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Animal Health
Monday, November 07, 2016 6:03:57 PM
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Merial's VAXXITEK HVT+IBD for poultry

 

Merial

 

 

1.  Introduction

 

Significant economic losses are attributed to immunosuppression in the poultry industry worldwide. Exposure to stressors in the poultry production environment, along with infectious diseases (viral or bacterial) that impair immunity, contribute to an overall reduction in flock health, causing a decrease in productivity. In one study, the immune function of vaccinated birds was monitored throughout the immunization process (1). The eff ect of VAXXITEK HVT+IBD on infectious bronchitis (IB) vaccine take was evaluated as well (2, 3). Recommendations of hatchery vaccination programs were updated, confi rming the compatibility of VAXXITEK HVT+IBD with inactivated adjuvanted Newcastle disease (ND) vaccines, regardless of the technique and equipment used for injections (4, 5); with Fowlpox H5 vector vaccines (6); and with Rispens CVI988 monovalent vaccines (7, 8).

 

Field work in broilers and layers was also published. Vaccine take of the HVT-IBD vector vaccine construct was monitored by qRT-PCR (quantitative reverse-transcription polymerase chain reaction) in target tissues in which the vaccine virus replicates, mainly the spleen, bursa and feather follicles (9).

 

It was shown that clinical protection against Gumboro and Marek's diseases induced by vaccination leads to improved performance which can be evaluated at the level of the fi nal product: the chicken carcass (10, 11, 12,13) or the egg (14,15,16). An increase in weight of broilers, in quantity of eggs, as well as in quality of fi nal products intended to Human consumption was observed.

 

2 13)VAXXITEK HVT+IBD: an immunization process that ensures bursa integrity and improves control of immunosuppression

 

Bursa integrity refl ects the overall immune status of chickens (19). Nevertheless, parameters related to the immunization process, such as B and T cell counts and stimulation, are useful for demonstrating bursal protection by vaccination with the HVT-IBD vector vaccine.

 

2.1. The process of immunization against IBD

 

The eff ects of the HVT-IBD vector vaccine and a live modi-fi ed IBD antibody complex vaccine on humoral and cell me-diated immunity were compared in commercial broilers (1). Following in ovo vaccination, broilers were inoculated with an inactivated ND virus vaccine at 12 days post-hatch, when the breakthrough titre of the IBD maternal antibodies (mAb) levels was reached, or at 15 days post-hatch. The diff erent eff ects of the two vaccines on IBD and ND antibody levels, the number of intra-bursal and circulating B and T cells and the mitogenic response of peripheral blood leukocytes were determined. Neither vaccine aff ected the peripheral T cell count. Birds vaccinated with the IBD complex vaccine had lower numbers of circulating B cells in comparison to those vaccinated with the HVT-IBD vector vaccine and the non-inoculated control group.

 

At 1 and 5-6 days post-hatch, 20 serum samples were tested for IBD antibodies to determine the estimated breakthrough titre for the IBD complex vaccine based on the Deventer formula. This tentative breakthrough time point was used to extrapolate the possible time at which the replicating intermediate IBD vaccine, which is released by the immune complex vaccine, may induce some transient immunosuppression. Based on serum antibody levels, day 12 post-hatch was chosen as the fi rst and day 15 as the second day of inactivated ND vaccination.

 

Necropsy was conducted at 12, 15, 21, 28, and 35 days post-hatch for vaccine induced lesions and other parameters used to evaluate the eff ects of the vaccines on the immune system and the IBD immune response. Gelatinization of the bursa of Fabricius was detected in 6 - 30% of chickens inoculated with the IBD complex vaccine between 15 and 28 days post-hatch. No other macroscopic lesions of the spleen, bursa of Fabricius or gut were detected at any time point in any of the investigated groups. While the bursa to body weight ratio was not aff ected in the diluent and HVT-IBD vector vaccine groups, birds inoculated with the IBD complex vaccine showed a reduction in bursal weight beginning at 21 days post-hatch (p < 0.05). Histopathological bursa lesions were detected only in birds inoculated with the IBD complex vaccine, starting at 12 days post-hatch.

 

A high variation between birds in the onset of lesion devel-opment was observed, suggesting that the levels of mater-nal antibodies varied between individuals in each group and infl uenced the onset of infection. The development of histological lesions in the IBD complex vaccine group was accompanied by infi ltrating T cells. No signifi cant changes in the number of intra-bursal CD3+ T cells were seen over time in the diluent and HVT-IBD vector vaccine inoculated groups (Figure 1).

 

 

 

C = controls, HVT-IBD = HVT-IBD vector vaccine vaccinates, Icx-IBD = IBD complex vaccine vaccinates, ph = post-hatch.

Figure 1 - Detection of CD3+ T cells in the bursa of Fabricius after vaccination with HVT-IBD and IBD complex vaccine. Cryosections were investigated at diff erent time points after hatch. The average number of cells ± standard deviation is indicated and the percent of birds with > 100 T cells/microscopic fi eld (400 x).

 

The eff ect of the vaccines on circulating B and T cell populations was investigated by fl ow cytometric analysis of the peripheral blood leukocytes. As shown in previous research, the IBD complex vaccine induced a signifi cant reduction in the number of circulating B cells at 21 days post-hatch (p < 0.05). CD4+ T cell numbers were aff ected at 21 days post-hatch in the group inoculated with the IBD complex vaccine. The percentage of this cell population increased signifi cantly in comparison to the control group inoculated with diluent (p < 0.05). The other T cell population was not aff ected in either group at any time point investigated; neither was the mitogenic response. These results demonstrate that neither vaccine had a strong eff ect on the investigated activity of systemic T cell populations.

 

At 12, 15 and 21 days post-hatch, bursa of Fabricius and spleen samples were further investigated by qRT-PCR for VP2 of IBDV. The detection rate of IBD virus in the bursa of Fabricius and spleen of birds inoculated with the IBD complex vaccine clearly increased over time. The detection rates for VP2 in HVT-IBD vector vaccine inoculated birds were lower, but VP2 was detected in spleen as well as in bursa of Fabricius at almost all investigated time points.

 

Both vaccines induced antibodies against IBD. Antibodies against ND were detected by ELISA after vaccination using an inactivated vaccine of both IBD-vaccinated groups. Due to high individual variation in the actual onset of vaccine take in birds inoculated with the IBD complex vaccine, no signifi cant diff erence in ND antibody levels was seen between vaccinated groups. However, the IBD complex vaccine group showed lower antibody titers in individual birds compared to the other two ND vaccinated groups. This group had received the ND vaccine at 15 days post-hatch, after IBD induced lesion development was demonstrated by histological and immunohistochemical investigation.

 

Overall, this study clearly shows that the IBD complex vaccine aff ects the humoral immune system by reducing circulating and intra-bursal B cells, and slightly suppressing the antibody response after ND vaccination. The impact of the vaccine on the humoral immune response was not found to be signifi cant at early time points following ND vaccination; however, this may be explained by a high variation in vaccine take between birds. Furthermore, this study demonstrated that lesions started to develop around 15 days post-hatch in birds inoculated with the IBD complex vaccine.

 

Interestingly, HVT-IBD vector vaccine was detected by qRT-PCR not only in the spleen but also in the bursa, although it induced no detectable eff ect on the T cell or the B cell immune response to inactivated ND vaccine. Both vaccines induced a strong humoral IBD antibody response by ELISA.

 

2.2. The eff ect of VAXXITEK HVT+IBD on live infectious bronchitis vaccine take

 

Evaluation of the performance of broiler chicks vaccinated with two diff erent IBD vaccines, the HVT-IBD vector vaccine and the IBD complex vaccine (used as the control vaccine), was performed under laboratory conditions (2).

 

Broiler chicks were divided into 3 groups in the hatchery; the fi rst group was kept unvaccinated, the second and third groups were subcutaneously vaccinated with HVT-IBD vector vaccine and control vaccine, respectively.

 

One of each HVT-IBD vector vaccine and control vaccine sub-groups was vaccinated with IB Mass H120 at one day of age and later with IB virus CR88 at 13 days of age. The other sub-group from each of the IBD vaccinated groups remained as controls. At 7, 14, 21, 28, 35 and 42 days post-vaccination, blood was collected for sera (for IBD and IB serology) and 5 chicks from each sub-group euthanized to determine bursa to body weight ratio (b/bw). In addition, trachea, lung, kidney, caecal tonsil and rectal tissues were collected for detection of IB virus by qRT-PCR. At 35 days post-vaccination, 10 chicks from each group were challenged with virulent M41 or virulent IB QX strain KG3P. Five days later, 5 chicks were euthanized for ciliary score and another 5 for collection of various tissues. Bursa integrity as determined by b / bw ratio was used to measure HVT-IBD vector vaccine effi cacy.

 

No diff erence in terms of clinical protection against respective IB virus challenges was observed. The main fi nding was decreased dissemination of the M41 virulent virus post-challenge in the trachea of challenged birds with protected bursas, as well as decreased dissemination of the QX virulent virus post-challenge in the kidneys of challenged birds with protected bursas, demonstrating the benefi t of obtaining bursa integrity after HVT-IBD vector vaccine application. Immunization against IB thanks to the vaccine program was equivalent in the vaccinated groups as evaluated by serology and clinical protection against challenge, with a decreased impact on live vaccine virus spread to the target organs as kidneys, leading to improved safety of the IB live vaccine application, if this may happen in fi eld conditions.

 

3.    Improved performances in broiler meat production

 

Broiler performance was evaluated during the rearing phase, as well as in the processing plant. It was found that control of IBD-induced immunosuppression results in a decrease in lesions caused by bacterial secondary infections and a decrease in carcass condemnation at the chicken processing plant.

 

3.1. Improved chicken carcass condemnation performance – Case study in Brazil

 

Condemnation rate is an important factor that aff ects the indexes and profi t of a chicken slaughter house. This parameter is directly linked to the overall health status of the chickens during the rearing period in the farms and their transfer to the slaughter house. Causes of condemnations are numerous: airsacculitis, arthritis, cellulitis, colibacillosis, cachexia and dermal lesions recorded as dermatosis and dermatitis. In the Brazilian condemnation recording system, the data are divided into full bird condemnations and partial bird condemnations. The 'Serviço Inspeção Federal' offi cially inspects carcasses immediately after the evisceration step and before market classifi cation: partially condemned carcasses are accepted by the internal market for processing, while only full bird carcasses are accepted for export (mainly to the European Union). The average percentage of carcass weight lost after partial condemnation depends on the reason for condemnation. It may range from 3% for a single abscess to 32% for airsacculitis (10).

 

This study looked at 840 fl ocks selected from the records of a broiler meat producer. Out of these, 397 fl ocks, representing approximately 9 million broilers, were vaccinated against IBD with an IBD complex hatchery vaccine. 431 fl ocks representing approximately 9.2 million birds, were vaccinated with the HVT-IBD vector vaccine. Flocks were quasi-contemporaneous. Total carcass condemnations and partial condemnations were recorded. The condemnation rates that could be directly related to immunity of broilers, bacterial contamination and skin resistance were analyzed. Condemnation rates resulting from the slaughtering process were disregarded.

 

Overall mean condemnation rates (total and partial) were 5.41% for the control fl ocks and 4.13% for the fl ocks vaccinated with the HVT-IBD vector vaccine. No signifi cant diff erence (p = 0.39) in the overall mean percentage of condemnation between populations was noted (Table 1).

 

However, there was a calculated economic savings of about $175K in carcass condemnations in the fl ocks vaccinated with the HVT-IBD vector vaccine compared to control fl ocks vaccinated with the IBD complex vaccine.

 

 

Table 1 - Condemnation rates and kilograms of meat condemned at the slaughterhouse in Brazil.

a Data from a total of 18,701,365 processed birds was analyzed showing the causes and weight in Kg of total and partial condemnations of two groups: 9,005,805 vaccinated with the control vaccine compared with 9,234,418 birds vaccinated with the HVT-IBD vector vaccine.

b Twelve fl ocks were not analyzed due to insuffi cient data.

 

3.2. Improved chicken meat yield – Case study in the United States of America

 

The eff ect of vaccination with the HVT-IBD vector vaccine on body weight, fl ock uniformity and virus shedding was investigated in a commercial broiler fl ock (11).

 

A fl ock of day-old broiler chicks was divided into 2 equal groups and placed in a commercial broiler house. The control group was administered Marek's disease vaccines in ovo at 19 days of incubation. The control group was not vaccinated for IBD. The treated group was administered HVT-IBD vector vaccine in ovo at 19 days of incubation. The fl ocks were weighed at 5 time points. At least 200 birds were individually weighed in each test group at each time point. Mean weights and coeffi cients of variation (CVs) were calculated for each test group. Flock uniformity was calculated as the number of birds within a range of ±15% of the mean weight. At each time point cloacal swabs were collected from 90 birds per group and tested for the presence of IBD virus using qRT-PCR. Serum samples were collected from 25 birds per group and tested for IBD antibodies using ELISA. Bursal tissue samples were collected from 6 birds per group. Fresh bursal tissue was tested for IBD virus using antigen recovery by capture ELISA. Formalin-fi xed bursal tissues were examined for histopathological IBD lesions. At 38 days of age, 120 birds per test group were selected for processing. Mean weights and CVs for each test group were determined for live weight, ready-to-cook (RTC) weight, and cut up parts weights (wings, breasts, tenders, legs and racks). Yields were calculated as a percentage of live weight and RTC weight.

 

 

Table 2 - Zootechnical performance results in standard broilers in China.

* Performance indexes usually observed across China for this type of chicken production.

 

 

Table 3 - Zootechnical performance results in coloured chickens in China.

* Performance indexes usually observed in chicken production of this type
of over 50 days, and up to 100 days or more for rearing.

  

In standard broilers vaccinated with the HVT-IBD vector vaccine, mortality rates were signifi cantly (p < 0.001) inferior compared to controls for the fi rst round of vaccination (4.90% vs 5.90%), and signifi cantly (p < 0.001) inferior for the second round, with an observed diff erence of 0.20%. In addition, an increased average live weight, increased daily weight gain, decreased feed conversion ratio, and overall increased performance index of production was observed in that population of chickens (Tables 2 and 3). Overall, vaccination with the HVT-IBD vector vaccine results in an economical benefi t of 0.35 Chinese Yuan (RMB) per bird for the fi rst round and of 0.28 RMB per bird for the second round in standard broilers, and 0.64 RMB per bird in the coloured bird populations. These observations demonstrate the interest of inducing an early protection of the bursa, the target organ of the vvIBD and Marek's disease. Feed cost for the production of chicken meat was diminished with a decreased feed conversion rate. The cost of medication was also decreased, thanks to improvement of bursa health and enhanced general immunity of the birds.

 

3.4. Improved broiler chicken growth performance – Case study in Egypt

 

HVT-IBD vector vaccine administration under fi eld  conditions in day-old broiler chickens in Egypt was compared to classical live IBD vaccination programs. Zootechnical parameters of fl ock performance were evaluated, and the economical eff ects were calculated. Field infection of the fl ocks by vvIBD virus and other respiratory viruses was confi rmed by serological and qRT-PCR examination of samples (13).

 

The zootechnical records showed no signifi cant diff erence in viability. The overall average daily weight gain (+ 1.13 g) of broilers processed at the same age showed a diff erence in favor of the HVT-IBD vector vaccine for the three successive cycles. The overall average bodyweight (- 0.002 Kg) was considered as equivalent in both populations. For birds vaccinated with the HVT-IBD vector vaccine, overall feed conversion was 0.05 points lower and the overall production index was 10 points lower, for the three successive cycles. These pointed diff erences led to an overall diff erence in live weight production at 35 days, with a price diff erence of 12 Egyptian Pounds (EGP) / Kg of bodyweight, equivalent to 0.245 $US/Kg of bodyweight. The increased immune protection off ered by the HVT-IBD vector vaccine therefore led to better economical results.

 

IB variant virus circulation was evidenced throughout the study, and the decision was taken to adjust the vaccination program according to this newcomer in the virus mix. The target bodyweight, 1.6 Kg per bird, was obtained earlier and more homogeneously using the HVT-IBD vector vaccine. This target weight was used in calculating the economic results of the study. Higher than average mortality rates were recorded for the period of this study. Feed conversion ratios decreased in birds vaccinated with the HVT-IBD vector vaccine, which also contributed to the economical benefi t in production (- 0.162 EGP per 1.6 Kg of bodyweight, equivalent to - 0.028 $US). The cost of medication, including vaccination, was decreased in birds vaccinated with the HVT-IBD vector vaccine (0.02 EGP per 1.6 Kg of bodyweight, equivalent to - 0.37 $US cents).

 

Egg production performance may be evaluated during the rearing phase, as well as in the egg processing plant. Control of immunosuppression by vaccination with the HVT-IBD vector vaccine results in higher uniformity of pullet bodyweight and increased egg production, especially during the peak of lay. Monitoring of bursal health is key to ensuring that pullet health status is ideal prior to placement for laying (14).

 

4.1. Improvement in egg production – Case study in Mexico

 

This comparative fi eld trial was aimed at monitoring the safety and effi cacy of the HVT-IBD vector vaccine compared to a live intermediate strain IBD vaccine within the same site of production, in contemporaneous fl ocks of commercial layers in Mexico. Mexico is one of the leading egg producers worldwide (15).

 

A total of 844,730 light replacement pullets were included in the study. Two groups were constituted: Group A (490,730 birds) vaccinated with the HVT-IBD vector vaccine and Rispens vaccine via the sub-cutaneous route at one day of age in the hatchery; and Group B (354,000 birds) vaccinated with HVT and Rispens vaccine at one day of age in the hatchery and two live IBD vaccines administered in the fi eld via drinking water. The following parameters were monitored: mortality rate during growth, pullet growth performances during the rearing period, bodyweight, uniformity of bodyweight, feed consumption and laying performances, mortality rate during lay, percentage of lay and total production of packed eggs.

 

Pullet mortality during the rearing period was signifi cantly reduced (- 0.9%) in the HVT-IBD vector vaccine group. Mean pullet bodyweight was slightly higher in pullets vaccinated with the HVT-IBD vector vaccine prior to transfer, with a gain of 29 g of bodyweight per bird. Uniformity was 18.5% higher in birds vaccinated with the HVT-IBD vector vaccine, at 84.4%, compared to the control group, at 65.9%. Total consumption during the rearing period was 3.353 Kg per layer in the HVT-IBD vector vaccine group compared to 3.409 Kg in the control group, with a decrease of 5.6 g per bird. Percentage of lay was signifi cantly higher in pullets vaccinated with the HVT-IBD vector vaccine, especially at the peak of lay, around 30 weeks of age, at 86.8% vs 83.4% in the controls with a diff erence of + 3.4%. A diff erence of 5 eggs more per bird at 65 weeks of age was recorded in the HVT-IBD vector vaccine group. Mortality during the laying period, around 10%, was due to management, harsh climate conditions and disease challenge factors typical of this region of Mexico. Nevertheless, mortality during the rearing period was signifi cantly inferior in pullets vaccinated with the HVT-IBD vector vaccine, probably due to improved bursa protection and enhanced immune function. Increased uniformity of pullet bodyweight led to improved overall general conditions prior to transfer to production.

 

 

Table 4 - Pullet performance monitoring in Mexico.

Group A: HVT-IBD vector vaccine vaccinates; Group B: controls.

s Signifi cant diff erence; alpha risk at 5%.

 

 

Table 5 - Monitoring of % of lay in Mexico.

Group A: HVT-IBD vector vaccine vaccinates; Group B: controls.

s Signifi cant diff erence; alpha risk at 5%.

 

4.2. Improvement in egg production – Case study in Hungary

 

The protective eff ect of the HVT-IBD vector vaccine against Marek's disease is comparable to the standard HVT vaccine. Total protection in layers is achieved against Marek's disease as well as IBD with the concurrent use of the Rispens vaccine (16).

 

Field experiments were carried out in two layer farms in Hungary. Two groups were formed on both farms: a control group of day-old chicks vaccinated with Rispens + HVT vaccine, followed by live vaccination in drinking water against IBD, and a trial group, vaccinated with HVT-IBD vector vaccine, as well as Rispens. The following parameters were recorded and compared during the fi eld trial: serological response against IBD (using classic commercial anti-IBD and IBD ELISA tests with improved affi nity to VP2), bursa/body weight ratio, losses during rearing and production, and egg production.

 

Mortality in the HVT-IBD vector vaccine trial groups was lower on both farms. Serology results showed a post-vaccination antibody response in the trial groups, which also produced more eggs per hen than the controls. Production parameters were increased in the trial groups on both farms, with 3.48 and 7.48 more eggs per hen housed on farm A and B, respectively.

 

 

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Article made possible through the contribution of Merial

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