False layer hens look and ovulate normally but do not produce eggs. Typically, hens appear healthy, with fully developed secondary sexual characters and an active ovary, but a nonfunctioning oviduct. They are found at the peak of egg production. The yolk is dropped into the abdominal cavity rather than being collected by the oviduct, and once in the abdominal cavity the yolk is absorbed.
Oviduct dysfunction can occur after infections with Escherichia coli or Mycoplasma gallisepticum, resulting in inflammation and obstruction of the oviduct and also from early infection (at 1–2 weeks of age) with an infectious bronchitis virus (IBV) variant that can cause hypoplasia of the ovary and oviduct. Originally reported in Asia in 1996-1998, where the virus was designated as QX, and in Europe in 2004, classified as D388 (genetically similar to QX), the virus has spread through a vast territory and has also been found in North and Latin America.