Infectious bursitis in chickens is a class of chicken infectious diseases mainly immunosuppressed caused by infectious bursal virus. According to the results of zhongke genetic testing, there is a trend of national epidemic. In this paper, the latest research on the prevalence of mutant strains, antigen changes, gene sequence analysis, etc., is discussed, and the changes of IBDV are more systematically understood, so as to better prevent and control the occurrence of such diseases.
<h1 class="pgc-h-arrow-right" > an overview of infectious bursal disease</h1>
Infection bursal disease virus (IBDV) belongs to the genus Of avian double RNA viruses in the family BiRNAviridae, and infects chickens with infectious bursal disease (IBD) that is mainly immunosuppressed. The IBDV genome consists of two double-stranded RNAs, A and B segments[1], where VP2-VP5 is encoded by A segments. VP2 and VP3 are structural proteins, VP4 has hydrolase activity, and VP5 is a non-structural protein. VP2 is a viral capsid protein and the primary host protective antigen, which is associated with the virulence, cellular appetite, and antigen variation of the virus [2-4]. In the aa206-aa350 region of virus VP2, known as hypervariable region (HVR), it is often used for genetic evolutionary analysis, and the VP1 protein is encoded by the B segment, which has RNA-dependent RNA polymerase activity.
< h1 class= "pgc-h-arrow-right" > prevalence of infectious bursal disease in chickens</h1>
Of the two serotypes of IBDV, serotype II is not pathogenic in chickens, and the main cause of IBD is serotype I[5]. Since the discovery of serum type I in 1957, IBDV has undergone two major major mutations, which can be divided into classic strains, variant strains, and super strong strains[1].
The IBDV super-strong strain (vvIBDV), which is characterized by acute, high fatality, has been endemic worldwide since the 1990s and has become one of several major epidemics plaguing the poultry industry [6-7]. The typical symptoms of sick chickens are fluffy feathers, decreased feeding intake, diarrhea, and the symptoms of autopsy are mainly symptoms such as bursal edema, bleeding, and necrosis. Through measures such as immunization and improved feeding management, the prevalence of vvIBDV was gradually controlled.
< h1 class="pgc-h-arrow-right" > prevalence of infectious bursal virus strains in mutant chickens</h1>
Historical prevalence
The IBDV variant was first discovered in the United States in the 1980s and has been endemic in the United States ever since, becoming a dominant IBDV strain that threatens the poultry industry [8]. Other countries, including China, also reported sporadically in the early 1990s of the IBDV variant.
Current domestic prevalence
The recent identification of new IBDV variants in China is the first report on the widespread prevalence of IBDV variants in the Asia-Pacific region in the past 30 years. The main ideas about the origin of the new IBDV variant are: first, because the wild strain has mutated or recombined under immune pressure; second, similar strains have long existed, but the monitoring of the mutant strain has been "neglected" due to the main concern about the occurrence of vvIBDV [9-10].
In 2017, the IBD variant strain began to circulate in areas with dense broiler farming in eastern China. The variant strain is quite different from the original vvIBD. First of all, the clinical symptoms of the variant strain are mild, showing only swelling of the bursa, a small amount of bleeding, or yellow exudate. Secondly, it causes fewer deaths, and the main hazard of the mutant strain will appear 1-2 weeks after the onset of the disease, and the bursa will show severe atrophy. Although it is weak in its own virulence, severe immunosuppression makes the flock extremely susceptible to other pathogens, which seriously affects production performance. In 2019, according to market feedback and the test results of Zhongke Gene, it was found that the IBDV variant strain in China's white feather broiler and egg chicken was prevalent.
Zhongke Gene has detected the prevalence of new IBDV variants from at least 15 provinces (cities) such as three northeastern provinces, central China, east China, and south China, and the main chickens are broiler chickens and breeding layer chickens over 25 days old, and show a trend of gradually expanding the age of the disease.
Antigen changes in novel IBDV variants
Sero-cross-neutralization test confirmed that there was a significant difference in antigenicity between the novel variant of IBDV and vvIBDV. Studies have shown that the novel IBDV variant has different monoclonal reaction profiles from vvIBDV, and the amino acid mutations at 318 and 323 positions of VP2 are one of the key factors in the neutralization activity of the VvIBDV antibody that escapes the novel IBDV strain [11]. This difference is an important reason why chickens with partially immune vvIBDV vaccines are still infected with the new IBDV variant, which is circulating nationwide [12].
Sequence analysis results of novel IBDV variants
Among the reported IBDV variants, they can be divided into two distinct subgroups, one subgroup of the early American variants and another subgroup formed in domestic reports. Fan Linjin et al. analyzed the amino acid sequence of the VP2 gene hypervariable region of the isolated IBDV novel variant and found that serum type I I IBDV was clearly divided into four branches: classical strain, super strong strain, attenuated strain and mutant strain. The homology of nucleotide sequences of Chinese mutant strains and early variants in the United States can reach a maximum of 94.8%, while the homology rate with classical strains, super strong strains and attenuated strains is less than 92%. The Chinese isolates had the characteristic amino acids 222T, 249K, 286I and 318D loci[13-15], and three new mutation sites of 221K, 252I and 299S were found, which proved that the newly popular IBDV variant in China belonged to the new IBDV variant [9].
In 2019, Huizhong Bio has begun tracking the new IBDV variant. The VP2 fragment sequencing (Hz001-Hz009) was collected from 9 strains on the market, and it can be seen from Figure 1 that the 9 isolates belong to the Chinese mutant strains.
Through the analysis of VP2 nucleotide sequence, Huizhong Bio found that the gene homology between the isolated market-endemic strains and the strains in the seed bank was between 96-99%, indicating that it could protect the infectious bursa fibrous strains of chickens circulating in the market.
Hazards of novel variants of IBDV
Histopathological sections showed that the filament structure in the atrophic bursa in the atrophic bursa was destroyed, the lymphocytes were significantly reduced, a small amount of heterophilic infiltration of individual follicular pulp could be seen, and local fibroblast hyperplasia was seen. Studies have also found that early infection causes swelling of the spleen and later on causes spleen atrophy.
The harm caused by the new variant of IBDV cannot be ignored. Although the lethality rate of the new IBDV variant strain is low, due to the irreversible bursal damage caused by it and its severe immunosuppression, the immunity of the chicken population is low, which increases the probability of infection of other pathogens, affects the growth of the flock, and leads to weight loss, poor uniformity, and increased ratio of feed to meat, which seriously affects economic benefits.
On the other hand, the new variant of IBDV interferes with the immune effects of other important disease vaccines. Antibody production from the avian influenza H5/H7 vaccine has been reported to be interfered with by infection with novel variants of IBDV [14].
<h1 class="pgc-h-arrow-right" > the control of mutant strains</h1>
Due to the emergence of mutant strains, the current vaccine protection rate on the market is not ideal, and there is a situation of post-immunization onset. Therefore, when selecting vaccines, try to select vaccines with similar gene sequences to the mutant strains for prevention and improve the protection of chickens after immunization.
At the same time, we should do a good job in the biosecurity measures of the chicken coop, improve the feeding management, and reduce the stress of the chicken population. Strict disinfection and emptying of sick chicken coops and improved immunization programmes. In the next batch of breeding, pay close attention to the occurrence of infectious bursal disease in chickens, and use high-free serum for treatment at the first time to reduce losses.
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