Laying hens are prone to spontaneous development of malignant tumors on their reproductive organs, in the form of ovarian and oviductal adenocarcinomas.
The high incidence of tumors (one study showed that 45% of 2 year old laying hens had tumors associated with the reproductive tract) are related to the extended maintenance of egg production in laying hen breeds. In the same study, 24% of all hens developed malignant ovarian adenocarcinomas by the time they were 2 years of age. The incidence of tumors increases with age, as hormone levels decrease.
As tumors enlarge, the lumen may become slit-like or filled by cribriform structures, often surrounded by dense fibrous connective tissue. It has often been described for it's resemblance to cauliflower, with irregular masses bulging outward from the central tumor and that usually replaces all of the normal ovarian structure. In advanced stages of the disease, tumor growth will extend into the abdomen, and grow on serosal surfaces of the oviduct, mesentery, and intestines, and on the pancreas. As the mesentery contracts and the bowel wall thickens, ascites develops, which appears as a soft, subcutaneous swelling around the hen's belly--also referred to as enlargement of the abdomen.