Joe Giambrone, Teresa Dormitorio, Tom Brown, and Ken Takeshita

Infectious bursal disease virus (IBDV) is a serious problem for commercial broiler producers throughout the world, costing Alabama poultry producers $1 million annually in production losses and treatment costs, and amounting to a hundred million dollars in losses each year worldwide.

IBDV affects the bursa of Fabricius, an organ that produces lymphocytes that help protect birds against disease and illness. IBDV attacks the bursa, killing lymphoid cells and suppressing the birds' immune systems. Clinical symptoms of IBDV include diarrhea, weight depression, paleness, and lameness. These symptoms result in increased condemnations of bird carcasses, above average mortality, and poor feed conversion and weight gain.

Freezing of poultry carcasses in these containers is an approved method for disposal of farm mortalities due to diseases such as IBDV.

Vaccines are available to battle this disease; however, IBDV (like many viruses) is able to rapidly produce mutated viruses, called variant strains, that are resistant to vaccines. These variants, sometimes combined with poor vaccination techniques, plague the poultry industry worldwide. Recent studies have shown that IBDV variants are common in U.S. broiler producing areas, and variant IBDVs have been isolated from Georgia, Mississippi, and Arkansas.

Vaccination for IBDV typically is accomplished through broiler hens, which are vaccinated at the pullet stage (before they begin to lay eggs) and then pass immunity on to their offspring via the egg yolk. This breeder vaccination effort represents the first line of defense against early IBDV infections.

The success of vaccines and vaccination programs can be determined by profiling blood sera of hens during egg production. Vaccinated pullets are tested using enzyme linked immuno-sorbent assay (ELISA) kits, which determine if antibodies are present in the birds' blood. However, these ELISA tests do not always identify the presence of IBDV, especially variant strains, nor can they predict the effect of IBDV on broiler flock performance.

Another means of determining the efficacy of vaccination programs is to measure the size of bursae as the birds age. A smaller than normal bursa could indicate that the bird is infected with IBDV and thus is unable to fight the virus. However, there are numerous infectious and noninfectious agents that can alter bursae size.

A University of Delaware researcher developed a method to predict the success of breeder hen vaccination programs by comparing bursa weight to body weight of two-week-old IBDV-challenged chickens. If the bursa is small in comparison to the size of the bird, the bird is susceptible to infection with IBDV. The greater the ratio between these two measurements, the more likely a bird is resistant to IBDV infection.

An ongoing AAES study, which was instituted in 1994, is helping determine the efficacy of broiler breeder vaccination programs in commercial southeastern broiler flocks using newly isolated variants from this region. Results were determined using bursae size of IBDV-challenged broiler progeny. The study's goal was not to determine which commercial vaccine or program is the best, but rather to determine whether current vaccines and programs commonly used in the industry would protect against newly isolated variant IBDV and if the new ELISA system containing both standard and variant IBDV antigens would correlate with resistance to infection with the variant IBDV at two weeks of age.

Four IBDV serologic (blood serum) and challenge (exposure to the virus) experiments were conducted using progeny from poultry producers located in the Southeast. All breeder flocks received at least one live and two inactivated vaccines, which contained both standard and variant IBDV strains. For each experiment, day-old progeny were collected from six breeder flocks. At two weeks, 15 progeny per flock were exposed to the standard virus, 15 per flock with a variant E virus from the Delaware study, and another 15 per flock were challenged with one of four new variants (GA, ARK-1, ARK-2, and MISS). Ten progeny per flock were not challenged.

At the same time, 10 birds from each flock were tested using a new commercially available ELISA. Seven days after exposure to the virus, the bursae of Fabricius from each bird was weighed. Any bird with a bursa-to-body weight ratio that was two standard deviations less than the average of the controls was considered unprotected. Flocks with at least 30% protection were considered well protected.

The table shows IBDV serologic and challenge data, based on bursa:body weight data, for the first experiment. Data showed that all flocks except flock number 6 were well protected against the standard virus. Results for the IBDV E and GA variants were similar to each other; all flocks except 5 and 6 were well protected. In the second experiment, results for the standard and MISS isolates were similar. All flocks except one were well protected against these two isolates. In contrast, only flock 3 was well protected against variant E. In the third experiment, results for the standard and ARK-1 isolates were similar. All flocks were well protected against these isolates. In contrast, only flock 2 was well protected against variant E. Data for the fourth experiment showed that only flocks 1, 2, and 3 were well protected against the standard isolate. Results for the variant E and ARK-2 isolate were similar. Only flock 2 was well protected against these isolates.

Results of the ELISA tests showed antibody titers (concentrations) ranging from a low of 14 to a high of 16,112 (see the table). Flocks with antibody titers of 1,500 or more were always adequately protected against the standard virus. However, correlation between ELISA analysis and resistance to the variant E virus was not consistent. Flocks with antibody titers as high as 16,112 were not well protected against this isolate. Flocks with low titers (below 1,500) were always poorly protected against the E isolate.

IBDV-induced immune suppression continues to cause significant economic losses in the U.S. broiler industry despite improved vaccine programs. All commercial broiler breeder pullets in the United States are now receiving combinations of live and inactivated vaccines containing both standard and variant viruses. Recent studies have shown that the incidence of variant IBDV is increasing throughout the United States and that resistance to IBDV variant exposure at two weeks of age is a better judge of immunity than the results of ELISA tests. Previous results showed that resistance to the variant viruses was significantly less than that to the standard virus.

The AAES study evaluated progeny from states that represent nearly 60% of the U.S. broiler production. The data confirmed previous results, which showed that producers were doing a good job of immunizing their flocks against the standard virus, but significantly less so against the variants. The data also agreed with previous work, which showed a lack of correlation between ELISA results and resistance to the variant E challenge viruses at two weeks of age. However, the new ELISA kit was an improvement over previous kits. Also, there was good correlation between ELISA titer and protection against the standard, MISS, and ARK-1 viruses.

Results of this study suggest that, since the incidence of IBDV infections continues to occur in the United States and new variant viruses continue to evolve, continual monitoring of IBDV pullet vaccination programs using a number of different IBDVs seems warranted. In addition, the study showed that MISS and ARK-1 variants are both distinct from the E, GA, and ARK-2 variants.