Some of this might sound a bit like high school biology — it is not meant to be a science lesson. However, bot flies are rather interesting and negatively impact most horses.
There are nine species of horse bot flies (Gasterophilus is the scientific name). Three are the most common in the North America.
- Gasterophilus intestinalis (DeGeer) – internal parasite of the gastrointestinal tract, and the most common
- Gasterophilus nasalis (Linnaeus) – nose bot fly
- Gasterophilus haemorrhoidalis (Linnaeus) – throat bot fly
Bot flies have a life cycle of four stages.The larvae stage is where the real damage is done. The larvae can attach to your horse’s mouth, cardiac area and stomach, causing multiple issues while they ‘host’ off your horse. To protect your horse, it is important to understand their life cycle as there are different ways to treat for or manage them in the different stages.
Adult (the actual fly) – The adult female lays her eggs on hosts (that would be your horse). And she can deposit 150 to 1,000 eggs on one horse. One generation is produced per year.
Eggs – The yellow ‘things’ that are attached to the hair on legs and body of your horse (shown in the picture below). Bot flies typically lay their eggs in early summer months; however, that can vary in regions due to climate.
Larvae – Developed from the egg after about seven to 10 days of being deposited by the female bot fly. These maggots are stimulated to ‘hatch’ by your horse licking or biting at them, then they are ingested by your horse or crawl into his mouth. In the mouth they burrow in the gums, tongue or lining of the mouth for an approximate 28 day stay. Then they molt and make their way to your horse’s stomach, or even to your horse’s cardiac regions. Inside your horse they ‘attach’ themselves again and continue to ‘host’ off your horse and continue their damage.
Pupae – The pupae are shed from the horse’s system into his manure. They incubate in the manure for one to two months.
What they do to your horse and you:
First, the bot fly is very annoying when it is flying around your horse, then they lay their eggs. Those yellow looking eggs on your horse are a bit unsightly, well that is just the start of it! Eggs can detach and get into your horse’s eye, or a person’s eye (ocular invasion). When handling a horse with bot eggs, do not touch your face or rub your eyes until you have washed your hands! Then, when the larvae are in your horse, they attach to your horse’s insides, just like a tick, and feed off him. They consume nutrients from the tissues inside your horse.
Signs that your horse may have bots:
- Inflamed mouth
- Gastric ulcers
- Stomach irritation
How to control/manage bot flies:
Grooming/Egg removal tips – Egg Stage
- Use a Bot knife to gently separate the eggs from your horse’s coat
- Pick off by hand (not really recommended – per the eye invasion possibility)
- Use a grill block to disconnect them from your horse’s skin
- Heat vinegar and wipe on the areas where the bot eggs are (the eggs will release from the hairs)
- Apply a layer of Vaseline over the eggs. Later in the day wipe off the Vaseline with a paper towel and the eggs will come off with the Vaseline. Throw the paper towel in the trash.
*Always wash your hands after removing bot eggs and handling or grooming a horse that has bot eggs.
Deworming – Larvae Stage — Ivermectin and moxidectin seem to be the most effective for the larvae stage. Research suggests that moxidectin is a bit more effective for killing the larvae than ivermectin. Deworm for bots in the spring and in the fall. Be sure to check with your veterinarian regarding your horse and the area in case there are some differences to adjust for.
Pasture management – Pupae Stage – Manure removal removes the pupae of the bot fly but removal is not entirely necessary. Breaking up the manure piles by dragging or knocking the piles of manure apart can destroy the pupae’s environment; therefore, destroying the pupae.
With egg removal and pasture management you have some opportunities to break up the cycle of bot flies, but do not forget the deworming. Fecal counts check for the shedding of parasites, and the timing of a fecal count with a bot infestation may not provide the right information at the right time. If you see bot flies and bot eggs on your horse, your best defense is to attack them through grooming, deworming and pasture management.
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