Why is reduced egg production a common and significant clinical sign, even though avian influenza virus infection is respiratory?

Although avian influenza viruses (AIV) are known to cause respiratory symptoms, reduced egg production is a common and significant clinical sign, even with strains that primarily affect the respiratory tract. Here is how this happens, based on the article above

1- Systemic effects of infection: Even if the primary site of replication is the respiratory tract, viral infection can lead to a systemic inflammatory response throughout the bird's body. This generalized disease, even without direct viral replication in the reproductive organs, can cause the following:

2- Stress: Any illness or severe stress can disrupt the delicate hormonal balance required for continued egg production.

3- Reduced feed and water intake: Sick birds often have reduced appetite and water intake, which directly affects the energy and nutrients needed for egg production.

4- Lethargy and depression: General malaise and inactivity can lead to reduced egg-laying activity.

Direct or indirect effects on reproductive organs:

6- Viremia and systemic spread: While some strains of avian influenza virus are more concentrated in the respiratory tract, many can spread systemically through the bloodstream (viremia) even at low levels. This means that the virus can reach and affect the reproductive organs, including the ovaries and fallopian tubes.

7- Fallopian tube/ovarian inflammation: Even low levels of virus or a systemic inflammatory response can cause inflammation (salpingitis or oophoritis) in the fallopian tubes or ovaries. This can lead to:

8- Cessation of egg production: The bird stops producing eggs completely.

9. Abnormal eggs: Eggs may be pale, thin-shelled, soft-shelled, or shellless due to damage to the shell-producing gland. They may also be malformed or of poor internal quality.

10. Egg yolk peritonitis: In some cases, damage to the reproductive tract can result in the release of yolk contents into the abdominal cavity, causing peritonitis, a severe and often fatal complication.

11. Reproductive organ atrophy: In more severe cases, especially with highly pathogenic strains of avian influenza (HPAI), complete atrophy (atrophy) of the ovaries and fallopian tubes may occur, leading to permanent cessation of egg production.