Normally at this time of year I would be doing the final “bot check” of my horses’ legs. I would be prepared to use my bot knife or sand paper to scrape them off or I might wipe the legs with warm water to force the eggs to open prematurely and die. This year – basically no bots!
I will assume it was our weather. It was a fairly cool summer here in upstate New York with many days in the low 70s and nights in the 50s. My favorite kind of summer though I know many lamented the scarcity of warmer days.
Back to bots. While in vet school, it was actually debated as to whether bots really caused horses any problems. Yes, post mortems with a stomach full of bots looked impressive but did they actually drain nutrients and cause horses discomfort? I know – that seems a bit amazing to us now!
Bots are actually flies (Gasterophilus species) that resemble bees a bit but they don’t sting or really “buzz.” Depending on the exact species, they will lay their eggs on the horse’s legs, mane, nose or jaw area. Nasal bot flies are especially irritating to a horse when the eggs are deposited. On a bad year, a horse with dark colored legs will have obvious solid patches of light yellow eggs.
Most eggs hatch with warmth and moisture – such as your horse nosing or rubbing the area or you using a warm, wet rag to wipe the area. The larvae then enter your horse’s mouth and hang out in those tissues for a couple of weeks. While in the mouth, some horses will develop reactive sores. The larvae are then swallowed or migrate to the stomach.
The next seven or so months see the bots attaching to the stomach or small intestine where they drain nutrients. They can cause blockages if they are numerous enough and in the “right” locations. Ulcers, anemia and colic have all been blamed on heavy bot infestations.
Eventually the larvae pass out in the manure where they sit, then pupate and turn into flies. The flies themselves do not bite but they can irritate horses simply by buzzing around them.
Until safe versions of ivermectin came along for horse deworming, there were separate dewormers just for bots. Most of these were organophosphates. They had to be used carefully and with consideration of any other chemical or medications given to your horse around the same time. Currently, most people simply rely on ivermectin preparations. Even with very few bot eggs noted, you may choose to deworm for them with your usual fall deworming. Simply use an ivermectin product in late fall to catch them in the stomach. Be sure to use a product labeled as effective against both second and third stage larvae, which will be present in the stomach.
It is important to realize that bot infestations won’t show up on a fecal examination like strongyles do. So your veterinarian will be relying on your observations of finding eggs on your horse. You need to carefully check your horse through late summer and into early fall.
Bot eggs are easy to spot on Crispy, our red dun Quarter Horse, or Spice, the donkey’s legs. Not so easy to find on Cinnamon the roan colored Appaloosa or Frodo the pinto Mini horse. Check manes and hairs around the nose and under the jaw as well as legs. Keep an eye out for any of the bot flies themselves.
I did see some bot flies around so I suspect our equines have at least a few of the larvae in their stomachs. So, yes, I will deworm with bot control in mind as well as the other internal parasites.