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Why is IBD a threat to the Layer industry?
IBD is a prevalent disease in chickens despite the wide use of vaccines and biosecurity. The virulence and resistance to environmental conditions allow the IBD virus to persist in the pullet houses despite proper cleaning and disinfection. The multi-age production system and genetic susceptibility also contribute to the virus persistency and economic losses for layer producers. The financial losses are associated with clinical (high mortality) and subclinical (immunosuppression) infection. This is especially important to layer-type chickens since they are more susceptible to the IBD virus than meat-type chickens (Tippenhauer,M. et al.;2013). Pullets flocks challenged with the IBD virus may experience low liveability, issues with other diseases and poor production performance.
IBD virus- A Pandemic Movement
Following the initial outbreaks in the US, clinical IBD was reported in different countries in the 1960s and 1970s. In the '80s, more virulent forms were reported in the US, associated with severe immunosuppression. These strains quickly spread to Central and South America. In the meantime, countries in Western Europe, North Africa, the Middle East and Asia started to report outbreaks caused by a more virulent IBD strain (vvIBD) (see map' Detection of IBD virus on a global level'). In the late 1990s and the early 2000s, the vvIBD started to be detected in Central and South America. Today, vvIBD and variant strains have been detected from layer flocks with clinical or sub-clinical forms, respectively, worldwide.
As forementioned, layer pullets are highly susceptible to IBDV infection in the first six weeks of their lives. During this period, the B-lymphocytes are in the full process of maturation and a target for IBDV. The maternally derived antibody against IBDV (MDAIBDV) will protect pullets against classical strains during the first 3 to 4 weeks. However, suppose the breeder flocks are not vaccinated with inactivated vaccines containing relevant variant IBD virus; in that case, variant IBDV strains may break through the passive immunity induced by classical IBD vaccine strains and cause infection. Due to this, the form of Gumboro to be developed after the infection is age-, strain- and dose-dependent and can be classified as Immunosuppressive, clinical and subclinical forms.
1.The immunosuppressive form occurs when pullets are infected within two weeks after hatch. Usually, variant strains are involved in this IBD form or when the MDAIBDV level is low enough, allowing classical strains to reach the bursa and replicate in the immature B-cell, leading to permanent immunosuppression.
2. An acute infection between 3 and 6 weeks leads to typical clinical signs of Gumboro diseases (bursa edema with or without hemorrhages, petechial or suffusions in the thighs and breast muscles). In severe cases involving vvIBD strains, the pullet mortality rate can increase significantly and reach 50-60%.
3. The subclinical form occurs when pullets are infected after 2-3 weeks of age, without typical clinical signs or mortality. The Bursa of Fabricius (BF) lesions are of variable intensity and persistent. Consequences depend on the pathogenicity of IBDV strain; most of the time, only poor or sub-optimal performances are detected (economical form' of IBD).