Salmonella organisms are an important cause of disease in humans and other animals. In dairy cattle, a variety of diseases has been attributed to Salmonella infections, including acute diarrhea in neonates, multi-systemic illness in adults, and invasive septicemia in animals of all ages.1 In some situations on commercial dairy operations, salmonellosis can adversely impact profitability and appropriate control measures are warranted. Moreover, salmonellosis (regardless of whether it is clinically apparent) may result in foodborne exposure of humans to Salmonella organisms through milk or via meat products from animals sent to slaughter because of poor productivity.1
The prevalence of Salmonella infection on dairy operations can be relatively high even without clinical evidence of salmonellosis. Investigators detected Salmonella spp on 30 of 96 (31.3%) dairy operations in a multistate project in which samples were collected at a single time point.2 In another study3 in which investigators used 129 dairies located in Minnesota, Wisconsin, Michigan, and New York, Salmonella organisms were recovered at least once from 120 (93.0%) operations. In that study,3 fecal and environmental samples were collected at each dairy at 2-month intervals during a 14-month period. Results of these studies illustrate that Salmonella spp are quite prevalent in dairy herds and that Salmonella organisms can be intermittently shed into the environment.
Control of Salmonella infection in dairy cattle could be important for animal health and productivity as well as for the impact on exposure of humans. Effective control of salmonellosis in commercial dairies involves sanitation; biosecurity; and, if possible, increasing the animal- and herd-level immunity to salmonellosis. Unfortunately, biosecurity is often less than ideal in modern, large-scale dairy production because frequent entry of replacement heifers or cows is common and because these animals may be a source of Salmonella organisms.4 Consequently, methods to improve immunity may be of more practical value than is biosecurity for control of Salmonella infections in large-scale dairy herds.
Some proteins on the outer surface of bacteria (such as Salmonella enterica) have antigenic properties and can be exploited through vaccine technology to induce effective immunity. The SRP proteins are critical for iron acquisition and are highly conserved among and within species of bacteria.5 A novel vaccine containing specific SRP proteins derived from Salmonella enterica serovar Newport has been conditionally approved to aid in the control of disease and fecal shedding caused by infection with Salmonella Newport. The premise for the vaccine is that inactivation of SRP proteins via antibody binding will restrict the ability of Salmonella Newport to acquire iron from the environment, which will place it at a disadvantage in the microbial consortia within the gastrointestinal tract. Although the authors are not aware of any peer-reviewed studies to support the effectiveness of this vaccine, many dairy producers have adopted the routine use of this vaccine to aid efforts to control salmonellosis.a Therefore, the objective of the study reported here was to determine the effects of vaccination with a Salmonella Newport SRP vaccine on milk production, SCC, and shedding of Salmonella organisms in female dairy cattle.
Materials and Methods
Animals—One hundred eighty Holstein cows and heifers at a 1,200-cow confinement dairy operation were enrolled in a prospective cohort study. This dairy had a history of salmonellosis (serotype not determined) in neonatal calves, but salmonellosis had not been reported in adult cattle. Approval for this study was obtained from the Kansas State University Institutional Animal Care and Use Committee (project No. 2454). Written consent for conducting this study and use of the trial data was obtained from the herd owner.
A convenience sample of nonlactating cows and pregnant heifers was enrolled in the study. These prepartum cattle were subsequently assigned to 2 groups in pairs, in which the first animal of a pair was assigned by a coin toss to one of the groups and then the other animal of the pair was automatically assigned to the other group. This process was repeated until all cattle had been assigned. Cattle in one of the groups were administered (SC injection) 2 mL of a commercially available Salmonella Newport SRP vaccine,b whereas cattle in the control group were administered 2 mL of a control solutionc (ie, vaccine adjuvant via SC injection). A second dose of vaccine and control solutions was administered 21 to 35 days after initial administration. The second dose was administered at least 14 to 21 days before anticipated parturition, which was in accordance with label directions for the commercially available vaccine. All cows and heifers that were purchased after initiation of the study and not enrolled in the study were vaccinated with the Salmonella Newport SRP vaccine at the time of arrival at the dairy. Newly purchased cows and heifers were commingled with cattle enrolled in the study in calving pens and pens for the various lactating cohorts (ie, arbitrarily designated as cohorts of early lactation, mid lactation, and late lactation). The dairy-unit manager and all dairy workers involved with management of the cattle were not aware of animal assignments for the study. The only person involved with the study who was aware of the animal assignments was the person who administered the injections of vaccine or control solution.
Procedures—Cattle enrolled in the study were managed in accordance with typical practices used on the dairy. They were housed in open-front, naturally ventilated buildings with 3-row free stalls filled with sand. Cattle were fed a total-mixed ration consisting of corn silage, alfalfa, wet corn gluten, milo, rolled corn, and a trace mineral and vitamin product. Bovine somatotropind was administered beginning at 70 days of lactation.
Daily milk production for each animal was monitored for the first 90 days of lactation by use of an electronic recording systeme that weighed milk on a continuous basis. At 45-day intervals, personnel from DHIA recorded the weight of milk produced by each animal. Estimates of milk production for a standardized 305-day lactation were made on the basis of at least 2 milk weight samples obtained at the dairy by DHIA personnel who used DHIA equipment.
Fecal samples were collected manually from the rectum of each cow or heifer on the day of first injection (approx 35 to 56 days before expected parturition) as well as at 7 to 14 days and 28 to 35 days of lactation. A new plastic sleeve was used for each sample collection. Fecal samples were placed in sterile collection bagsf and stored in coolers containing ice for transport to the Veterinary Diagnostic Laboratory at Kansas State University. Fecal samples were used for microbial culture, isolation of organisms, and serogroup determination. Laboratory personnel were not aware of treatment groups for any cattle in the study.
Once fecal samples arrived at the veterinary diagnostic laboratory, each sample was mixed separately until it was homogeneous. Then a swab specimen of each fecal sample was obtained and streaked onto plates containing blood agar, MacConkey agar, and Hektoen enteric agar. Plates were incubated at 37°C for 18 hours in an atmosphere of 5% carbon dioxide. Fecal swab specimens were also placed in Rappaport enrichment broth and incubated at 42°C for 24 hours. After incubation, a sample from the enrichment broth was streaked onto a plate containing Hektoen enteric agar and incubated at 37°C for 18 hours.
Samples of colonies that did not ferment lactose were collected from plates containing MacConkey agar and Hektoen enteric agar; these samples were streaked onto blood agar plates and incubated at 37°C for 18 hours in an atmosphere of 5% carbon dioxide. Isolates from the blood agar plates were then tested by use of spot oxidase and spot indole tests. When both of these tests had negative results, standard biochemical analysis of the isolates for confirmation of Salmonella spp was performed, and polyvalent Salmonella antisera were also used for testing. Biochemically confirmed, serogroup-positive Salmonella isolates were submitted to the National Veterinary Services Laboratory in Ames, Iowa, for serotyping.
Blood samples to determine antibody concentrations were collected via venipuncture of the coccygeal vein on the day of first injection and at 7 to 14 days and 28 to 35 days of lactation. Blood samples were centrifuged, and serum was harvested. Serum samples were submitted to a laboratoryg for determination of antibody response to vaccination. Personnel performing testing by use of an ELISA were not aware of group assignment for any of the cattle. Salmonella Newport–derived SRP antigen was coated on 96–well microtiter platesh at a concentration of 250 ng/well in carbonate-coating buffer (pH, 9.6); plates were then incubated at 4°C overnight. Plates were dried and blocked by the addition of 1% polyvinyl alcohol–PBS solution (200 μL/well); plates were then covered and incubated at 37°C for 2 hours. Two-fold dilutions (1:100 to 1:25,600) of serum samples were prepared in 1% polyvinyl alcohol–PBS solution. Samples were tested in duplicate. Plates were covered and incubated at 37°C for 1 hour. Plates were then washed 3 times with 0.05% PBS solution–Tween 20.i One hundred microliters of horseradish-conjugated sheep anti-bovine IgG (diluted 1:1,600 in 1% sheep serum–0.05% PBS solution–Tween 20) was added to each well. Plates were covered and incubated at 37°C for 1 hour. Plates were washed as described previously and developed by use of 100 μL of 2,2′ azino-di-3-ethyl-benzthiazoline-6-sulfonate. Optical absorbance was measured at 405 to 490 nm by use of an ELISA reader,j and sample-to-positive ratios were calculated.
Milk samples for use in measurement of SCC were collected on day 1 of lactation, between days 30 and 60 of lactation, and again between days 60 and 90 of lactation. Milk samples were transported to the DHIA laboratory, and SCC was determined by use of flow cytometry.k
Rectal temperature was monitored daily by trained personnel for the first 10 days after parturition. Cattle with a rectal temperature > 39.4°C on days on which the heat index was < 32.2°C and > 40°C on days on which the heat index was ≥ 32.2°C were considered febrile and treated in accordance with veterinary-directed protocols. Febrile cattle with no other clinical signs were classified as having fever of unknown origin. Metritis and mastitis were diagnosed in accordance with the definitions for the dairy's standard operating procedures and recorded by herd personnel.
Statistical analysis—Data were captured electronically, examined for errors, and then imported into a commercially available statistical software programl for analysis. Descriptive statistics were calculated and plotted in graphs or provided in tabular formats. Appropriate model diagnostics (eg, plots of Studentized residuals vs predicted mean) were used to aid model selection and statistical assumptions. Mixed-model methods were used to analyze data. Values of P < 0.05 were considered significant. First-order autoregressive covariance matrices were used to model within-animal dependency over time for the response variables daily milk production and Salmonella shedding (ie, variables appropriate for repeated-measures analysis).6 Continuous outcomes (daily milk production, estimated DHIA 305-day milk production, and SCC) and categoric responses (animal-level recovery of Salmonella organisms) were modeled by use of linear and logistic regression techniques, respectively. When appropriate, parity was forced into the models as a random variable and parity-adjusted least square means were computed. For repeated measures, the main effects of treatment and day and the treatment × day interaction were included in the models. When the interaction term was not significant, it was removed from the model and main effects were examined for significant effects.
Results
Of the 180 cattle enrolled in the study, 20 (11 control cattle and 9 vaccinates) were sold for poor production or disease or died prior to completion of the study. Cattle were culled when a terminal illness was diagnosed or milk production remained < 13.6 kg/d for 5 consecutive days with little chance for an increase. These decisions were made by personnel at the dairy. Data from an additional 7 cattle were insufficient to be included in the analysis because of multiple errors in identification or missing identification tags. Thus, sufficient data for analysis were available for 153 cattle.
Mean parity did not differ significantly between cattle vaccinated with the Salmonella Newport SRP vaccine (2.04 parturitions) and control cattle (2.02 parturitions). For the 78 cattle in the control group, 27 were in the first parity, 25 were in the second parity, 23 were in the third parity, and 3 were in the fourth parity or higher. For the 75 vaccinated cattle, 24 were in the first parity, 23 were in the second parity, 19 were in the third parity, and 9 were in the fourth parity or higher. Within each parity, there was no significant difference in the distribution between treatment groups.
No significant treatment × day interactions were detected. Therefore, only main effects were reported. Milk production varied on the basis of parity because daily milk production was lower in first-parity cattle than in multiparous cattle. Mean parity-adjusted average daily milk yield for the first 90 days of lactation was significantly (P = 0.01) greater in vaccinated cattle (40.4 kg/d), compared with mean average daily milk yield for control cattle (39.2 kg/d; pooled SEM, 9.9; Figure 1). However, the cumulative 90-day milk production did not differ significantly (P = 0.25) between treatment groups. Least square means for milk production were 3,472 and 3,569 kg for control and vaccinated cattle, respectively. Predicted DHIA 305-day milk production did not differ significantly (P = 0.27) between cattle in the 2 treatment groups (11,421 and 11,235 kg for vaccinates and control cattle, respectively).
Least squares mean ± SEM daily milk production for 75 cattle vaccinated with an initial dose and a booster dose of a Salmonella enterica serotype Newport SRP vaccine (white circles) and 78 control cattle injected with 2 doses of a control solution (black circles).
Citation: American Journal of Veterinary Research 69, 9; 10.2460/ajvr.69.9.1229
Prevalence of Salmonella-positive cattle at initiation of the study was 15.7% (24/153). Prevalence of Salmonella-positive cattle at initiation of the study did not differ significantly (P = 0.48) between vaccinates (10/75 [13.3%]) and control cattle (14/78 [17.9%]; Figure 2). All Salmonella isolates (n = 40) were identified as Salmonella enterica serotype Agona. There was no significant (P = 0.13) difference in prevalence of Salmonella recovery from vaccinated cattle, relative to the prevalence of recovery from control cattle, for any of the 3 fecal samples collected during the study. The percentage of cattle in both treatment groups that shed Salmonella organisms decreased significantly (P = 0.01) from 15% on the day of initial sample collection to 4% at days 7 to 14 of lactation. At days 28 to 35 of lactation, there was a 2-fold increase in the prevalence of Salmonella shedding in feces of control cattle, compared with the prevalence for vaccinated cattle; however, the prevalence of shedding at days 28 to 35 of lactation did not differ significantly (P = 0.25) between treatment groups.
Recovery of Salmonella enterica serotype Agona in fecal samples collected from 75 cattle vaccinated with a Salmonella Newport SRP vaccine (white circles) and 78 control cattle injected with a control solution (black circles). The first samples were collected at the time of injection of the first dose of vaccine or control solution (approx 35 to 56 days before parturition), whereas the second and third samples were collected at 7 to 14 days and 28 to 35 days of lactation, respectively.
Citation: American Journal of Veterinary Research 69, 9; 10.2460/ajvr.69.9.1229
Circulating antibody titers were similar between cattle of both treatment groups at the initiation of the study. Cattle administered the Salmonella Newport SRP vaccine had significantly (P = 0.01) higher concentrations of circulating antibodies at days 7 to 14 and days 28 to 35 of lactation, compared with antibody titers in control cattle at those same time points.
The SCC measured in milk samples collected at 30 to 60 days of lactation was significantly (P = 0.01) lower for cattle vaccinated with Salmonella Newport SRP vaccine, compared with the SCC for milk samples collected from control cattle (Table 1). The SCC did not differ significantly between treatment groups at any other time during the study.
Mean SCC in milk samples obtained from 75 cattle vaccinated with a Salmonella enterica serotype Newport SRP vaccine and 78 control cattle injected with a control solution.
| Sample collection (day of lactation) | Vaccinated cattle (× 1,000 cells/mL) | Control cattle (× 1,000 cells/mL) | Pooled SEM (× 1,000 cells/mL) | P value |
|---|---|---|---|---|
| 1 | 1,853 | 2,106 | 366 | 0.61 |
| 30–60 | 179 | 439 | 76 | 0.01 |
| 60–90 | 250 | 304 | 64 | 0.54 |
We did not detect significant differences in the proportion of cattle with a fever of unknown origin (16/75 [21.3%] and 14/78 [18.0%] for vaccinates and control cattle, respectively; P = 0.63), mastitis (14/75 [18.7%] and 17/78 [21.8%] for vaccinates and control cattle, respectively; P = 0.70), or metritis (16/75 [21.3%] and 15/78 [19.2%] for vaccinates and control cattle, respectively; P = 0.78).
Discussion
To our knowledge, the study reported here is the first to provide evidence that vaccinating cattle with Salmonella Newport SRP vaccine improves milk production in a herd without clinical signs of salmonellosis. The interactions between general health, disease, and milk production are complicated, multifactorial, and not completely understood. Total milk production is directly influenced by general health and mammary gland health,7 which can be correlated to SCC. In our study, vaccinated cattle produced more milk (1.14 kg/d) during the first 90 days of lactation, compared with milk production for the control cattle. There was not a significant treatment × day interaction in this study for milk production. However, the difference in milk production between the 2 treatment groups increased once they were at 30 or more days of lactation (Figure 1). The numeric difference of 90.9 kg of milk/animal between vaccinates and control cattle during the first 90 days of lactation was not significant. However, we believe it to be biologically relevant and of clinical importance. Furthermore, analysis of cumulative 90-day milk production likely led to a type II error in that variation attributable to time was not partitioned from residual error. Repeated-measures analysis is the most appropriate analysis for the type of data included in this study because we conducted repeated observations over time on an experimental unit.6 Because analysis of cumulative production is an inferior approach to the repeatedmeasures analysis, we place little value in it.
Cattle vaccinated with Salmonella Newport SRP vaccine had a lower SCC at 30 to 60 days of lactation, compared with results for cattle that received the control solution. Although the SCC differed significantly only at 30 to 60 days of lactation, the SCC was numerically lower for the vaccinates than for the control cattle at all time points in the study at which SCC was measured. Milk production decreases as SCC increases in dairy cows.8,9 Somatic cell count is directly related to mammary gland infection and has been associated with milk production of cows and the value of milk produced.9 Somatic cell count can be used as an indicator for subclinical mastitis. The day of lactation when a difference in SCC was detected was similar to the time point at which vaccinated cattle began to outperform the control cattle. Also, it is possible with the highly conserved nature of SRP protein fractions that cross-protection against other environmental gramnegative bacteria (such as Escherichia coli) could have been achieved with the SRP antigens from Salmonella spp.
The study reported here did not reveal differences in Salmonella shedding in the feces between cattle in the 2 treatment groups. However, Salmonella shedding in both treatment groups significantly decreased, despite the fact it was a warmer time of year. These results could be attributed to administration of a highly effective vaccine. Vaccination of half of the population with an efficacious vaccine could theoretically induce immunity in the entire herd and affect the ecologic aspects of Salmonella infection on a dairy by decreasing challenge exposure of all cattle, regardless of vaccination status of individual cattle. Shedding of Salmonella organisms into the environment creates a larger infectious dose for animals housed within the same facility.10,11 Also, the dairy at which our study was conducted was attempting to increase its number of cattle. All newly purchased cows and heifers were vaccinated with Salmonella Newport SRP vaccine at the time of arrival after initiation of the study. The new cattle were commingled with cattle enrolled in the study in calving pens and pens for the various lactating groups. Therefore, the only cows and heifers in the calving pens and pens for the various lactating groups during the 90-day study not vaccinated with the Salmonella vaccine were those cattle assigned to the control group. Finally, it is possible there was a decrease in Salmonella shedding in the herd at the time of this study that was independent of the vaccine effects.
Salmonella infection and clinical signs of disease are not synonymous. Disease (clinical or subclinical) is dependent on innate resistance, infectious dose, and infectivity or virulence of the organism.1 Salmonellosis in all animals (including humans) is more frequent during summer than during other times of the year. This is probably because the agent is able to replicate and survive longer outside hosts in the environment at summer temperatures.12,13 The study progressed into June and July (when most samples were collected), and the ambient temperature increased. Environmental contamination and minimizing the exposure dose are listed as major control points in the propagation of Salmonella organisms within a herd.1 Therefore, vaccination of the entire herd, except for the nonvaccinated control cattle, could have decreased environmental contamination sufficiently in our study to reduce exposure and subsequent shedding in the nonvaccinated cattle despite the progression into the warmer months of the year.
The only Salmonella organism identified in this study was Salmonella Agona. Serologic analysis revealed a significant antibody response to Salmonella Newport SRP vaccine in the vaccinated cattle, compared with the antibody concentrations in nonvaccinated control cattle. Reports of studies of vaccine effects on crossprotection among Salmonella serogroups or serotypes are rare. To our knowledge, only 1 controlled study14 in adult cows has involved the use of a modified-live Salmonella vaccine. In that study, no reductions were detected for serogroups other than the vaccine serogroup, as measured by a reduction in fecal shedding of organisms. Specific cross-protection in our study was not possible because only 1 Salmonella serotype was isolated and there was no difference in prevalence between treatment groups. Cross-protection between Salmonella Newport SRP contained in the vaccine and Salmonella Agona cultured from the dairy cattle may have influenced milk production in the vaccinates.
Milk production was significantly less for firstparity cattle than for multiparous cattle. This is similar to results in other reports.15,16 Production measures are significantly affected by parity.16 Therefore, effects of the Salmonella Newport SRP vaccine were analyzed on a within-parity basis. We did not detect treatment × parity interactions, which indicated that within a specific parity, all cattle had similar responses, regardless of vaccination status.
The study reported here has some limitations. First, the study was conducted until only 90 days of lactation. It appeared that the difference between treatments was more pronounced at the end of the 90-day period than during the early part of lactation (Figure 1). A study examining milk production during the entire lactation period would be warranted. Another limitation was that a high percentage of the cattle in the calving pen and in the early-lactation pen was vaccinated against Salmonella organisms because of the fact that dairy personnel began vaccinating cattle newly introduced to the herd during the study; none of those cattle were enrolled in the study. A study in which a similar number of vaccinated and nonvaccinated cows or heifers are placed in an environment with nonvaccinated cattle may be warranted. It is difficult to create an environment in a commercial setting that would replicate the effects of vaccination of all cows without biasing the results for one of the treatment groups. Finally, it would be interesting to evaluate concentrations of antibodies against Salmonella Newport in calves fed colostrum from vaccinated cattle. Salmonella outbreaks in calves could be a source of contamination for the remainder of the dairy cow population.
In this study, vaccinating dairy cows with Salmonella Newport SRP vaccine resulted in higher milk production for the first 90 days of lactation (1.14 kg/d), compared with milk production for nonvaccinated control cows. Vaccination with SRP antigens decreased the SCC and resulted in a positive serologic antibody response. A significant increase in milk yield may lead to improved profitability of dairies.
ABBREVIATIONS
| DHIA | Dairy Herd Improvement Association |
| SCC | Somatic cell count |
| SRP | Siderophore receptor and porin |
Sandstrom JD, Epitopix LLC, Willmar, Minn: Personal communication, 2007.
Salmonella Newport bacterial extract, Conditional license, Siderophore receptors and porins, provided by Agrilabs Inc, St Joseph, Mo.
Provided by Agrilabs Inc, St Joseph, Mo.
Posilac, Monsanto Co, St Louis, Mo.
FloMaster Pro, Alfa Laval Agri, Kansas City, Mo.
Whirl-Pak, Nasco, Fort Atkinson, Wis.
Epitopix LLC, Willmar, Minn.
Immulon II HB coated 96-well plates, Dynatech Corp, Chantilly, Va.
Tween 20, Sigma Chemical Co, St Louis, Mo.
BioTek Elx-800, BioTek Instruments Inc, Winooski, Vt.
Somacount 500, Bentley Instruments, Chaska, Minn.
SAS, version 9.1.3, SAS Institute Inc, Cary, NC.
References
- 1.↑
Gay JR. Bovine herd salmonellosis. Available at: www.vetmed.wsu.edu/courses-jmgay/FDIUHerdSalmonella.htm. Accessed May 21, 2008.
- 2.↑
USDA. APHIS. Salmonella and Campylobacter on US dairy operations, 2003. Available at: www.aphis.usda.gov/vs/ceah/ncahs/nahms/dairy/dairy02/Dairy02SalCampy.pdf. Accessed May 21, 2008.
- 3.↑
Fossler CP, Wells SJ, Kaneene JB, et al. Prevalence of Salmonella spp on conventional and organic dairy farms. J Am Vet Med Assoc 2004;225:567–573.
- 4.↑
Emanuelson U, Oltenacu PA. Incidences and effects of diseases on the performance of Swedish dairy herds stratified by production. J Dairy Sci 1998;81:2376–2382.
- 5.↑
Bullen DJ, Griffiths E. Iron binding proteins and host defense In: Bullen DJ, Griffiths E, eds. Iron and infection: molecular, physiological, and clinical aspects. 2nd ed. New York: John Wiley & Sons, 1999;327–368.
- 6.↑
Littell RC, Henry PR, Ammerman CB. Statistical analysis of repeated measures data using SAS procedures. J Anim Sci 1998;76:1216–1231.
- 7.↑
Gröhn YT, Bruss ML. Effect of diseases, production, and season on traumatic reticuloperitonitis and ruminal acidosis in dairy cattle. J Dairy Sci 1990;73:2355–2363.
- 8.
Miller RH, Norman HD, Wiggans GR, et al. Relationship of testday somatic cell score with test-day and lactation milk yields. J Dairy Sci 2004;87:2299–2306.
- 9.↑
Raubertas RF, Shook GE. Relationship between lactation measures of somatic cell concentration and milk yield. J Dairy Sci 1982;65:419–425.
- 10.
Huston CL, Wittum TE, Love BC, et al. Prevalence of fecal shedding of Salmonella spp in dairy herds. J Am Vet Med Assoc 2002;220:645–649.
- 11.
Potter AA, Klashinsky S, Li Y, et al. Decreased shedding of Escherichia coli O157:H7 by cattle following vaccination with type III secreted proteins. Vaccine 2004;22:362–369.
- 12.
Wells SJ, Fedorka-Cray PJ, Dargatz DA, et al. Fecal shedding of Salmonella spp. by dairy cows on farm and at cull cow markets. J Food Prot 2001;64:3–11.
- 13.
Gay JR. Epidemiology concepts for disease in animal groups. Available at: www.vetmed.wsu.edu/courses-jmgay/EpiMod2.htm. Accessed May, 21, 2008.
- 14.↑
House JK, Ontiveros MM, Blackmer NM, et al. Evaluation of an autogenous Salmonella bacterin and a modified live Salmonella serotype Choleraesuis vaccine on a commercial dairy farm. Am J Vet Res 2001;62:1897–1902.
- 15.
Laevens H, Deluyker H, Schukken YH, et al. Influence of parity and stage of lactation on the somatic cell count in bacteriologically negative dairy cows. J Dairy Sci 1997;80:3219–3226.
- 16.↑
Wagter LC, Mallard BA, Wilkie BN, et al. The relationship between milk production and antibody response to ovalbumin during the peripartum period. J Dairy Sci 2003;86:169–173.
