive cows on average tended to yield 7.43% less mature-equivalent milk than CF seronegative cows and these results are in agreement with Maddux and Williams ( 1975 ), who reported a substantial loss of milk production in a herd of 150 cows owing to CF infection. We believe that of the two indices of milk production used in our study, PERME is a relatively unbiased proxy of production efficiency in dairy cattle. PERME adjusts milk production to a common age, to 305-days duration of a given lactation, and to a common month of calving (van Vleck and Henderson, 1961 ).

The rationale for suspecting low milk production among CFETUS-positive cows derives from: ( 1 ) the inverse relationship of milk production to some of the reproductive efficiency parameters; and (2) subclinical mastitis due to a hypersensitivity reaction of exposed cows to CF toxins. In the present study, CF seropositivity was not significantly related to any of the three parameters of reproductive efficiency. Therefore, evidence in support of indirect association of CF seropositivity with milk production could not be obtained. How- ever, CF seropositivity was significantly related to PERME, one of the indices of milk production efficiency. This relationship may be the result of subclinical mastitis caused by a hypersensitive state due to subclinical CF infection in seropositive cows. We are unaware of data implicating hypersensitivity di- rectly to mastitis in cows and this aspect needs further investigation.

FARM4 (modeling the comparison between Farm 4 and Farm 1 in PERME response), DIMILK, CLACN and LEPTO were among the covariates strongly related with PERME. The variation in milk production efficiency of cows with respect to these covariates have been recorded in the literature (Carroll, 1965; Wood, 1967; Laben et al., 1982).

In our study LEPTO was positively associated with PERME. Carroll ( 1965 ) also reported improved milk production (from 8193 to 10 001 lb ) within 45 days of vaccination against bovine leptospirosis in a California dairy herd, while management and feeding practices were kept unchanged. The biological mechanism by which this association might have occurred has not yet been defined. Further exploration of the association of vaccinal titers with this production variable are needed.

We conclude that further research, if feasible using some prospective study design, is needed to determine the significance of persistent serological titers against CF and other pathogens with respect to their possible effects on productivity in dairy cattle.

EXPOSURE TO C. F E T U S A N D PRODUCTIVITY IN DAIRY CATTLE 13

ACKNOWLEDGMENTS

Th i s r e sea rch was s u p p o r t e d in pa r t by a g ran t f r o m the U n i t e d Sta tes De- p a r t m e n t o f Agr icu l tu re a d m i n i s t e r e d t h r o u g h the N a t i o n a l A n i m a l H e a l t h M o n i t o r i n g S y s t e m c o o p e r a t i v e a g r e e m e n t 12-16-98-019. T h e sen ior a u t h o r was a Fe l low o f the P a k i s t a n Agr icu l tu ra l R e s e a r c h Counc i l whi le th is s tudy was u n d e r t a k e n a n d he gra te fu l ly a c k n o w l e d g e s the i r suppor t .

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