Rotation strategy to cool poultry IBD challenges

An immune-complex vaccine has a delayed release which offers major advantages. Photo: Lex Salverda

When it comes to protecting flocks from infectious bursal disease (IBD), poultry producers and veterinarians have several strategies available, including immune-complex vaccines that may be included in a rotation strategy with recombinant HVT-IBD vaccines.

Immune-complex vaccines provide unique attributes in the IBD protection toolbox. These vaccines contain an IBD virus vaccine coated with antibodies, which must first degrade before the vaccine virus is released in the bird, delaying the immune response compared with classic live vaccines.

Often used in cases of high field-virus pressure, immune-complex vaccines are useful for ‘cooling down the house’ and keeping the field pressure low for subsequent flocks, said Leticia Frizzo da Silva, DVM, PhD, senior principal scientist, Zoetis. They do this by colonising the bursa and blocking early replication of the field virus. The introduction and repeated use of immune-complex vaccines can repopulate a house with the vaccine virus, creating a ‘diluting’ effect, she added. Over time, this approach can replace the field virus to the point where the farm mostly has the vaccine virus.

Vector and live vaccine comparisons

The immune-complex vaccine has a delayed release, therefore it cannot be immediately neutralised by maternally-derived antibodies as conventional live vaccines can, said Frizzo da Silva. This delayed release brings at least 3 major advantages:

The robust vaccine-induced active immune response reduces the high chance of neutralisation of vaccine virus by maternal antibodies.
The onset of immunity is naturally adapted to when each individual bird is ready to respond due to the active vaccine-induced immunity starting as passive immunity decreases.
The immunosuppression often seen with the use of conventional live vaccines is reduced. Whether the birds are vaccinated in ovo or at one day of age, the immune-complex vaccine gradually dissociates in the bird’s body and releases the live vaccine virus, which stimulates a robust immune response without being immediately neutralised by maternally-derived antibodies.

Vectored IBD vaccines – based on herpesvirus of turkey (HVT) – and immune-complex vaccines have complementary roles. While vectored HVT-IBD vaccines cushion the bursa better and tackle clinical disease effectively, they cannot replace field virus to reduce the environmental load or lower the overall infection pressure in the long run. This makes rotation of the 2 approaches a sensible option, said Frizzo da Silva.

Options for different field scenarios

The IBD virus is genetically diverse, with 7 different genogroups falling into classical virulent, antigenic variant and very virulent types. Different field challenges can call for different immune-complex formulations. A rotation strategy of immune-complex and vector vaccines could offer an option for controlling IBD. In broiler chickens, immune complex vaccines based on the Winterfield 2512 strain and the V877 strain can be used, while the V877 strain can be used in layer chickens.