Articles on Bursa of Fabricius | Gumboro Prevention

Bursa size, an easy tool to evaluate gumboro vaccination programmes

Adrià Martos — Fri, 11 Feb 2022 10:13:56 +0000

By conducting necropsies routinely, we can have a rough but quick idea of what is happening on our farms. However, most of the time we do not standardise or record the results of those necropsies. Recording the bursa size as a routine practice can be used to evaluate the Gumboro situation on farms and in particular the vaccination scheme we are using.

Bursa of Fabricius, the target organ of IBDV diagnosis

The bursa of Fabricius (BF) is a primary lymphoid organ that plays a key role in the differentiation of B-lymphocytes and thus the development of the humoral immunity. Its development begins during incubation and reaches maximum size between 8 and 10 weeks of age, when the regression process starts, and is completed by 6 to 7 months of age (Olah et al., 2014). The immature B-lymphocytes are the target cells for replication of the IBDV (Infectious Bursal Disease Virus, also known as the Gumboro disease virus).

This is why the bursa of Fabricius is the principal diagnostic organ for evaluation of the Gumboro situation on farms.

When the IBDV replicates in the lymphocytes, we will normally see an impact on the bursa size. This impact can range from an increased size because of oedema (at the beginning of the replication; Figure 1a), to a decrease in size or atrophy due to the disappearance of most of the lymphocytes in the bursa (at the end of replication; Figure 1b).

Figure 1. a) Impact on the bursa size due to the replication of IBDV: Oedematous bursas (increased size). Figure 1. b) Atrophic bursas (decreased size). Accordingly, live IBD vaccines will also have an impact on the bursa size as an effect of vaccine virus replication. In fact, this vaccine replication is one of the most important protective mechanisms of live IBD vaccines against the disease, thanks to the competitive exclusion principle. However, in this case, the impact on the bursa size is less marked, with only a certain decrease in size normally observed at the end of the replication. Building up an historical record of the bursa size in all our flocks can be very helpful in detecting the time when the bursas start to decrease in size as a consequence of the vaccine replication and its correlation with the MDAs. In this way, if we detect a delay in some flocks (or even that there is no atrophy at all), we can speculate that we are facing a problem or that the MDAs were initially higher than usual (Table 1). It is important to remember here that when using an HVT-IBDV recombinant vaccine, we do not expect to observe any impact on the bursa size (as there is no live attenuated IBDV; Table 1). So, when these vaccination programmes are used, any change in the bursa size might be indicative of a field Gumboro virus replication. Table 1. Impact on bursa size of different IBD vaccination programmes.

How is bursa size evaluated?

There are several methods of evaluation of the bursa size. We can score the size of the bursa using a bursa-meter (Figure 2) or we can simply measure the bursa with a ruler. When using a bursa-meter, we will obtain a score from 1 to 8 depending on the diameter of the bursa (Figure 2). Scores from 4 to 6 are considered normal for a healthy bursa (including live attenuated IBDV replication). Scores below or above that (1, 2 and 8) may be indicative of a pathological condition of the bursa. Figure 2. Images and scale of HIPRA bursa-meter.

Important points to take into account when using bursa size to evaluate a Gumboro vaccination programme

As has been explained, the size of the bursa can be a good indicator of the time when the bursa started to be atrophic as a consequence of the vaccine replication. However, it is important to take into account that, as a lymphoid organ, there are other causes (infectious and non-infectious conditions) that can also affect bursa size (Table 2). For this reason, a differential diagnosis will be crucial when the size and the time of the atrophy observed on the farm does not correlate with what is expected after vaccination. Table 2. Review of different conditions that can directly impact the size of the bursa of Fabricius. In addition, before using the bursa of Fabricius size as an indicator, it is also important to set size standards for a healthy bursa of Fabricius. Unfortunately, there are not many up-to-date publications that include all types of breeds, genders or farming conditions (Table 3). Table 3. Bursa of Fabricius height (mm) and width (mm) in male Cobb 500 broilers (Cazaban et al; 2015).

This is why it is so important for a historical record to be built up for each farm, as this parameter will probably just be comparable within this same unit.

References:

Casanova, S. et al. 2015. Ciência Rural, Santa Maria, 45: 64-67.
Cazaban, C. et al. 2015. Poultry Sci. 94:2088–2093.
Chang, S. et al. 2011. Avian Dis. 55:384–390.
Haridy, M. et al. 2012. J. Vet. Med. Sci. 74:757–764.
Hoerr, F. J. 2008. Diseases of Poultry, 12th edition: 1197–1230.
Olah, I. et al. 2014. Avian Immunology, 2nd Edition pp: 11-44.
Tarek, K. et al. 2013. Int. J. Poultry Sci. 12 (6): 377-378.
Wang, L. et al. 2007. Acta Microbiol. Sin. 47:492–497.
Wang, X. et al. 2019. Vet. Microbiol. 236: 108389.

La entrada Bursa size, an easy tool to evaluate gumboro vaccination programmes se publicó primero en Gumboro Prevention.

Competitive exclusion in the bursa of Fabricius: The hidden protection against IBD

Adrià Martos — Wed, 08 Sep 2021 10:51:54 +0000

Protection against IBD (Infectious Bursal Disease or Gumboro disease) by vaccines is normally evaluated through their capacity to create a good humoral response against the virus (IBDV). However, the protection provided in the case of live vaccines involves a step prior to the creation of specific antibodies against IBDV: The competitive exclusion effect in the bursa of Fabricius. This is not just the first barrier of protection that the live vaccine creates against IBDV, but also, probably, the most powerful one.

What is competitive exclusion in the case of Gumboro disease?

The competitive exclusion principle by Gause (1934) asserts that two species cannot exploit the environment in exactly the same way and coexist – one of the species will be excluded.

This principle is what we observe in the case of Gumboro disease in the competition for the bursa of Fabricius niche between two field IBDV, but also between a vaccine virus and a field virus.

In both cases, the virus that arrives first and replicates better in the bursa will block the multiplication of the other.

Why can competitive exclusion in the bursa of Fabricius be the most powerful protection against IBDV?

Basically, because the replication of the vaccine virus in the bursa of Fabricius will prevent not only the multiplication of the field virus (and the consequent viraemia) but it will also avoid shedding and possible mutations.

Moreover, the competitive exclusion protection does not depend on nucleotide similarities between the vaccine virus and the field strain since it is just a matter of physical occupation of the replication niche.

In fact, this viral competition is so powerful that it also explains why, in countries like United States, the endemic viruses are variant types instead of the vvIBDVs detected all over the world (Jackwood, 2011).

How can we measure the competitive exclusion protection provided by a live IBD vaccine?

The colonization of the bursa of Fabricius by the vaccine strain represents the start of protection of the bird by competitive exclusion and is established in each bird as soon as the first signs of replication of the virus in the bursa of Fabricius are observed.

For an optimal evaluation of the efficacy of vaccination with live vaccines, monitoring of the colonization of the bursa of Fabricius by the vaccine strain over time is recommended.

Molecular diagnosis (PCR + sequencing) is the tool that is normally selected for detection of the vaccine virus in the bursa.

References:

D. J. Jackwood. 2011. Viral competition and Maternal Immunity influence the clinical disease caused by very virulent infectious bursal disease virus. Avian Disease 55:398-406.

La entrada Competitive exclusion in the bursa of Fabricius: The hidden protection against IBD se publicó primero en Gumboro Prevention.

How do IBDV immune complex vaccine viruses arrive at the bursa of Fabricius on the optimal day?

Advertis Agencia — Fri, 18 Jun 2021 09:00:00 +0000

The IBDV immune complex vaccines were developed in the late 1990s with the aim of having an IBD vaccine that could be given in the hatchery and that could provide protection regardless of the maternal antibody level of the chicks. However, the use of IBD immune complex vaccines has proved to be much more, offering live vaccine protection with improved safety and increased potential for humoral response compared to the use of the homologous vaccine virus alone¹.

In addition, immune complex vaccines can provide the so-called “intelligent vaccination” against IBDV by adapting their onset of immunity to individual protective needs, colonizing the bursa of Fabricius on the optimal day for each chick. This adapted onset of immunity avoids the feared immunity gap that may happen when other types of vaccines are used². But what is the mechanism of action of IBDV immune complex vaccines? How is the neutralization by maternal antibodies prevented? Where is the vaccine virus retained until its replication at the bursa of Fabricius starts? Watch the following video for a better understanding of the mechanism of action of IBDV immune complex vaccines:

Video summary

The key point of IBDV immune complex vaccines is to ensure the correct coating of the vaccine virus with the specific IBDV antibodies (IgY). This correct coating of the virus is what will prevent the neutralization of the vaccine virus by maternally derived antibodies (MDAs). The immune complex (virus+IgY) will then be retained in the follicular dendritic cells of lymphoid organs (and probably on macrophages) which will lead to a continuous release of the virus without antibodies. If the protection provided by the mother is still active, the virus released will be neutralized by the MDAs and will not start to replicate. Once this maternal protection starts to decrease, the virus released will not be neutralized and will start to replicate in the bursa of Fabricius, which means that the bird will automatically be protected by competitive exclusion and the onset of immunity will start. This onset of immunity depending on the protection needs of each individual chick is known as “Gumboro intelligent vaccination”. References:

Whitfill et al. 1995. Avian Diseases 39 (4), 687-699.
Gelb et al. 2016. Avian Diseases 60(3), 603-612.

La entrada How do IBDV immune complex vaccine viruses arrive at the bursa of Fabricius on the optimal day? se publicó primero en Gumboro Prevention.