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.

"Subclinical IBD is hard to recognize”

Prof. Sjaak de Wit, Senior researcher, GD Animal Health

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.

“Subclinical IBD is more underestimated than in the past”

Prof. Sjaak de Wit, Senior researcher, GD Animal Health

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.

“Subclinical IBD is a global problem present everywhere”

Prof. Sjaak de Wit, Senior researcher, GD Animal Health

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 StepsActions
1. Macroscopic examinationObserve morphological changes such as atrophy, oedema, swelling and/or haemorrhaging
2. Microscopic examinationConduct histopathology to see if there’s loss of, or damage to, the lymphoid cells in the bursa
3. Molecular sequencingIdentify viruses present to see if they are vaccine strains or field strains (classical or variants).
4. Virulence assessmentEstimate the potential virulence of viruses to identify which vaccine program is likely to be most successful


"We do have the tools to monitor the potential impact of IBD"

Dr Guillermo Zavala, President of Avian Health International

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


"Make an attempt to include as many vaccine virus strains in your program”

Dr Guillermo Zavala, President of Avian Health International

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.

“The more different viruses you include in the killed vaccine gives you a better opportunity to broaden the spectrum of protection”

Dr Guillermo Zavala, President of Avian Health International

  1. Christensen NH. The cost to the meat chicken industry of the introduction of Infectious Bursal Disease to New Zealand. N Z Vet J. 33:191-3; 1985 Nov.
  2. Lasher HN, Davis VS. History of infectious bursal disease in the U.S.A.--the first two decades. Avian Dis. 41:11-9; 1997 Jan-Mar.
  3. McIlroy et al. Economic effects of infectious bursal disease on broiler production. 1989. Avian Pathology; 18: 465-480
  4. Tacken MGJ. 2003. Molecular Interactions of the Infectious Bursal Disease Virus Proteins; Chapter 1: 13
  5. Ohnmar Myint et al. Bursa atrophy at 28 days old caused by variant infectious bursal disease virus has a negative economic impact on broiler farms in Japan, Avian Pathology, 50:1, 6-17
  6. H. B. Aliyu et al. (2022) Comparative pathogenicity of Malaysian variant and very virulent infectious bursal disease viruses in chickens, Avian Pathology, 51:1, 76-86,
  7. Van den Berg, T. P. & Meulemans, G. Acute infectious bursal disease in poultry: protection afforded by maternally derived antibodies and interference with live vaccination. Avian Pathology 20, 409-421 (1991)

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