*****CAUTION: THIS POST CONTAINS PARASITE PICTURES WHICH MAY UPSET THE SQUEAMISH.*****
There have been some posts going around Reddit about a man who had a worm removed from his eye via laser. I definitely lost time imagining what it would be like to have a front row seat to watch an eye-dwelling worm attempt to out-run a laser. I had to sift through a few different reports before I could figure out which parasite they were suggesting was the culprit, but eventually I found a 
reference to raccoon feces, suggesting that they think the parasite is Baylisascaris procyonis. The definitive host of this nematode is the raccoon, in which it lives and reproduces in the intestine. The parasite’s eggs make it back into the environment in the raccoon’s feces. If they are accidentally ingested by another raccoon, then they return to the intestine and the cycle starts again. If they are ingested by anything else, they wreak havoc.
In any other host, Baylisascaris procyonis burrows through the intestine and begins a migration through the body. It has a particular propensity for the central nervous system (brain, spinal cord, etc.), which can have devastating implications for the host. This focus on the central nervous system serves to debilitate or kill the host, making it more likely to be eaten by a raccoon (thus returning the parasite to its favored host). Simply washing your hands regularly (and always before putting them or anything you’ve touched into your mouth) will go a long way towards preventing infection with this parasite.
Baylisascaris procyonis isn’t the only parasite which finds our eyes particularly hospitable. If you get bit by a horsefly in parts of Africa, then you may have the pleasure of hosting Loa loa, also known as the “eye worm”. While Baylisascaris infection can result in death, Loa loa usually just results in a serious case of the ookie mookies. The symptoms of infection are typically minor (localized itching and swelling, mainly), but the presence of a large worm in your eye is certainly enough to gross anyone out.
I was looking up information on removal of Loa loa and found this gem on the Stanford website. Apparently the worm is removed from the eye with forceps, but not until a 10% cocaine solution has been dripped onto the infected eye. I suppose a good cocaine drip would go a long way towards easing the tension associated with finding a huge worm living in your eye!
Trematodes in fish eye
Turns out, a lot of parasites infecting all sorts of hosts go for the eyes. The eye is actually a fairly hospitable environment for parasites as the immune system is fairly inactive in the eye. If you have a parasite in your eye that may eventually migrate out or isn’t always in your line of vision, it’s probably a better idea to ignore it than kill it. An immune response could result in blindness, or at least a blind spot where the dead parasite resides. More sight is probably salvaged by ignoring the parasite and focusing on avoiding contacting with future parasites than by attacking it. Here is an interesting article on trematode parasites infecting fish eyes.
Parasites like pinworms, (Enterobius vermicularis) that make themselves at home in your rectum have always been at the top of my “I hope that doesn’t live inside me” list.
Ooooh, that’s another good one! ~20-30% of US kids and ~16% of US adults are infected at any one time!
wreak havoc*
firstoff, just discovered your site, and it is wonderful!
second, due to your apparent interest in parasitic infections, have you investigated the Rabies virus?
while not quite your standard definition of a parasite, it is still a remarkable example of behavior modification in many species. Additionally, it does this by possessing only 4 genes!
Keep up the interesting work!
there is a House episode about this! I think it’s season 3… not sure what parasite exactly they found, but the kid got it from playing with/eating animal poop in the sand box!
omg, that is so disgusting.so they would rather you live your days with a moving worm in your eyeball, than remove it.omg……………….that is absolutely the worst news i’ve heard all day.
gross.
I found a reference to raccoon excrement, proposing that they think the parasite is Baylisascaris procyonis. The conclusive host of this nematode is the raccoon, in which it lives and replicates in the digestive system. The parasite’s eggs make it back into the environment in the raccoon’s feces. If they are accidentally ingested by another raccoon, then they return to the intestine and the cycle starts again. If they are ingested by anything else, they wreak havoc. This focus on the central nervous system serves to debilitate or kill the host.