Adenoviruses are ubiquitous in chickens and have been isolated from either sick or apparently healthy birds. Adenoviruses isolated from chickens are called fowl adenoviruses (FAdVs). FAdVs based on restriction fragment length polymorphism (RFLP) were grouped into five species FAdV-A to FAdV-E, and based on serum cross-neutralization tests were divided into 12 serotypes (FAdV-1 to FAdV-8a and FAdV-8b to FAdV-11). The most notable diseases associated with FAdV infection in chickens are the inclusion body hepatitis (IBH), hepatitis-hydropericardium syndrome (HHS) and gizzard erosions (GE). IBH is an acute disease usually caused by FAdV of groups D and E including FAdV-2, -3, -9, -11 and FAdV-6, -7, -8a, -8b serotypes, respectively. IBH outbreaks were distributed worldwide and the disease has been detected in different countries resulting in considerable economic losses.

In 2011, Newcastle disease virus (NDV) genotype VII reported in Egypt. During 2012 a new clade had emerged from infectious bronchitis virus within GI.23 and so-called Egyptian variant 2 (Egy var 2). While, in most of Middle East countries H9N2 virus introduced since 1998. These respiratory viruses circulated in the poultry flocks and became endemic, causing severe complicated respiratory infections with two or three viruses at the same time. The outcome of these mixed viral infections called respiratory complex disease caused severe clinical disease which negatively affects the performance and immunity of infected chickens. Moreover, the H9N2 virus previously known to have an immunosuppressive effect on infected chickens.

Generally, FAdVs alone cannot cause a disease in normal healthy birds, but when there are stress factors or previous infections, especially those compromise the immunity, this activate FAdV and can cause disease with significant mortality and losses. In Egypt and Middle East within last 10 years, an increased incidence and severity of FAdV infections in different chicken sectors specially broilers were observed. If we correlate between the timing of introduction of H9N2, NDV GVII, and IBV var2 and this observation regarding FAdV outbreaks, you can see the big role played by the respiratory viral infection(s) in exaggerating the outbreaks caused by FAdVs in chickens.

The climatic changes recorded last few decades worldwide should be considered also as a player in increasing problems observed in FAdV affections. The extreme temperatures in summer and winter and increased humidity levels act as stress factors on the birds that could negatively impact the bird immunity. Also, the remarked increase in the number of chickens reared in the Middle East region and increased density in individual farms that resulted in high level of ammonia and humidity in the house also acts as stress factors that make FAdV replicate to significant level in the bird body. The management and biosecurity practices applied on place also very important factor and any shortage will cause bird exposure to high viral load in its environment.

The coexistence of respiratory viral complex infections with the climatic changes and increased poultry populations with intensive rearing practices in the Middle East countries also in Asian countries, individually or collectively might resulted in the increased incidence and severity of FAdV infections over last several years.

It is noteworthy to mention that recently the novel variant IBD virus (nVarIBDV) detected in the Middle East countries that infect chicks in very young ages causing immunosuppression. Early infection by immunosuppressive viruses like chicken anemia virus (CAV) or IBDV may play a role in IBH lateral transmission. In Egypt, nVarIBDV detected in 2023, spread to different governorates infecting broilers, layers, and breeder flocks observed mainly before 14-days-old in most cases resulting in immunosuppression. Field observations indicated significant increase in the severity and mortalities recorded in IBH outbreaks after nVarIBDV introduction.

So, vaccination of chicken flocks against H9N2 AIV, NDV GVII, and IBV with effective vaccines should be considered to control these infections, this will reduce direct losses resulted from mortalities and lowered performance also to indirectly decrease the incidence and severity of IBH. In parallel with sound management and strict biosecurity measures. Also, massive vaccination of breeder flocks with inactivated vaccines containing nVarIBDV related to ones circulating in our region.