Fowl adenoviruses (FAdVs) belong to the family Adenoviridae. They are non-enveloped viruses containing double-stranded DNA (dsDNA). FAdVs comprise 8 species (designated A to E) and 12 serotypes (numbered 1 to 11, with serotype 8 divided into 8a and 8b). Among these, serotypes 4 and 8 are considered the most pathogenic and economically significant.

Classification into Three Groups

Fowl adenoviruses can be divided into three main groups based on the diseases they cause:

Group 1

Associated with:

• Inclusion Body Hepatitis (IBH)
• Hepatitis Hydropericardium Syndrome (HHS)
• Quail Bronchitis
• Avian Gizzard Erosion (AGE)

Group 2

Associated with:

• Turkey Hemorrhagic Enteritis (THE)
• Marble Spleen Disease (MSD) in pheasants
• Splenomegaly in chickens

Group 3

Associated with:

• Egg Drop Syndrome (EDS) in chickens

Transmission and Spread

1. Vertical transmission:
   From breeder hens to chicks via the egg; this is the most critical route due to early infection in young birds.
2. Horizontal transmission:
   Occurs through feces, aerosols, drinking water, contaminated equipment, or direct contact between infected and susceptible birds.

Susceptible Hosts

More than 40 genera of vertebrates are susceptible, including:

• Birds (chickens, turkeys, ducks, geese, quail, and others)
• Mammals
• Reptiles
• Fish

Clinical Signs

• Severe depression and ruffled feathers
• Pale comb and wattles
• Diarrhea and anorexia
• Uneven body weights within the flock
• Sudden mortality that may reach up to 90% in severe cases

Gross Pathology

(Lesions may vary depending on the viral strain, type of infection, and age of the birds.)

Liver:

• Congestion and enlargement
• Discoloration to yellowish-brown
• Presence of necrotic foci

Heart:

• Cardiomegaly
• Accumulation of yellow, gelatinous fluid within the pericardial sac (hydropericardium)

Kidneys:

• Enlargement and congestion

Hemorrhages:

• Subcutaneous hemorrhages
• Hemorrhages around joints
• Occasionally on muscle surfaces

Pathogenesis and Systemic Effects

• Fowl adenoviruses are potent immunosuppressive agents due to their destructive effects on B and T lymphocytes in the thymus and bursa of Fabricius. This leads to:
  • Reduced immune response to vaccination
  • Increased susceptibility to secondary viral and bacterial infections
• They also exert detrimental effects on the circulatory system, including the heart, blood vessels, and blood components.

Diagnosis

Accurate and early diagnosis (preferably before 3 weeks of age) is essential due to similarities with other poultry diseases.

• Histopathology: Detection of intranuclear inclusion bodies within hepatic tissue.
• Serology: Detection of specific antibodies.
• Molecular diagnosis: Polymerase Chain Reaction (PCR), which is the most rapid and accurate confirmatory method.

Prevention

• Strict biosecurity measures:
  Essential due to the wide host range and the high resistance of adenoviruses to heat and disinfectants.

Vaccination:
Use of inactivated FAdV vaccines or vector-based vaccines, selected according to the epidemiological situation and the predominant serotype in the region.