Abstract
Background
Pneumonia is the largest cause of mortality in Canadian lambs. Currently there are no licensed ovine vaccines in Canada to reduce economic losses from this production-limiting disease.
Objective, animals, and procedure
The effectiveness of an experimental subunit Mannheimia haemolytica leukotoxin A (LtxA) and transferrin binding protein B (TbpB) vaccine was evaluated in lambs for reduction of clinical disease in an experimental challenge study and in a controlled randomized field trial in a large commercial sheep operation.
Results
Following an experimental challenge of parainfluenza 3 virus and M. haemolytica, the subunit vaccine induced significantly higher LtxA and TbpB antibody titers at 48 d post-challenge compared to the adjuvant and Ovipast Plus bacterin (Merck Animal Health), but there were no significant differences in clinical signs or mortality among vaccine groups. Following vaccination of commercial ewes and their lambs at weaning, the only significant difference in health, growth, and carcass traits between vaccinates and non-vaccinates was a slightly higher pneumonia treatment rate in vaccinated preweaned lambs (25.7%) compared to unvaccinated preweaned lambs (23.4%) (P = 0.04).
Conclusion and clinical relevance
Although vaccination with the experimental subunit M. haemolytica vaccine induced high LtxA and TbpB antibodies, it did not reduce clinical disease in lambs following an experimental challenge study or in a controlled randomized field trial in a commercial sheep operation. Further research is required to identify additional protective antigens for a safe and effective ovine respiratory vaccine to reduce pneumonia losses in commercial sheep flocks.
RÉSUMÉ
Efficacité d’un vaccin respiratoire sous-unitaire expérimental de Mannheimia haemolytica ovin à réduire la pneumonie chez les agneaux
Contexte
La pneumonie est la principale cause de mortalité chez les agneaux canadiens. Présentement, il n’y a aucun vaccin ovin homologué au Canada pour réduire les pertes économiques associées à cette pathologie limitant la production.
Objectif, animaux et procédure
L’efficacité d’un vaccin sous-unitaire expérimental à base de la leucotoxine A (LtxA) et de la protéine B liant la transferrine (TbpB) de Mannheimia haemolytica a été évalué chez des agneaux pour la réduction de la maladie clinique lors d’une infection expérimentale et lors d’un essai de champs randomisé et contrôlé dans un grand élevage commercial de moutons.
Résultats
À la suite d’une infection expérimentale avec le virus parainfluenza 3 et M. haemolytica, le vaccin sous-unitaire a induit des titres d’anticorps significativement plus élevés contre LtxA et TbpB à 48 j post-infection comparativement à l’adjuvant et à la bactérine Ovipast Plus (Merck Santé Animale), mais il n’y avait aucune différence significative dans les signes cliniques ou la mortalité parmi les groupes vaccinés. À la suite de la vaccination de brebis commerciales et de leurs agneaux au moment du sevrage, la seule différence significative dans la santé, la croissance et les caractéristiques des carcasses entre les animaux vaccinés et non-vaccinés était un taux légèrement plus élevé de traitement de la pneumonie chez les agneaux vaccinés pré-sevrage (25,7 %) comparativement aux agneaux non-vaccinés au présevrage (23,4 %) (P = 0,04).
Conclusion et pertinence clinique
Bien que la vaccination avec le vaccin sous-unitaire expérimental M. haemolytica ait induit des taux d’anticorps élevés contre LtxA et TbpB, il n’a pas réduit la maladie clinique chez les agneaux à la suite d’une infection expérimentale ou lors d’un essai clinique randomisé contrôlé dans un élevage ovin commercial. Des recherches supplémentaires sont requises pour identifier des antigènes protecteurs additionnels pour un vaccin respiratoire ovin efficace pour réduire les pertes associées à la pneumonie dans les troupeaux ovins commerciaux.
(Traduit par Dr Serge Messier)
INTRODUCTION
Pneumonia is an important cause of mortality in lambs globally (1–4). In sheep, pneumonia is a multifactorial disease that may be caused by a combination of transportation, environmental, and nutritional stressors; various bacteria; and viruses (1–4). A large, 1.5-year mortality study was conducted in an Alberta lamb feedlot (1). Acute and chronic bronchopneumonia and embolic pneumonia were the most common forms of fatal pneumonia in weaned lambs. Bacteria commonly isolated in acute and chronic bronchopneumonia were Mannheimia haemolytica, Pasteurella multocida, and Mycoplasma arginini. In chronic pneumonias, Trueperella pyogenes bacteria were also cultured. Respiratory syncytial and parainfluenza viruses were not isolated, though a few lungs tested positive for ovine progressive pneumonia virus.
Currently in Canada, there are no licensed ovine vaccines against any viruses or bacteria that may cause infectious pneumonia in sheep. Small-ruminant practitioners try to control livestock losses from pneumonia through producer education on good animal husbandry, including housing and ventilation, nutrition, early and accurate disease diagnosis, and responsible treatment with antimicrobials. With the availability of long-acting macrolide antimicrobials over the last decade, these drugs are used more frequently to prevent and treat pneumonia in high-risk preweaned and weaned lambs. There is increasing pressure on the livestock industry and veterinarians to reduce unnecessary antimicrobial usage to reduce the risk of antimicrobial resistance development and ensure the long-term effectiveness of existing antimicrobials. Vaccination is one tool that can be used alongside other management practices to reduce disease risks.
Vaccines are available for many bacteria and viruses that cause respiratory disease in beef cattle, because bovine respiratory disease is the largest cause of morbidity and mortality in many North American feedlots (5). However, development of respiratory vaccines for sheep has been lacking, likely because the sheep industry in North America is a much smaller industry than the cattle industry and there may be limited economic drivers for the pharmaceutical industry to develop effective vaccines for sheep. Furthermore, it is unlikely that vaccines developed for cattle are effective in sheep, due to either different bacteria and viruses or different strains of bacteria and viruses involved in pneumonia in sheep (6). Out of desperation, some practitioners have used bovine respiratory vaccines in sheep to reduce disease losses, despite the lack of controlled vaccine field trials to indicate their effectiveness in reducing losses (7).
An ovine respiratory bacterin, Ovipast Plus (Merck Animal Health), is available in Europe (8,9). It is a whole-cell killed bacterin against various strains of M. haemolytica and Bibersteinia trehalosi. This vaccine was imported into Canada under an experimental research permit from the Canadian Food Inspection Agency (CFIA) and tested in a large, controlled, randomized field trial in an Alberta commercial sheep operation. Ewes were vaccinated twice pre-breeding to boost colostral immunity of their newborn lambs, and their lambs were vaccinated twice post-weaning. This was not effective in reducing lamb pre- or post-weaning pneumonia treatment or crude or pneumonia-specific mortality rates, or in improving growth performance or carcass traits (10–12).
Previous research conducted at the Vaccine and Infectious Disease Organization (VIDO; Saskatoon, Saskatchewan) identified 3 protective antigens against ovine isolated M. haemolytica (13). The objective of the current study was to conduct an experimental challenge study to obtain a research permit from CFIA to conduct a controlled randomized field trial in a commercial sheep operation using 2 of the 3 previously identified antigens, leukotoxin A (LtxA) and transferrin binding protein B (TbpB).
MATERIALS AND METHODS
Experimental challenge study
The experimental challenge study was conducted at VIDO and approved by the CFIA and the University of Saskatchewan Animal Care Committee (UACC AUP20210080). Thirty commercial, Arcott-Finn crossbred lambs aged 8 mo were purchased from a local flock near Saskatoon, Saskatchewan that had low LtxA antibody titers. Lambs were individually randomized to 1 of 3 vaccination groups: i) 30% Emulsigen-D (MVP Laboratories) adjuvant (negative control), ii) experimental subunit LMB-010 vaccine, or iii) Ovipast Plus bacterin. Based on previous research conducted at VIDO using the same experimental challenge model, this sample size was sufficient to detect a reduction in mortality from 87.5% in the placebo group to 30% in the vaccinated group, with a power of 80% and an alpha of 0.05 (13,14). The experimental subunit vaccine, LMB-010, contained 50 μg each of ovine M. haemolytica LtxA and TbpB formulated in 30% Emulsigen-D adjuvant. On Day 0 and Day 28, lambs were vaccinated SC with 2 mL of the vaccine by the VIDO research technicians. On Day 49, lambs were challenged by nebulization with 7.5 × 107 cfu of parainfluenza 3 virus; and on Day 56, they were challenged by intratracheal injection with 4 × 107 cfu of M. haemolytica.
Blood samples for ELISA testing for LtxA and TbpB antibodies were collected from lambs on Days 0, 28, and 56; and before euthanasia. Lambs were examined daily for clinical signs of respiratory disease by trained VIDO research technicians who were blinded to vaccine status. Lambs were followed for 21 d post-challenge. A clinical scoring system assessing respiration, depression, rectal temperature, and body weight was used (Table S1, available online from: Supplementary Materials). Any lambs that were moribund during the trial were humanely euthanized and necropsied by an experienced veterinarian who was blinded to the vaccine status of the lambs. At the end of the trial on Day 77, all remaining lambs were euthanized. A lung scoring system was used to assess lung lesions (Table S2, available online from: Supplementary Materials). Lungs were removed and washed in cold water to remove blood and blood clots from the surfaces. Photographs of the lungs were taken, ensuring the identification tag was visible. Scoring was done by an experienced veterinarian. Each area of each lung lobe showing consolidation was marked. The proportion of each affected lobe was estimated by palpation. Each of the estimated proportion values was multiplied by a factor to reflect the relative area of the lobe with respect to the total lung areas. The presence of pleural adhesions and pleural effusion and the proportion of lung affected in each lobe were recorded.
A cumulative clinical disease score was created by adding individual scores of depression, respiration rate, and fever. Differences in outcomes among the 3 vaccine groups were evaluated with the Fisher exact test, X2 test, and ANOVA. Nonparametric median and distribution tests were used when outcome variables were not normally distributed.
Commercial field trial
Study design
The trial was conducted at a large commercial sheep operation in southern Alberta; the facility had 9000 Rideau-Arcott crossbred breeding ewes and a conjoined finishing feedlot with a one-time feeding capacity of 25 000 head. A total of 3500 ewes were enrolled into the vaccine trial between April 15, 2022, and July 15, 2022. Trial sample size was determined using a sample-size equation to detect differences in proportions between 2 groups, with a 95% CI, 80% power, and reduction in pneumonia mortality of 0.5% (14). At this feedlot, the historic mortality rate from pneumonia was 1.0%. Assuming an average lambing rate of 180% and a 20% dropout rate, a total of 3333 ewes, or 1667 per vaccine group, were required. Ewe vaccine status was randomly assigned by the second author, in groups of 8 ewes, based on the capacity of the chute system on farm. For each group of 8 ewes, a poker chip was blindly selected from a bag, which determined the vaccination status of that chute. There were equal numbers of poker chips in the bag for both unvaccinated and vaccinated ewes, based on the number of animals in that specific group being allocated to the trial that day. The second author administered 2 mL of the LMB-010 vaccine, SC, in the left neck of each animal, using the 2-handed tented method to ensure the vaccine was properly administered SC.
Inclusion criterion for ewes was any ewe confirmed pregnant by ultrasound at 9 wk of gestation. Ewes were excluded from enrollment in the trial if they had aborted before the scheduled first dose of the vaccine. Inclusion criteria for lambs were lambs born to ewes enrolled in the trial that had survived to 2 d of age to receive a Canadian Sheep Identification Program (CSIP) ear tag. Lambs were excluded from the trial if they were born to litters of > 3 animals or were sent to the nursery to be raised on artificial milk. Lambs were also excluded from the trial if they were too unhealthy to undergo the neonatal procedures at 24 to 48 h of age. Lambs that survived the preweaning phase were followed and included in the postweaning phase of the trial.
Animal management and housing
Following the first dose of LMB-010 vaccine, ewes were returned to outdoor pens where they were housed together, commingling vaccinates and non-vaccinates, due to pen space limitations. Ewes were moved into the processing barn 2 wk before lambing, which was 4 wk after the first vaccine dose, to receive the second dose of the vaccine. They were separated into 2 groups by vaccine status and housed separately by vaccine status until the end of the trial. Ewes were brought into the lambing barn a few days before their expected lambing dates and housed in groups of 8 ewes until they lambed; thereafter, they were split into separate mothering pens called jugs.
Approximately 3 h after birth, lambs were weighed, their navels were sprayed with iodine, and they were administered selenium (Selon-E; Vetoquinol, Lavaltrie, Quebec), 2 mL, IM. Artificial colostrum was offered within 24 h after birth to all lambs born to litter sizes of ≥ 3 and to lambs that did not appear to have received colostrum from their ewe, based on gut fill. Lambs were offered LambGro KidGro Colostrum (Grober Nutrition, Cambridge, Ontario) containing ≥ 14% IgG bovine dried colostrum at a rate of 20 g powder per 2 kg body weight, to make 93 mL of total feeding volume per 2 kg body weight. A record was kept of lambs that received artificial colostrum. At 2 d of age, lambs’ tails were docked using an elastrator ring and each received 3 ear tags: i) a radio frequency CSIP ear tag, ii) a sex tag, and iii) a coloured trial tag that matched the ewe’s vaccine status. The colour significance of each vaccine tag was unknown to barn staff, who were blinded to the vaccine status of each lamb. Lambs were selected for the nursery and excluded from the trial if the litter size was > 3. The “odd lamb out” protocol was implemented, in which the 2 most similar lambs remained with the ewe, to prevent a larger lamb from outcompeting its sibling for milk consumption.
Ewes remained individually housed with their lambs for ~72 h after parturition, and then pens were opened into groups of 4 to 6 ewes. These groups remained together for ~1 wk. Ewes and lambs were then moved into Barn A, where they were placed in 6 pens of 44 ewes with their lambs and segregated by vaccine status. Animals remained in these pens for ~2 wk, at which time all trial lambs received toltrazuril (Baycox 5% Oral Suspension; Elanco, Mississauga, Ontario) and tulathromycin (Draxxin Injectable Solution; Zoetis, Kirkland, Quebec), as per label directions. These were standard metaphylactic treatments used at the farm to reduce the high risks of coccidiosis and pneumonia, based on the flock’s historic disease risks and a previously established veterinary health protocol. Immediately following administration of Baycox and Draxxin, ewes and lambs were moved into Barn B, where the 3 vaccine-associated pens were condensed into 2 pens of 134 ewes and housed separately by vaccine status. In this barn, ewes and lambs had access to an outdoor resting area. Animals remained in Barn B pens for 4 to 5 wk, and then the lambs were weaned.
Lambs were weaned abruptly at ~8 wk of age, which marked the end of the preweaning phase of the trial. During weaning, trial lambs were separated from ewes, weighed, and vaccinated with LMB-010 vaccine (2 mL, SC) if their mothers had also been vaccinated with the same vaccine pre-lambing; otherwise, lambs were not vaccinated. At weaning, trial lambs were divided into 4 housing groups by sex (ewes, rams) and vaccine status, and moved into outdoor feedlot growing pens where they were housed together until the end of the growing phase of the trial. Three consecutive weeks of weaned lambs were placed into each growing pen due to pen space limitations. The age gap meant the lambs remained together for ~3 to 5 wk in growing pens, at which time all lambs were brought back into the processing facility to be weighed, and previously vaccinated lambs received the second booster dose of the LMB-010 vaccine. This weight event marked the end of the growing phase.
At the end of the growing phase, lambs were sorted into finishing outdoor feedlot pens by body weight, vaccine status, and sex. Weight categories were as follows: i) < 40 kg, ii) 40 to 50 kg, and iii) > 50 kg. Lambs that weighed > 50 kg were moved into a shipping pen for immediate slaughter. Remaining lambs were sent to finishing feedlot pens where they were weighed periodically; individuals were sorted off for immediate slaughter when they reached ~50 kg of final live weight. All lambs were sent to slaughter at SunGold Speciality Meats (Innisfail, Alberta). Carcass data were collected on all slaughtered trial lambs by individual lamb CSIP ear tag numbers.
Lambs were placed on feed at weaning and fed to slaughter with a well-balanced ration of roughage, grain, and protein, with mineral and vitamin supplements, as per the flock nutritionist’s recommendations. The feeding program was the same for vaccinated and unvaccinated lambs. All lambs were examined daily by trained feedlot staff blinded to the vaccine status of the lambs. Any diseased lambs were treated by trained feedlot staff using a standardized treatment protocol developed by the flock veterinarian. The case definition of treatment for pneumonia was lambs exhibiting the following clinical signs: depression, anorexia, nasal discharge, ± cough, and no other clinical signs attributable to other body systems. All feeding records were stored in a computerized feeding-management software program (DeliverIT; ITS Global, Okotoks, Alberta); and individual animal health data, including vaccinations and treatments, were recorded in a chute-side animal health management software program (FeedIT; ITS Global) that allowed individual animal traceback from birth to slaughter.
All lambs that died were necropsied by the researchers or trained feedlot staff to determine the cause of death based on gross morphologic lesions (15), using a standardized necropsy procedure. If a lamb died of pneumonia, a lung score was assigned, and lung tissue samples were collected and frozen for bacterial culture. The following lung scoring system was used. Each lung lobe was allocated a score from 0 to 2 based on the percentage of lung lesions: 0, no lesions; 1, individual lobe with < 50% affected by pneumonic lesions; and 2, ≥ 50% pneumonic lesions. An additional point was assigned for pleurisy (3,16). Lung tissue samples were collected from pneumonic lungs, frozen, and sent to the Animal Health Laboratory at the Ontario Veterinary College (Guelph, Ontario), once all trial lambs went to slaughter, for bacterial culture to identify pathogenic bacteria associated with pneumonic lung lesions.
Analysis
Measures of frequency (frequency, percent), central tendency (mean, median), and dispersion (range, SD, 95% CI) were used for simple summaries of variables of interest. Normality and homoscedasticity for continuous variables were investigated using a Shapiro-Wilk test and visualized with a histogram and QQ-plot. A X2 test or Fisher exact test was used to assess the simple associations between categorical variables, such as vaccination status. Based on the outcome of the test of normality and test of variance, a P-value was generated using either a Studentized t-test for normal data with equal variance, a Welch t-test for normal data where variance was not equal, or a Wilcoxon rank-sum test for nonparametric data, to assess simple associations between continuous and categorical variables.
Five outcomes were evaluated within each phase of production (preweaning, growing, finishing) using regression analysis, because different groups of lambs were housed together by vaccine status during each phase of production. Differences in pneumonia treatment rates, crude mortality rates, pneumonia mortality rates, and weight gain were evaluated between vaccine groups. In the finishing phase, days spent in finishing was used as the production outcome instead of overall weight gain, because final weight was fixed in this phase of production, based on the operation’s selection of ~50 kg as a targeted finished live weight. Yield grade (YG) at slaughter was evaluated as the proportion of carcasses with YG 1 scores, because YG 1 received the greatest financial incentive. Categorical outcomes were analyzed using generalized linear mixed models, with birth weight controlled and the random effect of pen included. Linear outcomes were evaluated using linear mixed models, with birth weight controlled and the random effect of pen included. The P-value for significance in all statistical analyses was set at ≤ 0.05.
RESULTS
Experimental challenge study
The LMB-010 subunit vaccine induced significantly higher LtxA and TbpB ELISA antibody levels than the adjuvant placebo and Ovipast Plus bacterin (Table 1). Sixty percent (6/10) of the lambs died from pneumonia in the placebo group, 70% (7/10) in the Ovipast Plus group, and 40% (4/10) in the LMB-010 group before the end of the trial, when all trial lambs were euthanized. This difference in pneumonia mortality rates among vaccine groups was not statistically significant (P = 0.59). There were no significant differences among vaccine groups in clinical scores, rectal temperatures, body weight changes, or lung scores at necropsy. Based on the current experimental challenge study results, findings from a previous challenge study (13), and a concurrent vaccine safety trial [which did not identify any safety concerns with the experimental vaccine based on body weight measurements, rectal temperatures, and injection-site lesion temperatures or size (data not shown)], CFIA provided an experimental research study permit to conduct a commercial field trial using the LMB-010 experimental vaccine (CFIA PRBV 2022-12).
TABLE 1.
Leukotoxin A (LtxA) and transferrin binding protein B (TbpB) median ELISA antibody titers (IQR) following vaccination of lambs on Days 0 and 28 with a subunit LtxA and TbpB Mannhemia haemolytica vaccine (LMB-010), Ovipast Plus bacterin (Merck Animal Health), or an adjuvant placebo (30% Emulsigen-D; MVP Laboratories).
| Vaccine group | TbpB: Day 0c primary vaccination | TbpB: Day 28 booster vaccination | TbpB: Day 48 | TbpB: Post-challenge PI3 virus & M. haemolytica |
|---|---|---|---|---|
| Placebo | 66 330a (20 918) | 48 866a (45 024) | 71 140a (29 097) | 920 935a (1.30 × 106) |
| Ovipast Plus | 63 970a (58 715) | 52 993a (17 479) | 66 922a (212 615) | 911 584a (1.43 × 106) |
| LMB-010 | 69 269a (57 522) | 140 217b (203 972) | 1.18 × 106b (268 000) | 1.29 × 106a (677 000) |
| P-value | 0.51 | < 0.0001 | < 0.0001 | 0.20 |
|
| ||||
| Vaccine group | LtxA: Day 0 primary vaccination | LtxA: Day 28 booster vaccination | LtxA: Day 48 | LtxA: Post-challenge PI3 virus & M. haemolytica |
|
| ||||
| Placebo | 75 235a,b (48 091) | 64 556a (167 081) | 46 915a (17 631) | 748 924a (1.89 × 106) |
| Ovipast Plus | 77 174a (170 374) | 65 649a (186 000) | 54 847a (80 982) | 1.03 × 106a (2.20 × 106) |
| LMB-010 | 69 354b (5826) | 277 636b (555 665) | 725 344b (592 512) | 1.37 × 106a (1.62 × 106) |
| P-value | 0.03 | 0.004 | < 0.0001 | 0.85 |
IQR — Interquartile range; PI3 — Parainfluenza 3.
Kruskal-Wallis one-way nonparametric ANOVA; Dunn all-pairwise comparisons test.
P < 0.05.
Blood collection prior to initial vaccination.
Commercial field trial
Pre-weaning
A total of 3500 ewes were enrolled in the vaccine trial between April 15, 2022, and July 15, 2022, with 1751 in the vaccinated group and 1749 in the unvaccinated group. There was no difference in the number of ewe removals between vaccine groups (15.9% vaccinated, 16.7% unvaccinated; 95% CI: −0.02 to 0.03, P = 0.56). Reasons for ewe removal included not pregnant (49%), no lambs (either died or removed, 39%), lost in computer system (5%), lost ear tag and moved to non-trial pen (3%), lambed after last lamb was inducted on to the trial (2%), aborted (1%), or allocated to the wrong vaccine group at second dose (1%). The vaccine did not appear to cause any adverse effects in the commercial ewes following initial and booster vaccination, based on reasons for trial removal and daily observations of ewes by barn staff.
A total of 4874 lambs were enrolled in the vaccine trial between May 28, 2022, and September 8, 2022. Lambs were removed from the final data analysis if they were sent to the nursery (71%), if their records or the lambs were missing at weaning (14%), if they were mistakenly housed in a non-trial pen (13%), or if they were mistakenly assigned to the wrong vaccine group based on the vaccine status of their mother (2%). There was no significant difference between vaccine groups in the number of removals (Table 2). Nine percent of the lambs were stillborn and 6% died before tagging at 2 d of age. Thirteen percent of ewes had a litter size of 1 lamb, 47% had 2, 31% had 3, and 8% had ≥ 4. The average birth weight was 4.07 kg (95% CI: 4.04 to 4.10 kg) and lambs were 48 d of age when they were weaned (95% CI: 47 to 48 d). As litter sized increased by 1 lamb, average birth weight per lamb decreased from 0.3 to 0.5 kg (P < 0.0001). Fifty-one percent of lambs born were rams and 10% of lambs born were fed supplemental colostrum. The crude mortality rate in those supplemented with colostrum (12.7%) was higher than in those not supplemented (9.4%; 95% CI: 0.003 to 0.06, P = 0.01).
TABLE 2.
Simple associations in preweaning health and performance between lambs from unvaccinated ewes and those from ewes vaccinated during gestation with an experimental subunit ovine leukotoxin A (LtxA) and transferrin binding protein B (TbpB) Mannheimia haemolytica vaccine at a commercial sheep flock.
| Outcome | Unvaccinated (95% CI) | Vaccinated (95% CI) | P-value |
|---|---|---|---|
| Pre-weaning phase | |||
| No. trial lambs | 2453 | 2421 | — |
| No. removals | 86 | 99 | 0.32 |
| Colostrum supplementation (%) | 9.7 | 10.3 | 0.50 |
| Ram lambs borne (%) | 49.9 | 51.3 | 0.34 |
| Litter size (%) | |||
| 1 | 13.9 | 13.1 | 0.43 |
| 2 | 46.0 | 48.2 | 0.11 |
| 3 | 31.7 | 30.7 | 0.46 |
| ≥ 4 | 8.4 | 8.0 | 0.59 |
| Pneumonia treatment rate (%) | 23.4 | 25.7 | 0.06 |
| Crude mortality rate (%) | 9.0 | 10.4 | 0.09 |
| Pneumonia mortality rate (%) | 1.8 | 2.4 | 0.10 |
| Birth weight (kg) | 4.12 (4.07 to 4.16) | 4.02 (3.98 to 4.07) | 0.002 |
| Age at weaning (d) | 48 (48 to 49) | 47 (47 to 48) | 0.04 |
| Weaning rate (%) | 88.4 | 86.5 | 0.05 |
| Weaning weight (kg) | 15.6 (15.4 to 15.7) | 15.4 (15.2 to 15.6) | 0.17 |
| Average daily gain (kg/d) | 0.22 (0.22 to 0.22) | 0.22 (0.22 to 0.22) | 0.75 |
Lambs born to vaccinated ewes had a lower birth weight than those born to unvaccinated ewes (Tables 2 and 3). Thus, birth weight was controlled in the final statistical analyses of vaccine effectiveness (Table 3) because it differed between the vaccine groups and it is a known confounder of health and performance outcomes. Those that died from any cause (3.78 versus 4.10 kg; 95% CI: 0.23 to 0.43, P < 0.001) or from pneumonia (3.77 versus 4.08 kg; 95% CI: 0.10 to 0.52, P = 0.004) had significantly lower birth weights than those that did not die. Birth weight was positively correlated with weaning weight (r = 0.37, P < 0.0001) and average daily gain (ADG) (r = 0.13, P < 0.0001).
TABLE 3.
Effectiveness of an experimental subunit leukotoxin A (LtxA) and transferrin binding protein B (TbpB) ovine Mannheimia haemolytica vaccine at a commercial sheep flock.
| Outcome evaluated | Unvaccinated | Vaccinated | Odds ratio or coefficienta | 95% CI | P-value |
|---|---|---|---|---|---|
| Pre-weaning phase | |||||
| Birth weight (kg) | 4.12 | 4.02 | −0.11a | 3.94 to 4.30 | 0.001 |
| Treatment rate for pneumonia (%) | 23.4 | 25.7 | 1.15 | 1.01 to 1.31 | 0.04 |
| Crude mortality rate (%) | 9.0 | 10.4 | 1.15 | 0.95 to 1.40 | 0.15 |
| Pneumonia mortality rate (%) | 1.8 | 2.4 | 1.29 | 0.79 to 2.11 | 0.31 |
| Weaning weight (kg) | 15.6 | 15.4 | −0.01a | −0.32 to 0.30 | 0.97 |
| Weight gain (kg) | 11.4 | 11.4 | 0.05a | −0.24 to 0.35 | 0.73 |
| Growing phase | |||||
| Treatment rate for pneumonia (%) | 3.5 | 3.6 | 1.02 | 0.73 to 1.41 | 0.93 |
| Crude mortality rate (%) | 4.1 | 3.5 | 0.82 | 0.59 to 1.13 | 0.23 |
| Pneumonia mortality rate (%) | 1.7 | 1.9 | 1.09 | 0.69 to 1.72 | 0.71 |
| Final body weight (kg) | 26.3 | 26.2 | 0.02a | −0.57 to 0.61 | 0.95 |
| Weight gain (kg) | 10.7 | 10.7 | 0.13a | −0.27 to 0.52 | 0.53 |
| Finishing phase | |||||
| Treatment rate for pneumonia (%) | 0.8 | 1.1 | 1.43 | 0.59 to 3.49 | 0.43 |
| Crude mortality rate (%) | 3.4 | 3.6 | 1.09 | 0.49 to 2.45 | 0.83 |
| Pneumonia mortality rate (%) | 0.9 | 1.3 | 1.24 | 0.50 to 3.12 | 0.64 |
| Finished live body weight (kg) | 56.3 | 55.8 | −0.49a | −2.95 to 1.97 | 0.70 |
| Days on feed | 110 | 111 | −0.82a | 10.37 to 8.72 | 0.87 |
| Hot carcass weight (kg) | 28.0 | 27.7 | −0.21a | −3.00 to 2.58 | 0.88 |
| Yield grade 1 carcass (%) | 20.1 | 22.1 | 1.10 | 0.56 to 2.17 | 0.79 |
| Fat cover (mm) | 18.5 | 18.4 | −0.30a | −1.94 to 1.35 | 0.72 |
All odds ratios are reported using the unvaccinated group as the referent category, adjusting for random effects of pen, and controlling for the confounding effect of birth weight differences between vaccine groups.
Twenty-eight percent of lambs were treated with an antimicrobial, with 13% treated more than once. The treatment rate for pneumonia was 25%. Seventy-six percent of the treatments for pneumonia occurred by 14 d of age (median: 9, range: 2 to 83 d). Lambs treated for pneumonia had lower birth weights (3.98 versus 4.10 kg; 95% CI: 0.05 to 0.19, P = 0.0009), weaning weights (14.59 versus 15.78 kg; 95% CI: 0.92 to 1.47, P < 0.0001), and ADG (0.21 versus 0.23 kg/d; 95% CI: 0.02 to 0.03, P < 0.0001) than those untreated. There were more ram lambs treated for pneumonia (26.7%) than ewe lambs (22.4%; 95% CI: 0.02 to 0.07, P = 0.0006). Ram lambs had higher birth weights (4.18 versus 3.96 kg; 95% CI: −0.28 to −0.16, P < 0.0001), weaning weights (15.92 versus 15.07 kg; 95% CI: −1.08 to −0.62, P < 0.0001), and preweaning ADG (0.23 versus 0.22 kg/d; 95% CI: −0.02 to −0.007, P < 0.0001) than ewe lambs. During the preweaning period, 2.3% more lambs born to vaccinated ewes were treated for pneumonia than those born to unvaccinated ewes (P = 0.04) (Tables 2 and 3).
Between 2 d of age and weaning, 10% of the lambs died. The main causes of death were i) enteritis (22.4%), ii) starvation (21.9%), and iii) pneumonia (21.5%) (Table S3, available online from: Supplementary Materials). The mortality rate for enteritis was 2.2% and for pneumonia was 2.1%. Eighty percent of the enteritis deaths occurred by 5 wk of age (data not shown). Sixty-two percent of pneumonia deaths occurred after 4 wk of age (Figure 1). There were no differences in crude mortality rates, pneumonia mortality rates, days to weaning, weaning weights, or ADG between lambs born to vaccinated or unvaccinated ewes (Table 3).
FIGURE 1.
Occurrence of pneumonia deaths in lambs from birth to weaning by ovine Mannheimia haemolytica vaccination status at a commercial sheep flock.
LMB-010 vaccine = leukotoxin A (LtxA) and transferrin binding protein B (TbpB).
Growing phase
The last trial lambs were weaned on October 19, 2022. A total of 4215 lambs were weaned and entered the growing phase of the trial. Lambs were removed from the final dataset if they were moved to a non-trial pen, shipped early, given a different clostridial bacterin at weaning, or given the wrong trial vaccine at weaning. There were significantly more removals of unvaccinated lambs (2.2%) than vaccinated lambs (0.68%), mainly due to lost ear tags (Table 4).
TABLE 4.
Simple associations in postweaning health and performance between unvaccinated lambs and those vaccinated with an experimental subunit ovine leukotoxin A (LtxA) and transferrin binding protein B (TbpB) Mannheimia haemolytica vaccine at a commercial sheep operation.
| Outcome | Unvaccinated (95% CI) | Vaccinated (95% CI) | P-value |
|---|---|---|---|
| Growing phase | |||
| No. weaned lambs | 2146 | 2069 | — |
| No. removals (%) | 2.2 | 0.7 | < 0.001 |
| Pneumonia treatment rate (%) | 3.5 | 3.6 | 0.82 |
| Crude mortality rate (%) | 4.1 | 3.5 | 0.26 |
| Pneumonia mortality rate (%) | 1.7 | 1.9 | 0.61 |
| Days on feed | 35 (34 to 35) | 35 (34 to 35) | 0.96 |
| Final weight (kg) | 26.3 (26.0 to 26.6) | 26.2 (25.9 to 26.5) | 0.67 |
| Average daily gain (kg/d) | 0.30 (0.30 to 0.31) | 0.31 (0.30 to 0.31) | 0.47 |
| Finishing phase | |||
| No. grower lambs | 2008 | 1984 | — |
| No. removals (%) | 1.0 | 0.8 | 0.33 |
| Pneumonia treatment rate (%) | 0.8 | 1.2 | 0.32 |
| Crude mortality rate (%) | 3.4 | 3.6 | 0.68 |
| Pneumonia mortality rate (%) | 0.9 | 1.3 | 0.34 |
| Days on feed | 111 (109 to 112) | 111 (109 to 113) | 0.83 |
| Finished live weight (kg) | 56.0 (55.8 to 56.3) | 55.5 (55.2 to 55.7) | 0.001 |
| Average daily gain (kg/d) | 0.27 (0.27 to 0.28) | 0.27 (0.27 to 0.27) | 0.008 |
Lambs were 51 d of age when they entered the growing phase of the trial, and they weighed 15.5 kg (95% CI: 15.4 to 15.6 kg). They were on feed for 35 d, their ADG was 0.31 kg/d (95% CI: 0.30 to 0.31 kg/d), and their body weight at the end of the growing phase was 26.5 kg (95% CI: 26.3 to 26.7 kg). Ram lambs were the same age as ewe lambs at weaning, but they weighed more at weaning (15.9 versus 15.1 kg; 95% CI: −1.08 to −0.62, P < 0.0001), they were on feed 2 d longer (36 versus 34 d; 95% CI: −2.74 to −1.54, P < 0.0001), and their ADG was higher (0.33 versus 0.28 kg/d; 95% CI: −0.06 to −0.05, P < 0.0001); thus, they weighed more at the end of the growing phase (28.2 versus 24.7 kg; 95% CI: −3.94 to −3.12, P < 0.0001).
Seven percent of the lambs were treated with an antimicrobial and 9% were treated more than once. The treatment rate for pneumonia was 3.6% and it represented 49% of all antimicrobial treatments. The treatment rate for other diseases was 3.9% and it included antimicrobial treatment for pinkeye, arthritis, listeriosis, abscesses, laryngitis, and injuries. Pinkeye accounted for 83% of these other treatments. Lambs treated for pneumonia had lower weaning weights (13.3 versus 15.6 kg; 95% CI: 1.55 to 2.95, P < 0.0001), ADG (0.23 versus 0.31 kg/d; 95% CI: 0.06 to 0.12, P < 0.0001), and body weights at the end of the growing phase (21.8 versus 26.6 kg; 95% CI: 3.20 to 6.40, P < 0.0001) compared to untreated lambs, but they were on feed 3 d less (32 versus 35 d; 95% CI: 1.42 to 5.46, P < 0.0001).
The crude mortality rate during the growing phase was 3.8%. The leading cause of death during the growing phase was pneumonia, causing 47% of all deaths (Table S3, available online from: Supplementary Materials). The pneumonia mortality rate was 1.8%, with 77% of all pneumonia deaths occurring within 3 wk after weaning (Figure 2). There were no differences in pneumonia treatment rates, crude mortality rates, pneumonia mortality rates, days on feed, body weight, or ADG between vaccinated and unvaccinated lambs (Table 3).
FIGURE 2.
Occurrence of pneumonia deaths in lambs from weaning to slaughter by ovine Mannheimia haemolytica vaccination status at a commercial sheep flock.
LMB-010 vaccine = leukotoxin A (LtxA) and transferrin binding protein B (TbpB).
Finishing phase
A total of 3992 lambs entered the finishing period. Lambs were 87 d of age when they entered the finishing phase of the trial, and they weighed 26.5 kg (95% CI: 26.3 to 26.7 kg). They were on feed for 111 d (95% CI: 110 to 112 d), their ADG was 0.27 kg/d (95% CI: 0.269 to 0.273 kg/d), and their finished live weight was 55.8 kg (95% CI: 55.6 to 55.9 kg). Ram lambs weighed more on entry (28.3 versus 24.7 kg; 95% CI: −3.95 to −3.14, P < 0.0001) and at finishing (56.1 versus 55.4 kg; 95% CI: −1.03 to −0.34, P = 0.0001), than ewe lambs, their ADG was higher (0.28 versus 0.26 kg/d; 95% CI: −0.02 to −0.01, P < 0.0001), they were on feed fewer days (102 versus 120 d; 95% CI: 16 to 20, P < 0.0001), their hot carcass weight (HCW) was higher (28.5 versus 27.2 kg; 95% CI: −1.55 to −1.19, P < 0.0001), they had more YG 1 carcasses (27 versus 13%; 95% CI: −0.25 to −0.18, P < 0.0001), and their fat cover was lower (16.3 versus 20.6 mm; 95% CI: 3.93 to 4.63, P < 0.0001).
During the finishing phase, there were very few treatments with an antimicrobial (1.5%), and only 11% were treated more than once. The treatment rate for pneumonia was 0.9% and it represented 59% of all antimicrobial treatments. Although lambs treated for pneumonia during the finishing phase had lower ADG (0.20 versus 0.27 kg/d; 95% CI: 0.02 to 0.12, P = 0.006) and finished body weights (49.8 versus 55.8 kg; 95% CI: 0.75 to 11.2, P = 0.03) than untreated lambs, there were no differences in days on feed, HCW, YG, or fat cover (Tables 3 and 5).
TABLE 5.
Simple associations in carcass data between unvaccinated lambs and those vaccinated with an experimental subunit ovine leukotoxin A (LtxA) and transferrin binding protein B (TbpB) Mannheimia haemolytica vaccine at a commercial sheep flock.
| Outcome | Unvaccinated (95% CI) | Vaccinated (95% CI) | P-value |
|---|---|---|---|
| Lambs shipped to slaughter (%) | 95.6 | 95.5 | 1.00 |
| No. carcasses | 1919 | 1895 | 1.00 |
| Hot carcass weight (kg) | 28.0 (27.9 to 28.1) | 27.7 (27.6 to 27.8) | 0.002 |
| Fat cover (mm) | 18.5 (18.2 to 18.8) | 18.4 (18.1 to 18.7) | 0.52 |
| Yield grade 1 (%) | 20.1 | 22.1 | 0.14 |
| Yield grade 2 (%) | 31.2 | 30.7 | 0.74 |
| Yield grade 3 (%) | 11.7 | 11.5 | 0.91 |
| Yield grade 4 (%) | 37.0 | 35.7 | 0.41 |
The crude mortality rate during the finishing phase was 3.6%. The leading cause of death was pneumonia, causing 31.7% of all deaths (Table S3, available online from: Supplementary Materials). The pneumonia mortality rate was 1.1%. The crude mortality rate in ram lambs (5.3%) was higher than in ewe lambs (1.7%; 95% CI: −0.05 to −0.03, P < 0.0001). Ram lambs also had higher pneumonia mortality rates than ewe lambs (1.6 versus 0.6%; 95% CI: −0.02 to −0.004, P = 0.001). There were no differences in pneumonia treatment rates, crude mortality rates, pneumonia mortality rates, days on feed, final live weights, ADG, HCW, fat cover, or YG between vaccinated and unvaccinated lambs after controlling for birth weight differences (Tables 3 and 5, Figure 2).
The most common forms of pneumonia, based on gross morphologic diagnoses, were acute and chronic bronchopneumonia, followed by fibrinous pneumonia and, less frequently, only pleuritis. Bacterial culture results from pneumonic lungs are presented in Table S4 (available online from: Supplementary Materials). Mannheimia haemolytica was the most common bacteria cultured, followed by M. arginini and M. ovipneumoniae. There was no difference in isolation rates of M. haemolytica or other bacteria between vaccinated and unvaccinated lambs.
DISCUSSION
Pneumonia was a major cause of treatment and death in lambs in the preweaning, growing, and finishing phases of life, as in previous years, based on the flock’s historic animal health records (1). The most common form of pneumonia observed was bronchopneumonia, followed by fibrinous pneumonia. Although M. haemolytica was the most frequent bacteria cultured from pneumonic lungs, mixed infections were common. Other bacteria isolated in pneumonic lungs were M. arginini, M. ovipneumoniae, P. multocida, and B. trehalosi, which have been associated with ovine pneumonia (1,2,11,12). Lambs that were treated for pneumonia or died from pneumonia had lower birth weights than untreated lambs, as previously reported (2,4,11,12,17–21). Lambs treated for pneumonia in the preweaning, growing, and finishing phases of life had poorer body-weight gains compared to untreated lambs, demonstrating that the costs of this disease include not only drug and labor costs and mortality losses, but also feed and yardage costs; thus, the economic impact of this disease may be underestimated in sheep (22). Although ram lambs grew faster and had heavier body weights and HCW and more YG 1 carcasses than ewe lambs, they were at higher risk of treatment and death from pneumonia at various stages of production. This was previously reported in the Ovipast Plus bacterin field trial at the same sheep operation, which also reported higher treatment and mortality rates for pneumonia and increased lung condemnations and lung lesions at slaughter in ram lambs compared to ewe lambs (10–12). It has been speculated that the increased risk of pneumonia in males compared to females may be due to differences in sex steroid hormones that directly affect immune responses to infection and vaccination (23).
Although the LXB-010 vaccine induced high LtxA and TbpB ELISA antibody titers to M. haemolytica, it did not significantly reduce pneumonia in the experimental challenge model, perhaps due to a lower-than-anticipated disease rate in the placebo group, a lower-than-anticipated vaccine efficacy, or a sample size too small to detect a significant vaccine effect (if one truly existed). The current experimental challenge study had a 60% mortality rate in the placebo group; an earlier experimental challenge study had a mortality rate of 87.5%, which was used to determine sample size (13). Based on the current mortality rate in the placebo group, the vaccine would have had to be at least 90% effective to produce a significant statistical difference between vaccine groups, based on an alpha-level of 0.05 and a power of 80% (14).
Potential causes of failure to detect a significant vaccine effect in the commercial field trial included the following: i) additional protective vaccine antigens from M. haemolytica are necessary to significantly reduce pneumonia caused by M. haemolytica, ii) ovine pneumonia is a multifactorial disease with mixed infections and other pathogenic bacteria and viruses may be needed in an effective ovine respiratory vaccine to prevent naturally occurring disease, iii) ewes had preexisting naturally acquired antibodies to M. haemolytica before vaccination and vaccination did not significantly increase specific colostral immunity, iv) newborn lambs did not receive sufficient amounts of colostrum within 24 h after birth to improve health and performance, v) metaphylactic treatment with tulathromycin at 3 wk of age for all lambs reduced pneumonia risks and vaccine effects, vi) postweaning disease risks occurred before primary and booster vaccination of weaned lambs could induce a protective immune response, and vii) high disease levels overwhelmed vaccine immunity.
Whereas the LXB-010 vaccine contained LtxA, a secreted protein, and TbpB, a transferrin binding protein shown to be important in reducing pneumonia caused by M. haemolytica (13,24), other vaccine antigens may be necessary to significantly reduce naturally occurring pneumonia in sheep caused by M. haemolytica (25,26). Experimental research previously conducted at VIDO identified another protective antigen of M. haemolytica, transferrin binding protein A (TbpA) (13). It was not included in the LMB-010 vaccine used here due to the difficulty and expense in producing sufficient volumes of TbpA for a commercial trial vaccine. Previous experimental challenge studies in lambs at VIDO using the same challenge model described here reported a significant reduction in pneumonia mortality with an LtxA, TbpA, and TbpB combination vaccine: 12.5% mortality in the vaccinated group compared to 87.5% mortality in the adjuvant group (13). Further research is needed to substantiate this earlier experimental research and determine whether TbpA can be produced cost-efficiently in sufficient volumes to be included in a commercial ovine respiratory vaccine.
Based on culture results from pneumonic lungs (Table S4, available online from: Supplementary Materials), although M. haemolytica was the most-isolated bacteria, other pathogens were cultured, indicating mixed infections were common. Previous research in the same sheep operation determined the Ovipast Plus bacterin (10–12), which contains both M. haemolytica and B. trehalosi, was ineffective in reducing clinical disease, perhaps due to failure to induce leukotoxin antibodies (Table 1). Leukotoxin is an important vaccine protein in reducing M. haemolytica respiratory infections in sheep (6,13) and cattle (24). Although there are commercial vaccines for M. haemolytica, P. multocida, H. somni, and M. bovis for cattle, their consistent effectiveness in beef feedlots to reduce bovine respiratory disease is not well-established (5). Currently, there are no commercial vaccines in Canada for M. ovipneumoniae or M. arginini, Mycoplasma arginini was more frequently isolated in this study than M. ovipneumoniae, and it has been isolated previously at this sheep operation (1,11,12). Mycoplasma ovipneumoniae and M. arginini were pathogenic in sheep and goats in Italy, causing atypical pneumonia (27). Younger animals were at higher risk of infection than adults, with infections most common in the summer (27). Furthermore, M. haemolytica infection may increase M. bovis disease in cattle (28). Therefore, additional research is needed to better understand the occurrence, pathology, and significance of M. arginini in ovine pneumonia and whether M. haemolytica infection increases Mycoplasma diseases in sheep.
Blood samples were not collected from ewes and lambs in the commercial field trial before and after vaccination, which would have enabled evaluation of preexisting M. haemolytica antibody titres and humoral immune responses following single and double vaccination. Since serology was not done, it is not known if the ewes and lambs had high preexisting antibody titres to M. haemolytica before vaccination, which could have reduced comparative differences in antibody titres between the 2 vaccine groups following vaccination and the ability to detect vaccine effectiveness. In commercial operations, since M. haemolytica is a normal commensal bacterium of the upper respiratory tract of sheep, it is expected that there will be preexisting antibodies present in some animals before vaccination and that any commercial vaccine must be effective under these variable conditions.
Immunoglobin G concentrations were not measured in newborn lambs after birth and colostrum ingestion to determine if failure of passive transfer was a significant factor in reducing colostral-specific immunity to M. haemolytica, which would have reduced vaccine effects on lamb health and performance. There was no difference between vaccine groups in the number of preweaned lambs that were fed supplemental colostrum. The mortality rate was higher in lambs that received supplemental colostrum, likely because it was given to all lambs born to litters of sizes ≥ 3 and to lambs that did not appear to have received colostrum from their ewes, based on gut fill. These lambs may have been less vigorous at birth; and thus, at higher risk of death. Individual birth weights were lower in larger litters and birth weights were lower in those that were treated for pneumonia or died from pneumonia.
All preweaned lambs were treated metaphylactically at 3 wk of age for coccidiosis, because coccidiosis was an important disease in this sheep operation. Parasitic infections can suppress immunity and increase the risk of other diseases, such as pneumonia (2,29). Metaphylactic treatment with tulathromycin likely reduced pneumonia treatment and mortality rates, reducing the ability to detect differences observed between vaccine groups. At the current disease rates and sample sizes after metaphylactic treatment, this vaccine study would have been able to reliably detect a vaccine effectiveness of at least 50%. Vaccines with < 50% effectiveness may not be a cost-effective solution for disease control in commercial sheep operations due to costs of the vaccine and the extra labor and handling needed to vaccinate animals.
In future pneumonia vaccine studies, lambs should not be treated metaphylactically with antimicrobials, because this reduces disease risks and thus the ability to detect vaccine differences between vaccinates and non-vaccinates. This research study was conducted at a commercial sheep operation and there were no financial incentives available to pay the producer for increased disease losses if he had not treated the lambs metaphylactically with tulathromycin. Before future vaccine studies are conducted, controlled metaphylactic field trials are needed in commercial sheep operations to determine the economic value of metaphylaxis at various lamb ages and disease risks, so that the financial benefits of metaphylaxis can be determined. Sheep producers can then be financially compensated for potential losses from failure to treat lambs metaphylactically with antimicrobials in vaccine trials; otherwise, producer participation in field trials will be unlikely due to the financial risks. From an evidence-based perspective for small-ruminant practitioners, running controlled vaccine field trials under naturally occurring disease conditions is the most reliable research approach to determine vaccine cost-effectiveness in commercial sheep operations.
Lambs were vaccinated at weaning and boosted the next time they were handled, which was when they were weight-sorted and entered the finishing phase, ~3 to 5 wk later. Pneumonia deaths started occurring shortly after weaning, and 77% of them occurred by 3 wk post-arrival (Figure 2). If 2 doses of the vaccine are required for a protective immune response, then booster vaccination occurred too late. Due to the 3- to 5-week timeframe to fill growing pens, there was an age difference when lambs within a pen received their vaccine booster, which may have increased variability in immune responses and the ability to detect a vaccine effect. The issue of vaccine timing and disease occurrence is a common challenge in feedlot cattle. In this setting, primary vaccination typically occurs on feedlot arrival, and if the vaccine is boosted, this usually occurs the next time the cattle are handled for another reason, which is typically past the time of most respiratory deaths (30). To reduce disease risks shortly after weaning, vaccination would be required before weaning or in lambs before 8 wk of age. It is unknown if colostral immunity from ewe vaccination would interfere with vaccination before weaning. To assess this, lambs from unvaccinated and vaccinated ewes would have to be vaccinated either once pre-weaning and boosted at weaning or twice pre-weaning, to evaluate vaccine effectiveness post-weaning. Further vaccine trials are required to determine the most effective preventive vaccination regimes based on natural disease occurrence once cost-effective ovine respiratory vaccines are developed.
ACKNOWLEDGMENTS
The authors thank VIDO Animal Services for their participation in the experimental challenge study and VIDO personnel for help in vaccine formulation. VIDO receives operational funding from the Government of Saskatchewan through Innovation Saskatchewan and the Ministry of Agriculture and from the Canada Foundation for Innovation through the Major Science Initiatives Fund. The North American Lamb Corporation staff are acknowledged for their hard work and participation in the field trial, which would not have been possible without their support. We thank Merck Animal Health for providing the Ovipast Plus bacterin for the experimental challenge study. The Canadian Food Inspection Agency is thanked for providing a permit to conduct the commercial field trial using the experimental ovine vaccine. Dr. Maria Spinato from the Animal Health Laboratory at the Ontario Veterinary College is thanked for culturing lung samples. This research was supported by grants from Results Driven Agriculture Research and Alberta Lamb Producers. Published as VIDO manuscript series no. 1052. CVJ
Funding Statement
This study was funded by Results Driven Agriculture Research, Alberta Lamb Producers, and VIDO.
Footnotes
Unpublished supplementary material (Tables S1–S4) is available online from: Supplementary Materials
This study was funded by Results Driven Agriculture Research, Alberta Lamb Producers, and VIDO.
Copyright is held by the Canadian Veterinary Medical Association. Individuals interested in obtaining reproductions of this article or permission to use this material elsewhere should contact Permissions.
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