Abstract
A study was conducted in Alberta to determine the seroprevalence of Neospora caninum in feedlot calves purchased from various auction markets throughout western Canada. Four feedlots (1 feedlot from each of the Airdrie and High River areas and 2 feedlots from the Strathmore area) were selected for sampling. At each feedlot, a random 10% sample of feedlot steer and bull calves entering the feedlot from September 2001 to December 2001 were enrolled in the study until there were a maximum of 500 animals enrolled per feedlot.
Blood samples were collected from 1976 male animals at the time of entry to the 4 study feedlots. The animals represented 375 groups purchased from 70 sale points throughout British Columbia, Alberta, Saskatchewan, and Manitoba. Of the 1976 animals tested, 128 animals tested positive for antibodies to N. caninum. The prevalence and adjusted 95% confidence limits for N. caninum in beef calves on entrance to the feedlot in western Canada were 6.5% (95% CI, 5.1 to 8.2).
There were no significant (P ≥ 0.05) associations between the risk of treatment, the risk of designation as “chronic,” and the risk of death and antibodies to N. caninum either before or after adjusting for feedlot, entry weight, entry date, and clustering of disease within lots at each feedlot. In addition, there was no significant (P ≥ 0.05) association between serological status and feedlot entry weight or average daily gain. Note that there was no information available on feed conversion because the calves were mixed within existing commercial feedlot pens and the actual feed intake of each animal could not be determined. Adjustment for the concentration of antibodies to bovine viral diarrhea virus on arrival did not change any of the examined associations between N. caninum status and calf health or performance.
The results of this study demonstrated that the prevalence of N. caninum in feedlot calves in western Canada was less than the prevalence reported in the United States. Additional studies are required to determine whether the inferior rate of gain and feed efficiency observed in the southern United States with animals testing positive for antibodies to N. caninum also occurs under the management conditions used in western Canada.
Abstract
Résumé — Détermination de la séroprévalence de Neospora caninum chez des bouvillons en parcs d’engraissement en Alberta. Une étude a été menée en Alberta afin de déterminer la séroprévalence de Neospora caninum chez des veaux en parcs d’engraissement achetés à divers encans dans l’ouest du Canada. Quatre parcs d’engraissement (1 parc chacun pour les régions de Airdrie et de High River et 2 pour la région de Strathmore) ont été retenus pour l’échantillonnage. Dans chacun des parcs, 10 % des bouvillons et veaux mâles ont été choisis au hasard à leur entrée, de septembre à décembre 2001, pour faire partie de l’étude jusqu’à ce qu’un maximum de 500 têtes par parc d’engraissement soit atteint.
Des échantillons de sang ont été prélevés chez 1976 animaux mâles au moment de leur entrée dans l’un des 4 parcs. Les animaux provenaient de 375 groupes achetés dans 70 points de vente répartis entre la Colombie Britannique, l’Alberta, la Saskatchewan et le Manitoba. Sur les 1976 animaux testés, 128 étaient positifs aux anticorps de N. caninum. La prévalence et les limites de confiance ajustées à 95 % pour N. caninum chez les veaux de boucherie à leur entrée au parc d’engraissement dans l’ouest du Canada étaient de 6,5 % (95 % LC, 5.1 à 8.2). Il n’y avait pas d’associations significatives (P ≥ 0,05) entre la probabilité de traitement, la probabilité d’une désignation dite «chronique», la probabilité de mort et les anticorps à N. caninum à la fois avant et après ajustement pour le parc d’engraissement, le poids à l’entrée, la date d’entrée et l’ensemble des maladies entre les lots dans chacun des parcs. De plus, il n’y avait pas d’associations significatives (P ≥ 0,05) entre le statut sérologique et le poids à l’entrée dans le parc ou la moyenne du gain quotidien. Il n’y avait pas de renseignements disponibles sur la conversion alimentaire puisque les veaux étaient dispersés entre les différents enclos commerciaux et que la prise alimentaire individuelle ne pouvait être mesurée. Des modifications tenant compte de la concentration d’anticorps contre le virus de la diarrhée virale bovine à l’arrivée ne changeaient aucune des associations étudiées entre le statut par rapport à N. caninum et la santé ou la performance des veaux.
Les résultats de cette étude démontrent que la prévalence de N. caninum chez des veaux en parc d’engraissement dans l’ouest du Canada était inférieure à la prévalence rapportée aux États-Unis. d’engraissement dans l’ouest du Canada était inférieure à la prévalence rapportée aux États-Unis. Des études supplémentaires sont nécessaires pour déterminer si les taux inférieurs de gain de poids et d’efficacité alimentaire observés dans le sud des États-Unis chez des animaux positifs aux anticorps et d’efficacité alimentaire observés dans le sud des États-Unis chez des animaux positifs aux anticorps de N. caninum surviennent également dans les conditions d’élevage qui prévalent dans l’ouest du Canada.
(Traduit par Docteur André Blouin)
Introduction
The impact of Neospora caninum infection on the reproductive performance of dairy cattle has been extensively described in the veterinary literature. A substantial link between N. caninum infection and reproductive losses in beef herds in Alberta (abortion, stillbirth, and culling) was first reported in 1998 (1). A severe outbreak of abortion in a northern Alberta beef herd caused by N. caninum has also been described (2). During the outbreak, 157 of 354 (44.4%) cows in this herd did not become pregnant or experienced fetal loss. Initially, 81.3% of cows in the herd were positive for antibodies to N. caninum, and the seropositive cows were 6.2 times less likely to be pregnant than the negative cows. No other potential causes of abortion were identified during the intensive investigation. More than 82% of the surviving calves born to positive cows during the spring following the abortion outbreak were infected with the parasite. Additional work has been done to determine the long-term consequences of N. caninum infection in this herd. The year after the abortion outbreak, 48% of the cows and heifers were still serologically positive for N. caninum. Both the positive heifers and cows were 1.8 times less likely to be pregnant after the subsequent breeding season than were their negative herd mates, suggesting an ongoing economic cost to the herd (3).
Subsequent studies have been conducted to determine the seroprevalence of N. caninum in beef cows in western Canada. In 1 study, blood samples were collected from 1806 pregnancy-tested cows from 174 herds at a northern Alberta auction market in the fall of 1998. One hundred and sixty-two (9.0%) of these samples were positive for antibodies to N. caninum. Cows that were positive for antibodies to N. caninum were less likely (OR, 1.7; 95% CI, 1.3 to 2.2) to be pregnant than were the negative cows (4). In a 2nd study, 260 randomly selected, lyophilized sera from a bank of 1608 samples collected from the same area during the 1980s were analyzed for antibodies to N. caninum. Thirty-five (13.5%) of these samples were serologically positive for N. caninum (4).
While the reproductive losses in dairy and beef cattle have been extensively described, the effects of N. caninum infection on performance have not been well defined. A study of dairy cattle in California found that Neospora-seropositive heifers produced approximately 4% to 5% less milk over the lactation period than did their seronegative herd mates (5). However, Canadian studies have shown inconsistent relationships between N. caninum status and milk production (6,7). Recent studies of beef cattle in the United States suggest that there may be an association between infection with Neospora and post-weaning rate of weight gain, feed efficiency, carcass traits, and economic profits in the feedlot (8–10). Seropositive calves entering a Texas feedlot had significant (P < 0.05) reductions in average daily gain (ADG), live body weight (BW) at slaughter, and increased cost of treatment for a projected economic loss of $15.62/ animal as compared to seronegative animals (8,9). A follow-up study in 34 weaned Texas beef calves found significant (P < 0.05) negative effects on short-term weight gain and feed efficiency associated with the presence of antibodies against N. caninum (10).
The feedlot productivity losses described above cannot be directly extrapolated to western Canada. Both feedlot studies were conducted under management conditions that are different from those used in western Canada, and both studies used a serological test to determine N. caninum infection status that is different from the test used in western Canada. In 1 study that examined weight gain prior to weaning, the presence of precolostral antibody titers did not have an effect on calf performance (11). Fall calf weights and ADG for calves in Alberta with antibodies to N. caninum at birth were not significantly (P ≥ 0.05) different from the fall calf weights and ADG of calves with no antibodies (difference, −9.2 lbs; 95% CI, −31.3 to 12.8), after adjusting for calf sex, dam age, calf birth weight, calf age at weighing, and sire group. In addition, available evidence suggests that the seroprevalence of N. caninum is lower in western Canada than in the United States, although the Canadian data are available only for mature beef cows (4,10,12). At the present time, there is no information that describes the seroprevalence of N. caninum in feeder cattle in Alberta. As a result, the relative importance of N. caninum infection in western Canada is unknown.
The objective of this study was to determine the seroprevalence of N. caninum in feedlot calves purchased from various auction markets in western Canada. With this information, the need for additional research on the potential cost of N. caninum to the feeding industry in Alberta can be evaluated.
Materials and methods
Study facilities
Four feedlots in Alberta (1 feedlot from each of the Airdrie and High River areas and 2 feedlots from the Strathmore area) were selected for sampling. These feedlots have capacities ranging from 16 000 to 28 000 animals and feed a total of more than 60 000 feedlot calves annually. The basic design of each feedlot was representative of the standard design used in western Canada. The animals were housed in open-air, dirt-floor pens arranged side by side with central feed alleys and 20% porosity wood-fence windbreaks. Each feedlot pen holds approximately 200 to 300 animals. Hospital and processing facilities are located in each feedlot. Each hospital facility has a hydraulic chute, an electronic scale, a chute-side computer for the collection of animal health data (Feedlot Health Animal Record Management (FHARM); Feedlot Health Management Services (FHMS), Okotoks, Alberta), and separation alleys to facilitate the return of animals to designated pens. Each processing facility has a hydraulic chute, an electronic scale, and several adjacently located receiving pens.
Study animals
The animals sampled in this study were crossbred beef steer and bull calves purchased from auction markets throughout western Canada. Animals were transported by truck to the feedlot after assembly at the auction market. The animals were approximately 6 to 8 mo of age and weighed between 141 and 404 kg. Upon arrival at the feedlot, the animals were moved through a hydraulic chute for a group of procedures known collectively as processing. At processing, animals received unique, individual animal identification; an infectious bovine rhinotracheitis virus, parainfluenza-3 virus, bovine virus diarrhea virus (BVDV), and bovine respiratory syncytial virus vaccine; a multivalent clostridial bacterin/toxoid; a Mannheimia haemolytica bacterin/ toxoid; a Haemophilus somnus bacterin; prophylactic parenteral long-acting oxytetracycline; an anabolic growth implant; and a topical external and internal parasite control product. In addition, all bulls were castrated.
Experimental design
A study in the United States reported that the prevalence of antibodies to N. caninum in feedlot calves was 13% (6). Based on this prevalence estimate, on logistic constraints related to the collection and transport of serum samples, on financial constraints, and on the prerequisites that the feedlots sampled procure a large number of feedlot calves from throughout western Canada and use individual individual animal data collection systems, it was determined that approximately 500 serum samples from each of 4 feedlots would be appropriate. The financial and practical constraints resulted in a decision to use an allowable error of 1.5% and confidence limits of 95% to determine the final sample size. The feedlot prerequisites were defined so that a representative sample of feedlot calves from throughout western Canada would be available for sampling and to ensure that the feedlots selected for the study would be able to provide the data for individual animal health and purchase location.
Sample collection
At each feedlot, a random 10% of animals meeting the criteria described for study animals were selected for inclusion in the study. A blood sample was obtained by jugular venipuncture from the 1976 animals enrolled on the study to a maximum of 500 per feedlot. The blood samples were collected in 13-mL SST Gel and Clot Activator Venous Blood Collection Tubes (Vacutainer; Becton Dickinson, Franklin Lakes, New Jersey, USA) and refrigerated. Subsequent to being transported by truck to FHMS, the samples were centrifuged within 48 h of collection at 3200 RPM for 10 min. Serum was harvested from the centrifuged samples, placed in 2-mL microcentrifuge tubes, and shipped on ice via overnight courier to the diagnostic laboratory (Prairie Diagnostic Services, Regina, Saskatchewan) for analysis.
Sample analysis
The serum samples were analyzed with a commercially available N. caninum antibody diagnostic kit (IDEXX, Westbrook, Maine, USA). The mean optical density (OD) values of the positive and negative serum controls and the test sera were recorded. From these values, the sample to positive (S/P) ratio for each sample was calculated. Samples with an S/P ratio greater than 0.5 were designated as positive. Recently, a sensitivity of 97.6% and specificity of 98.5% has been reported for this test (13). The first 105 samples that were considered positive for N. caninum were also analyzed for antibodies to BVDV by using a serum neutralization procedure. For every positive sample that was tested for antibodies to BVDV, the next 2 N. caninum-negative samples in the sampling -series were also tested for antibodies to BVDV. In addition, BVDV isolation was attempted for all samples selected for determination of antibody titer to BVDV within 4 wk of sample collection.
Data collection and management
The results of the analyses of antibody to N. caninum and the purchase location data were entered into an electronic spreadsheet (Microsoft Excel 97; Microsoft Corporation, Redmond, Washington, USA) and verified. Animal health information was extracted from FHARM, collated, and verified for the first 100 d of the feeding period. Morbidity data included initial treatment of animals diagnosed as sick on arrival (undifferentiated fever [UF]/bovine respiratory disease [BRD] at arrival with a rectal temperature > 40.0°C), initial treatment of animals diagnosed as UF (UF/BRD with a rectal temperature > 40.3°C subsequent to arrival), initial treatment of animals diagnosed as no fever (UF/BRD with a rectal temperature ≤ 40.3°C subsequent to arrival), and initial treatment of animals diagnosed as gastrointestinal disease. Additional information extracted included animals designated by feedlot personnel as “chronic,” animals that died, an anonymous designator for feedlot, lot number (pen), source city (auction market), arrival ticket number, entry weight, days on feed, and terminal implant weight. These data were merged with the results of the N. caninum and antibody to BVDV analyses to form a single data set.
Statistical analysis
The prevalence of animals with positive antibody titers to N. caninum were estimated with 95% confidence intervals (95% CI) to adjust for the expected clustering of binomial outcomes (Epi Info, version 6; Centers for Disease Control and Prevention, Atlanta, Georgia, USA). The design effect was calculated from the apparent prevalence observed in the animals from each source (arrival ticket number) and by using the following formula:
 |
The associations between various health outcomes and serological status were analyzed by using a generalized estimating equation (GEE) method to account for over dispersion within the data resulting from clustering of disease and production outcomes (14). For outcomes measured at arrival, clustering was controlled for within arrival ticket number in the analyses, and for outcomes measured subsequent to arrival, clustering was controlled for within feedlot pen number (lot) in the analyses. Data were analyzed by using a statistical computer software program (SAS System for Windows, version 8.2 [PROC GENMOD]; SAS Institute, Cary, North Carolina, USA). Model specifications included a binomial distribution, logit link function, and an exchangeable correlation structure. Variables, including potential confounders, such as entry weight, were screened manually for consideration in a final model. A manual backwards strategy was used to further evaluate any variables where P < 0.25. Only the variables remaining in the final multivariable model at P < 0.05, based on the robust empirical standard errors produced by the GEE analysis, were considered statistically significant. Biologically plausible first-order interaction terms were evaluated for importance.
Similar analyses were repeated to examine the association between serological status and entry weight and ADG by using the identity link function and assuming a normal distribution. Finally, all analyses were repeated by using the subset of samples that were also tested for antibodies to BVDV. All continuous outcomes were also examined by using a random effects model (PROC MIXED; SAS Institute) to determine if the results were similar across modeling approaches. As described above, for outcomes measured at arrival, clustering was controlled for within arrival ticket number in the analyses, and for outcomes measured subsequent to arrival, clustering was controlled for within feedlot pen number (lot) in the analyses. These latter results are reported only when a difference in interpretation from the results obtained through the marginal model (PROC GENMOD; SAS Institute) was noted.
Results
Blood samples were collected from 1976 male animals at the time of entry to the 4 study feedlots (Table 1). The animals represented 375 groups (arrival tickets) purchased from 70 sale points (auction markets) throughout British Columbia, Alberta, Saskatchewan, and Manitoba. On 5 occasions during the study, groups of animals were purchased from multiple sales in the same area. The exact origin of each animal was not recorded; therefore, the point of origin of each animal was assigned to the central auction market. The average number of study animals per arrival ticket for each group was 5.3 animals (s = 4.2). Where arrival ticket number was not available, a group designation was assigned for all animals purchased from the same source on the same date. Because the risk of infection appeared to be clustered slightly by source, the 95% CI for prevalence of antibodies were adjusted for a calculated design effect of 1.9. Of the 1976 animals tested, 128 tested positive for antibodies to N. caninum (Figure 1). The prevalence and adjusted 95% confidence limits for N. caninum in beef calves on entrance to the feedlot were 6.5% (95% CI, 5.1 to 8.2). The geographic distribution of the titers is shown in Figure 2.
Table 1.
Summary of origin and placement of study animals by feedlot
| Feedlot | |||||
|---|---|---|---|---|---|
| A | B | C | D | All | |
| Number of animals per source province | |||||
| AB | 354 | 260 | 283 | 110 | 1007 |
| BC | 24 | 26 | 12 | 2 | 64 |
| MB | 9 | 32 | 27 | 197 | 265 |
| SK | 112 | 161 | 177 | 190 | 640 |
| Total | 499 | 479 | 499 | 499 | 1976 |
| Number of source sale barns represented | 30 | 26 | 34 | 36 | 70 |
| Number of lots receiving study animals | 24 | 28 | 23 | 19 | 94 |
AB — Alberta; BC — British Columbia; MB — Manitoba; SK — Saskatchewan
Figure 1.
Distribution of sample to positive ratios (S/P) for antibodies to Neospora caninum detected by a commercial enzyme linked immunosorbent assay (ELISA) for 1976 animals from 4 feedlots in Alberta.
Figure 2.
Geographic distribution of antibodies to Neospora caninum traced to the location of purchase for each group of animals prior to entry to the feedlot. Map indicates percentage of calves with antibodies to N. caninum from each source.
Animals were placed into 94 lots or pens at the 4 study feedlots. The risk of treatment, the risk of designation as “chronic,” and the risk of death are summarized in Table 2. There were no significant (P ≥ 0.05) associations between the risk of treatment, the risk of designation as “chronic,” or the risk of death and antibodies to N. caninum either before or after adjusting for feedlot, entry weight, entry date, and clustering of disease by lot (Table 2). Also, there were no significant (P ≥ 0.05) associations between serological status and feedlot entry weight or ADG (Table 3).
Table 2.
Summary of associations between animal health variables and enzyme linked immunosorbent assay (ELISA) antibody to Neospora caninum status adjusted for clustering by lot within feedlot
| Cumulative prevalence | ELISA antibody to N. caninum status | |||
|---|---|---|---|---|
| Animal health variable | Total | Positive | Negative | Odds ratio (95% CI) |
| Initial sick on arrival treatment | ||||
| Yes | 97 (4.9%) | 10 | 87 | 1.75 (95% CI, 0.85 to 3.62) |
| No | 1879 | 118 | 1761 | P = 0.13 |
| Initial undifferentiated fever treatment | ||||
| Yes | 436 (22.1%) | 22 | 414 | 0.70 (95% CI, 0.41 to 1.22) |
| No | 1540 | 106 | 1434 | P = 0.21 |
| Initial no fever treatment | ||||
| Yes | 245 (12.4%) | 22 | 223 | 1.49 (95% CI, 0.95 to 2.38) |
| No | 1731 | 106 | 1625 | P = 0.83 |
| Initial treatment for gastrointestinal disease | ||||
| Yes | 7 (0.4%) | 0 | 7 | P = 0.99 |
| No | 1969 | 128 | 1841 | |
| Overall chronicity | ||||
| Yes | 55 (2.8%) | 5 | 50 | 1.39 (95% CI, 0.60 to 3.26) |
| No | 1921 | 123 | 1798 | P = 0.45 |
| Overall mortality | ||||
| Yes | 53 (2.7%) | 1 | 52 | 0.27 (95% CI, 0.04 to 1.92) |
| No | 1923 | 127 | 1796 | P = 0.20 |
Table 3.
Summary of associations between initial weight and average daily gain and enzyme linked immunosorbent assay (ELISA) antibody to Neospora caninum status
| Animal health variable | Overall mean | Difference between N. caninum antibody-positive and antibody-negative calves (mean and 95% CI) |
|---|---|---|
| Initial weight (kg)a,b | 261.1 (s = 36.7) | 0.35 (−4.6, 5.3) P = 0.89 |
| Average daily gain (kg/day)a,c | 1.53 (s = 0.33) | −0.02 (−0.6, 0.03) P = 0.50 |
| Initial weight (adjusted for log BVDV antibody concentration) (kg)b,d,e | 259.6 (s = 34.0) | −0.14 (−6.8, 6.6) P = 0.97 |
| Average daily gain (adjusted for log BVDV antibody concentration) (kg/day)c,d,e | 1.55 (s = 0.42) | −0.04 (−0.10, 0.02) P = 0.19 |
CI — Confidence interval; BVDV — Bovine viral diarrhea virus
an = 1861
bAdjusted for occurrence of initial sick on arrival treatment
c Adjusted for initial weight, initial sick on arrival treatment, initial undifferentiated fever treatment, initial no fever treatment, and designation as “chronic”
dn = 283
e Adjustments for presence or absence of antibodies to BVDV at various cutoff values did not change the interpretation or results
For all of the 299 samples tested for antibodies to both BVDV and N. caninum, 65 (21.7%) had a titer to BVDV greater than 1:300 and 14 (4.7%) had a titer greater than 1:4000. Only 1 of the samples was positive for BVDV. The concentration of antibodies to BVDV is shown in Figure 3. Of 105 N. caninum antibody-positive samples, 23 had BVDV titers greater than 1:300; of 194 N. caninum antibody-negative samples, 42 had BVDV titers greater than 1:300. There was no association between the N. caninum status and the presence of antibodies to BVDV in this group of calves (P > 0.05). There were 3 animals with antibody to BVDV concentrations above 1:4000 in the N. caninum-positive group and 11 animals in the N. caninum-negative group. The presence or absence of antibodies to BVDV at various cutoff values (present/absent, > 300, or > 4000), the inverse titer, or the log of the inverse titer did not change the association between N. caninum and any measure of feedlot health or performance (Tables 3 and 4).
Figure 3.
Distribution of inverse titers for antibodies to bovine viral diarrhea virus (BVDV) for 105 and 194 Neospora caninumpositive and -negative samples from 4 feedlots in Alberta.
Table 4.
Summary of associations between animal health variables and the log enzyme linked immunosorbent assay (ELISA) antibody to Neospora caninum S/P ratio adjusted for the log of antibody titer to bovine viral diarrhea virus (BVDV)a
| Animal health variable | Parameter estimate (95% CI) |
|---|---|
| Initial sick on arrival treatment | 0.10 (−0.66, 0.87) P = 0.79 |
| Initial undifferentiated fever treatment | 0.27 (−0.06, 0.61) P = 0.11 |
| Initial no fever treatment | 0.42 (−0.19, 1.05) P = 0.18 |
| Initial treatment for gastrointestinal disease | no estimate availableb |
| Overall chronicity | −0.19 (−1.56, 1.18) P = 0.78 |
| Overall mortality | −0.12 (−1.11, 0.87) P = 0.81 |
a Models using N. caninum as a categorical variable and adjustments for presence or absence of antibodies to BVDV at various cutoff values would not converge because of the smaller sample size. Therefore, both antibody levels were considered as log transformed continuous variables
b Of the 299 cases with data on both BVD and N. caninum antibodies, only 1 was treated for gastrointestinal disease
Discussion
The prevalence of N. caninum in feedlot calves purchased from auction markets in western Canada is less than that reported from mature cows in Alberta or from similar studies in the United States (1,4,8,12). In a study in Texas, approximately 13% of 1009 beef calves tested had antibodies against N. caninum (8). Beef cattle in Texas were at increased risk of exposure to N. caninum as a result of the abundance of wild canids and the density of beef cattle (15). There is no apparent geographic pattern in the samples collected from calves sourced from across western Canada. Sanderson et al (12) reported an overall seroprevalence to N. caninum of 24% for beef cows from the northwestern United States. Both of the United States feedlot and cow-calf studies used different serological assays than those available in western Canada, and the study in Texas included Bos indicus as well as Bos taurus cattle (8,12). Serological data reported to date for beef cows from Alberta have shown the prevalence of cows with antibodies to N. caninum to be greater than 9.0% (1,2,4). Since most seropositive cows are expected to produce seropositive calves, the prevalence in this survey might have been expected to be slightly higher than that observed. The difference in the observed prevalence may reflect that previous surveys in Alberta were limited in geographic distribution and the oldest survey used a different serological test.
With respect to the effect of levels of antibody to N. caninum on feedlot entry weight and subsequent animal health, the results of the current study agree with the previously reported data from Texas. The costs of treatment were not examined for the calves from western Canada. Barling et al (8) did not find an association between antibody levels and the risk of treatment or mortality in the feedlot. In addition, there was no association between feedlot entry weight and antibody levels to N. caninum (8). However, Barling et al (8) reported an increased cost of treatment and decreased rate of gain in cattle with antibodies to this parasite. The findings of the most recent study in the United States suggested that seropositive feedlot steers required 2.16 kg more feed (100% dry matter basis) for each 1.0 kg of live BW gain than did seronegative steers (10). Although there was no obvious association with rate of gain in the present data set, the interpretation of this result should be limited, because the calves were mixed within existing commercial feedlot pens and the actual feed intake of each animal animal could not be determined. Note that the study in Texas mal did not find an association with feed intake, but it did with rate of gain and feed conversion. Additional studies are required to investigate whether the inferior feed efficiency and rate of gain observed in the southern United States also occurs under the management conditions used in western Canada.
Finally, there is no evidence in the present data set that antibody titers to BVDV on entry to the feedlot influence the effect of N. caninum infection on feedlot performance. However, due to the fact that the seroprevalence of N. caninum was relatively low in the sample studied, the power to examine this association was limited. CVJ
Footnotes
This project was supported by research grants from the Alberta Cattle Commission — Matching Grants Program and Intervet Canada, Regina, Saskatchewan.
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