Dr. Barbara Oglesbee
Crop stasis refers to a condition where the crop, which is a diverticulum of the esophagus, stops emptying and becomes distended with fermenting food and fluids. This is a serious, life-threatening condition and needs to be treated by a veterinarian immediately. After being swallowed, food moves into the crop. The function of the crop is to moisten, soften and store food.
Normal Digestive Process
The upper gastrointestinal of pet birds has several unique features and when food is ingested, it goes through the following process:
Food then moves from the crop into the first stomach, called the proventriculus. The proventriculus contains glands that secrete enzymes and acid to begin the chemical digestion of the ingested food.
Once these digestive chemicals have been added, the food then moves into the second stomach, the ventriculus. The ventriculus contains two opposing sets of muscles, which grind and macerate the food. Food typically moves back and forth between the proventriculus and ventriculus several times before moving on into the intestines.
Movement of food from the crop to the proventriculus to the ventriculus is dependent on highly coordinated contractions (peristalsis). If the proventriculus is empty, food will move immediately into the proventriculus without being retained in the crop. If food is present in the proventriculus, it will be stored in the crop.
In birds with normal gastrointestinal tract motility, peristaltic waves can be observed moving across the surface of the crop. These waves are easy to see in baby birds, since the crop lacks a covering of feathers. When the crop is filled with food, 1-3 peristaltic waves should move across the crop per minute. In normal birds, peristaltic waves are coordinated throughout the entire gastrointestinal tract, and the waves observable in the crop reflect movement of the gastrointestinal tract as a whole. Any disease process that inhibits peristalsis or disrupts its coordination will cause crop stasis.
Many diseases, both within the intestinal tract and in other organ systems, disrupt or inhibit peristalsis. Additionally, any physical obstruction occurring along any point within the gastrointestinal tract will cause food to back up into the crop. There is a tendency to think of crop stasis as a problem with the crop itself. Although disease within or surrounding the crop can cause crop stasis, the disorder more often lies in the lower intestinal tract or disease in other systems. It may help to think of the crop as a kitchen sink. When the kitchen sick backs up, the problem is in the plumbing, not in the sink itself.
There are many causes of crop stasis in birds. Crop stasis can be caused by diseases of the digestive tract or can occur as the result of toxicities, disease of the nervous system or metabolic diseases. Many contagious diseases cause crop stasis, so it is important to inform your veterinarian of any potential contact – direct or indirect – with other birds. Possible causes of crop stasis in birds include:
Viral infection. Several different avian viruses may cause crop stasis. Proventricular Dilatation Disease is caused by a virus (Avian Bornavirus) that attacks the nerves that coordinate peristalsis in the intestinal tract. When this nerve supply is disrupted, the coordination of movement from the proventriculus to the ventriculus is interrupted. Initially food that is passed appears undigested in the droppings. Later in the course of the disease, movement of food through the intestinal tract slows or stops completely, causing food to back up into the crop. Other viruses, such as polyoma virus and herpes virus may cause ileus (gastrointestinal tract stasis). With these viruses, however, crop stasis will usually be only one of several symptoms. Viruses can be transmitted by direct exposure to another bird, shared food or water dishes, or on your hands or clothing, depending on the type of virus.
Obstruction. Ingested foreign objects may block the intestinal tract. Psittacine birds (parakeets, cockateils and parrot like birds) have powerful beaks and a strong instinct to chew. Objects that commonly obstruct the intestinal tract include string toys, bark or wood shavings, and pieces of rubber toys. Birds that are provided grit as a dietary supplement will occasionally overeat the grit, resulting in gastrointestinal tract obstruction.
Growths. Neoplasia (cancer) or papillomas (wart-like structures) may occur anywhere along the gastrointestinal tract.
Toxins. Heavy metal toxicity, caused by ingestion of objects containing lead or zinc, is one of the most common diseases seen in pet birds. These toxins affect the nerves that supply the gastrointestinal tract and coordinate peristalsis. Not all birds with heavy metal toxicosis have crop stasis, and most have other symptoms, especially neurologic signs.
Bacterial infection. Bacterial infections may come from other birds, from an overgrowth of dangerous bacteria on dirty food or water bowls, or spoiled foods. Often, small amounts of potentially dangerous bacteria will live in the intestinal tract without causing harm. This population of bacteria can overgrow and cause disease if the bird's immune system is not functioning properly, as may occur during times of stress. An overgrowth of harmful bacteria may also occur when antibiotics are used improperly.
Yeast infection. Candida is a type of yeast that normally lives in small quantities in the intestinal tract. Stress or antibiotic use can also cause an overgrowth of Candida. These yeast commonly invade the wall of the crop itself, and can also invade the lower intestinal tract. Yeast infections are a common cause of crop stasis in baby birds.
Parasites. If large numbers of intestinal parasites such as roundworms and tapeworms infest the intestinal tract, complete obstruction may occur. Trichomonas may occasionally cause crop stasis in small psittacine birds, especially budgerigars.
Metabolic disorders. Liver disease, diabetes mellitus, renal disease
Dehydration. When birds become dehydrated, fluid may be pulled from the contents of the intestinal tract, resulting in obstruction from thickened, dry food.
Goiter. Enlargement of the thyroid gland commonly occurs in budgerigars with iodine deficiencies. Birds on diets consisting only or solely of seeds are especially prone to goiter. The thyroid glands are located beside the crop and can sometimes become so large as to obstruct outflow from the crop.
Improper feeding formula in neonatal birds. Crop stasis is a common problem in neonatal birds being hand-fed. Dietary formulas that are fed too hot or too cold, or are mixed to an improper consistency can cause crop stasis.
Improper environmental temperature or humidity. Neonatal birds that are chilled or overheated often develop crop stasis. If the ambient humidity is too low, baby birds may dehydrate.