What is subclinical IBD?

Infectious bursal disease (IBD) is associated with significant immunosuppression, increased mortality, and poor economic performance ^{1, 2}.

In the subclinical form of IBD, susceptible chickens younger than two weeks of age don’t show, or hardly show, any clinical signs when they become infected by classical or variant IBD strains.

The importance of subclinical IBD

Subclinical IBDV infections are responsible for growth depression, reduced feed conversion efficiency and, because of IBDV’s immunosuppressive effect, an increase in reactions to vaccinations as well as a reduction in the efficacy of vaccine programs.

This makes affected flocks more susceptible to many common poultry pathogens, resulting in increased mortality, poor performance and an increased requirement for antibiotic use.

Economic impact of subclinical IBD

These infections are economically important because of the severe, and long-lasting, immunosuppression they cause.  A significant net income drop of 10% per 1,000 birds (p>0.001) has been reported from flocks in IBDV infected houses compared to concurrent flocks in houses in which the virus was not present³.

The global distribution of IBDV infections

An OIE survey in 1995 confirmed IBD as a truly international problem with 95% of the 65 countries responding to the survey declaring cases of infection, including New Zealand which had been free from the disease until 1993.⁴

Variant IBD strains were firstly reported in US and some Latin American countries and they are currently being reported in Japan⁵, Malaysia⁶ and other countries worldwide.

Diagnosis of subclinical IBDV infections

Most companies don’t look for IBD before 21 days of age (or later) because, in healthy flocks, IBDV infection of the bursa of Fabricius should not be evident before 21-24 days of age.

To prevent premature bursal damage caused by subclinical IBDV infections, broiler producers should score and evaluate the integrity of the immune system at strategic ages starting from 14 days. Macroscopic (field) and microscopic (lab) examination of the bursa of Fabricius will detect morphological changes such as atrophy, oedema, and haemorrhaging that are consistent with IBDV infection. The next step in diagnosis should be to apply molecular techniques to identify and genetically sequence any IBD viruses present.

Key diagnostic steps

Diagnostic Steps	Actions
1. Macroscopic examination	Observe morphological changes such as atrophy, oedema, swelling and/or haemorrhaging
2. Microscopic examination	Conduct histopathology to see if there’s loss of, or damage to, the lymphoid cells in the bursa
3. Molecular sequencing	Identify viruses present to see if they are vaccine strains or field strains (classical or variants).
4. Virulence assessment	Estimate the potential virulence of viruses to identify which vaccine program is likely to be most successful

IBD Prevention

IBDV is a highly contagious and resistant virus that is spread horizontally. As such, the priorities for prevention are:

1. High biosecurity standards with intensive cleaning and disinfection of houses between cycles
2. Vaccinating breeder hens so that high levels of maternally derived antibodies (MDA) are vertically transmitted to young broiler chicks
3. Vaccination of the chickens once the MDA have depleted

Role of inactivated IBDV vaccines to provide high level of MDA

High levels of MDA can fully protect newly hatched chicks against IBDV infections in their first weeks of life. For this reason, it has become a common practice to prime parents with one or two live vaccines and subsequently apply an inactivated oil emulsion vaccine (OEV) before laying to hyper-immunize them and induce high and uniform levels of MDA⁷. The quality of MDA is influenced by the antigenic diversity of the vaccine administered

Inactivated IBDV vaccines with broad antigenic spectrum

Although all IBDV strains belong to serotype 1, different subtypes have been found. All these subtypes share part of the protective antigenicity, but clear differences can be found between several subtypes. For this reason, it is important that killed vaccines contain a high antigenic mass and antigenic diversity by including various vaccine strains in order to broaden the antigenic spectrum, and protection, provided by the vaccine.