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    Beware venom ophthalmia

    An ophthalmologist’s guide for avoiding snake venom

    Things that strike terror in the hearts of many Americans—spiders, earthquakes, Ebola virus disease, and politicians with plans to “fix” healthcare—don't faze me much. But, for as long as I can remember, I have had this visceral negative reaction to snakes.

    This strong objection to these fusiform reptiles was recently reinforced when I learned the story of Bayan Chingsang. He served in Kublai Khan’s army as it invaded China in the 13th century. In the cinematic version of the story, Genghis Khan orders that the man be blinded as a punishment for defying the Khan’s orders. Subsequently known to Marco Polo as “Bayan Hundred Eyes,” he became a general who led his troops with great success on the battlefield.

    The way in which he is blinded is remarkable. His captors hold a cobra in front of him, and the snake spits fluid into the eyes of the prisoner who cries out in pain. The result is scarred and inflamed lids and bilaterally opaque corneas.

    Is this story true?

    Apparently the person did exist, was blind, and was a gifted general. How he came to be blind and other details of his life are uncertain or disputed. But what is known with scientific precision is that there are indeed cobras that can blind animals by spitting into their eyes.

    According to National Geographic1, the cobra shoots its toxin from its fangs and precisely targets the eyes of large animals it considers a threat. The venom contains not only the neurotoxin we—or at least, I—think of with snakes like this, but also a very cytotoxic agent described as being similar to mustard gas. Relatively innocuous when applied to skin, when it hits the ocular surface the spit causes de-epithelialization of the cornea with stromal inflammation, necrosis, and scarring. Steroids are reportedly not helpful at preventing corneal tissue destruction and scarring in rabbit corneas first treated with the spitting cobra venom.

    The snakes are accurate. One scientist wearing goggles tested the snake; from a distance of 6 feet the Mozambique snake hit the investigator's eyes ten out of ten times, while the black necked spitting cobra was on target 8 of 10 times. This spitting is instinctive, with the little rascals documented to spit at herpetologists (the scientists who study snakes) literally at the moment they emerge from their shells.

    Why do the snakes inject their venom into some animals and spit at others? According to scientists, they inject and eat smaller animals while they spit at large animals that they perceive as threats but too large to eat. Pretty logical.

    What have we learned?

    Peter J. McDonnell, MD
    He is director of The Wilmer Eye Institute, The Johns Hopkins University School of Medicine, Baltimore, and chief medical editor of ...

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