Infectious bursal disease (IBD) is one of the most important and common immunosuppressive diseases affecting poultry, along with chicken infectious anemia, Marek’s disease, and some mycotoxicoses and various forms of stress. Each of these diseases can impair the ability of chickens to cope with ordinary infectious disease agents, either independently or in concert.

Specifically, infectious bursal disease virus (IBDV) replicates in gut-associated macrophages, causing infection and destruction of proliferating immature B cells in the bursa of Fabricius, compromising antibody responses against a variety of poultry pathogens.^1,2

A permanent threat

IBDV is one of various poultry pathogens that is always present in poultry houses. Good sanitation reduces virus concentration in the environment and may help to delay and reduce early challenge, but it never eliminates the virus in commercial settings.

Consequently, all poultry operations must establish robust vaccination programs to protect breeder flocks and their progeny against early challenge.³

Breeders are typically immunized with live and inactivated (killed) vaccines, whereas chicks are usually vaccinated at the hatchery with live traditional or recombinant vaccines, or in the field using live attenuated vaccines. Commercial layers are vaccinated mostly with recombinant herpesvirus of turkey (HVT)-IBD vaccines at the hatchery, or with live attenuated vaccines in the field.^4-6

One practical and effective approach for early protection is to rely on maternal immunity against IBDV until about 14 to 21 days of age, combined with a vaccination approach capable of addressing possible early IBDV challenge in the progeny, broilers or layer pullets.

Live attenuated vaccines have been used for many years to immunize young birds. However, live vaccines display wide variation in their degree of attenuation, and some can potentially cause bursal atrophy.⁷

Recombinant HVT-vectored IBD vaccines are only mildly sensitive to the effects of maternal immunity⁸ and have been adopted by the broiler and layer industries precisely because they do not cause bursal atrophy.⁹

They are also favored by producers due to their convenience, ease of application in ovo or subcutaneously or at hatch,¹⁰ overall efficacy against early and late challenge,^11,12 and the protection they provide against a variety of antigenic types and pathotypes.^13-15

Getting the best from live attenuated vaccines

Live attenuated vaccines may display some spectrum of protection against a variety of field viruses,^16-19 but there may be difficulties in successfully immunizing young chickens against IBDV when using them.

First, maternal antibody titers must not be high at the moment of vaccination so that a live attenuated vaccine may be able to infect and replicate in the bursa without interference.²⁰

Live attenuated vaccines should be able to trigger an immune response directed against the vaccine virus and against a variety of field viruses, which is always a challenge because the vaccine must not be interfered with by maternal antibodies, and it must not cause significant bursal damage.

A particular feature of HVT-vectored recombinant IBD vaccines (rHVT-IBD) is that they can be given at hatch or in ovo without maternal immunity causing interference.²¹

In addition, rHVT-IBD vaccines can cross-protect against both the so-called standard or classic strains of IBDV, and against a wide spectrum of variant viruses, including genogroup 2 variant viruses, regardless of the level of maternal antibodies against IBDV.^22-25

Narrowing the gap of susceptibility

It is important that any vaccine can induce early protection against IBDV, particularly at the interphase between declining levels of maternal antibodies and rising active immunity.

One of the benefits of rHVT-IBD vaccines is early onset of immunity. rHVT-IBD-vaccinated chickens may develop a significant level of protection against classical and genogroup 2 variant viruses as early as 14 days of age,²⁶ regardless of whether the vaccine was administered in ovo or at hatch. In the case of very virulent IBD, protection has been observed as early as 12 days of age using either route of administration.²⁷

Late protection is just as important, particularly in commercial layer pullets, which can remain susceptible to IBDV for several weeks. IBDV challenge with classic and variant strains at 63 days of age has suggested that protection against classic strains of IBDV can be as high as 96% to 98%, whereas moderate to high levels of protection were shown against variant strains.²⁸

A wide spectrum of protection

IBDV contains a genome consisting of two segments of double-stranded RNA, which facilitates mutations potentially resulting in the emergence of numerous antigenic types and pathotypes.²⁹

Antigenic drift is common in RNA viruses and is the result of gradual, cumulative mutations that lead to a slow drift of the antigenic makeup of IBDV.³⁰

Occasionally, more drastic genetic reassortments can result in antigenic shift and the sudden emergence of antigenic variants or pathotypes, which could cause a serious threat to young poultry.

So far, rHVT-IBD vaccines have been shown to be effective against classic genogroup 1 and variant genogroup 2 viruses, two drastically different group types of IBDVs.^31-33

This wide spectrum of protection is essential to cover broiler and layer pullets in multiple geographic locations, considering that every geographic region might tend to favor the emergence of local strains of IBDV.

In summary, rHVT-IBD vaccines represent an attractive alternative for protection against IBDV in young chickens. rHVT-IBD vaccines can provide early and late protection, and they can also provide protection against a wide variety of viruses representing classic and variant field strains.