When I was in grad school, I was always a little disappointed that I didn’t need to travel much further than northern Florida. Other students in my graduate cohort were busy going off to do research in interesting and exotic places like Jamaica, New Guinea, Brazil, Peru, or Gabon, and I was driving down to Tallahassee and picking up roadkill.
But sometimes I’m reminded of exactly how much I missed. One of my friends, now a full-fledged professional systematist, came back from one of his field seasons in Africa with a passenger.
Loa loa is also known as the African eyeworm. It’s a filarial nematode. Fundamentally, that means it’s a long, thin worm that molts periodically as it grows. There are lots of parasitic nematodes, but Loa loa is the only one where the adults occasionally swim across your eye.
I thought I had flicked something into my left eye. There was something in there that was bothersome, but not sharply painful. So I went to the bathroom mirror and pulled down my lower eyelid. There I saw (with both my eyes, I suppose) a semi-transparent worm, about 1 cm in length and no thicker than a silk thread, making its way across the white (sclera) of my eye, below the iris and beneath the conjunctiva…
Of course, by the time that adult was having a swim somewhere visible, John had turned into a worm habitat. Humans are the only known host of adult Loa loa. They typically live in subcutaneous tissue. But they also breed like bunnies, pumping out thousands of tiny threadlike larvae that ride through the bloodstream on a daily basis. At night, they collect inside lung tissue. During the day, they move out to the veins under the skin and wait on the off chance that they’ll get sucked out by their second host – a biting fly.
Once the larvae are inside the fly, they bore through its gut tissue, migrate to its muscles, molt a couple of times, then finally move into the fly’s proboscis. They wait for the fly to bite someone else, and crawl into the wound. New host!
It takes the larvae about 6 months to become adults. But they can live inside human tissues for 15 to 17 years. So it’s not entirely clear when John was infected. He’s been studying fish in central Africa for nearly a decade. He does know that it had been two years since his last research visit, so the worms had him for at least that long.
So, what was it like to have a more-active-than usual adult swimming around?
When it was active, it was quite annoying and my eye teared profusely. I remember hoping it would just go back to wherever it had come from and leave my eye alone. It did, eventually. My eye was a little red where it had broken some of those blood vessels. It never made a reappearance.