An IBDV and IBV survey of North Georgia broilers in early 2016
By D. Sandu, M.W. Jackwood, H.S. Sellers, C.S. Roney, and S.M. Williams
Poultry Diagnostic and Research Center, Department of Population Health,
University of Georgia, Athens, GA
In fall and winter of 2015, the number of respiratory cases within broiler submissions to PDRC was on the increase. A noteworthy variation within the same flocks submitted, was a smaller than average bursa size indicating involvement with infectious bursal disease virus (IBDV). Worldwide, IBDV and infectious bronchitis virus (IBV) continue to be problematic in commercial poultry (1, 2).
IBDV is a bisegmented double-stranded RNA virus from the Birnavirus family (3). It is a very hardy virus due to its non-enveloped nature and tends to persist in the environment despite cleaning and disinfection. There are five viral proteins that have been recognized and of those, VP2, the capsid protein plays an essential role in the production of neutralizing antibodies (4, 5). IBDV causes severe immunosuppression and mortality in young chickens. To control IBDV, the broiler industry’s vaccination strategy is to hyperimmunize broiler breeders who would ultimately pass the antibodies via passive transfer to the broiler chick (5). Vaccination programs vary between each company, but ultimately, breeders are administered a vaccination series composed of live attenuated vaccines for priming and inactivated vaccines to prolong immunity. Breeder programs and field challenge in progeny is monitored to determine efficacy of vaccination programs. However, despite immunization programs, variants of Del E strain that are antigenically different from the vaccine, continue to display immunosuppressive effects in broilers (6,7)
IBV is a single-stranded, enveloped RNA virus in the Coronaviridae family. It is a highly contagious virus with a very short incubation period. IBV is very difficult to control because of numerous antigenic types and serotypes. The spike or S glycoprotein is involved in host cell attachment, membrane fusion and entry into the host cell (8). The tissue tropism of IBV was thought only to be localized to the upper respiratory tract. However, some IBV viruses can affect the female reproductive tract causing egg losses while other strains have a preference for renal tissue causing renal disease and increased mortality in young birds (9, 10). IBV infections are often complicated by secondary invasion of opportunistic bacterial organisms (11). The fast genetic mutation and recombination capabilities of IBV have made it extremely difficult to control. The use of live attenuated vaccines in the broiler industry has been able to provide some level of protection; however with the increase in new variants, vaccination programs are under constant challenge (8,11).
The goal of this study was to survey commercial broiler flocks from multiple companies located in North Georgia in order to identify circulating IBDV and IBV field viruses.
A total of 5 companies located in North Georgia participated in this survey. Over a period of 3 months, 420 birds were surveyed. Each company provided a current vaccine program for IBDV and IBV. Birds were submitted to the PDRC diagnostic lab and necropsied.
Survey Results and Discussion
IBD-XR Titers. IBD-XR (IDEXX, Westbrook ME) geometric mean titers (GMT) and the average coefficient of variance (CV) per company (A, B, C, D, and E) for birds between 18 and 21 days of age were obtained and compared to North Georgia historical average titers for birds around the same age. Each company submitted 10 birds from 5 flocks. In this survey, multiple combinations of IBDV breeder vaccinations were utilized by different companies. Three out of 5 were on recombinant in-ovo vaccine and 2 out of 5 were on a live IBD vaccination in-ovo. It should be noted that the sample size in this survey is relatively small and only represents a snapshot within each company sampled. Based on historical data from GPLN (https://www.gapoultrylab.org), the ELISA IBD-XR titers for broilers that are 14 to 27 days of age in Georgia is between 730 and 1300. The titers for companies A through E in this survey were well below this value and were considered negative. The differences in titers for companies in this survey may be due to varying degrees of field challenge and vaccination programs.
Histopathological Lesion Scores. The average bursal lesion score for birds submitted at ages 18-21 is summarized in figure 1. The combined average bursal score per company was below 2.5. On a scale of 0 (no lesions) to 4 (severe lesions), the lowest scoring company, C, had an average score of 2.08 and the highest score was observed in company D with an average of 2.48. The averages for the each company’s bursal lesion scores were considered normal (below 3); however the individual flock bursal lesion score was suggestive that there was a level of challenge within each farm. Within each company there were flock variations of bursal lesions as indicated in table 2.
IBDV rRT-PCR, +/- sequencing. The individual bursal scores ≥3 (scale of 0 to 4) and/or showed necrosis on histological evaluation were submitted for virus isolation and sequenced as needed. The highest percent identity to variant reference isolates is reported in table 3. The DMV-4952-07 VP2 isolate was the most predominant variant. DMV-4952-07 VP2 was isolated from two different companies, B in two flocks and D in one flock. IBDV AL2VP2 and EDEL were isolated from companies B and A respectively. At the time, these were the most commonly isolated variants.
IBV Titers. The IBV geometric mean titers (GMT) and the average coefficient of variance (CV) per company (A, B, C, D, and E) for birds between 32 and 38 days of age is shown in table 4. Each company submitted 10 birds from 5 flocks with the exception of company A (5 birds/flock). The GMT for companies A and B is slightly high for vaccinated broilers within this age range possibly suggesting field virus exposure; however the number of birds sampled was small. Titers will also vary depending on vaccination program and whether challenge is present.
IBV rRT-PCR. Strains of IBV isolated from choanal swabs in birds 32-38 days of age in companies A through E are represented in figure 2. Based on these survey results, the most common isolates were ARK DPI, GA-13, GA-08 and Mass. However, vaccination programs for 3 out 5 companies included ARK, 2 out 5 included GA-08, GA-98 and Del 072, and 1 out 5 GA-13.
In this survey, IBDV histological bursal scores varied regardless of vaccination program between companies. However, variation in flocks within companies was also observed with some flocks being more affected than others. This could indicate a higher level of IBDV challenge in those particular flocks or could suggest that the virus is antigenically different from the vaccine (6). An interesting finding was the isolation of DMV-4952-07 –like IBDVs from companies B and D with different vaccination programs. Field isolate DMV-4952-07 was previously characterized by Gelb et. al., (2007) as a Del E variant. DMV-4952-07 from a 4 week- old commercial broiler farm located in the Delmarva Peninsula with a history of poor performance. In their study, IBDV bursal lesions were observed at 4 weeks of age in birds affected by DMV-4952-07; however, bursal lesions were observed as early as 3 weeks of age. Similarly, in birds from our survey where DMV-4952-07- like virus was isolated, bursal lesions could be seen microscopically as early as 20 days of age. Field isolates from our survey 113754 and 113756 contained 2 amino acid substitutions in the hypervariable region of VP2 compared to the DMV 4952-07. Amino acid changes in this region could change the antigenic properties of the virus, which may affect the efficacy of the Del E vaccine. In the broiler industry, the progeny relies heavily on maternal antibodies for protection against IBDV. Future progeny challenge studies with isolates obtained from this survey would be useful to determine whether current the vaccination programs are likely to provide sufficient levels of protection against field challenge.
Throughout the survey months, the most common IBV serotypes circulating in the same regions where the surveyed flocks were present were GA13, ARK, GA08 and Mass. Similarly, our survey showed that the most common IBV isolates in some these flocks were ARK, GA-13 and GA-08. However, it is difficult to distinguish based on nucleotide sequence whether the isolates were from vaccine or true field challenge in the absence of IBV related clinical signs. During necropsy and collection of tissues for IBV surveying at 32 to 38 days of age, most flocks were healthy with no signs of respiratory disease. However, 1-2 flocks from 3 out 5 companies (A, B, and C) had varying degrees respiratory disease ranging from mild to moderate upper airway noise and airsacculitis. In companies where the birds showed respiratory clinical signs, 3/3 were on a vaccination program that included Ark, but no GA 13 and 1/3 companies included GA 08, but GA 08 was detected in 2/3. Of interest was the result from Company E, where Ark was not detected via rRT-PCR; however, Ark was not included in their vaccination program at that time.
For control of IBV, uniform vaccination through the use of attenuated live IBV vaccines continues to be the method of choice. Application of the vaccine, as well as matching of field serotype to vaccination program is recommended. However, changes in the spike glycoprotein leading to new serotypes may result in loss of protection (10, 11, 12). Regardless of combination of vaccination programs, companies should test against the variant challenge in question to determine the level of protection.
IBD and IBV continue to be a challenge in the poultry industry. This survey showed that as companies continue to vaccinate for IBD and IBV, the virus challenge in the field remains and varies throughout different companies and even within flocks. As companies begin to switch their vaccination programs this winter, it is important to consider the level of challenge as well as the most appropriate vaccination program to follow.
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Article courtesy of The Poultry Informed Professional
Published by the Department of Population Health, University of Georgia
Editor: Dr Stephen Collett, Associate Professor