By Bob Zink
Early one morning, I saw a headline in the National Deer Association online newsletter that read: “Freak buck had corneal dermoids.”
In my pre-caffeinated daze, I thought to myself, why would a buck have cornmeal in his, wait, what are dermoids? The article by Lindsay Thomas Jr. was, in a word, fascinating.
I’d just finished lecturing about the human genome, and how cool it is that in all, or most, of our cells we have the DNA blueprint to make anything we want, anywhere on our bodies. How even cooler it is that the genes for making fingernails are turned on only in the cells at the tips of our fingers (and toes, in the case of toenails), muscle cells in the right places, and genes for our complex eyes in the sockets in our skulls. This process is a matter of gene regulation – so that we don’t have fingernails on the tops of our heads, eyeballs on our knees, etc.
The process, however, sometimes goes awry. In the fruit fly, some mutations occur that result in the antenna being turned into legs – cool, but not if you’re that fly. Other flies can have two sets of wings. Sometimes the developmental process gets really screwed up, and we’ve seen a cow with two heads, turtles with heads at each end, and people with tails. Yes, some human infants have tails (fewer than 50 known cases), which are usually surgically removed. The term for expression of genes and growth of the wrong structure in an abnormal place is an atavism.
The freak buck mentioned above, who should be named “Damn Unlucky,” (DU, with apologies to Ducks Unlimited), was shot as a 1-1⁄2-year-old in eastern Tennessee in late August 2020, as it was wandering around in a daze, obstructing traffic.
No wonder DU was wandering around; he had hair growing all over an eyeball.
Remember that vertebrate eyes have an iris (the colored part), a pupil (the dark, open center), and a lens, all of which are covered by the cornea – that thing that if scratched hurts like the devil.
DU’s cornea was the site of the accidental expression of skin genes, which did their due diligence and made hair, like they’re supposed to, just not there.
Contributing to DU’s condition was EHD, epizootic hemorrhagic disease. Unlike chronic wasting disease – in fact, very much unlike – EHD is caused by a virus that is spread by those annoying “no-see-ums.” Populations often are hit hard by EHD, but the viral infection in the population goes away after time.
CWD, caused by a pool of ever-increasing abnormal protein, a nonliving entity, builds up in the soil like an environmental toxin and keeps reinfecting deer irrespective of how many deer are in the area. A deer’s genotype can impede the onset of the disease, but it is considered 100% fatal at present.
EHD and CWD are very much unalike.
Still, DU had EHD and showed the symptoms of disorientation, much like deer with CWD. Whether DUs eyeball issue was related to EHD is unknown. Lots of deer have had EHD with no reports of accompanying corneal dermoids.
The term for this eyeball atavism is a corneal dermoid. The term dermoid is derived from dermis (skin), and is a piece of functional skin growing where it typically would not be – in this case, the cornea of DU. The genes can produce glands and other features in addition to skin. Many other animals have had this affliction, including dwarf rabbits, hairless guinea pigs (now that’s ironic), cattle, and at least one human.
Mr. Thomas reports that a pathologist suggested that DU had the affliction from birth and likely survived for as long as it did because it was in a relatively safe urban area and was attended to, at least early on, by its mother. It’s not clear whether the dermoid increased in coverage and severity over time, or whether DU was effectively blind from birth or it got progressively worse.
Damn Unlucky is apparently one of only two known white-tails to have had this affliction. The other known whitetail with corneal dermoids was a 11⁄2-year-old doe harvested in St. Mary Parish, La., in 2007, and was described in an article by LaDouceur et al. in the Journal of Wildlife Diseases.
Unlike DU, this doe had three relatively small spots on the left eye only. Upon dissection, it was found that these three masses on the deer’s cornea were composed of hair follicles, sebaceous (oil-producing) and apocrine (sweat) glands, and well-differentiated cartilage. Most surprisingly to me, the authors reported that they found a cystic tarsal gland in the eyelid! Talk about misplaced genes! A tarsal (leg) gland showing up in an eyelid …
These corneal dermoids are stark reminders of how well coordinated the vertebrate developmental systems are – going from a single fertilized cell to millions of cells all developing in the correct places to produce a complex body like that of a white-tailed deer, with only two known instances – out of millions of harvested deer – of hairy eyeballs.