New study finds iron, bromine may help explain mysterious black lesions in cats' eyes

A black or dark-brown lesion that appears on the eyes of some cats has long puzzled veterinarians. Now, researchers at the Hebrew University of Jerusalem have identified 2 elements that distinguish affected corneal tissue from healthy tissue, offering new clues about the condition.¹

A stubborn, poorly understood lesion

The study, led by Oren Pe'er, DVM; Lionel Sebbag, DVM, PhD, DACVO; and Ron Ofri, DVM, PhD, DECVO, of the Koret School of Veterinary Medicine at Hebrew University, examined feline corneal sequestrum, a condition in which part of the cornea becomes damaged and necrotic, forming a characteristic dark lesion. The condition can cause chronic eye discomfort and sometimes requires surgical treatment.¹

Over the years, researchers have proposed several explanations for the dark discoloration, including melanin and iron accumulation, while factors such as chronic corneal irritation, tear-film abnormalities, and viral infection have been investigated for a possible role in the disease. But both the source of the discoloration and the condition's underlying pathogenesis have remained incompletely understood.¹

"The uniqueness of this study is that it looks at a phenomenon we see in cats, essentially necrotic tissue on the cornea, from a different angle," the study's researchers said in a release announcing the findings.² "There have been various theories about what causes the black color, including pigmentation, iron or other changes. Using a highly sensitive technique for elemental analysis, we identified two elements that clearly distinguished affected tissue from healthy corneas."

Elemental analysis flags 2 suspects

The researchers examined corneal tissue removed from 11 affected eyes belonging to 9 cats and compared it with healthy corneal samples from 4 control cats. They used particle-induced X-ray emission (PIXE), a highly sensitive technique capable of detecting trace elements in very small biological samples.¹

Iron and bromine were detected in the corneal sequestra but were absent from the healthy control samples. Other elements tested, including calcium, phosphorus, sodium, potassium, zinc, chloride, magnesium, and sulfur, showed no statistically significant differences between the groups.¹

The findings suggest that the lesions have a distinct elemental profile, although they do not establish what role iron or bromine may play in the disease. Iron is of particular interest because it can participate in reactions that generate molecules capable of damaging cells and tissue. The researchers suggest that such processes could potentially contribute to the corneal degeneration seen in affected cats and may help explain the lesion's characteristic dark color. Bromine may provide another clue, although its role in feline corneal sequestrum remains unclear.¹

What it means for practice—for now

The researchers cautioned that the findings do not yet prove iron or bromine cause feline corneal sequestrum; it remains possible that the elements accumulate only after the cornea has already been damaged. The study also involved a relatively small number of cats, and PIXE can identify which elements are present without determining their exact chemical form or location within the tissue.¹

The researchers said the findings could eventually have implications beyond simply explaining the disease's appearance.² "These findings open a new direction for investigating how the disease develops and may ultimately help identify new diagnostic or therapeutic targets," they said.

Further studies will be needed to determine where the elements come from, how they accumulate in the tissue, and whether they play an active role in the disease's progression.¹

References

1. Pe'er O, Sebbag L, Ofri R. The role of trace elements in the formation of feline corneal sequestra. Vet Ophthalmol. 2026;29(5):e70242. doi:10.1111/vop.70242
2. Hebrew University of Jerusalem. What causes mysterious black eye lesions in cats? Researchers find new clues. EurekAlert! Published August 16, 2026. Accessed August 27, 2026. https://www.eurekalert.org/news-releases/1140283