Horse Dewormer Paste Hardened into a 4-Centimetre Mass Blocking a Man’s Small Intestine

A 79-year-old man bought a tube of ivermectin paste formulated for horses after his primary care doctor refused to prescribe the drug. Several weeks later, he arrived at a Connecticut emergency department with two weeks of worsening nausea and abdominal pain, and no bowel movement for seven days.

Surgeons eventually opened his abdomen and found a hard, mobile mass about 4 centimetres across lodged in the distal ileum. Laboratory testing at Cornell University's Animal Health Diagnostic Center later confirmed what it contained: ivermectin.

The case, reported by surgeons from Yale School of Medicine and the West Haven VA, was published in May in Cureus. The authors believe it is the first reported bezoar formed from congealed ivermectin paste, and the first case of ivermectin ingestion requiring surgery.

The Key Fact Emerged Only When Surgery Was Being Considered

The diagnosis was delayed twice, and both delays came from the patient.

He had already been treated at another hospital several days earlier, where a CT scan showed constipation but no obstruction, and he was sent home. When he was admitted to Yale, his heart rate was in the 100s, his white blood cell count was 14, and his abdomen was distended and diffusely tender. Because his kidney function had fallen below baseline, doctors ordered a scan without contrast. It showed dilated small bowel with a transition point in the mid-pelvis. He was admitted for bowel rest.

He vomited on the second day of his hospital stay, removed his nasogastric tube and left against medical advice. He returned the following day with worse pain, followed by faeculent vomiting. A repeat CT scan using a small-bowel protocol showed a persistent obstruction and focal thickening in the distal ileum.

Only then, as doctors discussed surgery, did a family member mention that he had swallowed a tube of ivermectin horse paste shortly before his symptoms began. He had no psychiatric history. He told doctors that he had watched online videos about ivermectin's supposed benefits, and that embarrassment was one reason he had waited two weeks before seeking care.

The Blockage Was Only the First Problem

Surgeons removed the bezoar through a 2-centimetre incision in the bowel wall, and he initially recovered uneventfully.

Then the bleeding began. Blood-tinged fluid appeared in the nasogastric tube on the day of surgery. By the fourth day after the operation, he was passing both dark and bright red blood. His haemoglobin fell from 12.2 to 8.9 and then to 7.1, while his systolic blood pressure dropped into the 80s. He received three units of packed red blood cells.

A colonoscopy on the sixth day found blood throughout the colon and extending into the terminal ileum, sending him back to the operating room. Endoscopy of the small bowel during surgery revealed ulcers that appeared ischaemic, extending through the distal jejunum and the entire ileum, and most concentrated just above the original incision. Surgeons removed just over 50 centimetres of small bowel.

Pathology showed mucosal pustules and haemorrhagic mucosa throughout the specimen, with normal tissue at the margins. He was taken off the ventilator and weaned from blood pressure medications by the third day after the second operation, and required no further transfusions.

Surgeons Propose a Mechanism, Not a Proven Cause

The team hypothesises that the paste congealed into a mass, blocking the bowel, while the remaining paste stayed in contact with the upstream intestinal lining for weeks, causing toxic injury and ulceration.

However, the surgeons stress that this is an interpretation rather than a proven explanation. The paper lists alternative possibilities, including pressure injury from the bezoar itself and localised ischaemia caused by bowel distension. It also notes that the bleeding may have been worsened by clopidogrel, which the patient was taking for coronary artery disease.

That distinction is important. A single case cannot establish that ivermectin caused the ulcers; it can show only that the two occurred together in a patient with several possible contributing factors.

US Regulators Have Warned About Animal Formulations for Years

Ivermectin formulated for horses is dosed for animals that weigh many times more than humans. The Food and Drug Administration states that these formulations have not been tested in people, so their safety in humans is unknown. The agency advises people never to use medicines intended for animals and says it has received multiple reports of patients needing medical care, including hospitalisation, after doing so.

The US federal response intensified as demand surged during the pandemic. In late August 2021, the FDA sent a letter to veterinarians and retailers asking them to help provide consumers with safety information, and publicly summarised the step the following day. A CDC health advisory issued that month described a sharp increase in reports of people suffering adverse effects after taking ivermectin intended for horses, cattle and sheep.

The report also cites research showing that non-prescription ivermectin use has been concentrated among older adults and people facing high social vulnerability, a pattern that matched this patient. He was discharged to a skilled nursing facility. Anyone taking or considering a medicine outside a prescription should speak with a clinician, while anyone experiencing ongoing abdominal pain, vomiting or an inability to pass stool should seek medical attention.

Key Questions Answered

What is a bezoar?

A bezoar is a hardened mass of material that accumulates in the digestive tract and does not pass. Bezoars usually form from plant fibre, hair or medicines. In this case, the mass formed from congealed veterinary ivermectin paste.

How did doctors confirm what it was made of?

The mass was sent to the Animal Health Diagnostic Center at Cornell University's College of Veterinary Medicine, where mass spectrometry confirmed the presence of ivermectin in the extracted bezoar.

Did ivermectin cause the intestinal bleeding?

The surgeons believe prolonged contact between the trapped paste and the intestinal lining caused the ulceration, but they identify other possible causes, including pressure from the bezoar, ischaemia caused by bowel distension and the antiplatelet drug the patient was taking. One case cannot prove causation.

Why did he take horse paste?

He had seen online content promoting ivermectin, and his primary care physician had refused to prescribe it. He therefore bought a veterinary formulation online without a prescription.

What does the FDA say about veterinary ivermectin?

It says people should not use it. The agency states that these formulations have not been tested in humans and that their safety in people is unknown. It has repeatedly warned against the practice.

How did the patient do?

He required two operations, three units of blood and removal of more than 50 centimetres of small bowel. He recovered and was discharged to a skilled nursing facility.

Originally published on Medical Daily