Bot Fly Larvae in Horse Manure: Essential Facts and Prevention Tips

Bot flies are a common concern for horse owners, as their larvae can cause health problems in horses.

The horse stomach bot fly, Gasterophilus intestinalis, is a predominant species found in North America that attacks horses, and can lead to a stomach bot larvae infestation within the animal’s system source.

These flies lay their eggs on the lower forelegs of horses, which then hatch into larvae within five to ten days source.

Once hatched, the larvae make their way into the horse’s mouth and eventually to its digestive tract, causing discomfort and potential health issues.

Aside from horses, bot flies can also impact other mammals as their larvae could live inside a host source.

Proper manure management is essential in controlling bot fly populations and preventing infestations.

By minimizing moist manure surface area and storing manure in a fly-tight area during warm months, horse owners can greatly reduce the risk of bot flies laying their eggs near their animals source.

Bot Fly Life Cycle and Infestation

Life Stages and Reproduction

The bot fly life cycle consists of three stages: egg, larva, and adult.

During the spring and summer months, adult female bot flies lay yellow eggs on the lower forelegs of horses.

The eggs hatch within a few days, allowing larvae to infest the host.

Egg

• Yellow eggs laid on horse forelegs
• Hatch within days

Larva

• Migrate through the horse’s body
• Responsible for myiasis infestation

Adult

• Adult flies emerge in spring and summer
• Lay eggs, starting the cycle anew

Infestation Process

The infestation begins when horses lick or bite the area where eggs are laid. The larvae enter through the horse’s mouth and migrate to the gastrointestinal tract.

Myiasis, the infestation of host skin for larvae nutrition, occurs as the larvae grow inside the host.

Infestation steps

1. Horse licks or bites area with eggs
2. Larvae enter the horse’s mouth
3. Larvae migrate to the gastrointestinal tract
4. Myiasis occurs as larvae grow

Migrating Larvae

The migrating larvae cause discomfort and potential health issues for the horse. In some cases, they may even migrate through the lungs or cause blockages in the intestine.

After approximately six weeks, the larvae mature and are expelled from the host.

Larvae effects

• Discomfort and potential health issues
• Possible migration through lungs or intestine blockages

In conclusion, understanding the bot fly life cycle and infestation process can help better manage and prevent myiasis in horses.

Effects of Bot Fly Larvae on Horses

Horse bot flies can cause several health issues in horses.

This section will discuss the impact of bot fly larvae on horses, focusing on the following sub-sections:

Gastrointestinal Issues

• The most common species affecting horses is Gasterophilus intestinalis, which develops in the horse’s stomach¹.
• In high numbers, these larvae can cause irritation, ulcers, and colic, leading to appetite loss, weight loss, and diarrhea².
• In severe infestations, complications can be life-threatening.

Mouth and Oral Cavity Irritations

• Bot fly larvae can also affect the horse’s mouth and oral cavity.
• G. nasalis, also known as the nose bot, targets the horse’s molars³.
• Some common symptoms include:
  • Swollen tongue
  • Gum irritation
  • Difficulty chewing

Throat and Esophageal Problems

• G. haemorrhoidalis, or the throat bot, mainly affects the esophageal lining⁴.
• Common issues include:
  • Throat irritation
  • Esophageal ulcers
  • Esophageal paralysis⁵

Comparison Table:

Bot Fly Species	Affected Area(s)	Symptoms and Issues
G. intestinalis	Stomach	Irritation, ulcers, colic, diarrhea
G. nasalis	Mouth, molars	Swollen tongue, gum irritation, difficulty chewing
G. haemorrhoidalis	Esophagus	Throat irritation, esophageal ulcers, esophageal paralysis

Bot Fly Larvae in Horse Manure: Detection and Diagnosis

Signs and Symptoms

Detecting bot fly larvae in horse manure may indicate an infestation in your horse. Some of the symptoms include:

• Irritation around the mouth and lips
• Colic signs or digestive upset
• Coughing or respiratory distress

These symptoms may indicate bot fly larvae have entered the horse’s body.

Veterinarian Examination and Diagnosis

To accurately diagnose bot fly infestations, a veterinarian will perform several checks:

1. Physical examination: The vet will examine the horse’s mouth, lips, and nose for any signs of irritation or bot fly eggs.
2. Fecal examination: The vet will check the horse’s manure for larvae presence.

If the vet detects the presence of larvae, they will recommend appropriate treatment to control the infestation.

Method	Pros	Cons
Physical examination	Non-invasive and quick	May not detect larvae in early stages
Fecal examination	Directly identifies larvae	May require lab facilities for confirmation

Remember, early detection is crucial in managing bot fly infestations. By noticing symptoms and seeking veterinarian assistance promptly, you can help prevent more severe issues and improve your horse’s health.

Treatment and Management

Deworming and Medications

Deworming is an essential part of managing bot fly larvae in horse manure. Horse owners should establish a worming schedule, usually in the autumn, to keep their horses healthy.

Common dewormers include ivermectin and moxidectin, which are effective in eliminating bot fly larvae.

Ivermectin and moxidectin have specific pros and cons:

Dewormer	Pros	Cons
Ivermectin	Broad-spectrum treatment, cost-effective	Some resistance, not effective on some worms
Moxidectin	Effective against resistant strains, longer-lasting	More expensive, not suitable for all horses

Prevention Strategies

Prevention is the key to managing bot fly infestations. Some strategies include:

• Regularly inspecting horses for eggs and removing them with a bot knife
• Incorporating manure management to reduce fly development sites
• Ensuring proper timing of deworming treatments
• Providing adequate nutrients for a healthy immune system

Fly Control

Fly control methods can significantly reduce bot fly infestations in horses. Here are some effective methods:

• Using fly sprays, which repel flies and reduce larvae transmission
• Applying fly sheets, which physically prevent flies from laying eggs on the horse
• Implementing fly control measures in the stable, such as proper sanitation and fly traps

A combination of deworming, prevention strategies, and fly control can effectively manage bot fly larvae in horse manure and protect horses from the associated health risks.

Parasitic Infections and Related Health Risks

Internal Parasites and Infections

Horses can be affected by various internal parasites that can lead to infections. Some common internal parasites are:

• Ocular myiasis: Flies lay their eggs in the horse’s eyes, causing inflammation and other issues.
• Intestinal parasites: Worms like roundworms, tapeworms, and strongyles can infect a horse’s gastrointestinal tract, leading to malnutrition and other digestive issues.

Skin and External Parasites

Horses can also be affected by parasites that thrive on their skin or in their excrement, which includes cutaneous myiasis.

This condition occurs when fly larvae infest a horse’s skin, causing itchy, painful wounds.

Moreover, bot flies are attracted to horse manure, and their larvae can mature in the feces or on the horse externally.

This can lead to severe irritation and infection if not properly managed.

Impact on Horse’s Nutrition and Wellbeing

Parasitic infections in horses can have long-term consequences on their overall health, such as:

• Nutritional deficiencies due to internal parasites consuming nutrients
• Weakness and lethargy from decreased nutrient absorption
• Infections and abscesses from open wounds caused by external parasites

Parasite Type	Impact on Horse’s Health
Internal	Malnutrition, digestive issues, poor weight gain
External	Skin irritation, infections, potential for internal infestation

Preventing and managing parasitic infections in horses is crucial for maintaining their overall health and wellbeing.

This can be achieved through regular grooming, promptly removing manure, and providing proper nutrition and veterinary care.

Conclusion

In summary, the common horse stomach bot fly is a parasite that infests horse manure.

The female lays eggs on horses’ lower forelegs, and eventually, the larvae enter through the horse’s mouth.

To reduce bot fly larvae infestations, proper manure management is essential. This includes composting and not overstocking pastures.

Here are some key points to consider:

• Bot flies can infest horse gastrointestinal tracts
• Proper manure management helps control these parasites
• Composting kills weed seeds and fly larvae

Comparing two manure management methods:

Method	Pros	Cons
Composting	Locks in nutrients, kills fly larvae	Requires time and effort
Spreading	Easier to do	Could lead to more flies and weed growth

In conclusion, understanding the life cycle of bot flies and practicing good manure management can help prevent infestations on horse farms.

Bot flies can also infect squirrels and household pets like cats.

Footnotes

1. Bot Flies | Livestock Veterinary Entomology – Texas A&M University ↩

2. Worms Cause Problems for Horses – Veterinary Medicine at Illinois ↩

3. ENY-283/IG139: External Parasites on Horses – EDIS ↩

4. Worms Cause Problems for Horses – Veterinary Medicine at Illinois ↩

5. ENY-283/IG139: External Parasites on Horses – EDIS ↩

Reader Emails

Over the years, our website, whatsthatbug.com has received hundreds of letters and some interesting images asking us about bot flies. Scroll down to have a look at some of them.

Letter 1 – Human Bot Fly Larva from Belize

Human Bot Fly larva pics
April 29, 2010
Hey Bug Guys! I have some pictures of the Human Bot Fly larva that I picked up in Belize. (Perhaps more literally, they picked me…). I’d be happy to send them to you if you would like (and enjoy gross pictures).
Sierra

Hi Sierra,
By all means, please send them.  Please keep the subject line the same and include all relevant information.

June 4, 2010
Hi,
Sorry it took me a while to get back to you.  The first picture is the back of my neck, where the larvae were growing (about 1 month after infection) and the second one is one of the larva next to a ruler (CM scale). 

I had a miserable time convincing American doctors that I actually had these insects as parasites– they wanted to treat me for staph infections and paranoia.   I was only able to remove/kill them by covering their air supply in the skin with superglue left on over night. 

I hope that anyone who thinks they may also have a bot fly infection finds this page and tries the superglue- nothing else I tried (suggestions from the internet of using meat or petroleum jelly) worked.
Thanks and enjoy!
Sierra Blatter

Hi Sierra,
Thanks so much for providing this wonderful account and accompanying images of an encounter with a Human Bot Fly in Central America.

Letter 2 – Bot Fly Larvae emerge from Mouse

Subject: Two Larva Eat and Kill Mouse
Location: Mass USA
September 3, 2013 5:08 pm
I caught a mouse in my kitchen. I come to find I was able to because it was injured. I put it into a clear cage to show the kids. I come to find the mouse has a hole in its stomach and Two protruding round items imbedded inside. Which I thought were ticks. The mouse is almost dead anyways.

So I decide to keep it in the container and wait to see what happens. I check the next day after work and find the bugs detached from the mouse. Each about a half inch long wiggling around. Not really moving in any direction but just wiggling.

I also showed this to a friend who’s an exterminator and he says he’s never seen anything like it, also the mouse may be a rat. If that helps. I’ve always been into bugs and snakes, etc. I have never seen anything like this before. Should I be worried seeing as I caught the mouse in the house?
Signature: Matt

Dear Matt,
Bot Fly or Warble Fly Larvae in the family Oestridae are common endoparasites in mammals, especially rodents.  The adult Bot Fly resembles a bumble bee. 

From what we have read, the larvae do not kill the host, so perhaps your mouse died of other causes, or perhaps in the case of small animals, the Bot Fly Larvae can do significant damage.  We will copy Bot Fly specialist Jeff Boettner to see if he can add any information.

Letter 3 – Bot Fly Larva

upstate ny, came out of dead rabbit
Location: Greene County, NY
July 24, 2011 4:44 pm
Have no clue.. it is alive and crawls around.. found it while cleaning a dead rabbit can you help
Signature: FoX83

Hi FoX83,
This is the larva of a Rodent Bot Fly in the genus Cuterebra.  The larvae are endoparasites found on certain mammals.  The Bot Fly larva forms a warble, generally on the neck of the infested host, and though they look quite nasty, they are alleged to not harm the host.

Letter 4 – Mysterious Thing found in bedroom may be Little House Fly Larva

Subject: Found this in a bedroom
Location: Braintree, Essex, England
December 16, 2016 3:23 am
Would really appreciate your help. My daughter found this in her bedroom and having looked at a few websites I have no idea what it is.
Signature: Thanks, Neil

Dear Neil,
This thing reminds us of a Bot Fly larva, but we have never seen an image of a Bot Fly larva with spiny projections along its body.  We will do additional research and get back to you.  It is possible it gained entrance to your daughter’s bedroom because of a family pet.

Thank you for tour prompt email.
I hope you have luck finding it, one thing that may help is that we don’t have family pets other than guinea pigs but they live outside.
Thanks again for your help, its much appreciated
Neil

Many Bot Flies are endoparasites on rodents, so the Guinea Pigs may be playing host.

Eric Eaton provides a correction:  Little House Fly Larva
Daniel:
Pretty certain the fly larva is Fannia sp., family Fanniidae.  They used to be in the Muscidae, but are now in their own family.  Known as “Little House Flies.” 

Hm-m-m, I may have to do more digging, but the “habitat” would sure fit for that, too…. I’d like permission to use the Fannia larva images so I can do a blog post.  I found an adult here in Colorado Springs just a couple weeks ago.  I’m attaching an image, in fact.
Happy holidays, safe travels!
Cheers,
Eric

Hi Daniel,
I have attached both images I took as my phone doesn’t keep a record of emails sent for some strange reason.

I am more than happy for him to use the images I did have a look on line at the one he thinks it is and it does look similar other than the one online mentioned 5-8 mm and a black head, I think you can see from the images this was approx 15mm and didnt have a black head. ..

TBH Im more concerned about if its dangerous, how and why it was in my daughter’s bedroom and to stop it happening again.
Thanks again for all your help
Neil

Letter 5 – Bot Fly Larvae: Internal Parasites may affect pets!!!

cuterebra
Sorry to hassle you again after sending the glowing scorpion
from the Grand Canyon, but I thought you might want some cuterebra
photos too. I am a veterinarian in Colorado Springs,
CO, and I commonly see cuterebra larvae (my personal favorite
clinical entity) in dogs, cats, ferrets, and rabbits from
late July to early September. The pet usually has a
mound on the skin with a perfectly round hole on top through
which you can see the larva moving.

Sometimes there
is a little discharge, but in general it does not seem to
be too irritating. Imagine the surprise when the owner
squeezes at the mound and out pops this little larva!
It sure is dramatic, but they don’t really cause any
serious health problems. When I see them I use a little
local anesthesia around the opening and then enlarge it with
a scalpel blade before gently squeezing the larva out because
the hole is often smaller than the larva and if you are too
forcefull you can squeeze the larva inside out, leaving the
cuticle to fester under the skin. It is unusual for me to
see more than one on a pet at one time, since most pets
are not the preferred hosts, but this little Yorkie had
10 of them.

The owners squeezed most of them out themselves
(thus taking away all my fun) and fortunately all the larvae
came out intact for them. Cattle also have a cuterebra
species that affects them and the common name for the
condition is ‘Warbles’. The quick and dirty way to get
rid of the larvae in cattle is to place a soda bottle upside
down with the mouth over the opening and then quickly hit
the bottom of the bottle so that the larva shoots up into
it.
Anne

Hi Anne,
Getting such an informative letter would never be a hassle.
Thanks for your expert account of a Bot Fly Larvae infestation
and treatment recommendation. The typical hosts for North
American Bot Flies are rodents like rabbits and squirrels.
There is a Human Bot Fly found in Central America.

Letter 6 – Sheep Bot Fly Larva from Egypt

I have no idea what this bug is please help
Fri, Jan 16, 2009 at 12:38 PM
Hey,
Me and my dad slaughtered a sheep and when after we skinned the head we split ot open and there was this little guy in there. pretty gross right? I couldn’t believe it and have no idea what it is? how it got there? what it eats? and if it transforms into anything? How does it effect the sheep?
Thanks alot Mariam
Egypt

Hi Mariam,
We believe this is a Sheep Bot Fly Larva, an endoparasite. We searched Bot Fly Egypt and came up with this online article on the species Oestrus ovis: “Ophthalmomyiasis caused by the sheep bot fly Oestrus ovis in northern Iraq.
Gregory AR ,Schatz S ,Laubach H .
U.S. Army 101st Airborne Division, Fort Lauderdale, Florida, USA.

Myiasis is the feeding of fly larvae on vertebrates. The sheep bot fly larva of Oestrus ovis is a mammalian parasite of the skin, nose, ears, and eyes. When the larvae infest and feed on the structures of the eye, the condition is termed ophthalmomyiasis.

Most often this infestation is limited to the external structures of the eye and is referred to as ophthalmomyiasis externa. The features of this condition are severe local inflammation, positive foreign body sensation, erythema, and lacrimation. Vision may or may not be reduced, depending on involvement of the cornea. A 20-year-old white male soldier sought treatment for an inflamed eye and an irritated cornea OS.

His eyelids were swollen with marked periorbital edema and conjunctival erythema OS. On slitlamp examination, small whitish organisms were viewed on the conjunctiva OS. The organisms were removed, preserved, and sent to Nova Southeastern University where they were identified as O. ovis first-stage larvae. The patient was treated with antibiotic ointment, and the inflammation resolved within 1 week. O. ovis has a worldwide distribution, and although sheep are the preferred host, humans may also serve as an intermediate host in the organism’s life cycle.

This case represents one of several reports of ophthalmomyiasis in the Middle East caused by O. ovis. U.S. troops stationed in Iraq and surrounding areas are vulnerable to eye infestation by fly larvae, and health care providers need to include this condition in their differential diagnosis of anterior segment inflammatory disorders. ”  You may also want to look at Dennis Kunkel’s Microscopy image of the head of a Sheep Bot Fly Larva.

Letter 7 – Presumed Bot Fly Larva found in girl’s chin!!!!

Name that larvae
location:  New York
September 15, 2012:  6:07 PM
A friend of mine’s daughter had a bite on her chin a few weeks ago and the pediatrician said it was a spider bite. Last week there appeared to be a hole in her skin. And today, this came out. What is it???
Christie Mierzwa

Hi Christie,
This appears to be a Bot Fly Larva.  You did not use our regular submission form and there is no location listed for your sighting.  Please provide us with a location.  Your verbal description fits what one would expect of a Bot Fly, though in North America, the typical hosts are rodents.  We will try to copy Jeff Boettner to see if he can provide any insight.

Thank you. I used an old email address from a few years back and did not know there was a submission form. We live in NY. In this case, the host was a 3 yr old child!!

Thanks for the update Christie.  We eagerly await input from Jeff Boettner.

Letter 8 – Bot Fly Larva

Subject: Parasitic Larva
Location: Chihuahuan Desert, Far West Texas near the Rio Grande
April 19, 2013 4:01 pm
Hi there! This morning I set out to doctor what I thought was an infected thorn stuck in my dog’s side. Imagine my shock when instead of a thorn, I pulled out a wiggling larva!

It didn’t look like a normal fly maggot to me, and a quick search of the internet pointed me to the Bot Fly. I’d really appreciate your expertise to clear up this baby bug’s identity.
Signature: Sara

Hi Sara,
We are really happy you identified your dog’s Bot Fly Larva, and even though they are allegedly not a threat to the health of the host, your dog is probably relieved to have had it removed.

Letter 9 – Bot Fly Larva

Subject: Bot fly larva found in house!
Location: Cherokee County, NC
July 24, 2013 7:28 am
We found this fat little fellow slowly inching across the floor of our family room earlier this morning. Last night our cat decided to entertain herself by bringing in a dead mouse, so I think the little maggot likely came from the unfortunate rodent.

It measured about an inch when scrunched up in the shape pictured, and a little longer when tying to move.
The mouse appeared to be a common house mouse, so I’m thinking it might be a Rodent bot fly, perhaps?
Signature: Jacob

Dear Jacob,
Your Bot Fly Larva photo is a welcome addition to our site.  Your speculation that it came from the mouse your cat brought in is most likely correct.

Letter 10 – Rodent Bot Fly Larva removed from dissected mouse in Canada

Subject: A botfly in the far North?
Location: Far North, Ont., Can.
September 15, 2013 9:23 am
I caught a mouse one night and found that there were four huge bumps on its back. I looked closer and saw what appeared to be botfly larvae in holes on each bump.

I froze it and gave it to our local science teacher who dissected it with her class. Here’s a picture of what they dissected. Sure looks like a botfly to me!

I live in Fort Albany First Nation, Ontario, Canada, and I am surprised that there are botflys this far North! But is it really a botfly?
Signature: FAFN Resident

Dear FAFN Resident,
We concur that this is a Rodent Bot Fly Larva.  According to BugGuide Data, Bot Flies are found in Canada.

Letter 11 – Bot Fly Larva

Subject: Bot Fly Larva
Location: North Bay, Ontario, Canada
August 21, 2014 10:25 pm
Hi there,
I am located in North Bay, Ontario, Canada. I have recently found a mouse inside my house walking around pretty slowly. I put gloves on and picked him up to put him outside and when I looked at him I saw a weird brown thing protruding from his side.

Upon closer examination I determined it was alive and I recognized it as a bot fly larva that I had read about online a while ago while researching animal parasites. I pulled it out carefully with tweezers, plus about 5 other ones. They were quite large. I have a video of this extraction.

I estimate the larger ones were roughly 3cm, maybe slightly larger. Definitely matched the description of rodent bot fly larva. I kept the mouse in a container and fed him until his wounds healed and let him go.

A couple days later (before I let the other mouse go) I was cleaning out and removing a big work tent that was in our backyard that had been used for our house renovations. It was damp, lots of wood scraps etc. I emptied a basket of garbage wood and a mouse emerged from the stuff I was dumping. He was slow and you could actually see two huge bot flies hanging out of him. Very disturbing.

Due to the fact that I have worked extensively in that gross work tent, plus the other mouse was found in our house full of the parasites, some serious questions have come up.

Firstly, how concerned should I be regarding bot fly infections on/in me or my two cats? Is there something I should be looking for on the three of us (obviously a gross black worm thing, but I would prefer to catch it waaaay before that).
Secondly, is this normal??? Are bot flies common this far north? Should I be reporting this, and if so, then to who?

Lastly, how do I avoid coming into contact with the eggs? Are there common types of material they are laid on or environments I could perhaps minimize in order to dissuade them from being laid near my house?
Thank you for your help with this.
Signature: Kate Griese

Dear Kate,
Thank you for your thorough and engaging request.  You are correct that this is the larva of a Rodent Bot Fly.  A link from that posting is no longer valid, however we did quote from what might have been the College of Veterinary Medicine at Michigan State University which stated: 

“Cuterebra is a normal bot fly of rodents and rabbits, but can also infect cats, dogs, and man. ”  This online library seems to support that cats can become hosts to Rodent Bot Fly larvae.  Companion Animal Parasite Council indicates:  “Cats and dogs are accidental hosts.” 

VCA Animal Hospitals indicates:  “Cats are accidental hosts of Cuterebra larvae. They are most commonly infected when they are hunting rodents or rabbits and encounter the botfly larvae near the entryway to a rodent’s burrow. Most cases of warbles in cats occur around the head and neck.” 

BugGuide data on sightings indicates that you are in the normal range for Rodent Bot Flies.  We believe it is highly unlikely that a human will be parasitized by a Rodent Bot Fly.  We will attempt additional research on this when time permits.

Letter 12 – Bot Fly Larva

Subject: Larvae?
Location: Central MN
August 7, 2016 7:52 am
Hi, Bugman! What on earth is this, apparent, larvae? About an inch and a half long, VERY chubby, no ‘legs’. Looks like Jabba the Hutt! Black spotty body, found in a garage.
Signature: Confused naturalist

Dear Confused Naturalist,
This looks to us like the larva of a Rodent Bot Fly.  See this BugGuide image for comparison.

THANK YOU!!  Yes, I found that on your amazing website…..I refer to it often. Thank you for your quick response. I have never EVER seen this before…and I’ve been on a frenzy trying to figure it out. Now I can sleep…hehehehee! (Total bug nerd)
Kelly