A hairball is a small collection of hair or fur formed in the stomach of animals (uncommon in humans) that is occasionally vomited up when it becomes too big. Hairballs are primarily a tight elongated cylinder of packed fur, but may include bits of other elements such as swallowed food. Hairballs are sometimes mistaken for other conditions of the stomach such as lymphosarcoma, tuberculosis, and tumour of the spleen. Felids are especially prone to hairball formation since they groom themselves by licking their fur, and thereby ingest it. Rabbits are also prone to hairballs because they groom themselves in the same fashion as cats, but hairballs are especially dangerous for rabbits because they cannot regurgitate them. Because the digestive system of a rabbit is very fragile, rabbit hairballs must be treated immediately or they may cause the animal to stop feeding and ultimately die due to dehydration. Cattle are also known to accumulate hairballs, but as they do not vomit, these are found usually after death and can be quite large.
A trichobezoar is a bezoar (a mass found trapped in the gastrointestinal system) formed from the ingestion of hair. Trichobezoars are often associated with trichotillomania (compulsive hair pulling). Trichobezoars are rare, but can be fatal if undetected. Surgical intervention is often required.
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Although uncommon in humans, some hairballs have been reported. These hairballs occur when hair strands collect in the stomach and are unable to be ejected as a result of the friction on the surface of the gastric mucosa. Hairballs are often seen in young girls as a result of trichophagia, trichotillomania, and pica. In 2003, a 3-year-old girl in Red Deer, Alberta, Canada had a grapefruit-sized hairball surgically removed from her stomach; in 2006, an 18-year-old woman from Chicago, Illinois, had a 4.5 kg (9.9 lb) hairball surgically removed from her stomach; and in 2014, a 9-pound hairball was removed from the stomach of an 18-year-old in Kyrgyzstan. Hairballs can be quite hazardous in humans, since hair cannot be digested or passed by the human gastrointestinal system, and (assuming it is identified) even vomiting may be ineffective at removing the hair mass. This can result in the general impairment of the digestive system.