A new study, conducted by the researchers from the University of Liverpool and published in JAMA Ophthalmology, identifies the specific characteristics of Ebola retinal lesions, which provide further clues as to how the virus travels to the retina and causes damage.

Viruses like Ebola can stay hidden in the body by exploiting a vulnerability in the immune system. This vulnerability is called “immune privilege,” and comes from an old observation that foreign tissue transplanted into certain parts of the body don’t elicit the usual immune response. This includes the brain, spinal cord, and eyes. Scientists believe this is because the brain, spinal cord, and eyes are simply too delicate and important to withstand the inflammation that’s typical of an immune response to infection.

In 2016 an eye team led by Dr Paul Steptoe, compared eye examinations of Ebola survivors in Sierra Leone and a control population. A total of eighty two Ebola survivors who had previously reported ocular symptoms and one hundred and five unaffected controls from civilian and military personnel underwent ophthalmic examination, including widefield retinal imaging. The team identified a unique retinal scar in 15% of Ebola survivors who reported eye symptoms after recovering.

The University of Liverpool reports in the latest study the researchers used a non-invasive imaging test called optical coherence tomography (OCT) to take pictures of the retinas of the survivors with this Ebola retinal scar. These images provide a high resolution, cross-sectional view of the retina.

These images help demonstrate that within these Ebola retinal scars there are multiple, microscopic focal areas of damage to the retinal layers corresponding to the photoreceptors with a collapse of the overlying retinal structures. This pattern of scarring supports a theory that the virus travels along neuronal structures within the retina.

The researchers also identified areas of abnormal retina surrounding the Ebola retinal scars in 89% of cases. The affected areas appear darker than the normal retina and are due to a change in the reflectance of a layer of the retinal layer called the ellipsoid zone which contains a high concentration of mitochondria which are the structures that create energy to run the cell.

Paul Steptoe said “This is the first time this appearance has been associated with a retinal infection and implies the virus may be causing a wider effect to the surrounding retinal cells beyond the boundary of the scars. Our observations of these areas continue as part of a long-term study Ebola survivors. In the cases we have identified, the position of the scars did not involve the part of the retina responsible for fine central vision, but in survivors where scars are close to this area, we have detected permanent visual field defects. The retina offers a landscape on which to observe and better understand the long-term complications of Ebola virus disease that include ophthalmic and neurological problems.”