I'm sorry that your question wasn't answered in a timely manner and I’m sorry to hear of this with Ethel. Poultry keepers who keep genetically improved breeds bred for high egg production might see swollen abdomens more frequently in their flocks as production breeds – most of which are usually kept for just 12-18 months in commercial settings – don’t have the genetics to support 3-4 years of egg laying. The risk of reproductive disorders increases with age in layers. These afflictions aren’t exclusive to production breeds, however; they can show up in any small-scale flocks. Although it’s best to have a vet examine Ethel, here’s a short list of differential diagnoses:
1) Impacted (“egg-bound”) oviducts
These oviductal disorders are seen in obese hens, older hens, or pullets that come into lay too early. The oviduct becomes blocked by an egg or a mass of broken eggs and eventually eggs are pushed back in the ceolom (“abdomen”) as the hen continues to lay. Affected hens walk like penguins when the eggs in the abdomen are excessive. In chickens the obstruction can result from several lodged eggs or a mass of broken shells, shell membranes, or a mass of yolk and egg white. When the obstruction occurs in the uterus (the usual case), eggs enclosed by shell membranes might be found in the abdominal cavity. The indicates that eggs continued to form but were refluxed back into the coelom. The prognosis for affected hens is poor. The use of antibiotics might prolong an affected chicken’s life for a few months but it will eventually die from the condition.
This inflammation of the oviduct occurs frequently and can be introduced through the cloaca (vent”( by various means, including pecking. The most common infection is by E. coli bacteria. In the later stages of the condition, the oviduct and abdomen become distended due to masses of foul-smelling, cheesy contents in the oviduct. The cheesy masses are sometimes mixed with egg contents; as a result, salpingitis can frequently be confused with an impacted oviduct. A chicken with salpingitis can remain healthy for a long time until the late stage when oviductal contents start to impinge on vital organs. The chicken then becomes sick, refuses to eat and slowly declines. Antibiotics seem to help only temporarily and while a vet might attempt surgery, it’s unlikely to be successful and these birds will die.
Fluid can accumulate in the coelomic cavity secondary to heart disease or tumors in the heart. Fluid in the abdominal cavity is usually accompanied by respiratory distress and cyanosis (bluish/greyish color) combs and wattles indicating lack of oxygen to the tissues. There’s little treatment of value for these birds.
4) Cystic oviduct
Normally only the left ovary and oviduct of a hen is functional but sometimes the right oviduct is functional and becomes cystic. Overly large cysts can distend a hen’s coelom and compress internal organs. A vet might be able to drain the cyst using a sterile syringe and needle.
Several diseases, such as Marek’s disease (herpesvirus), lymphoid leukosis, and various adenocarcinomas can be found in the internal organs and might, in turn, enlarge the coelom. Tumor -causing diseases tend to be chronic and affected chickens slowly suffer weight loss and inappetence. There’s no treatment for these birds.
6) Fat deposition
Extremely obese hens have a thick fat pad that can distend the lower abdomen. Obesity, normally caused by high-energy diets, also predisposes chickens to a condition called fatty liver hemorrhagic syndrome where the liver is infiltrated with fat and contribute to abdominal distention. The syndrome causes acute death in chickens when blood vessels in the liver rupture and cause internal bleeding. It’s seen with increased frequency in backyard and pet chickens fed table scraps high in calories. It’s also very common in small-scale flocks fed free-choice via feeders.
In summary, while draining can improve respiratory function, it should be considered to be palliative only. Please respond with further questions or concerns if you wish.
Thank you to Dr. Lyle McNeal for his nicely concise synopsis of swollen abdomen in chickens from which I freely excerpted here.