Page 76 - May 2021

EQUINE HEALTH
BATTLING BOTS
by Heather Smith Thomas
Most internal parasites of horses are worms, but bots are actually the larvae of a fly that needs a horse to complete
its life cycle. This parasite spends most of its life fastened to the inside of the host animal’s stomach, spending the winter robbing nutrients from the blood of the host via the stomach lining, and causing irritation to that lining. Bot larvae occasionally perforate the stomach wall, which may lead to fatal complications.
If a horse has only a few bot larvae, they may not cause serious problems in the stomach. But
in large numbers, they can create a lot of damage and sometimes colic. The adult bot flies are a seri- ous nuisance, however, when laying their eggs.
The adult fly is brown, hairy (fuzzy-look- ing), and about the size of a small bee. These flies live only a few days for the sole purpose
of mating and laying eggs. The female hovers around horses, preparing to deposit eggs. The flies are most prevalent in summer and full hatching out occurs in hot weather. In northern climates the bot season may be short, with flies
hatching in late August and into September-- until the first hard frosts. In the southern U.S., the adult flies may keep appearing year-round.
Erika Machtinger, PhD, Assistant Professor of Veterinary Entomology at the Pennsylvania State University, says there are actually several different kinds of bot flies that parasitize other animals besides horses. “There are human bots, and some that live in squirrels and other mam- mals,” she says. Related heel flies (also called warble flies or gadflies) parasitize cattle.
“Bot flies don’t sting and they don’t bite, but the horses certainly know they are around!” she says. These flies apparently produce an unpleasant sensation and startle the horses when hovering and laying eggs, because horses try to get away from the flies.
“The female fly seeks horses to lay her eggs, and some species can lay up to 1,000 eggs—usu- ally on several different horses. In a group of horses, the fly will go from horse to horse and attach eggs. The eggs may be laid in different areas of the horse’s body, depending on the species of
If a horse has only a few bot larvae, they may not cause serious problems in the stomach. But in large numbers, they can create a lot of damage and sometimes colic.
fly. We typically see the tiny eggs attached to the inside of the forelegs,” says Machtinger.
“Sometimes you’ll see eggs on the horse’s neck or flanks, or even the shoulder, but gener- ally the eggs are laid in clusters in areas that the horse might be able to reach to rub or lick, such as the forelegs. The eggs remain stuck to the hairs until the horse stimulates their hatching by licking or rubbing, providing the necessary warmth,” she explains.
“Bots are very common in southern states,
but you can find them anywhere,” she says. In warmer regions the adults emerge early in the summer and may be active late into the fall (sometimes year-round), while in northern areas they may not emerge until the hottest days of late summer and only last until the first killing frosts. There are three species of horse bots in the U.S. Gasterophilus intestinalis is the most com- mon, laying eggs on the legs and flanks of their host. Each of these female flies can lay 150 to 500 yellow eggs, one to a hair. Eggs from this fly must incubate for one to two weeks before the larvae are ready to hatch; eggs hatch only if the horse licks or bites that part of his body. Moisture, warmth and friction from licking stimulate the eggs to hatch into tiny, spiny larvae. It only takes a little warmth to facilitate hatching. Machtinger says a person needs to be careful when removing bot eggs from the horse’s hair. “Wear gloves and make sure you are not inadvertently causing those eggs to hatch because of your warm hands or warm breath or use of warm water.”
The adult fly is brown, hairy, and about the size of a small bee. These flies live only a few days, for the sole purpose of mating and laying eggs.
74 SPEEDHORSE May 2021