P curate predictor of passive transfer status in young

Transcription

1 J Vet Intern Med 1998;12:79-83 Use of Serum Protein Concentration to Predict Mortality in Mixed-Source Dairy Replacement Heifers Jeff W. Tyler, Dale D. Hancock, Steve E. Wiksie, Susan L. Holler, John M. Gay, and Clive C. Gay The relationship between serum protein concentration in the 1st week of life and survival to 16 weeks of age was examined in 3,479 Holstein replacement heifers over a period of 10 years on a farm with endemic salmonellosis. Thirty-four percent of calves studied had serum protein concentrations <5.0 g/dl and 60.5% of calves had serum protein concentrations <5.5 g/dl. Cumulative mortality was 7.9%, indicating that calves with marginal passive transfer status can be reared successfully under conditions of endemic salmonellosis. Optimal survival was observed in calves with serum protein concentrations >5.5 g/dl. Calves with serum protein concentrations of g/dl had only a slightly increased relative risk (RR) of mortality (RR = 1.3) compared to calves with serum protein concentrations >5.5 g/dl. The highest RR was experienced by calves with serum protein concentrations <4 g/dl (RR = 4.6) and g/dl (RR = 3.1). Calves with inadequate passive transfer (serum protein concentration <5.0 g/dl), experienced increased mortality until at least 10 weeks of age, indicating that failure of passive transfer has an effect on calf health that extends into the juvenile period. Models in which serum protein concentration was treated either as a continuous variable or as a categorical variable failed to demonstrate any significant interaction between baseline mortality and the RR of mortality. This finding suggests that the RR derived in the present study should be applicable to farms with dramatically different baseline mortality rates. Key words: Calves; Colostrum; Passive transfer; Refractometry; Serum proteins. revious studies have shown that refractometry is an ac- P curate predictor of passive transfer status in young calves. Reported r' values for models predicting serum immunoglobulin G (IgG) concentration as a function of serum protein concentration range from 0.6 to OX.'-' Accuracy of refractometry in prediction of calf health and survival has been less clear. Although calves with low serum protein or immunoglobulin concentrations generally are accepted to be at greater risk for morbidity and mortality,4-i1 some studies have failed to identify any significant increase in risk associated with lower serum protein or immunoglobulin concentrations.12j3 In some instances, small sample size was hypothesized as a primary cause for the failure to identify increased risk of m0rta1ity.l~ The purpose of this study was to characterize the association between passive transfer status, as determined by refractometric determination of serum protein concentration, and mortality in mixed-source dairy replacement heifers. Furthermore, this study examined whether the relationship between passive transfer status and mortality was dependent or independent with regard to baseline mortality risk in calves. Lastly, the present study attempted to determine the upper age limit at which inadequate passive transfer status was related to the ongoing risk of mortality. Materials and Methods Study Site The study farm was a contract calf-rearing operation located in central Washington. During the 10 years of the study, the operation ap- From the Department of Veterinary Medicine and Surgery, University of Missouri, Columbia, MO (Tyler); the Department of Veterinury Clinical Sciences, Washington State University, Pullmun, WA (Huncock, Holler, Gay, Guy); and the Department of Large Animal Medicine and Surgery, Texus A&M University, College Station,?x (Wiksie). Reprint requests: Jeff W. Tyler, DVM, Department of Veterinary Medicine and Surgery, University of Missouri, Columbia, MO 65211; jtyler@vets.vetmed.missouri.edu. Accepted September 15, Copyright by the American College of Veterinary Internal Medicine /98/ /$3.00/0 proximately doubled in size, and a number of changes in management procedures and source dairy herds occurred. From 20 to 60 calves were received weekly from approximately 25 dairy herds located in northeastern Washington. With very few exceptions, these calves were Holstein heifers aged 1-7 days. Calves were housed in individual hutches and fed 1.8 L of a commercial milk replacer twice daily. A grain concentrate was fed beginning in the 1st week of life. Calves were weaned at approximately 7 weeks of age and moved to small group pens at approximately 8 weeks age. Six endemic strains of Sulmonella dublin were identified during the 10-year study period, 3 of which were associated with 2 sharp-onset outbreaks, one beginning in 1986 and another in late Additional periods of less severe morbidity and mortality associated with S dublin isolations were observed throughout the study period, and other Salmonella serotypes also were isolated occasionally. Other causes of death common to calf-rearing operations (eg, undifferentiated diarrheal disease during the 1st 2 weeks of life, pneumonia of calves of all ages) were observed throughout the study period. Sample and Data Collection Data collection was begun in September 1986 and completed in November Entry and death dates were recorded carefully by the owners as is necessary for billing purposes in a contract calf-raising operation, and copies of these data were obtained several times per year and entered into a computer database. In conjunction with routine farm visits, blood samples were collected from all calves that had arrived in the facility in the preceding 7 days. The frequency of these visits varied from year to year. Blood was collected via jugular venapuncture, refrigerated, and transported to the laboratory for sample processing. Serum was harvested after centrifugation, and serum protein concentration was determined using a temperature-compensating refractometer (TS Meter, American Optical, Buffalo, NY). The same refractometer was used for the duration of the study. Health records, which included calf identification, entry date, and mortality date, were collected in conjunction with routine farm visits. Data and Analysis Cumulative incidence of mortality to 16 weeks postentry was computed for each yearly entry cohort from 1986 through This was done for all entering calves (including those not sampled) and separately for the subset of calves from which refractometry results were available. The percentages of sampled calves having total protein con-

2 ~~~ 80 Tyler et al Table years. Descriptive statistics for mortality and passive immune transfer for a large calf-rearing operation followed for 1986' ' Total Number sampled ,479 Total entered 640 1,207 1,529 1,467 l,81 I 1,856 1,978 2,983 2,931 1,286 17,688 Mean total protein (g/dl) I 5.3 5% < 4.5 g/dl % < 5.0 g/dl % < 5.5 g/dl c Died (sampled) I % Died (total) Death age (sampled)h Death age (total) (days) Partial years. Median age at death (days) centrations of C4.5, C5.0, <5.5, and 25.5 g/dl also were computed for yearly entry cohorts. Separate survival curves to 16 weeks postentry were constructed for calves with serum protein concentrations <5.0 g/dl and for those with concentrations 25.0 g/dl using the product limit method.ii The relative risk (RR) of mortality associated with low passive transfer level was computed by dividing the cumulative mortality incidence of calves with serum protein concentration <5.0 g/dl by that of calves with concentrations 25.0 g/dl.lh Confidence intervals were computed for RR using an epidemiologic computer program (Epi-Info, Version 6, Centers for Disease Control, Atlanta, GA). Relative risks associated with low passive transfer were computed for each 2-week time increment postentry beginning with the 1st and 2nd weeks and ending with the 15th and 16th weeks. RR associated with low passive transfer also were computed for all calves entering during the 5 years with above median mortality and for those entering during the 5 years with below median mortality. Logistic regression was used to describe the relationship between serum protein concentration and mortality risk on a continuous scale and to test the hypothesis that the relationship differed in low and high mortality years.'" For this analysis, a variable was created to denote whether a calf entered in a yearly cohort that experienced above or below median mortality incidence. Linear and quadratic effects of serum protein concentration on the risk of mortality were fit after subtracting the mean serum protein concentration. The variable specifying whether the calf entered in a cohort with above 30% w 25% Serum protein concentration (g/dl) Fig 1. Cumulative mortality incidence and relative risk of mortality (reference group: calves with serum protein 26.0 g/dl) among groups of calves with different serum protein concentrations upon entry into a calf-rearing operation. Numbers in sidebars represent numbers of calves. median mortality was offered to the model to test the effect that the intercept differed between high and low mortality years and interaction cohort mortality, and serum protein concentration was offered to test the hypothesis that the slope differed. Results A total of 3,479 calves were sampled and tested for serum protein concentration during the study period. These calves comprised 19.7% of the 17,688 calves entering the calf-rearing farm during the 10-year study period. The number of calves entering and the number sampled during each study year are shown in Table 1. Cumulative incidence of mortality to 16 weeks of residence was 8.2% among the sampled calves and 7.9% among all calves entering the operation. As shown in Table 1, the yearly mortality incidences varied from 1.5 to 23.0% among sampled calves and from 4.7 to 22.8% among all calves. During the 5 study years with above median mortality incidence ( , 1993), the mortality incidences ranged from 6.5 to 23.0% among sampled calves and from 8.6 to 22.8% among all calves. During the 5 years with below median mortality incidence ( , ), the mortality incidence ranged from 1.5 to 4.7% among sampled calves and from 4.7 to 6.1 % among all calves. The percentage of calves with low serum protein concentrations varied greatly over the study period (Table 1). The percentage of calves with serum protein concentrations <4.5 g/dl varied from a high of 22.9% in 1987 to a low of 4.8% in The percentage of calves with serum protein concentrations <5.0 g/dl varied from a high of 47.3% in 1987 to a low of 12.1% in The percentage of calves with total protein concentrations <5.5 g/dl varied from a high of 72.6% in 1987 to a low of 31.5% in As shown in Figure 1, the mortality risk to 16 weeks postentry was incrementally lower for each 0.5-g/dL increase in serum protein concentration up to the g/dl category. Above this level, the mortality risk appeared to reach a plateau or to even increase slightly. The greatest incremental effect on mortality risk was observed in comparing the <4.0 g/dl category to the next highest level. Calves in this lowest category were at 4.6 times greater risk of mortality than were calves with serum protein concentration 26.0 g/dl. Calves with serum protein concentra-

3 Mortality in Dairy Replacement Heifers m > 2 a m 92% ~ 90% RR = 2.31 RR= % - 86% Days from entry Fig 2. Survival curves for calves having serum protein concentrations G.0 g/dl or 25.0 g/dl tions in the next highest category ( g/dl) had a 3.1 RR of mortality. The mortality risk of calves with serum protein concentrations of g/dl was identical to that of calves with serum protein concentrations 26.0 g/dl (RR = l.o), and the risk for calves with serum protein concentrations of g/dl was only slightly higher (RR = 1.3). Survival curves for calves with serum protein concentrations <5.0 g/dl and those with concentrations 25.0 g/dl diverged widely with increasing age (Fig 2). The incremental increase in divergence was greatest during the first 2-weeks postentry. At 14 days postentry, 99% of calves with serum protein concentrations 25.0 g/dl were alive, whereas approximately 95% of calves with serum protein concentrations G.0 g/dl were alive. In an effort to determine the maximal age at which serum protein concentration was associated with different mortality risk, the RRs of mortality for low (<5.0 g/dl) versus high serum protein concentration calves were computed at 2-week age intervals (Fig 3). As expected from the increasing divergence of the survival curves, the RR associated with low serum Log( 10) Relative risk Relative risk 1 I o.8 1 jli r T T 16.3 I o Weeks post-entry Fig 3. Interval relative risk of mortality for calves having total serum protein concentrations c5.0 g/dl compared to calves with serum protein concentrations 25.0 g/dl..o. lmortality, avg = 14.3% ~ ~ mortality, avg = 3.6% 1 Fig 4. Relative risk of mortality for calves having total serum protein concentrations (5.0 g/dl (reference group 25.0 g/dl) and entering the calf-rearing operation during high and low mortality years. protein was >1.0 for all periods to 16 weeks postentry. A significantly higher RR (P <.05) was observed through the first 6 weeks postentry and for the 9-10-week period postentry. The RR of mortality associated with low serum protein concentration (<5.0 g/dl) was not affected by the base mortality incidence. The RR for mortality and the 95% confidence intervals of this risk for calves entering the calfrearing operation during high and low mortality years is shown in Figure 4. The RRs were similar in magnitude (2.31 and 2.14, respectively) and did not differ significantly. A logistic model demonstrated that both serum protein concentration and cohort mortality (high versus low mortality period) were significantly associated with mortality (Fig 5; Table 2). The quadratic effect (serum protein concentration X serum protein concentration) also was significant as anticipated from the failure to see additional reductions in mortality risk at serum protein concentrations 26.0 g/dl as shown in Figure 1. However, no significant interaction was found between serum protein concentration and cohort mortality. Discussion In contrast to previous studies, data in this study were collected from a large population of calves over a period 40%, 1 30% >.I-.- m r 20% P 10% t (86-89,93) Total protein (G/dl) Fig 5. Relationship between serum protein concentration and mortality risk In high and low mortality years (using the model from Table 2)

4 82 Tyler et al Table 2. Results of a logistic regression model predicting mortality as a function of linear and quadratic effects of serum protein concentration, baseline cohort mortality, and interaction of serum protein concentration and baseline cohort mortality in 3,479 comingled dairy replacement heifers. Stan- Coeffi- dard Student's Predictor Variables cient Error f P Value Constant ,0000 Serum protein concentration Serum protein concentration squared ,0004 Cohort mortality status ,0000 Interaction ,4491 of several years. This prolonged data collection permitted enrollment of adequate numbers of calves to more precisely estimate the risk associated with different serum protein concentrations. The present study supports the conclusion that serum protein concentration as measured by refractometry in calves <1 week of age is associated with mortality risk of dairy heifer calves under a variety of baseline mortality conditions up to at least 10 weeks of age. Refractometry provides rapid and inexpensive test results, and as such is a useful tool for monitoring passive transfer status. Questions remain, however, about the value of refractometry compared to other measures of passive immune status and with regard to the boundary that should be used to designate adequate from inadequate passive transfer status. It could be argued that refractometry, as an indirect measure of immunoglobulin c~ncentration,]~ is inferior to other measures of passive transfer, such as immunoglobulin concentration measured by radial immunodiffusion. Studies that have compared the ability of glutaraldehyde gelation, sodium sulfite and zinc sulfate turbidity, and refractometry to predict serum immunoglobulin concentration as measured by radial immunodiffusion have demonstrated that refractometry is equivalent or superior to other available assay proced~res.~~~*.~~ Furthermore, if passive immunity is defined as the functional ability to resist naturally occurring infectious diseases, no direct measures of this ability exist. The choice as to which indirect tests to use must be made on the basis of technical ease and expense, on the degree to which results can be standardized, and on the degree of association each test has with functional outcomes such as mortality, morbidity, rearing costs, and future performance. With respect to technical ease and expense, refractometry is superior to radial immunodiffusion. On the other hand, refractometry has no clear technical or cost advantage over the semiquantitative zinc sulfate or sodium sulfite tests. However, refractometry is more easily standardized among operators and laboratories than are many semiquantitative methods. The present study did not compare the degree of association between mortality risk and test results for the various passive immunity tests. Such comparisons have been reported previousiy.'*j3 The present study demonstrates that serum protein concentration, as measured by refractometry, has 2 desirable features in its association with mortality risk. First, the relationship was found to be consistent. Serum protein concentration was negatively associated with mortality risk during every study year and periods of high and low base mortality and the magnitude of this association was similar under conditions of both low and high mortality. Second, the relationship was monotonic up to a moderately high serum protein concentration. For each increment in serum protein concentration (up to approximately 6.0 g/dl) there appeared to be a decrease in mortality risk. Such a monotonic relationship is required of a good indirect measure of functional passive immunity. The similarity of relative mortality risks in high and low mortality years, the absence of a significant interaction between baseline cohort mortality and serum protein concentration, and the absence of a significant decrease in mortality risk at serum protein concentrations >5.5 g/dl has direct bearing on our understanding of the relationship between passive transfer status and calf health. We hypothesize that on a given farm, management, environmental, and pathogen factors combine to determine the baseline mortality risk experienced by calves with optimal passive transfer status (>5.5 g/dl). Furthermore, this baseline mortality risk varies farm to farm. Calves with less than optimal passive transfer will experience an increased risk for mortality relative to the individual farm baseline mortality rate, but optimal passive transfer will not compensate for inadequate hygiene. The RR of mortality may be decreased by improving passive transfer, but farm baseline mortality rate will be affected by additional factors. Improved passive transfer will not compensate for inadequate hygiene. No specific boundary between adequate and inadequate passive immune status can reliably identify which calves will die and which calves will survive. The majority of calves in the lowest serum protein concentration category (<4.0 g/dl) survived, and, in low mortality years, nearly all these calves survived. However, deaths occurred even among calves having serum protein concentrations >5.5 g/dl. The observed effect was one of decreasing risk of death for each increment in serum protein concentration up to 5.5 g/dl. Defining 5.5 g/dl as the boundary between adequate and inadequate passive transfer is not possible based on the data from the farm evaluated in the present study. Previous studies have proposed a serum protein concentration of 5.5 g/dl as a boundary," but our data suggest this boundary probably is unreasonably high for dairy calves. More than 60% of the calves we studied had serum protein concentrations C5.5 g/dl. Unrealistic goals can have a negative impact on client compliance by engendering a sense of failure and futility. Given that the RR of mortality was much higher for calves in lowest serum protein strata (<4 g/dl, RR = 4.6; g/dl, RR = 3.1; and g/dl, RR = 2.2), it is probably best to reduce the number of calves in these categories. Only 34.5% of the study population had serum protein concentrations -6.0 g/dl and, during some study years ( ), substantially lower failure rates were observed. Consequently, a threshold serum protein concentration of 5.0 g/dl probably is a reasonable choice that would be associated with minimal mortality risk while setting client goals that are reasonable and attainable.

5 Mortality in Dairy Replacement Heifers 83 Selection of a single boundary is probably rendered less important by the manner in which we apply the results of passive transfer tests. The goal of most monitoring programs is not prediction of the fate of individual calves. Rather, it is to monitor the success of passive transfer management in individual herds or, as for the study farm, in the herds from which calves originate. In this regard, a summary herd measure is needed. As shown in Table 1, a general relationship exists between the percentage of calves having serum protein concentrations <4.5 g/dl, calves having serum protein concentrations G.0 g/dl, and calves having serum protein concentrations <5.5 g/dl during various years. Using such summary measures in a monitoring program, in conjunction with intervention when goals are not met, would be expected to result in improved passive transfer status in calves and thereby reduce overall mortality risk. Acknowledgments We thank the owners and staff of the Reuble Calf Ranch, Ellensburg, WA, for technical assistance. References 1. Naylor JM, Kronfeld DS. Refractometry as a measure of the immunoglobulin status of the newborn dairy calf Comparison with the zinc sulfate test and single radial immunodiffusion. J Am Vet Med Assoc 1977; 171: Perino LJ, Sutherland RL, Woollen NE. Serum gamma-glutamyltransferase activity and protein concentration at birth and after suckling in calves with adequate and inadequate passive transfer of immunoglobulin G. Am J Vet Res 1993;54: Tyler JW, Hancock DD, Parish SM, et al. Evaluation of 3 assays for failure of passive transfer in calves. J Vet Intern Med 1996;lO Blom JY. The relationship between serum immunoglobulin val- ues and incidence of respiratory disease and enteritis in calves. Nord Veterinaermed 1982;34:27& Besser TE, Gay CC. Colostral transfer of immunoglobulins to the calf. Vet Annu 1993;33: Besser TE, Gay CC. The importance of colostrum to the health of the neonatal calf. Vet Clin North Am Food Anim Pract 1994;lO Gay CC. The role of colostrum in managing calf health. Bovine Pract 1984;16: Hancock DD. Assessing efficiency of passive immune transfer in dairy herds. J Dairy Sci 1985;68: McEwan AD, Fischer EW, Selman IE. Observations on the immune globulin levels of neonatal calves and their relationship to disease. J Comp Pathol 1970;80: Tyler JW, Parish SM. Strategies to maximize the health of embryo transfer or exceptional calves, Compend Contin Educ Pract Vet 1995; 17: Carry F, Adams R, Aldridge B. Role of colostral transfer in neonatal calf management: Current concepts in diagnosis. Compend Contin Educ Pract Vet 1993;15: Barber DML. Serum immune globulin status of purchased calves: An unreliable guide to viability and performance. Vet Rec 1978;102: Rea DE, Tyler JW, Hancock DD, et al. Prediction of calf mortality by use of tests for passive transfer of colostral immunoglobulin. J Am Vet Med Assoc 1996;208: Rice DH, Besser TE, Hancock DD. Epidemiology and virulence assessment of Salmonella dublin. Vet Microbiol 1997;56: Dixon WJ. BMDP Statistical Software Manual. Los Angeles, CA: University of California Press; 1992: Afifi AA, Clark V. Computer-Aided Multivariate Analysis. Belmont, CA: Lifetime Learning Publications; 1984: Radostits OM, Blood DC, Gay CC. Veterinary Medicine, 8th ed. Philadelphia, PA: Bailliere Tindall; 1994: Parish SM, Tyler JW, Besser TE, et al. Prediction of serum IgG, concentration in Holstein calves using serum gamma-glutamyl-transferase activity. J Vet Intern Med. 1997;11: Tyler JW, Besser TE, Wilson L, et al. Evaluation of a whole blood glutaraldehyde coagulation test for in calves. J Vet Intern Med 1996; 10:82-84.