Apparently there are round worms and parasites in soil, etc., that if accidentally ingested (which is quite easy, if you don’t wash your hands after touching soil and then eat something, you may ingest it,), that head for the human eyes after being ingested.
I don’t quite understand the pathway, but it involves being ingested into the stomach, maybe intestine, but then the worms and whatnot somehow end up in the eyes, and cause blindness or other issues.
How do they get there - there are a lot of obstacles in the way from going from the stomach to the eye? (blood, organs, tissue, muscle, membranes?) And why in the eyes, of all places?
Searching a bit on this I found one candidate. Onchocerca volvulus, but it comes in through a bite, and blindness is only one late stage symptom. There doesn’t appear to be any “heading for the eyes”, just that as the infection spreads, through the blood. The eyes are among the parts that can be noticably affected.
IANAD though, so maybe there are worms that head for the eyes.
There is a show ‘the monsters inside me’ that go over these things case by case from assumed infection, method of transport, treatment and outcome. Most do not go for the eyes, but can wonder around and may arrive there. Many will get through the intestine wall into the blood to find some happy place to stay and have young’uns. Others will get in through the skin and borough their way around with no real direction known. Others will travel via the lymphatic system. Some do have preferred organs such as the heart (like the heartworm)
Parasites have a set of hosts. The roundworms that I know in humans tend to go to the eyes are not our parasites. They are parasites we accidentally got, but those parasites would much rather develop in the guts of dogs and/or raccoons. Unfortunately, we humans ingest the infective stage, and it starts to develop… Only we are not its host, so instead of going on the pathway to the gut (and reproduction and happy parasite life), it gets sidetracked and does a different migration pattern. That way it gets into our eyes.
How can it get there? Well, once they develop in the small intestine, they can dig their way to the mucosal layer of the intestine and into a blood or lymphatic vessel. From there, they just follow the flow. They are supposed to be following a set of signals, sadly they don’t get the right set of signals from our bodies (we are not dogs or raccoons), so they get basically lost and wander around. And then they eventually make it to the eyes.
Actually the immune system has a way to deal with parasites, even bigger ones. They do have ways of responding and attacking, and can destroy/neutralize some parasites that way. But it all depends on the species. Some are good in masking/escaping the immune system. In other cases our immune system responds, but not fast enough.
"Ocular larva migrans
From Wikipedia, the free encyclopedia
Ocular larva migrans is the ocular form of the larva migrans syndrome that occurs when toxocara canis larvae invade the eye. They may be associated with visceral larva migrans. Unilateral visual disturbances, strabismus, and eye pain are the most common presenting symptoms."
KarlGrenze gave a nice “why”.
I have heard people state that parasites are “simple” creatures, based on their relatively simple, stripped-down, anatomy. I think that if you consider their often complex life-cycles, they are anything BUT simple.