"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

February 4, 2019

Acanthamoeba spp.

Today we're featuring a guest post by Sally O'Meara - a student from 4th year class of the Applied Freshwater and Marine Biology' degree programme at the Galway-Mayo Institute of Technology in Ireland. This class is being taught by lecturer Dr. Katie O’Dwyer and this post was written as an assignment about writing a blog post about a parasite, and has been selected to appear as a guest post for the blog. Some of you might remember Dr. O'Dwyer from previous guest post on ladybird STI and salp-riding crustaceans. I'll let Sally take it from here.

This blog post today is dedicated to all you visually impaired contact lens wearing folk out there! Before I begin, I just want to say that I truly hope all of you adhere to the instructions your optometrist gives you with regards to using contact lenses (washing hands before and after handing them, taking them out while showering/bathing). If not, I’m afraid you are running the risk of meeting my new acquaintance; Acanthamoeba spp., also known as the cornea guzzling free-living protozoa from hell!

Acanthamoeba in its two forms: (A) trophozoite, (B) impenetrable cyst
Image by Jacob Lorenzo-Morales, Naveed A. Khan, and Julia Walochnik, used under CC BY 2.0
Acanthamoeba spp. are microscopic organisms that can be found just about anywhere, from soil to water, to the air we breathe. They are the direct culprits of Acanthamoeba keratitis (AK) a relatively rare but sight-threatening disease which is actually caused by at least eight species of Acanthamoeba: A. castellanii, A. culbertsoni, A. polyphaga, A. hatchetti, A. rhysodes, A. lugdunesis, A. quina, and A. griffin. Ocular trauma and contaminated water are also associated with AK infections but it has been found that contact lens wearing accounts for > 80% of the cases. If found early the infection can be cured, but this gets progressively more difficult the longer it remains untreated. The difficulty lies with the life cycle of the Acanthamoeba species which consists of two stages: the trophozoite and the cyst.

The trophozoite is the vegetative form which feeds on organic matter and ranges in size from 10 to 25µm. When the going gets tough, the tough get going... tough being the trophozoite. When conditions become unfavourable, like under extreme heat or lack of nutrients, the trophozoite transforms itself into a double walled cyst which is almost invincible. The cyst remains unscathed by repeated cycles of freeze-thawing, and incredibly high doses of UV and even GAMMA RADIATION. Cue the Terminator and his infamous catchphrase…. “I’ll be back”.

Characteristics of AK include eye pain, redness, itchiness, and a general feeling of something being stuck in your eye. Sounds like most eye infections, right? One extra feature is the presence of a stromal ring-like infiltrate in the eye. Basically, an ulcer forms on the cornea of the infected eye as a result of the hungry Acanthamoeba. It has been discussed that contact lenses serve as vectors for transmitting Acanthamoeba trophozoites, and to make matters worse studies have shown that wearing lenses results in mild corneal trauma which alters the surface of your eye making it even more susceptible to infection!
Healthy human eye (left) vs infected eye with Acanthamoeba keratitis (right). Arrow indicating stromal ring-like infiltrate.
From Figure 1 of the paper
Scientists have tried to create vaccines to prevent AK by terminating the Acanthamoeba trophozoite or the cyst, but these have proved unsuccessful. However, it was discovered that using a vaccine composed of dead trophozoites stimulates the production of antibodies in the tears, and these block adhesion of the trophozoites to the ocular surface which in turn prevents the development of AK.

Now, before you all go destroying your contact lenses in a panic-stricken state let me inform you that over 30 million Americans wear contact lenses, yet remarkably the incidence of AK in contact lens wearers is less than 33 cases per million. Acanthamoeba species are found in virtually every environmental niche on our planet ranging from thermal springs to solid ice, yet why are AK cases so far and few between? Scientists believe the host’s immune system plays an important role in successful AK infections.

Serological analysis of IgG and tear IgA (both of which are antibodies found in blood) revealed that 50-100% of healthy individuals with no history of AK possessed antibodies against Acanthamoeba antigens. What’s more, the serum IgG and tear IgA levels were significantly lower in patients with AK compared to the cohort of normal individuals with no history of AK, suggesting a prominent role of the mucosal immune system in preventing AK.

In 1939, Winston Churchill referred to Russia as “… a riddle, wrapped in a mystery, inside an enigma” … one might classify Acanthamoeba and the infections it produces in the same way! Although scientists have a clearer understanding of Acanthamoeba keratitis and the parasite which causes it, there is still much to be learned about its cunning and conniving ways.

Neelam S. and Niederkorn J.Y. (2017) Pathobiology and Immunobiology of Acanthamoeba Keratitis: Insights from Animal Models
. The Yale Journal of Biology and Medicine. 90:261-268.

This post was written by Sally O'Meara


  1. We have studied Acanthamoeba sp. in lecture and the parasite piqued my interest initially as I am a contact wearing individual. The thought of acquiring a “cornea guzzling… protozoa from hell” really makes me reconsider my poor contact lens habits. I found it very interesting that there is such a high percentage of individuals with antibodies for the parasite, suggesting it is fairly prevalent but not always very virulent. It is amazing all the little creatures our bodies can defend against right under our noses… or eyebrows in this case!

  2. I think this species is super interesting because 80% of cases are due to wearing contact lenses, which I thought was a huge number. I used to wear contacts, and it is possible that I could have been infected if the water was contaminated. I also think that it’s cool that the parasite can cause a stromal ring-like infiltrate in the eye. This ring is due to an ulcer, but when you look at it, it looks much more interesting than painful. It’s crazy to me that scientists discovered a vaccine made of dead trophozoites that stimulates the production of antibodies in the tears, blocking the adhesion of the trophozoites to the ocular surface. I had no idea this was even a possibility and I wonder what made them think to use dead trophs

  3. As a wearer of glasses who has in the past tried and desperately failed at wearing contact lenses, this post made me somewhat that contacts didn’t work out for me! It is very fascinating how such small organisms can be so dangerous and complex. I find it especially cool how the cyst is radiation resistant. It is also very interesting that many people have a natural immunity to this parasite.

  4. As a contact wearer myself, I find this article very interesting! I was actually wondering in class why AK cases were so rare if the parasites were so abundant in the environment and this article answered that question for me. It’s quite fascinating that individuals with no history of AK possessed antibodies capable of fighting off the parasite. This article also gave me the motivation I needed to start taking my contacts off before I go in the shower!

  5. I count myself lucky as I wear glasses and not contact lenses and I have often thought of using contact lenses several times. After learning about Acanthamoeba, I believe I would not ponder over that again. I love the way this article presents the information as not only does it inform the public on this crazy critter, but it calmly reinforces safe and hygienic practices pertaining to the removal and insertion of contact lenses for those who use them. I always love reading articles that are reader friendly in that they incorporate humor and keeps the audience engaged.

  6. This was super interesting, particularly as somebody who has tried and considered contacts before... now I really don't want to wear them.

  7. When learning about the life cycle stages of Acanthamoeba spp., I found it very impressive that the cysts are resistant to gamma radiation! I also found it interesting how researchers are trying to develop a vaccine against Acanthamoeba spp. which could be a very helpful preventative agent in the future. It was fascinating to see how many species of Acanthamoeba cause keratitis. Although, as a contact lens wearer myself, I found it concerning how my optometrist never warned me not to shower with my contacts in!

  8. There are several qualities of Acanthamoeba spp. that I find intriguing, especially since I work for an eyecare facility and this is a parasitic infection that causes damage to the eyes. The first characteristic being how durable the cyst form is as it is capable of persevering after various types of treatments such as freezing and gamma radiation. The other surprising quality of Acanthamoeba keratitis (AK) is how similar the symptoms of this infection is to any other type of eye infection and the “cardinal” symptom being a stromal ring-like infiltrate that arises after. I imagine misdiagnosis of AK is quite frequent if it were based on symptoms and not serological analysis

  9. this is very nice to know does it affect people who wear glasses rather than contacts. like if you shower with your glasses can they live on them and affect you. thanks