infectious bursal disease

Chickens, turkeys, ducks, pigeons and guinea fowl may be infected with IBDV, but clinical disease only occurs in chickens. Mortality is higher in lighter breeds than heavier breeds (Berg, 2000). Serotype 1 viruses mainly infect fowl, and turkeys to a lesser extent, although the widespread use of serotype 1 vaccines makes it difficult to determine the true prevalence.

IBDV Type 2 viruses are widely distributed in turkeys (McFerran, 1993). In several other species, IBDV or IBDV-specific antibodies have been detected, such as in coturnix quail, pigeons, pheasants, village weavers (Ploceus cucullatus), pied cordon bleus (Uraeginthus bengalus), magpie geese (Anseranas semipalmata), shearwaters (Puffinus carneipes [Ardenna carneipes], P. pacificus [A. pacifica]), soothy terns (Sterna fuscata), common noddy (Anous stolidus), silver gulls (Larus novaehollandiae) and black ducks (Anas superciliosa) (Wilcox et al., 1983; McFerran, 1993).

Classical IBD viruses occur worldwide, with the probable exception for New Zealand (Lukert and Saif, 1991; McFerran, 1993; Becht, 1994). In the USA, antibodies against IBDV serotype 2 are widespread in chicken and turkey flocks, indicating the common prevalence of the infection. Since the recognition of very virulent IBD viruses (vvIBDVs), in the late 1980s in Europe, acute forms of the disease have been described in Japan during the early 1990s. Currently, vvIBDVs have been isolated in Asia, Central Europe, Russia, the Middle East and South America (Yamaguchi et al., 1997; Fabio et al., 1999; Liu et al., 2001; Meir et al., 2001). To date, Australia, New Zealand, Canada and the USA are unaffected with vvIBDVs (Proffitt et al., 1999). It is estimated that vvIBDVs are present in 95% of the World Organisation for Animal Health (OIE) member countries (Berg, 2000).

For current information on disease incidence, see OIE's WAHIS database.

In summary, the diagnosis can be achieved by observation of (1) clinical signs (2) gross pathological lesions in the cloacal bursa (3) microscopic analysis of the bursa for B cell depletion in the follicles (4) laboratory diagnosis using RT-PCR for detection of VP2 viral gene in bursa tissues (5) Sequence alignments and phylogenetic analysis of the VP2 region to characterize the isolated viruses into genogroups.

Clinical and gross pathological evaluation: In fully susceptible flocks, acute clinical disease may easily be recognized. The disease manifests itself with a rapid onset, high morbidity, spiking mortality and rapid recovery (Lukert and Saif, 1991; Eterradossi and Saif, 2008). At necropsy, the bursa will show changes depending on the stage of the disease. In early stages, the bursa is enlarged, whilst in later stages, the bursa is reduced in size due to atrophy.

Subclinical infections occur in very young chickens, or chickens carrying maternal antibodies, or are due to variant IBD viruses.

Differential diagnosis for sudden onset and ruffled feathers should include coccidiosis, especially when there is some blood in the droppings. However, an enlarged oedematous bursa and haemorrhages in the muscle tissue would suggest IBD. Some strains of infectious bursitis virus, in particular vvIBDVs can cause nephrosis, and should be considered when changes in the kidneys are observed. Changes in the bursa will readily point towards IBD (Lukert and Saif, 1991; Berg, 2000).

Laboratory diagnosis:

For detection of the antigen, the cloacal bursa or the spleen are the tissues of choice (Lukert and Saif, 1991). IBDV-specific antigen can most economically be demonstrated using an agar gel precipitation test. Alternatively, impression smears of frozen sections of the bursa may be tested by immunofluorescence.

Reverse transcription PCR (RT-PCR), followed by restriction enzyme digestion or restriction fragment length polymorphism (RFLP) analysis of the amplified fragment is common for the detection of IBDV and for the differentiation of pathotypes (Ghorashi et al., 2011; Hernández et al., 2011; Tomás et al., 2012). Nucleotide sequencing is commonly used to confirm RFLP analysis and for epidemiological studies, including the study of the evolution of the virus in different geographic locations (Cortey et al., 2012). FTA cards have been used successfully for the collection of nucleic acid of IBDV and for the safe transport of the samples, including internationally, to diagnostic laboratories (Moscoso et al., 2006).

Virus isolation can be performed in embryonated eggs of CEC, however, this is not necessary for routine diagnosis. Bursal tissues should be macerated in an antibiotic-containing medium and centrifuged to remove larger tissue particles. The supernatant fluid is then used to inoculate embryonating eggs or cell cultures.

For serological diagnosis, the presence of IBDV antibodies could be determined using IBDV-specific commercially available ELISAs (Becht, 1994). The presence of IBDV antibodies in chicks is not always an indication of infection because most young chicks have maternal antibodies. ELISA is mainly used to evaluate the flock immunity (Lukert and Saif, 1991). The antibody profile may be performed with breeders or with day-old progeny, in the progeny sera, the titres are usually about 60-80% lower than in those of the breeders (Lukert and Saif, 1991). Another serological method is virus neutralization test (VNT) which can differentiate between serotype 1 and 2 of IBDV strains. Given the antigenic variation between the IBDV strains, the reference strain used in the VNT is of utmost importance (Jackwood et al., 2004).

Management procedures

Disease prevention aims at prevention of infection by strict hygiene measures and vaccination. The stability of the virus largely contributes to the likelihood that transmission will occur for prolonged periods of time and from an infected premises towards uninfected farms. Therefore, routine sanitary precautions must rigorously be followed for IBDV. Although disinfection may be difficult, thorough disinfection with appropriate disinfectants will reduce the virus load and therefore will reduce the risk of transmission. Eradication of mechanical vectors such as mosquitoes, mealworms and smaller rodents must also be pursued (Lukert and Saif, 1991). On farms where IBDV outbreaks have occurred, the virus may be considered endemic. Young birds will be exposed to the virus at a very early age, when cleaning between broods is not thorough (Lukert and Saif, 1991).

Immunization

The control of IBD is largely dependent upon the use of effective vaccines and vaccination strategies resulting in protection against the disease. Humoral immunity has a major role in protection against IBD. Indeed, there is a close correlation between neutralizing antibody titres and protection against the disease. To induce high levels of maternal antibodies in the progeny, parent stocks are usually vaccinated between 4 and 10 weeks of age with live vaccine and again at approximately 16 weeks with inactivated oil-adjuvanted vaccine. Maternal antibody provides protection in young chicks, and this depends on antibody titre. Therefore, determination of antibody titre in chicks is important for planning an effective vaccination strategies as maternal antibodies can interfere with vaccination.

Another consideration in vaccine development against IBV is point mutations and recombination events, which have led to the antigenic drift and antigenic variation in IBDV strains. Thus, selecting an effective vaccine has become critical to controlling IBDV strains which have undergone antigenic divergence. Because of antigenic drift in different geographical sites, development of a universal IBDV vaccine which can be used worldwide is impractical (Hon et al., 2008; Jackwood and Sommer-Wagner, 2011; Jackwood, 2012).

An early method of control involved intentional exposure of young chickens to IBDV (Lukert and Saif, 1991; Lasher and Davis, 1997). This technique lowered IBD mortality but often resulted in immunosuppression and further dissemination of the field virus. Live attenuated vaccines were developed, based on mild field isolates passaged in specific-pathogen-free eggs. They are still widely used today in breeders as a primer and in the control of very virulent IBD in many countries. Until the 1980s, mortality caused by IBD was effectively controlled by vaccination. However, the effects of immunosuppression and the tremendous economic impact of the disease were just starting to be appreciated. Recognition of Delaware variants in the USA in the mid-1980s and the emergence of very virulent forms of the condition in Europe and Asia in 1989 illustrated the continuing importance of IBD (Lasher and Davis, 1997; Lütticken, 1997; Berg, 2000; Eterradossi and Saif, 2008).

It is essential to prevent infection at an early age, so that the immunosuppressive effect of IBDV can be circumvented. This can be achieved by immunization of the breeders. When oil-adjuvanted vaccines are used to boost the immune response, the maternal immunity may be extended to 4-5 weeks. Normally maternal immunity lasts 1-3 weeks, protecting the chicks from early immunosuppressive infections (Lukert and Saif, 1991). However, maternal antibody can interfere with the replication of live attenuated vaccines. If the vaccines are administered too early, the vaccine virus will not replicate effectively, while if it is administered too late, the birds will become vulnerable to virulent IBDV (Mahgoub, 2012). Therefore, monitoring of the antibody level in a breeder flock or its progeny can aid in determining the right time to vaccinate (Lukert and Saif, 1991; Eterradossi and Saif, 2008).

Vaccines may be administered by intramuscular injection, by spray or by drinking water. When maternally derived antibodies are not present, vaccination is possible at 1 day of age. If maternally derived antibodies are suspected, serological monitoring is required to determine the right time of vaccination (OIE, 2000).

Attenuated vaccines are referred to as mild, intermediate, or ‘intermediate plus’ (hot) vaccines. Mild vaccines do not cause bursal damage in chicks but may be poorly efficacious in the presence of certain levels of maternal antibody or infection by vvIBDV. Vaccines of greater intrinsic pathogenicity (intermediate, or ‘intermediate plus’ (hot)) may break through high levels of maternal immunity but may also cause damage to the bursa, with subsequent immunosuppression. In addition, they may also not protect against infection with vvIBDV (Rautenschlein et al., 2005) or antigenic variants.

Mild vaccines are frequently used to prime broiler breeders prior to vaccination with an inactivated, frequently oil-adjuvanted vaccine. Intermediate and ‘hot’ vaccines are mostly used to overcome the maternally derived antibodies in young broilers (OIE, 2000).

The reverse genetics (RG) system opens a new avenue for the development of IBDV live attenuated vaccines which are highly immunogenic while the resulting live attenuated vaccine have low chance of revert phenotype. Several recombinant attenuated IBDV have been rescued and characterized for live attenuated vaccine purpose. Rational attenuation strategies using RG may lead to a new generation of safer, more widely applicable live attenuated vaccines for IBDV (Yang et al., 2020).

Given the difficulties of achieving effective vaccination with live IBDV vaccines in the face of differing levels of maternal immunity, challenge strains of different pathogenicity, and the danger of inducing immunosuppression with ‘hotter’ vaccines, several different approaches to the development of IBDV vaccines have been explored. These include genetic modification of IBDV to attenuate pathogenicity very precisely; subunit vaccines based on the production of IBDV recombinant protein subunits in heterologous expression systems such as plants, yeast, bacteria and insect cells. Other vaccines include DNA vaccines; immune complex vaccines (Icx), viral like particles and live viral vector vaccines. All these have been reviewed by Müller et al. (2012).

An Icx vaccine comprises live pathogenic IBDV mixed with anti-IBDV antibodies derived from hyperimmunized chickens. It can be administered subcutaneously at day-old in the presence of various levels of maternal antibody, resulting in active immunity without causing any vaccine-induced immunosuppression (Haddad et al., 1997; Iván et al., 2005). Icx vaccines are also used to vaccinate in ovo at day 18 of incubation using automated technology to achieve very precise vaccination.

Live virus vector vaccines comprise a gene from one pathogen e.g. VP2 of IBDV, within the genome of another, live virus - the vector. Vectors investigated with VP2 include Newcastle disease virus, fowlpox virus, Marek’s disease virus, herpesvirus of turkeys (HVT) and avian adenovirus. Of these, the HVT-vectored VP2 vaccines have been introduced commercially in some countries. HVT vaccines are widely used, having been used successfully for decades to control Marek’s disease. Unlike live IBDV vaccines, HVT vaccines do not cause immunosuppression and these vaccines are not interfered by maternally-derived antibody. HVT-VP2 vaccines, depending on the manufacturer, can be applied in ovo or subcutaneously in day-old chicks. A single injection of HVT based vaccine, which is not influenced by MDA, induces protection in 1-day. However, the main limitation to its wide use is the need of cold storage in liquid nitrogen, which increases the cost of vaccinations (Bublot et al., 2007; Gros et al., 2009; Perozo et al., 2009; Jackwood, 2017; Rage et al., 2020).