Infectious bronchitis is an acute and highly contagious viral respiratory disease in chickens, characterized by tracheal rales, coughing, and sneezing. Can cause weight gain and decreased feed efficiency in chickens; Participate in mixed infection causing cystitis; Causing a decrease in egg production and quality, thus having significant economic implications.

Pathogenology

1、 Category:

Infectious bronchitis virus (IBV) belongs to the coronavirus family and the coronavirus genus.

2、 Morphology:

The infectious bronchitis virus is circular to polygonal in shape, with an envelope. It is the largest known RNA virus in terms of genome, with a diameter of 80-120nm. It is a linear single stranded RNA virus with rod-shaped spikes on its surface, approximately 20nm long. The arrangement of IBV filaments is not as tight as the rod-shaped filaments of paramyxovirus. In most cases, the observed RNP is a rope like structure with a diameter of 1-2nm, but occasionally curled structures with a diameter of 10-15nm can also be seen. The density of IBV strains in sucrose gradient is different. The density of virus particles with complete spike components is 1.18g/ml, while the density of virus particles with fewer spikes can be reduced to 1.15g/ml. When centrifuging the virus, it is best not to exceed 100000g of centrifugal force, otherwise it may cause loss of the spike or at least easily lose the S1 subunit that makes up most of the globular tip of the spike protein.

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  Chicken infectious bronchitis virus particle, with rod-shaped protrusions as shown in the picture

3、 Chemical composition:

The genome is approximately 27-32kb in length and mainly encodes four structural proteins: spike protein (S), nucleocapsid protein (N), and membrane protein (M), Small envelope protein (E). The virus binds to the host cell receptor (ACE2) protein through the receptor binding domain (RBD) on the spike protein (S protein); There is a Furin protease cleavage site between the S1 and S2 subunits of S protein. S1 protein is associated with viral infectivity, pathogenicity, virulence, and tissue affinity; The adsorption of viruses onto cells can induce the production of antibodies and cellular immunity, which determines the serotype specific antigenic determinant cluster. Genes are the most variable genes, with nucleotide differences reaching over 50%. According to the full-length sequencing of the S1 gene, the transmission viruses can be divided into two large groups (Group I and Group II, with S1 gene homology below 70%), 29 lineages (genotypes), and 9 genotypes have been reported in China. The main prevalent strains in China are Group I and Group II, which have only been isolated in recent years. Serological diversity: There are at least four serotypes in China, H120， 4/91 type, QX type, TW1,TW2。

4、 Resistance to physical and chemical factors

Most IBV strains can be activated at 56 ℃ for 15 minutes and 45 ℃ for 90 minutes. Infectious tissues are well preserved in 50% glycerol, and IB virus can survive outdoors for 12 days in spring and 56 days in winter.

Freeze dried infectious bladder fluid can be stored in a vacuum sealed refrigerator for at least 30 years. 10% glucose has a stabilizing effect on the freeze-drying and freezing of IBV.

IBV is sensitive to common disinfectants, such as 1% Lysol, 1% Carbolic acid, 0.1% Potassium permanganate, 1% formalin, and 70% alcohol, all of which can kill it within 3-5 minutes.

5、 Pathogenicity

Infectious bronchitis can replicate in the respiratory tract, intestines, kidneys, and fallopian tubes.

Five days after IBV infection, cold stress and Mycoplasma synovialis infection at the same time will lead to the occurrence of gasbag inflammation, but the incidence rate and disease severity vary with the virulence of IBV strains. Mycoplasma gallisepticum can exacerbate the condition of IBV infection.

Co infection of different IBV strains with Escherichia coli via nasal drip route can cause death in young chickens, while infection with any of these factors alone is not lethal. The mortality rate of co infection varies from 14% to 82% depending on the IBV strain. This indicates that there are indeed differences in virulence among IBV strains.

The pathogenicity of IBV strains to the reproductive tract also varies. Maternal antibodies can prevent damage to the fallopian tubes during early IBV infection. Among susceptible laying hens, the pathogenicity caused by different IBV strains varies greatly. Some only cause changes in eggshell pigment without a decrease in egg production, while others cause a 10% -50% decrease in egg production.