Avian influenza A viruses belong to the Orthomyxoviridae virus family and have -ssRNA genome. AIVs depending on their disease severity can be classified into high and low pathogenic viruses (HPAIVs and LPAIVs).
In the early 1990s, the H9N2 subtype was first identified in China and is now considered to be endemic in poultry throughout Asia, Europe, North Africa and the Middle East.
Almost, all governments in Egypt recorded a positive cases of H9N2, the new Egyptian H9N2 viruses belonged to a new subcluster related to the strains that had been circulating since 2015 (Egy-2 G1-B lineage pigeon-like), the reassortant viruses acquired four genome segments from the classic Egyptian H9N2 viruses (HA, NA, NP, and M) and four segments from Eurasian AIVs (PB2, PB1, PA, and NS).
LPAIVs cause mild symptoms and low mortality rate, decrease egg production and are limited to respiratory and intestinal tracts.

The H9N2 subtype increase possibility of secondary infections especially affect the respiratory system e.g. (Mycoplasma & IB) with potentially higher mortality rates. It is also considered zoonotic.
Several studies have emphasized the importance of the gut microbiota in developing immunity against viral infections in chickens.
It is well known that the poultry entero-tropism of LPAI is higher than that of HPAI, which is more linked to the respiratory epithelium, So intestinal tract is the 1ry site of LPAIVs replication & shedding 1ry from cloaca.
The H9N2 subtype cause severe intestinal damage through raises the expression of proinflammatory cytokines (IFN-g, IFNa, IL-17A and IL-22), increases the proliferation of members of proteobacteria, particularly Escherichia coli and decreases the proliferation of certain beneficial bacteria In addition, decreases the expression of primary gel-forming mucin, this reflects on the broiler FCR negatively.
Commercial inactivated vaccine of H9N2 play a vital role in shedding control that makes the vaccine more valuable even in high MDAs levels besides the protection against H9N2 infection.
Economically, if we say the risk factor (RF) of H9N2 could be represent 10% approximately vary between its negative effect on immune response, mortality rate & FCR of broiler flocks.
for example,
Nowadays, broiler flock 10.000 bird costs 27k $, So RF of H9N2 = 2.7K $ LOSSES/cycle.
RF of H9N2 for a year =17.55k $/year. (Approximate equation)

Therefore, being able to treat and control the spread of H9N2 AIV in poultry populations is important to SAVE/reduce economic losses and protecting public health throughout using a suitable H9N2 inactivated vaccine.