Crop stasis, also known as "sour crop", is a condition where the chicken's crop doesn't empty properly. This delays or prevents the passage of fluid and feed materials through the chicken's gastrointestinal tract, leading to dehydration and rapid loss in body condition. If the chicken continues to eat or drink, it will just collect in the crop and cause it to increase in size. In cases where this occurs, the chicken's weight on a scale may be deceiving due to the weight of the crop contents
The stagnant crop content often causes a yeast, fungal or bacterial infection of the crop which worsens the existing problem. It is a common misconception that sour crops are always caused by a yeast infection. This is just one of numerous reasons why a chicken's crop isn't emptying. Chickens with sour crop can go downhill very quickly so it's important to promptly seek veterinary care.
The crop is a round-shaped out-pocketing of the chicken's esophagus. It is located at the base of the neck (it may be easier to locate when it's contents are full, which is most likely to occur at the end of the day). The crop acts like a temporary storage pouch for food, and is where the initial stages of digestion occurs in chickens.
Normally, when chickens ingest food, it should only remain in the crop temporarily, before moving through to the proventriculus (referred to as the glandular stomach, where digestion primarily begins), and from there through to the rest of the gastrointestinal tract. The movement of the food through the digestive system is referred to as gastrointestinal motility, and is controlled by highly coordinated waves of contractions.
Any disruption in the gastric motility, or obstruction of the intestinal tract will eventually cause food to back up into the crop. If the food remains in the crop, it will start to decompose and cause a crop infection (in addition to the pre-existing problem that caused crop stasis).
Crop stasis can be caused by a primary issue (involving the crop) or occur as a secondary result of a disease somewhere else in the chicken's body. Most cases of crop stasis in chickens are secondary.
Primary crop issues that can cause crop stasis include:
- Crop infection: Yeast, fungal or bacterial.
- Crop impaction: May be caused by eating tough, fibrous vegetation (such as that from grass clippings), long pieces of string or baling twine, certain potted plants, bedding, sand and other foreign materials that get stuck in the crop.
- Trauma
- Worms: There are a few species which invade and reproduce inside the mucosa of the chicken's crop. These include Capillaria spp and Gongylonema ingluvicola.
Secondary issues that lead to crop stasis:
- Reproductive disease: Any disease of the reproductive tract (egg yolk peritonitis, internal laying, oviduct impaction, ovarian tumors and salpingitis) can compress, displace, or damage a hen's GI tract which can impair it's motility.
- Tumors: Tumors of the GI tract can cause adhesions, partial or complete obstructions or strictures which impact GI motility. Tumors in nearby organs can metastasize into the GI tract. There are several cancer-causing viruses that affect chickens that can cause lymphomas. The marek's disease virus can damage the nerves that control GI motility due to the development of lymphoproliferative lesions in peripheral nerves.
- Infections: Infections of the GI tract (e.g., mycobacteria, cocci, clostridium), lower respiratory tract (lungs and air sacs) and reproductive tract can affect the GI system.
- Ingestion of foreign bodies: The same materials that can cause a crop impaction can obstruct and damage other organs in the chicken's GI tract.
- Heavy metal poisoning: Ingestion of foreign bodies made with lead or zinc can impact the GI tract motility.
- Intestinal parasites: There are several roundworms (Ascaridia galli, Capillaria spp, and Tetrameres spp) that can damage the GI tract and cause obstructions.
