Since the first description of the bursa, by Hieronymus Fabricius ab Aquapendente in the 16^th century, this peculiar organ has probably enjoyed the most debate and controversy in poultry research, especially in the last 60 years, since the identification of a virus (Infectious Bursal Disease virus) specifically targeting the bursa of Fabricius.

Bursa of Fabricius - Some Facts:

• B-lymphocytes are responsible for antibody (Ig) production during an immune response, and hence for the humoral immunity.
• Development of the bursa starts at 8 days of embryonation by colonisation of the bursa by the pre-bursal stem cells. These cells proliferate and differentiate in the bursa to become B-lymphocytes (Davison, 2008).
• B-lymphocytes start to migrate from the bursa to peripheral lymphoid tissues at 18 days of embryonation. This process continues to 3-4 weeks of age.
• Uninterrupted, the bursa follows normal growth, plateau and regression phases (Taylor & McCorkie, 2009), as shown in Figure 1.

Figure 1: Normal evolution of the Bursa of Fabricius

• The age when normal bursal regression starts, depends mainly on age at sexual maturity (Jolly, 1914 & Glick, 1956).
• Described in various publications, normal bursa size is further influenced by sex, breed, rearing conditions, age etc. In fact, any stress related situation i.e. too low or too high temperature, too much or too little ventilation, other diseases, mycotoxins etc. can also leads to a decrease in bursa size and therefor, variability in individual bursa sizes in a flock.
• Complete removal of the bursa (bursectomy) after 3 weeks of age has a minimal impact on immunocompetence (Blackmore et al, 2001). 
• In the case of an IBDV field infection, the size of the bursa follows a specific pattern (Figure 2):

o  4 days post infection, the bursa can be 2-3 times bigger than when not infected.

o  9 days post infection, the bursa can be 3-6 times smaller than when not infected.

o  35 days post infection, the bursa will have returned to the same size as when it was not infected.

Figure 2: Evolution of the bursa size or weight after IBD infection.

 

Bursa Size:

The size of the bursa is usually expressed, as either measuring the diameter of the bursa (bursometry in mm), weighing the bursa (g) or the weight of the bursa expressed as a fraction of the chicken’s body weight (Bursa to Bodyweight Ratio or BBR).

In a study by Moraes in 2004, it was stated that: “Bursometry (diameter) [...] of the bursa of Fabricius [is] considered inadequate to evaluate vaccine pathogenicity. There was no correlation between BF diameter and bursal histopathological lesion scores. [...] Similarly, no correlation was found between BF diameter and disease or vaccination”.

Bursa weight is difficult to evaluate and interpret under field conditions. As shown in Figure 2, the “normal” increase and then decrease in bursal size after infection, can only be used if the date of infection is known. Even when vaccinating, or in other words infecting, the chicken with an attenuated live IBD virus on a specific day, the exact date of infection in the specific chicken under evaluation is not known, because of the variability of maternally derived antibodies in a flock. 

Right or wrong, BBR, or Bursal Bodyweight Ratio, is the most common way in evaluating vaccination efficacy in the field. This in itself is problematic, for exactly the same reason as only looking at bursal weight, not knowing the date of the infection. As can easily be understood from what has been stated above, consensus on a standard BBR at a specific age is also not available and will probably never be. Figure 3, shows the BBR of control broilers at various ages in 11 studies.

Figure 3: BBR in the control broilers of 11 studies performed from 1979 to 2009.

At 28 days, the usual age when BBR’s are calculated, the values varied between the studies from 0.1351 to 0.328. Compared to the “standards” as claimed by three prominent poultry researchers, McMullin, Douglas and Bennett, it is clear that even the broiler controls in most instances do not have “acceptable” BBR figures.

The same picture is seen when evaluating control SPF chickens, in 22 studies between 1979 and 2009 (Figure 4).

Figure 4: BBR in the control SPF chickens of 22 studies performed from 1979 to 2009

 

IBD Vaccination:

IBD protection of poultry, through vaccination, is based on:

• Early protection (First 2-3 weeks of life) by Maternally Derived Antibodies (MDA) resulting from vaccinating the parent stock with a combination of live and inactivated IBD vaccines (Passive Immunisation using the hyperimmunisation technique).
• Later protection (2- 3 weeks and older) by active colonisation of the bursa with a live IBD vaccine strain (Active Immunisation).

Active colonisation is crucial for protection against IBD field infection. Schröder & Mundt, in the COST 839 Proceedings of 2002, clearly demonstrated that in challenge studies conducted in previously vaccinated chickens, with virulent IBDV, in the birds where the vaccine virus caused bursal lesions, no virulent IBD challenge virus, or lesions caused by it, was detectable. Conversely, in birds where the vaccine virus did not cause any detectable lesions, severe acute lesions were observed, caused by the virulent challenge virus.

Various IBD vaccines are commercially available to vaccinate all types of poultry. All the live IBD vaccines, except the recombinant IBD vaccines, are classified by their ability to colonise the bursa and also the level of bursal lesions caused by this colonisation. This classification also indicates the ability of the vaccine to overcome various levels of maternally derived IBDV antibodies and consequently its ability to protect the bursa against a field infection from either Classical IBDV or Very Virulent IBDV:

Table 1: Live IBD vaccines classification

 

Conclusion:

• Evaluating bursa size to determine immune competence and / or evaluating IBD involvement and / or evaluating IBD vaccination, is definitely not absolute. Various other factors, including environmental stress and other diseases, play a big role in bursa size.
• Other measures that must be used to evaluate IBD and IBD vaccination are:

o  IBD RT-PCR & Sequencing – distinguishing what virus, field strain, vaccine strain or no IBD virus, is present in the bursa.

o  Serology – not only IBD but also ND and IB. Very  low titres against ND and IB, can be the  indication of a compromised immune system (but keep in mind it can also be due to faulty vaccination practice).

o  Performance – mortality, mortality pattern, weight, FCR and abattoir condemnations. A lot of published studies, not concerning IBD, have found smaller bursa’s and below average performance, but, no studies have shown that a bigger bursa equals better performance.

o  Histopathology – lymphoid depletion, as evaluated by histopathology, must be used in conjunction with IBD PCR, to determine the cause of the lymphoid depletion.

• When evaluating bursa size, it is imperative to consider all factors involved. The inputs of a veterinarian are critical in arriving at the right conclusions.
• “Is bursa size still relevant today?” – the answer is yes and no. Yes, when using bursa size as an indicator in a more comprehensive, overall health plan and considering all possibilities including other diseases and other environmental stressors. No, when considering only IBD.