There's perhaps nothing more likely to bring a smile to a cat parent's face than seeing their kitty bat around a ball of yarn. Unfortunately, string and yarn are very dangerous for cats.
Signs That Your Cat Ate String
Often, pet parents aren't present to see that their cat swallowed string. So how can you tell if your cat ingested it? Vomiting is the most common sign that your cat has eaten string. Also, look for signs of abdominal pain, which your cat may show by hiding or being unusually aggressive when picked up. You may also see bloody diarrhea.
Dangers of a Cat Swallowing String
If your cat eats string, it will only cause an issue if a piece of it gets stuck somewhere between their mouth and gastrointestinal tract, while the other part is trailing farther ahead into the intestines. For example, a piece of yarn can get lodged at the base of a cat's tongue.
One of the biggest concerns of cats eating string is a condition veterinarians refer to as linear foreign body. The reason why this condition can cause so much damage is due to the way it can cause an intestinal obstruction says Manhattan Cat Specialists. "Usually, one end of the string becomes anchored, either by wrapping around the base of the tongue, or by getting lodged in the pylorus (the part of the stomach that leads to the small intestine). The peristaltic waves (peristalsis: the involuntary constriction and relaxation of the muscles of the intestine) generated by the intestines try to propel the free end of the string along the intestinal tract. But the leading end is lodged, so the string can't be pulled along." When this happens, the intestine can become folded, or plicated, which makes it not possible to pull the string out. As a result, the string can tighten and further exacerbate the risk of the intestines perforating (piercing a hole in the intestines).
Swallowing string can also cause a serious condition called intussusception. Intussusception results in the vigorous contraction of a specific bowel segment in an effort to move along an object that isn't passing through. If untreated, this part of the bowel can collapse into a nearby segment and lead to partial or complete GI obstruction, compromised blood flow to the affected section of the bowel, and tissue death. Intussusception can be fatal, according to the Merck Veterinary Manual.
A cat suffering from a linear foreign body creates a fairly difficult situation for a veterinarian due to the risk of perforations and the difficulty of removing the string says VIN. Cats with chronic linear foreign bodies (meaning they have them frequently) may be suffering from malnutrition, dehydration, or peritonitis, as well as potentially extended surgeries that can put a cat's life at risk.

What to Do If Your Cat Swallowed String
If you know that your cat ate string, even if it's only been partly swallowed, resist the urge to pull it out yourself. While string is dangerous to ingest, the danger isn't immediate — you have time to seek veterinary care. If you try to remove the string yourself, you risk damaging the esophagus and causing choking or vomiting that could result in aspiration pneumonia (a lung infection).
Head to your vet's office promptly. Call them while you're on your way to give them a heads-up about the situation. After you arrive, your vet will examine your cat. Be prepared for them to ask to anesthetize your pet; this allows them to completely examine the oral cavity, including underneath the tongue.