Wednesday, 12 March 2014

Human Bot Fly Dermatobia hominis myiasis

It's been a couple of weeks since I hopped onto the Cessna Caravan at Chan Chich, and there's no doubt that once again I have brought back with me as souvenirs of Belize more than the usual couple of bottles of Marie Sharp's Fiery Hot Habanero Pepper Sauce.

I've picked up Human Bot Flies elsewhere, but Belize is a particularly good place to encounter Dermatobia hominis. Fortunately, I have always had them on my legs where they cause only minor discomfort and are relatively easily extracted: I have heard tales of scalp infestation and can well imagine that this would be a serious inconvenience. Indeed, in rare cases, larvae living in the scalp have burrowed through the soft fontanel and lodged in the brain, killing the host.

Bot flies are native to tropical America where they parasitise mammals such as deer, tapir, pigs, dogs and cattle, as well as Homo sapiens. I am not sure why they should be so common in Belize, since there are far more tapirs and cows in the Venezuelan llanos or the Brazilian Pantanal and I have never picked them up there. The larvae inhabit the subcutaneous layer of their hosts, where they form a pustule within which they feed on tissue fluids. Stiff, black, backward-pointing bristles anchor them in place as they scrape away with a pair of fang-like mandibles at the host tissue, while a breathing tube allows them to take up oxygen through a hole in the skin which opens after a couple of weeks. The larva is well adapted to its task. But the most extraordinary aspect of the bot fly's life cycle is its devious strategy for getting the larva into place. The adults capture a blood-sucking insect, such as a mosquito or deerfly, and deposit their mature eggs on the ventral side a behaviour known as phoresy. Upon the vector's contact with the mammalian skin, body warmth triggers the eggs to hatch and the newborn larva gains access to the host through the bite wound or a nearby hair follicle. The larvae spend about two months within their host, during which time they pass through several instars before crawling out to pupate in the soil. The University of Florida provides a good introduction to the Human Bot Fly.

What happens if you (or your family members) do not intend to allow the larval bot fly to complete its natural span within your body? Going to a medic in the USA or Europe will almost certainly result in minor surgery and perhaps counselling for the physician. As the US CDC stipulates, "Fly larvae need to be surgically removed by a medical professional". In fact, there's a much simpler solution: I prefer to sit it out until the larvae become uncomfortable and large enough to physically expel. From about the third instar onwards, mine have tended to move around and rasp away in the early hours, causing pain and lost sleep, so this is the time to get on with the task. The first time I had them, I tried taping bacon over the holes, then ham, then beef, but simply woke up night after night with a curious sort of "breakfast in bed". Much better is to apply a thick layer of petroleum jelly to the hole and wait a few hours. Starved of air, the larva will work their way out of their breathing hole and if you are quick you can deftly apply pressure to the surrounding skin and force the intruder out. Since by this time they are flask shaped, with the bulk of the body in the depths of the pustule, they typically pop out with some force not unlike a miniature champagne cork!

To be continued... here


  1. I'm really looking forward to their "coming out" to see what they look like. How long is incubation? Presumably they become larger and more painful? You've had several before so you know what you're doing. How long are you going to give them before you apply the petroleum jelly?

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  3. The data given on incubation period in the literature varies widely from 4 to 12 weeks. I've usually got mine out within 3 weeks of realising what they were, although that may take a fortnight until a hole develops and you can feel them moving about and feeding. I probably picked this batch up around 20 February. I put on the petroleum jelly last night, but it just made a mess! War has been declared though: I have no desire to be a martyr or to see them through their life cycle - I know what they do and what they look like (pretty neat, I have to say). It's just that they are hard to get out when small.

  4. How are your parasites doing? Have you managed to extract any of the Fantastic Four?
    Let's see some photos! (OK, it didn't go viral on Facebook but it's interesting me anyway.) Are they active pupae as in Tsetse flies? or passive like sheep bots?