Prevalence of Neospora caninum and persistent infection with bovine viral diarrhea virus in dairy-breed steers in a feedlot

Bruce R. Hoar Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95616

Search for other papers by Bruce R. Hoar in
Current site
Google Scholar
PubMed
Close
 DVM, PhD
,
Alana C. McQuarry Country Veterinary Clinic, 4839 E Butte Rd, Live Oak, CA 95953

Search for other papers by Alana C. McQuarry in
Current site
Google Scholar
PubMed
Close
 DVM, MPVM
, and
Sharon K. Hietala Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95616

Search for other papers by Sharon K. Hietala in
Current site
Google Scholar
PubMed
Close
 PhD

Click on author name to view affiliation information

Abstract

Objective—To determine the prevalence and effect of Neospora caninum infection and persistent infection (PI) with bovine viral diarrhea virus (BVDV) on weight gain, morbidity, and mortality rate in dairy-breed steer calves located on a feedlot in California.

Design—Prospective cohort observational study.

Animals—900 dairy-breed steer calves in 2 pens.

Procedures—The 3- to 4-month-old calves were evaluated for serum antibodies against N caninum and infection with BVDV at entry to the feedlot. Five months later, sera were again analyzed for anti–N caninum antibodies; calves that were determined to have BVDV infection initially were retested to evaluate PI status. Average daily gain, morbidity, and deaths were recorded for all calves.

Results—Among 900 calves, prevalence of N caninum infection was 16.7% (95% confidence interval, 14.3% to 19.3%); prevalence of BVDV-associated PI was 0.2% (95% confidence interval, 0.03% to 0.9%). Morbidity rate and time to first illness were not significantly different between calves that were seropositive or seronegative for N caninum.Atthe second sample collection, weight and average daily gain of calves that were seropositive for N caninum was less than that of seronegative steers in 1 pen, whereas these measures did not differ between groups in the other pen. Statistical power was insufficient to evaluate the effect of BVDV PI on any outcome measurement.

Conclusions and Clinical Relevance—Although N caninum serostatus had no significant effect on morbidity rate, some seropositive calves had reduced growth, compared with seronegative calves, 5 months after entry to the feedlot.

Abstract

Objective—To determine the prevalence and effect of Neospora caninum infection and persistent infection (PI) with bovine viral diarrhea virus (BVDV) on weight gain, morbidity, and mortality rate in dairy-breed steer calves located on a feedlot in California.

Design—Prospective cohort observational study.

Animals—900 dairy-breed steer calves in 2 pens.

Procedures—The 3- to 4-month-old calves were evaluated for serum antibodies against N caninum and infection with BVDV at entry to the feedlot. Five months later, sera were again analyzed for anti–N caninum antibodies; calves that were determined to have BVDV infection initially were retested to evaluate PI status. Average daily gain, morbidity, and deaths were recorded for all calves.

Results—Among 900 calves, prevalence of N caninum infection was 16.7% (95% confidence interval, 14.3% to 19.3%); prevalence of BVDV-associated PI was 0.2% (95% confidence interval, 0.03% to 0.9%). Morbidity rate and time to first illness were not significantly different between calves that were seropositive or seronegative for N caninum.Atthe second sample collection, weight and average daily gain of calves that were seropositive for N caninum was less than that of seronegative steers in 1 pen, whereas these measures did not differ between groups in the other pen. Statistical power was insufficient to evaluate the effect of BVDV PI on any outcome measurement.

Conclusions and Clinical Relevance—Although N caninum serostatus had no significant effect on morbidity rate, some seropositive calves had reduced growth, compared with seronegative calves, 5 months after entry to the feedlot.

Since its first description in 1984, Neospora caninum has become recognized worldwide as an important cause of abortion in both dairy and beef cattle.1 Many of the early descriptions of the disease caused by this organism involved dairy cattle in California.2–5

The effect of congenital infection on calves is not clearly established. Infected live-born calves can have clinical signs that range from paralysis and musculoskeletal stunting to mild neurologic deficits. However, calves infected in utero typically do not have apparent clinical signs. As many as 95% of calves born to cows with serum antibodies against N caninum are clinically normal, yet yield positive results for infection when evaluated.6 Infected heifer calves will subsequently pass the infection on to most of their offspring. Infection in calves is thought to be chronic and persistent.

The potential for subclinical disease is of great importance to producers, particularly if growth, performance, or health is adversely affected, because economic returns may be impacted. Reports7–9 of the effect of N caninum infection on growth and weight gain in feedlot animals vary. In 1 study7 of 1,009 beef-breed steer calves in Texas, N caninum infection had a negative impact on weight gain, carcass measurements, and economic return. A study8 of 1,976 beef feedlot steers from Alberta, Canada revealed no significant association between serologic status for N caninum and risk of treatment, feedlot entry weight, or average daily gain. In another investigation,9 667 animals were examined in 2 feedlots in Iowa and no significant association between serologic status and morbidity rate, mortality rate, production variables, or carcass measurements was identified.

Given that dairies in California are large (in 2004, mean number of cows per dairy was 825) and that seroprevalence increases with increased cattle density,10–12 we were interested in undertaking a study to determine the prevalence and effect of N caninum infection on weight gain, morbidity, and mortality rate in dairy-breed steer calves located on a feedlot in California. Also, the prevalence of PI with BVDV in those calves was investigated.

Materials and Methods

The study protocol was reviewed and approved by the University of California-Davis Animal Care and Use Committee. A prospective cohort observational study was carried out on a feedlot in central California in fall 2004; informed consent from the owner of the animals was obtained prior to starting the study. The 900 dairy-breed steer and bull calves enrolled in the study arrived at the feedlot in 2 shipments of 450 calves on 2 days (21-day interval between shipments). Calves originated from a single calf ranch, which bought day-old bull calves from several local dairies. At entry into the feedlot, the calves were 3 to 4 months old. A 10-mL blood sample was collected (by BRH or ACM) from each calf at that time; after clotting, blood samples were centrifuged on-site to provide serum for analysis. Calves were individually identified, weighed, and processed according to a standard feedlot protocol, which included castrating bulls (3 calves arrived as bulls; the others had been castrated at the calf ranch) and vaccinating all calves against infectious bovine rhinotracheitis virus, BVDV, bovine respiratory syncytial virus, parainfluenza-3 virus, Mannheimia haemolytica, and Clostridia spp. Each group of 450 calves remained together in a single feedlot pen. Only study calves were kept in these 2 pens, which were adjacent to each other. Other groups of cattle had fenceline contact on either side of the study pens. A second 10-mL blood sample was obtained (by BRH or ACM) from all surviving calves at 1 time point, which was either 149 or 170 days after entry to the feedlot. At this time, calves were weighed, allowing for calculation of individual average daily gain (second weight minus initial weight divided by number of days that the calf received feed).

Serum samples were stored and transported on ice and were submitted to the California Animal Health and Food Safety Laboratory, Davis branch within 6 hours of collection. Sera were assessed for antibodies against N caninum the following day by use of a kinetic ELISA.13 The S:P ratio was reported for all samples. Briefly, the optical density of the sample is compared to that of the positive control sample, and a ratio is created. A ratio greater than a predefined cutoff is considered positive. Bovine viral diarrhea virus infection was detected by use of a commercially available antigen capture ELISA.a Calves that had BVDV infection at entry to the feedlot were retested at the second visit to confirm that calves had PI with BVDV.

Calf health was monitored daily by feedlot personnel. Calves suspected of being ill were moved to an animal handling facility and treated according to a standard protocol. Calves that died underwent a postmortem examination, and cause of death was assigned. The disease status of a calf was defined as chronic if it was treated ≥ 3 times for disease on separate occasions. Individual health records were maintained, including diagnosis, treatments, and outcome; only selected data were analyzed in this study.

Statistical analysis was performed by use of commercially available software.b,c Prevalence of infection was analyzed separately by agent and by pen. Prevalence of illness and death for calves with positive or negative test results was calculated by pen. Differences in initial weight, final weight, and average daily gain between calves with positive and negative test results were compared on a pen-level basis with a 2-sided t test. The association between infection status and morbidity events and between infection status and mortality rate were examined by use of a stratified 2 X 2 table. If the P value for the homogeneity of odds ratio test was > 0.05, a common odds ratio across pens was calculated; otherwise, separate results for each pen were presented. A Cox regression analysis was performed to determine differences in time to first morbidity event between calves with positive and negative test results (with pen as a covariate). For all comparisons, P < 0.05 was considered significant.

Results

The proportion of cattle that were seropositive for N caninum at entry to the feedlot for pens 1 and 2 was 16.9% and 16.4%, respectively. There was no significant difference in seroprevalence between the 2 pens, so results were combined to give an overall seroprevalence of 16.7% (150/900 calves; 95% CI, 14.3% to 19.3%). At the second sample collection approximately 5 months later, seroprevalence was 15.9% (133/836 calves; 95% CI, 13.5% to 18.6%; Table 1). Again, there was no significant difference in seroprevalence between pens. The distribution of S:P ratios for all samples was calculated (an S:P ratio > 0.45 is reported as positive for N caninum13; Figure 1).

Table 1—

Number of dairy-breed steer calves kept in 2 pens at a feedlot in California that did or did not have antibodies against Neospora caninum in serum samples obtained at entry to the feedlot and after an interval of approximately 5 months.

PenInitial sampleSecond sample*
NegativePositiveNot knownTotal
1Negative3302222374
Positive1849976
Total3487131450
2Negative337732376
Positive1855174
Total3556233450
1 and 2 (combined data)Negative6672954750
Positive3610410150
Total70313364900

Second sample was obtained at 149 or 170 days after collection of the initial sample at time of entry to the feedlot.

Sixty-four calves died or were shipped for slaughter before the second sample was obtained.

Figure 1—
Figure 1—

Distribution of S:P ratios for serum antibody against Neospora caninum among 900 dairy-breed steer calves at entry (gray bars) to a California feedlot and approximately 5 months later (black bars).

Citation: Journal of the American Veterinary Medical Association 230, 7; 10.2460/javma.230.7.1038

The effect of serologic status for N caninum on calf health variables was assessed (Table 2). Calves in pen 1 were 4.5 (95% CI, 2.6 to 7.9) times as likely to be ill at arrival, 1.7 (95% CI, 1.3 to 2.2) times as likely to be treated after arrival, and 2.8 (95% CI, 1.0 to 7.9) times as likely to develop chronic disease as calves in pen 2. There was no difference in overall mortality rate between pens (odds ratio, 1.2; 95% CI, 0.7 to 2.1). After stratifying by pen, there was no significant difference between seronegative and seropositive calves with respect to detectable illness on arrival, treatment after arrival, or overall chronicity; therefore, common odds ratios were calculated for these variables. However, in pen 1, the overall mortality rate was significantly greater for seropositive calves than it was for seronegative calves, whereas in pen 2, the converse was true.

Table 2—

Association between health variables and serologic status for N caninum in 900 dairy-breed steer calves kept in 2 pens at a feedlot in California, stratified by pen (common odds ratios presented where appropriate).

PenVariableProportion of seronegative calves (%)Proportion of seropositive calves (%)Odds ratio (95% CI)
1Sick on arrival54/374 (14.4)10/76 (13.2)0.9 (0.4–1.9)
Sick after arrival139/320 (43.4)31/66 (47.0)0.9 (0.5–1.5)
Overall chronicity11/365 (3.0)3/70 (4.3)1.4 (0.4–5.3)
Overall deaths20/374 (5.3)9/76 (11.8)2.4 (1.0–5.4)
2Sick on arrival13/376 (3.5)3/74 (4.1)1.2 (0.3–4.2)
 Sick after arrival119/363 (32.8)20/71 (28.2)1.2 (0.7–2.2)
 Overall chronicity5/358 (1.4)0/73 (0)NA (0.0–5.3)
 Overall deaths23/376 (6.1)1/74 (1.4)0.1 (0.01–0.7)
1 and 2 (combined data)Sick on arrival67/750 (8.9)13/150 (8.7)1.0 (0.5–1.8)
Sick after arrival258/683 (37.7)51/137 (37.2)1.0 (0.7–1.5)
Overall chronicity16/723 (2.2)3/143 (2.1)1.0 (0.3–3.4)

NA = Not applicable.

Time to first detected morbidity event was evaluated via Cox regression analysis (Figure 2). There was a significant (P < 0.001) pen effect. The hazard ratio for pen was 1.8 (95% CI, 1.5 to 2.3), indicating that at any point in time, the risk of a calf having a morbidity event was 1.8 times as great for calves in pen 1 as it was for calves in pen 2. No significant differences in time to first morbidity event were evident between seropositive and seronegative calves in either pen.

Figure 2—
Figure 2—

Cox regression survival curves for days until first treatment for 2 pens of dairy-breed steers (450 calves each) that were seropositive or seronegative for N caninum.

Citation: Journal of the American Veterinary Medical Association 230, 7; 10.2460/javma.230.7.1038

The effect of serologic status on weight and average daily gain was dependent on pen (Table 3). In pen 2, there was no significant difference in initial weight, final weight, or average daily gain between seropositive and seronegative calves. However, in pen 1, seronegative calves weighed 12.8 kg (28.2 lb) more at the second sample collection (P = 0.005) and had a 0.05 kg/d (0.1 lb/d) advantage in average daily gain (P = 0.02), compared with seropositive calves.

Table 3—

Mean ± SD incoming and final weights (kg) and average daily gain (kg/d) for dairy-breed steer calves from a California feedlot by serologic status to N caninum and by pen. To convert kilograms to pounds, use a factor of 2.2.

PenVariableseronegative calves (n)seropositive calves (n)P value
1Initial weight127.1 ± 14.4 (374)123.7 ± 14.3 (76)0.06
Final weight367.5 ± 34.2 (352)354.7 ± 32.0 (67)0.005
Average daily gain1.41 ± 0.18 (352)1.36 ± 0.16 (67)0.02
2Initial weight123.8 ± 11.6 (376)125.2 ± 11.3 (74)0.32
Final weight333.0 ± 36.4 (344)333.0 ± 35.1 (73)0.99
Average daily gain1.40 ± 0.21 (344)1.40 ± 0.21 (73)0.82

Results of the antigen capture ELISA for BVDV were positive for 3 calves at entry to the feedlot (0.3%; 95% CI, 0.07% to 1.0%): 2 were in pen 1, and 1 was in pen 2. All 3 calves did not have serum anti–N caninum antibodies. One of the BVDV-positive calves from pen 1 died (cause of death reported as liver abscesses) 2 months after entry to the feedlot. The other 2 BVDV-positive calves remained healthy throughout the trial, and both had positive BVDV test results again at the second sample collection, confirming PI with BVDV (BVDV PI prevalence, 0.2% [2/836 calves]; 95% CI, 0.03% to 0.9%). These 2 calves had average daily gains of 1.16 kg/d (2.5 lb/d) and 0.79 kg/d (1.7 lb/d).

Discussion

Results of the present study have indicated that the prevalence of N caninum infection among dairy-breed steer calves in a feedlot in California was greater than that reported from beef-breed steer calves in Texas, Alberta, and Iowa7–9; in those regions, seroprevalence of N caninum infection at entry to the feedlot was 13%,7 6.5%,8 and 5.9%,9 respectively. We were not able to determine the source of calves in our study; however, the calf ranch that provides calves to the feedlot is located in an area of California where the mean number of milking cows per herd is > 1,000. Those dairies have a drylot design in which feed is provided in bunks adjacent to the pens. Such animal density may provide greater opportunity for horizontal transmission of N caninum through contaminated feed or environment.

No relationship between serologic status for N caninum at entry to the feedlot and risk of illness on arrival, receiving treatment after arrival, or overall disease chronicity was detected in calves of the present study. This is in agreement with results of other feedlot studies.7–9 Marked differences between pens with regard to risk of treatment and risk of chronicity were evident. Compared with pen 2, pen 1 had significantly more calves that required treatment (either at or after arrival) and had more calves that were culled because of chronic illness. Morbidity rates can vary widely among pens of feedlot cattle (from 15% to ≥ 45%), depending on exposure to viral and bacterial infectious agents, intrinsic host defenses, and environmental stressors.14 It is possible that the calves in pen 1 had either greater exposure or increased stress during the immediate period before arrival to the feedlot, which led to increased illness in this group.

In the present study, the effect of serologic status for N caninum on growth rates was dependent on pen. Although seropositive calves in pen 1 weighed less at the second sample collection and had lower average daily gain than seronegative animals, there was no difference in weight or average daily gain between seropositive and seronegative calves in pen 2. These differences between pens may be associated with their significantly different rates of morbidity. Other researchers have hypothesized that stressful conditions, such as weaning or respiratory tract illness, may result in dissemination of N caninum tachyzoites to various body systems, resulting in the development of inflammation and impairment of body function.7 This could help explain the finding that the pen with higher morbidity rate had reduced growth in seropositive calves, even though there was no within-pen difference in morbidity rate between seropositive and seronegative calves. This is also consistent with the finding that the risk of death for seropositive calves in pen 1 was 2.4 times as great as that for seronegative calves in pen 1.

It is interesting to compare results of our study with other investigations7,8 that have examined the effect of infection with N caninum on calf growth rates. Compared with seronegative calves, a reduction in average daily gain was detected in seropositive calves in a study7 of 1,009 calves from Texas (0.05 kg/d less for seropositive calves), as well as in a Canadian study8 of 1,976 calves (0.04 kg/d [0.09 lb/d] less for seropositive calves), although the latter reduction was not significant. In the present study, there was a significant reduction in average daily gain of 0.05 kg/d in seropositive calves in 1 of 2 pens. Given the similarity in numerical difference in average daily gain in all 3 studies, it would appear that N caninum has a negative impact on growth in feedlot cattle. Further studies to investigate this relationship are warranted, particularly in pens with high morbidity rates. The study7 performed in Texas revealed that seropositive calves were associated with a loss in income because of lighter hot carcass weight and increased cost of treatment ($15.62/calf), compared with seronegative calves.

By use of an S:P value of 0.45 as a cutoff point, it was determined that 29 seronegative calves became seropositive during the interval between sample collections and that a crude incidence of seroconversion was 8.3/100 calf-years at risk. The California Animal Health and Food Safety Laboratory reports results of N caninum testing as probabilities of infection.15 An S:P ratio between 0.20 and 0.45 is consistent with a 9% probability of infection, and the result is interpreted as negative. An S:P ratio > 0.45 but ≤ 0.70 corresponds to a 68% probability of infection, and the result is interpreted as positive. An S:P ratio > 0.70 is consistent with a 100% probability of infection, and the result is also interpreted as positive. Of the 29 calves that seroconverted, 23 had values that were between 0.45 and 0.70 and 6 had S:P ratios > 0.70 at the time of the second sample collection. If the conservative value of 6 seroconversions is considered, then the crude incidence of seroconversion becomes 1.7/100 calf-years at risk. A study6 of dairy heifers in the United Kingdom revealed an overall incidence of seroconversion of 1.9/100 dairy heifer-years at risk, whereas a study16 in California revealed an incidence of seroconversion of 8.5/100 pregnant dairy cows/y. Results of another study17 of 2 dairies in California indicated that the estimated postnatal infection rate was < 1%/y. Thus, it appears that seroconversion may be occurring at a low level during all phases of animal growth.

The changes in S:P ratios identified in the present study may indicate false-positive reactions, true sero-conversion, or transient serologic responses. The kinetic ELISA has a stated sensitivity of 88% and specificity of 97% when a value of 0.45 is used as the cutoff for interpretation based on a seroprevalence of 40%13; therefore, some of the results may represent false-positive reactions. By assuming that sensitivity and specificity are constant at various levels of prevalence, it is possible to calculate the estimated true prevalence in the study sample as 16.1%, the predictive value of a positive test is 0.849, and the predictive value of a negative test is 0.977. On the basis of these calculations, 84.9% of calves that have positive test results are truly infected and 97.7% of calves that have negative test results are truly not infected; therefore, it is likely that there were several false-positive reactions as well as a few false-negative reactions in our study. If the calves truly seroconverted as a result of postnatal acquisition of infection, the source of oocysts is uncertain. Transient serologic response to N caninum in beef steers has been previously reported18; in that study, a significant impairment in feed efficiency was detected following a positive serologic result.

Conversely, 36 of 140 (25.7%) calves that were seropositive for N caninum initially were seronegative at the second sample collection. These results may represent loss of colostral antibodies, false-negative results, or transient serologic responses. Calves in our study entered the feedlot at approximately 3 to 4 months of age. Therefore, some of the initial positive test results for N caninum could have occurred because of the acquisition of antibodies through ingestion of colostrum. Decay of anti–N caninum antibody concentration follows first-order decay kinetics with a mean half-life of 19.6 ± 5.2 days.17 Colostral antibody concentration decreases to negative S:P ratios by 128 days in 100% of calves and by 33 days in 50% of calves,16 making it unlikely that most of the conversions from seropositive to sero-negative were attributable to loss of colostral antibody. Given the imperfect specificity of the N caninum kinetic ELISA, a small number of false-negative results is possible. As stated previously, transient serologic responses have been reported,18 which may also help explain some of the seroconversions among calves in our study.

Persistent infection with BVDV was found at a low prevalence (0.2%) in the calves of our study. This is consistent with findings of other feedlot studies9,19–21; PI was detected in 0.3% of 667 calves, 0.3% of 2,000 calves, 0.2% of 5,041 calves, and 0.4% of 21,743 calves. These data appear to indicate that PI with BVDV develops in dairy-breed steer calves at a low prevalence similar to that found in beef-breed calves.

Overall, although N caninum serostatus had no significant effect on morbidity rate or chronicity of infection among calves in the present study, seropositive calves in 1 pen that had a high morbidity rate had gained less weight at approximately 5 months after entry to the feedlot than the seronegative calves in that pen. Further investigation into the mechanistic processes behind this finding is warranted.

ABBREVIATIONS

PI

Persistent infection

BVDV

Bovine viral diarrhea virus

S:P

Sample to positive control

CI

Confidence interval

a.

IDEXX HerdChek bovine viral diarrhea virus Ag/serum, IDEXX Corp, Portland, Me.

b.

SPSS, version 14.0 for Windows, SPSS Inc, Chicago, Ill.

c.

StatXact 7, Cytel Software Corp, Cambridge, Mass.

References

  • 1

    Dubey JP. Neosporosis in cattle. Vet Clin North Am Food Anim Pract 2005;21:473483.

  • 2

    Anderson ML, Blanchard PC, Barr BC, et al. Neospora-like protozoan infections as a major cause of abortion in California dairy cattle. J Am Vet Med Assoc 1991;198:241244.

    • Search Google Scholar
    • Export Citation
  • 3

    Thurmond MC, Anderson ML, Blanchard PC. Secular and seasonal trends of Neospora abortion in California dairy cows. J Parasitol 1995;81:364367.

    • Search Google Scholar
    • Export Citation
  • 4

    Anderson ML, Palmer CW, Thurmond MC, et al. Evaluation of abortions in cattle attributable to neosporosis in selected dairy herds in California. J Am Vet Med Assoc 1995;207:12061210.

    • Search Google Scholar
    • Export Citation
  • 5

    Pare J, Thurmond MC, Hietala SK. Congenital Neospora caninum infection in dairy cattle and associated calfhood mortality. Can J Vet Res 1996;60:133139.

    • Search Google Scholar
    • Export Citation
  • 6

    Davison HC, Otter A, Trees AJ. Estimation of vertical and horizontal transmission parameters of Neospora caninum infections in dairy cattle. Int J Parasitol 1999;29:16831689.

    • Search Google Scholar
    • Export Citation
  • 7

    Barling KS, Lunt DK, Snowden KF, et al. Association of serologic status for Neospora caninum and postweaning feed efficiency in beef steers. J Am Vet Med Assoc 2001;219:12591262.

    • Search Google Scholar
    • Export Citation
  • 8

    Waldner C, Wildman BK, Hill BW, et al. Determination of the seroprevalence of Neospora caninum in feedlot steers in Alberta. Can Vet J 2004;45:218224.

    • Search Google Scholar
    • Export Citation
  • 9

    Schmidt PL, O'Connor AM. Neospora caninum seroprevalence in Iowa feedlots and its association with morbidity, mortality, production parameters and carcass traits, in Proceedings. Am Assoc Bov Pract 2004;37:164165.

    • Search Google Scholar
    • Export Citation
  • 10

    Pare J, Thurmond MC, Hietala SK. Congenital Neospora infection in dairy cattle. Vet Rec 1994;134:531532.

  • 11

    Barling KS, Sherman M, Peterson MJ, et al. Spatial associations among density of cattle, abundance of wild canids, and seroprevalence of Neospora caninum in a population of beef calves. J Am Vet Med Assoc 2000;217:13611365.

    • Search Google Scholar
    • Export Citation
  • 12

    California Agricultural Statistics, 2004. Available at: www.nass.usda.gov/ca. Accessed Dec 8, 2006.

  • 13

    Pare J, Hietala SK, Thurmond MC. An enzyme-linked immunosorbent assay (ELISA) for serological diagnosis of Neospora sp. infection in cattle. J Vet Diagn Invest 1995;7:352359.

    • Search Google Scholar
    • Export Citation
  • 14

    Kelly AP, Janzen ED. A review of morbidity and mortality rates and disease occurrence in North American feedlot cattle. Can Vet J 1986;27:496500.

    • Search Google Scholar
    • Export Citation
  • 15

    Thurmond MC, Johnson WO, Munoz-Zanzi CA, et al. A method of probability diagnostic assignment that applies Bayes theorem for use in serologic diagnostics, using an example of Neospora caninum infection in cattle. Am J Vet Res 2002;63:318325.

    • Search Google Scholar
    • Export Citation
  • 16

    Pare J, Thurmond MC, Hietala SK. Neospora caninum antibodies in cows during pregnancy as a predictor of congenital infection and abortion. J Parasitol 1997;83:8287.

    • Search Google Scholar
    • Export Citation
  • 17

    Hietala SK, Thurmond MC. Postnatal Neospora caninum transmission and transient serologic responses in two dairies. Int J Parasitol 1999;29:16691673.

    • Search Google Scholar
    • Export Citation
  • 18

    Barling KS, Lunt DK, Snowden KF, et al. Association of serologic status for Neospora caninum and postweaning feed efficiency in beef steers. J Am Vet Med Assoc 2001;219:12591262.

    • Search Google Scholar
    • Export Citation
  • 19

    Loneragan GH, Thomson DU, Montgomery DL, et al. Prevalence, outcome, and health consequences associated with persistent infection with bovine viral diarrhea virus in feedlot cattle. J Am Vet Med Assoc 2005;226:595601.

    • Search Google Scholar
    • Export Citation
  • 20

    O'Connor AM, Sorden SD, Apley MD. Association between the existence of calves persistently infected with bovine diarrhea virus and commingling on pen morbidity in feedlot cattle. Am J Vet Res 2005;66:21302134.

    • Search Google Scholar
    • Export Citation
  • 21

    Fulton RW, Hessman B, Johnson BJ, et al. Evaluation of diagnostic tests for detection of bovine viral diarrhea virus and prevalence of serotypes 1a, 1b, and 2a in persistently infected cattle entering a feedlot. J Am Vet Med Assoc 2006;228:578584.

    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 97 0 0
Full Text Views 514 495 217
PDF Downloads 143 129 5
Advertisement