Sunday, May 5, 2013

Dermatobia hominis: The Human Skin Bot

D. hominis larva
For those of you who haven't already heard, I made a journey across the state yesterday to my home for the summer.  I'll be spending the next three months working at a biological field station owned by the university I am attending.  Yesterday while sitting in the dining hall, I saw an adorable little rabbit hopping around the cedar trees.  The first thing I thought about was how I scared my roommate last week with a video of a bot fly emerging from a recently killed rabbit.  (You can view this video yourself here.)  When I awoke this morning and began thinking about what to write for today's post, I realized I had never really talked about bot flies.  So here we are! There were lots and lots to chose from, but I decided we would take a human perspective and look exclusively at the human skin bot (Dermatobia hominis) for this post.

As you probably already know, flies are animals belonging in phylum Arthropoda under the class Insecta. All flies belong to the order Diptera, meaning "two-wing"; this is in reference to the fact that these insects have only one pair of wings as opposed to two pairs like most other insects.  The second pair of wings in most flies have been reduced to vestigial structures called "halteres".  These structures now function mostly as balancing organs and have a knob-like appearance.  This order houses about 120,000 species representing 140 families.  The family that contains bot flies is the family Oestridae.  This family is split into four subfamilies: Cuterebrinae (skin bot flies), Oestrinae (head maggots), Hypodermatinae (cattle grubs, ox warbles, and heel flies), and Gasterophilinae (stomach bots flies infecting horses and their relatives, elephants, and rhinos).  Our D. hominis is found in subfamily Cuterebrinae and is common from Southeast Mexico down through the South American continent, extending to Argentina and Chile. Though we call it the "human skin bot", it will actually develop beneath the skin of just about any warm-blooded animal...mammals or even birds.

Adult human skin bot fly
Life Cycle,
The life cycle of this fly is similar to other flies in that it involves the laying of an egg what will hatch into a larva, which will go on to form a pupa from which an adult fly will eventually emerge.  Unlike most flies that infect their hosts (a condition known as "myiasis" which we will discuss in a moment), this fly does not lay eggs directly onto their hosts' bodies.  Nope, these guys are much more devious. An adult female fly will catch another parasitic insect, like maybe a mosquito, and glue her eggs onto the insect's body with the opercula (little flaps that open to release the eggs' contents) of the eggs facing downward.  It is unknown how many different types of insect carries are used by D. hominis, but there are at least 48 different types of flies and on tick species that have been known to be carriers.  Once one of these carriers lands on warm skin, the opercula open releasing the larvae onto their new hosts.  The larvae then burrow into unbroken skin and make a home in the dermal layers of the skin, where they stay for about 6 weeks before burrowing out of the skin and falling into the soil where they form a pupa and undergo metamorphosis, emerging as adult flies ready to mate and restart the cycle.

Bot fly larva being removed from a leg
This is the clinical term for an infection of fly maggots.  This can occur either opportunistically, as is the case in facultative myiasis...a condition in which  a non-parasitic maggot accidentally winds up in a host, or it can occur as part of a parasitic fly's life cycle, which is the case with obligatory myiasis.  If one becomes infected with D. hominis, that is obligatory myiasis.  Myiasis can occur just about anywhere there is exposed skin for a carrier to land on.  In fact, I even ran across a case of penile myiasis...that's right, some poor schmuck got infected with a bot fly on his penis.  No matter the site, these types of infections cause painful lesions, but almost never results in secondary infections.  In fact, there have been reports that fly maggots actually secrete antibacterial substances that are alkaline in nature, making wounds unfavorable environments for bacterial growth!  How cool is that??!!!

Human skin bot fly larva

No matter how awesome it is that these guys make antibacterials from their spit, they are not fun to have burrowing into your dermis.  The best way to remove a bot fly larva from yourself is to suffocate the little guy and then pull him out by squeezing or using tweezers.  There are several ways to suffocate the larva.  One way is to put tape over the wound, but this is not recommended as it can actually rip the larva causing problems with total removal.  Another, less damaging way is to apply several coats of nail polish to the wound, but this can be problematic when removing the nail polish and is easily applied incorrectly due to the nature of skin.  The best, most effective way with the easiest cleanup is to apply a generous coating of petroleum jelly to the wound.  Which ever method you use, leave the applied substance on the wound for about a day or more, then you can remove the substance and subsequently remove the fly.  Some sources say that a venom extractor (found in snake-bite first aid kits) can be very effective at removing these larvae.

Moral of the Story
I think if you get a bot fly,
someone should buy you this shirt.
If you travel south for research or for funzies, be sure to take both insect repellent and petroleum jelly.  A snake-bite kit wouldn't be a bad idea either! Also keep in mind that though these things seem scary, they have tremendous potential in the field of medicine.  Perhaps if we could isolate some of these secretions, we could get new forms of antibacterial medicines or even substances that can be used to treat necrosis (dying tissue).  Heck, maybe someone already has figured out how to do either or both of those things! The point is if you do get a bot fly, remember that it is easily treated and that you aren't likely to get any weird secondary infections. Easy enough to deal's all mind over being freaked out by a parasite! :p

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