Botflies (Gasterophilus spp) are parasitic flies that affect the horse’s digestive tract and can cause negative health consequences.
Botflies lay eggs on the horse’s coat in the summer. Some of these eggs, known as horse bots, are ingested as the horse licks and grooms itself.
The bot eggs hatch and the larvae develop in the horse’s mouth before migrating to the stomach where they attach to the gastric mucosa.
Once mature, they detach and are passed through the manure. They pupate into flies, and the cycle repeats with new botflies seeking out horses to host their eggs.
Prevention and treatment of bot invasion are essential to keeping your horse healthy. If you suspect that your horse is affected by botflies, consult with your veterinarian for a diagnosis and treatment options.
What are Horse Botflies?
Botflies are flying insects that belong to the Oestridae family. These winged insects can be identified by their fuzzy yellow, brown or gray bodies that give them a bee-like appearance. 
Horses are susceptible to over 60 common parasites that can affect their behaviour, health and performance. 
Unlike other parasites, such as worms (helminths), bots are larvae that grow and develop within the horse. Botflies use horses as hosts for most of their life cycle. 
Where are Botflies Found?
Bots are common pests in North America, the United Kingdom, the Mediterranean and China.  It is believed that imported horses originally brought bots to North America from Europe. 
Horse bots have extremely strong host specificity and cannot survive in the digestive tracts of other livestock.  They will travel long distances to find an appropriate equid host. 
These seasonal parasites can be found at most farms where horses are present, making them pervasive pests for horse owners.
Types of Botflies
Three types of horse botflies can be found in North America. Each species has a similar life cycle, apart from where they attach to the horse.
The common botfly (Gasterophilus intestinalis)
The common botfly deposits her eggs on the forelegs, chest, shoulders, belly or flanks during late summer or early autumn.
These eggs incubate on the body for 1-2 weeks. The horse must stimulate the eggs to hatch by licking or biting the larvae during grooming. 
The throat botfly (Gasterophilus nasalis)
The throat botfly deposits her eggs on the hairs under the jaw, or near the throat latch during late spring and early summer. Each egg attaches to a long, singular hair.
Unlike the other bots, hatching can occur without friction from the horse’s mouth within 4-6 days. 
The nose botfly (Gasterophilus haemorrhoidalis)
The rarer nose botfly is smaller than the other two types. She deposits her eggs on the hairs of the lips and nose.
These eggs are black and have stalks. They hatch in response to moisture from the lips within 2-4 days.
Unlike the other species, nose bot larvae will attach to the rectum when excreted before dropping to the soil to pupate. 
Botfly Life Cycle
Understanding the bot life cycle is important for prevention and treatment. Each species requires 10 – 12 months to develop within the digestive tract. 
Eggs (2-10 days)
During the summer months, the female botfly mates and then seeks out a host for her eggs.  Once a suitable host is found, the fly will hover over the horse and attach 150 – 1000 small eggs onto the horse’s coat.
These eggs range from cream and yellow to orange in colour and are extremely sticky. The flies and eggs cause irritation and itchiness, prompting the horse to lick and bite at affected areas to soothe the skin. 
Common botfly eggs require friction and moisture from the horse’s licking to hatch. 
Larvae (8-12 months)
The hatched larvae are ingested into the oral cavity where they develop over a period of three weeks to a month. 
During this time, the bots bury themselves in the gums, tongue and lining of the oral cavity using their strong mouth-hooks. 
Common bots migrate into the stomach and attach to the mucosa of the non-glandular region, near the esophageal entry site. The nose bot attaches near the pylorus at the exit from the stomach. 
Once attached, they become immobile for eight to twelve months while they grow.  This parasitic infestation is known as myiasis.
Pupal Stage (3-8 weeks)
After developing in the gut wall, the larvae detach from the lining and pass through the digestive tract. They are then excreted in the horse’s feces, where they enter the pupal stage.
In early summer, they burrow into the pasture for 3-5 weeks before emerging as an adult botfly. 
Adult (7-10 days)
The adult botfly does not have a mouth and is unable to feed. She only lives for a few days to reproduce, find a horse host and deposit her eggs.
Symptoms of Botflies in Horses
How do I know if my horse has botflies? Botfly infection can go undetected for long periods of time as most horses demonstrate no outward signs of illness.
Severe mouth or gastrointestinal tract infection due to bot invasion is rare. Large numbers of larvae can be present in the body without symptoms.
Adult bots tend to fly around and dive at the horse’s head and face, causing annoyance and stress. They have been known to prevent horses on pasture from grazing due to their aggressive flight patterns. 
Adult bots do not bite or sting, but they can cause significant distress and spook the horse.
In some cases, horses infested with bots will demonstrate the following clinical signs: 
- Sinus irritation and infection
- Irritation of the stomach, legs and mouth
- Rubbing of the face and cribbing to relieve mouth irritation
- Bit shyness
- Mild to severe ulcers in and around the mouth
- Weight loss
- Peritonitis (inflammation of the peritoneum)
- Colic and intestinal blockages due to large numbers of bots in the digestive tract
- Gastric rupture
If your horse displays any of these signs, consult with your veterinarian to determine if your horse has a botfly infection or another condition.
Testing for Botflies
Fecal egg count (FECs) is a diagnostic test performed on horse manure samples to measure parasite load. This test can identify the type and amount of parasitic eggs the horse carries in the body.
However, this test cannot be used to screen for bots since the eggs are laid outside the body and hatch inside the mouth.
No eggs pass through the digestive system, and healthy larvae rarely do. Excreted dead larvae are often too decomposed to identify under examination. 
Bot infection is difficult to identify in horses. Infestation is usually diagnosed by the presence of eggs on the horse’s body or based on observation of clinical signs.
If one horse in a herd is suspected of having a bot infection, it can be assumed that all of the horses in the herd will need treatment.
Prevention and Management
Preventing horse botflies can be tricky. Bots rely on invading horses to survive, making them very effective at infiltrating a host.
Unlike other livestock pests, botflies are not attracted to traps or easily repelled. Fortunately, these parasites can be deterred by interrupting their life cycle and preventing egg attachment and hatching.
It is recommended that physical prevention be used in collaboration with a deworming plan to protect your horse from bots, unless otherwise stated by your veterinarian.
Manual Egg Removal & Body Checks
Conducting frequent bot checks on your horse is essential to interrupting their parasitic cycle. The eggs are laid and hatched only days later.
Some tools, such as a botfly knife, can be used to remove eggs from the hair coat. These knives have a curved, serrated edge that gently scrapes the eggs off of body hairs.
In breeds with longer body hair, regular clipping of the legs may be recommended in the summer and early fall to facilitate egg removal.
Bot eggs can also be removed by bathing affected areas with warm water or applying Vaseline to loosen the sticky eggs before wiping them off with a paper towel.
Fly Covers and Spray
While egg removal is the best way to prevent bot infections, horses with sensitive skin and anxious horses may require extra protection from flies.
Protective gear may be beneficial if your horse shows signs of irritation – including tail swishing and stomping – due to the presence of flies. Hovering and diving adult bots can be deterred by fly boots, fly sheets and fly spray.
Treatment and Deworming
Anthelmintic drugs, or dewormers, are used to treat parasitic infections and invasions in horses.  Regular administration of dewormers helps to control and eliminate parasite populations and prevent reinfection.
Anthelmintic treatment should be considered for all horses. Deworming plans should be adjusted for different life stages and risk factors, such as foals, pregnant mares and geriatric horses.
Avermectins (ivermectin, moxidectin)
Avermectins were introduced in 1981 as a new class of anthelmintic drugs with a broad spectrum of efficacy against internal and external parasites. 
Currently, two parasiticides on the market are effective against horse bots: ivermectin and moxidectin.  These dewormers block nerve transmission in parasites, leading to paralysis and eventually death.
Ivermectin is potent and safe for animals to consume, with product remaining in the body for long periods of time. Oral deworming pastes that include ivermectin are 98% effective against bots and are highly recommended to include in your deworming schedule. 
Moxidectin is an oral gel that has been commercially available since 1997. While it is approved for use against bot larvae, recent data suggests it is only 90% effective against bots. 
However, Moxidectin can be effective against other parasites such as small strongyles. 
When Should I Deworm my Horse?
Deworm your horse against bots once a year after the first frost.  Adult bots cannot survive in cold weather, so deworming near the end of fall eliminates this parasite from the body and prevents reinfection before winter. 
Botfly season has been known to extend into December in the southern United States. Consult with your veterinarian about when to administer dewormers as the best treatment varies depending on climate. 
Some horses require treatment for bots in the fall and in the spring. If your horse has signs of bot infection during the winter, a second round of treatment may be recommended by your veterinarian.
How to Administer Equine Dewormers
Accurate body weight is needed to determine the correct dewormer dosage for your horse. If a scale or weighbridge is unavailable, a weight tape can be used to get an approximate weight.
Avermectin dewormers (ivermectin, moxidectin) usually come in paste or gel form and are given to the horse orally once a year.
Common avermectin trade names include EquiMax, Zimerectin and Quest.  Individual product instructions should be followed when administering dewormers.
Horses of all ages are susceptible to botfly infection. Close monitoring of foals for signs of infection is recommended.
Foals should be dewormed for the first time at 1-2 months of age, with repeated treatments every 1-2 months.  Foals should receive a minimum of four anthelmintic treatments in the first year of life. 
Ivermectin is safe for foals, but moxidectin should not be used for those younger than 4 months.
Treating with avermectins when the foal has not been dewormed regularly can lead to a sudden, massive elimination of parasites in the gut. This could cause gut obstruction and colic, which can be fatal.
Consult with your veterinarian to develop an effective parasite control program for your foal.
Ivermectin has been remarkably successful in treating parasites for decades. Due to its efficacy, there has been limited research into discovering and developing new drugs. 
However, there are growing reports of anthelmintic resistance in livestock worldwide.  New generations of botflies are increasingly resistant to deworming drugs such as ivermectin and moxidectin.
Several factors have led to this large increase in drug resistance, including over-use of the drugs, undertreatment with low dosages and changing parasite genetics. 
When it comes to deworming bots, proper timing and dosage are crucial. Ask your veterinarian about how to lower the risk of anthelmintic resistance in your horse and ensure treatment is being administered properly.
Is Your Horse's Diet Missing Anything?
Identify gaps in your horse's nutrition program to optimize their well-being.
- Akele, Y. et al. Equine Myasis Caused by Gastrophilus Flies: A review. Acta parasitol glob. 2018.
- Attia, M. M. et al. The prevalence of Gasterophilus intestinalis (Diptera: Oestridae) in donkeys (Equus asinus) in Egypt with special reference to larvicidal effects of neem seed oil extract (Azadirachta indica) on third stage larvae. Open Vet J. 2018.
- Barragry, T. B. A Review of the Pharmacology and Clinical Uses of Ivermectin. Can Vet J. 1987.
- Bishopp, F. C. & Schwartz, B. Horse Bots and Their Control. Yearb Agric. 1942.
- Cogley, T. P. and Cogley, M. C. Field observations of the host-parasite relationship associated with the common horse bot fly, Gasterophilus intestinalis. Vet Parasitol. 2000.
- Equine Guelph. Parasites- Botflies & Tapeworms. Equine Guelph. 2013.
- Hu, D. et al. Effects of Gasterophilus percorum infestation on the intestinal microbiota of the rewilded Przewalski’s horses in China. PLoS One. 2021.
- Holden-Dye, L. & Walker, R. J. Anthelmintic drugs and nematicides: studies in Caenorhabditis elegans. Wormbook. 2014.
- Ihler, C. F. Anthelmintic resistance. An overview of the situation in the Nordic countries. Acta Vet Scand. 2010.
- Kaufman, P. E. et al. Pest Management Recommendations for Horses. Cornell University. 2000.
- Lyons, E. T. & Tolliver, S. C. Macrocyclic lactones for parasite control in equids. Curr Pharm Biotechnol. 2012.
- McCue, P. M. Deworming Foals. Colorado State University.
- Nielsen, M. K. et al. Internal Parasite Control Guidelines. American Association of Equine Practitioners (AAEP). 2019.
- Reinemeyer, C. R. Parasitism and Colic. Vet Clin North Am. 2009.
- Shilkin, J. Internal parasites of the horse. Journal Dept Agri Western Australia. 1956.
- Stoltenow, C. L. & Purdy, C. H. Internal Parasites of Horses. NDSU Extension Service. 2003.
- Yan, L. et al. Evolutionary history of stomach bot flies in the light of mitogenomics. Syst Entomol. 2019.
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