After a tough battle with Ebola, Ian Crozier’s blood was finally cleared of the virus and his doctors thought he was cured. What they hadn’t counted on was that some of the virus might survive deep within the immuno-privileged recesses of the eye. When Ian woke one morning to find that his normally blue left had turned green, he knew something wasn’t quite right.
The doctors injected steroids above his eye, and gave him an antiviral drug which they hoped might reach the interior of his eye in sufficient concentration to wipe out the raging virus. Luckily, Ian’s blurred vision gradually came back and he was able to make a recovery. Lurking beneath this apparent success story lies what is arguably the most critical question one can ask in medicine — did the anti-viral drug work, or did Ian make his recovery himself?
The doctors still aren’t sure of the answer to this conundrum. Right now, several trials to find effective medicines and vaccines for Ebola are underway. Although it is not the same panic-generating threat it was just a few months ago, Ebola is still very much a concern. It is now known to survive for many months in certain parts of the body, like semen, where the immune system can’t touch it. Where doctors once said things like ‘the tears and surface of the eye are Ebola-free so there is no danger to others’ we now know better than to take such claims at face value.
Now that Ian seems to be doing better, many might be asking themselves, what was up with that eye? Ebola-green may never go viral as a new brand of colored contacts, but changing eye color naturally, and reversibly, may have a certain appeal to some. To do it, it seems you need to fiddle with the internal pressure of the eye and adjust the spacing in the proteins responsible for scattering light. At the height of his infection, the pressure in Ian’s eye rose precipitously, and then fell off, leaving him with a soft and spongy green eye. But pressure alone isn’t enough, you also need adjust the composition of the eye. So what exactly is doing the scattering here?
The answer is a whole of host of things. In darker eyes, pigments including various kinds of melanins absorb much of the stray light giving a dark appearance. In lighter eyes, color is affected by the presence of different fatty deposits, so-called lipochromes, and even trace elements like copper. However to really sound definitive, you need to mix in some physics. If you imagine that the eye is like the sky, then the clear blue sky eye color would come from the process of Raleigh scattering, where the shorter wavelengths are scattered more. In this case, scattering off of smaller particles (at least as compared with the wavelength), within the connective tissue or stroma of the iris. To get grey colored eyes, as we would have for the thickening clouds of a humid day, you need a touch of frequency-independent Mie scattering from collagen.
If instead you ask a geneticist for insight, you quickly find that all you really need to know about eye color is two genes, namely HERC2 and OCA2. Even that may be more than you need, because HERC2 is the direct regulator of OCA2. Using just six variants (single nucleotide polymorphisms) of these genes it is possible to predict human eye color with 90% accuracy, although we don’t know how many color bits that corresponds to.
While interesting, eye color obviously isn’t the real story here. The larger picture of Ebola infectivity and transmission is still emerging as we speak. The current concern is sexual transmission of the virus months after it was thought to have been eradicated. For inquiring minds that just got to know, there is a good reason why the testes are privileged whereas the ovaries are not: sperm are immunogenic. Since they mature after the critical period of immune learning where the body first gets to know itself, they need to be walled off and protected.
Appreciating some of these finer details — like how and where the virus can hide out in the body — will be critical to learning how to stop it.