The BB index assists on assessing flock health

To circumvent the shortcomings of the bursa weight to body weight ratio (BB ratio, expressed in %), the BB index seems to be a better tool. This is defined as follows: BB index = BB ratio of infected (or vaccinated) birds/BB ratio of the controls. It enables to remove several variability parameters, like the age of the birds.

Standards of BB index do exist. They enable to classify the IBD viruses, or the conventional live IBD vaccines as shown in Table 1. The BB index may be useful in experiment conditions, where it is common to keep unvaccinated control birds. This is not an option in field conditions, however.

Under field conditions, a routine monitoring of the flock health by sampling chickens at a fixed date (say at 28 days of age) to record the BB ratio is mostly helpful if a farm BB ratio database has been set up. If the observed BB ratio is much smaller (or higher) than an accepted variability range around the mean, it may indicate a change in the environment of the chickens, which could be related to the overall stress (temperature, density, food quantity and quality, access to drinking water), or to diseases. It does not give much more information in case of infection, since contrary to experimental conditions, nobody would know the time at which the birds were exposed to the pathogen. As a consequence, nobody actually knows if the BF is in swelling stage (very early infection), on the process to get atrophied (turning into chronic stage), or already in a recovery process. It is only a kind of snapshot.

Invasion necessary
Would a relative atrophy of the BF be beneficial after vaccination using a conventional live IBD vaccine? Researchers Schroeder and Mundt compared conventional live IBD vaccines of different degrees of residual virulence. They managed to demonstrate that only chickens showing some bursa lesions due to the vaccine take were able to stand a challenge (using the classical strain Faragher 52/70).

On the contrary, chickens with intact BF following vaccination (hence without vaccine take, probably because of the use of a too mild vaccine strain) were severely affected by the challenge infection. In other words, it means that an invasion of the BF is necessary, and even desirable, by the IBD vaccine virus, to be able to stand a challenge with a pathogenic field IBDV later on.

Process of proliferation
Would a colonisation of the BF beyond two weeks of age by a Gumboro virus be harmful?

In the B cell ontogeny, two major steps can be identified during the embryonic life:

·         Embryonation days (ED) 8 to 14: the BF is colonised by pre-bursal stem cells.

·         ED 18: B lymphocytes start to be released by the BF into the peripheral bloodstream: they will reach the secondary lymphoid organs (eg, the spleen).

The very first immune cells appear from ED 12: IgM-bearing B lymphocytes can already be detected in the BF. At the end of the embryonic development, some isotype switching did already occur, since IgG-bearing B lymphocytes can be detected in the BF. By the 2nd to the 3rd week of age, the process of proliferation and emigration of the B lymphocytes is over.

These various experiments demonstrate that any lesion of the BF beyond two weeks of age is of no major consequence upon the bird’s immune competence. Note that the immunosuppression, due to the infection by a virulent Gumboro disease virus (eg, vvIBDV) is actually the result of the damage to a wider spectrum of target cells: immature B lymphocytes in the BF, but also mature B lymphocytes in the secondary lymphoid organs (spleen, caecal tonsils), and macrophages. Pathogenic IBDV is also damaging the cellular immune response through a transient, but marked atrophy of the thymus, inducing T cell apoptosis.

Atrophy is subjective
The BF appearance, size, and weight can be evocative of some physiological disorders (stress, disease). Such an investigation in field conditions is helpful, but does not tell the whole story. There is no standard of size for a given age, in a given breed. To conclude, atrophy, normal, or enlargement is therefore subjective.

A bursal atrophy can be observed in the field and in experimental conditions, following vaccination with a live IBD vaccine (using a conventional vaccine). But a BF atrophy does not necessarily indicate infection (or vaccination) by IBDV. A live (conventional) IBD vaccine take does induce a BF atrophy which is relative and transient. This atrophy is necessary to get the protection and has no consequences on the bird’s ability to mount an immune response.

Assessing flock health
How to properly assess a flock health status, in relation to IBD vaccine take? It should be made using a combination of observations and analyses, not through one criterion only. Recommendations in this respect are:

·      Compare the flock performances to the standard or the target.

·      Record BF size and BB ratio, with the limitations earlier described. This is useful if a database of the farm already exists.

·      Collect blood samples at the end of the growing period, and ask for a broad serology testing (IBD, and in case of suboptimal performances, immunosuppressive viruses like CAV and Reo).

·      Request histopathology analysis as well as virology testing (PCR and sequencing).

Additional studies are certainly needed to better describe and update the bursa size dynamics, according to the current standards of rearing.