Bot Fly Eggs – They attach to the inside of the mouth… Ewww. Really? YUK. Get rid of them!

Monday, October 24th, 2011 | Filed under Uncategorized

During my epic horse grooming day yesterday, I came across a zillion bot fly eggs dangling from the legs and trunks of my horses.

I did what I always do.  I brought out my bot knife (I need a new one) and my bot block.  I started with my dull knife.  What eggs I didn’t get with my dull knife, I caught with my block.  The hangers-on I pulled with my fingernail.  Eww.

And than I discovered some things I should have done when I did some research…

These are bot eggs on Wrigley's chest and shoulder.


Yuk.  This is gonna gross you out.

OK.  So, the female bot fly hovers around the horse – I’m sure you’ve seen this – and lands long enough to deposit 400 – 1000 eggs.  On a single strand of hair you will see one yellow, cylindrical egg attached.  It kinda looks like miniature, yellow rice.

Anyway, that is the easy part.  You have all probably seen the eggs on your horse.

The gross part is what happens next.

You see, the eggs itch (gotta love Mother Nature) so the horse bites at them.  The saliva and warm carbon monoxide from the horse’s mouth stimulates the egg to hatch.  But, that isn’t the gross part.  The upsetting part is that the bot fly life cycle continues in your horse’s mouth.

Yup.  The horse bites at the egg.  The egg ends up in his mouth.  The warmth and wet of the mouth encourage hatching.  The hatched larvae burrows into the horse’s tongue or gums.  A time later it emerges – itchily… (imagine having a bit in during this time – ouch).  The newly emerged bot larvae is then swallowed and lives in your horse’s intestinal wall, eating what your horse eats.

Eventually, the final bot larvae is pooped out and hatches.  It then flies away – a mature bot fly – ready to do this all over again.

I feel sick.

Life cycle


Eggs. Eggs are attached to hair on the host’s body; the site of egg laying varies depending on the species of bot. Superficially the eggs resemble louse eggs, but the location and time of deposition are characteristic for bot species.

Common bot. Eggs of the horse bot are stalkless and are generally glued near the end of the hairs. The eggs are greyish yellow to yellow in color and about 0.05 inch long. Two flanges along the lower half of the egg encircle the hair and serve to attach the egg to the hair. The non-flanged half extends from the hair at about a 30 degree angle.

Throat bot. Eggs of the throat bot are also stalkless and are usually laid near the skin. For this reason, they are often obscured by overlying hair. The flanges which attach the egg to the hair extend almost the entire length of the egg. The color is whitish-yellow and the egg is approximately 0.05 inch long. The long axis of the egg extends parallel to the hair.

Nose bot. Nose bot eggs are stalked and the general shape is barnacle-like. The connecting flange extends from the stalk upward toward the top of the egg. The general color is brownish-black and the egg is about 0.06 inch long.

Here you can see the eggs easily on the leg of a dark horse.

Larvae. Horse bot larvae are well adapted to life in the digestive tract of the horse. Larvae are equipped with mouth hooks, setae and spines to damage and irritate submucosal tissues in the mouth of the horse and to attach to the lining in the stomach and intestines of the horse. The last stage larva (Figure 16) is robust and yellowish in color. The feeding larva is the overwintering stage. Larvae of all three species are similar in appearance.

Pupae. The pupae of horse bots are all similar. Pupation takes place on the ground, after the last stage larvae have left the horse. The pupal period lasts approximately one month before the emergence of the adult fly.

Adults. The adults of all three species are similar in appearance and superficially resemble honey bees (Figure 17); they are hairy-bodied and about the same size as honey bees. Adult flies have non-functional mouthparts and do not bite.

One type of bot block.


First off, don’t fret because Ivermectin will kill bots.  So, after the first frost that kills all the adult flies – or December to be safe – worm you horses with Ivermectin.

Then, use Ivermectin again in the Spring, if you feel you need to.  I usually use Strongid in the Spring.  Of course, you can easily and inexpensively have your manure tested to know for sure what wormer is needed.  Here is the link for Horseman’s Lab Fecal Kit.

Ivermectin kills bots.



Well, I didn’t really make any mistakes, but I could have removed the eggs in a more efficient and safe manner.  This list is more complete:

1)  Get rid of the bot eggs as soon as you see them.  You may have to do this everyday for a couple of weeks.  The egg is ripe for ingestion as soon as it is deposited.

2)  Use a sharp bot knife or you will become frustrated.  They are cheap so if you can, buy a new one each season.

3)  Have a bot block nearby to scrub off the difficult areas.  It looks like a block of lava rock.  They are very inexpensive and easy to haul around because they weigh almost nothing.

4)  Don’t pick off the bot eggs where the horses eat or they will ingest them off of the ground where you dropped them.  The eggs will live on the ground.

5)  If you use your fingernails, DO NOT rub your eyes.  The bot eggs will grow in  your eyes.  Ugh.  Totally gross.

The average, inexpensive bot knife


A bot fly ingestion is not often lethal.  It is like any other parasite…  it compromises your horse’s constitution, but can be remedied.  Only in severe cases is it lethal.


OK, if it is possible to be even more ooogey, there is a HUMAN FORM of bot fly.  Yup.  We won’t go there but you can look it up on the internet.  Yeesh.


Just do it.  Get rid of them.  A sharp knife and a bot block should cost under $10 total.

I have never used this but it was advertised as a bot knife... Have any of you used this?


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HORSE AND MAN is a blog in growth... if you like this, please pass it around!

9 comments have been posted...

  1. Cristy Vandenhende

    You did a great job on this post. Very informative. Everyone remember to use Fenbenzadole not too long after you shave off the eggs. Use the fenbenzadole again later near beginning of Summer if you have seen bot eggs again. That has worked for me for a long time.

    But I didn’t know anything about nose bot eggs. So thank you for the post!

  2. Barbara Wood

    I love this blog and pass it on to my daughter frequently. She is the horse owner in the family. Ordered the calendar yesterday. Thanks for helping Hope!

  3. Linda Horn

    I usually read your posts earlier in the day. Letting this one go until dinnertime was a huge mistake!

  4. RiderWriter

    PS. I have one of those shavers that I purchased for use on my doggie! She didn’t like it much so it’s languishing in the bottom of my grooming kit. We don’t de-whisker the friend’s horses so no use there, but if I do happen to spot bot eggs I guess I’ll be ready!

  5. RiderWriter

    Ewwwwwwww….. ugh. Especially the human part. I have seen photos in the past and I am *not* rushing off to check them out again. I wish someone had told me when I was a kid to be careful around my eyes, that freaks me out! The reason I say “when I was a kid” is I distinctly remember bot eggs being an issue on the horses in NJ when I was growing up. Interestingly, I don’t think I’ve ever seen one on a horse here in MO. Maybe they aren’t here? Or maybe they were already removed at the barn where I used to take lessons, before I was grooming the lesson horses? That still leaves my friend’s horses that I’ve been riding for a year and a half and I could swear I’ve never seen bots on either one of them… but maybe SHE’S removing them (I just asked her). Anyway, thanks also for the link on the fecal analysis lab. I just brought that up to my friend since her two horses never go anywhere or have contact with other equiines; I think they are prime candidates for “worming as needed.”

  6. Melissa

    I have used that shaver, but not as a bot knife. It’s super handy for trimming whiskers on the go, cheap and disposable, and very comfy on a horse with sensitive or ticklish skin on the nose and mouth who would be irritated by electric clippers. I wouldn’t have guessed it would be sharp enough for bot removal, but I’ll keep it in mind!

  7. Jen

    Yes I have used Win by a Nose horse shaver & just plain shaving razors to shave off bot fly eggs. Also heard of using vaseline on the areas where eggs are prone so the eggs can’t stick to skin.


    YES! I just used that shaver as a bot knife yesterday and wondered if anyone else ever did! Freaky! I’ve also used disposable razors.

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