Controlling Gumboro Disease, is what I am doing enough?- Part 1

Gumboro Disease or Infectious bursal disease (IBD) is a disease of chickens which targets the Bursa of Fabricius, an important organ in the young chicken's developing immune system.

The causative agent, a Birna virus, destroys immature B-lymphocytes in the Bursa of Fabricius, resulting in immunosuppression. Very virulent strains of IBDV can result in mortality of up to 40%. Control is best achieved by improved biosecurity and vaccination.

There is no IBD vaccination schedule that can be routinely recommended and there are many influencing factors (type of chicken, level and uniformity of MDAs, MDA breakthrough titre of the vaccines; field pressure;…).

For this reason, in order to monitor vaccination effectiveness, it will be important to implement a follow-up protocol by which different variables should be evaluated over time. This will help to establish a baseline of results as a reference for the farm.

Some basic parameters should be considered in the monitoring routine:

· Serological response

· Macroscopic evaluation of the bursa

· Detection of vaccine virus in the bursa

· General productive parameters

Serological response

Taking blood samples at marketing age or at specific points in the bird’s life is essential in order to know the situation of the flocks and to be able to compare this with the mean for previous flocks. In this way we can also detect virus pressure and have a full and better understanding of the situation.

Macroscopic evaluation of the bursa

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 standardize or record the results of those necropsies. This routine practice can be used to evaluate the Gumboro situation on farms and to evaluate the vaccination scheme we are using.

By the macroscopic evaluation of the bursa we can assess:

· The size of the bursa: This parameter is not an accurate measure but it gives us a quick idea of the condition of the bursa. We can score the size of the bursa using a bursa-meter (Table 2) or a ruler. Depending on the diameter of the bursa we will obtain a score from 1 to 8. The main aim is to build up an historical record of all the flocks, in order to be able to detect the time when the bursa started to be atrophic as a consequence of the vaccinal replication and its correlation with the MDAs.

· Ratio of bursa: body weight (bursa weight X 1000 / body weight): This is a standardized way of determining the level of bursal atrophy, whether post vaccination or after a challenge with IBDV. The main objective is the same as that of the size of the bursa.

· Appearance of the bursa: In birds vaccinated with live vaccines, it is expected that bursas observed will be homogenous in size and appearance and without major signs of external or internal lesions. Small petechial haemorrhages and some degree of atrophy are the signs that can be seen in bursas where a live attenuated virus is replicating.

If poor condition of the bursa is suspected, and for further investigation, a histopathology study could be recommended.

Detection of vaccine virus in the bursa

The colonization of the bursa by the vaccine strain represents the onset of immunity of the vaccine 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, 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 quick detection of the vaccine virus in the bursa.

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