Page 77 - May 2021

EQUINE HEALTH
Typically, the tiny eggs are seen attached to the inside of the forelegs.
As the flies dart around the head to lay eggs up under the jaw, the horse will usually jerk its head up or strike out with
a front foot in an effort to keep the fly away.
The egg capsules remain in place on the hair after the larvae emerge and enter the mouth. Then they burrow into the tissue. The empty egg capsules are small and flat instead of plump. The tiny first stage larvae either crawl to the mouth or are ingested and then burrow into the tongue, gums, or lining of the mouth, where they remain for about three or four weeks. Then they molt into second stage larvae and emerge from the mouth tissues. “The horse then swallows those larvae, and they attach to the lining of the stomach,” says Machtinger.
The second and third stage larvae typically attach to the lining of the stomach in the non- glandular portion near the junction with the esophagus or near the outlet into the intestine. The larvae remain there, immobile but grow- ing, for the next eight to 12 months. If large numbers attach near the stomach outlet, they may obstruct food passage.
“They go through those three different stages inside the horse, until they are mature enough to detach and go out with the manure,” she explains.
The next spring or summer, they detach from the stomach, migrating through the intestines and change to the next stage (pupae), passing out with manure. These pupae burrow into the ground for three to five weeks. After maturing into adult flies, they emerge from the ground and seek out horses to lay their eggs on, and the cycle begins again.
G. nasalis is the species of bot that lays yellow eggs along the lower jaw of the horse. This must create an uncomfortable sensation, because horses being pestered by these egg-laying flies often run wildly to get away from them. As the flies dart around the head to lay eggs up under the jaw, the horse will usually jerk its head up or strike out with a front foot in an effort to keep the fly away. When bot flies are around, horses can be danger- ous to work with because they may strike at a fly and inadvertently hit you, or they may charge over the person handling them in an attempt to avoid
the fly. Horses in groups may bunch together, rub- bing their chins on each other’s backs.
Eggs under the jaw are deposited up between the hairs and are harder to see than the eggs on legs and flanks. Once attached to the hair, the eggs incubate for four to six days before hatching, needing no external stimula- tion. These larvae simply migrate along the jaw and crawl into the mouth and burrow into the gum tissue. After about a month, they follow the same cycle as G. intestinalis. They emerge from the gums, pass down into the stomach and find a place to attach and grow.
G. haemorrhoidalis is a different species that lays black eggs on the hairs near the horse’s lips. These eggs hatch by themselves in two to four days before penetrating the inner lip membrane in front of the incisors. Then they follow the same path as their relatives, after five to six weeks in the mouth.
Bot larvae cause stomach inflammation at the site of attachment. A heavy load of bots may cause indigestion, mild colic, unthrifti- ness and lack of appetite. Large numbers in the gums may create pus pockets and irritation in the horse’s mouth.
CONTROL OF BOTS
The first line of defense and the least invasive strategy is simply to remove the bot eggs from the horse’s hair coat. This does not involve use of chemicals. You can use clippers, or the specially designed tools that scrape off the eggs. You can also pluck the eggs off with your fingers. Yellow eggs on the hair coat are very easy to see, especially on a dark-colored horse. The main thing is to keep track of those eggs or clipped-off hairs and destroy them or have the horse in an area where if they drop onto the ground, they will never get into a horse’s feed or be consumed with grass.
Removing the eggs is a very effective way to control bots if you are handling the horses
every day. Eggs should be removed from the hair coat as soon as they are observed. Eggs
on legs, shoulders and flanks are easily seen and picked off with your fingers, trimmed
off with scissors or a safety razor, or scraped off with sandpaper. Washing will not remove them. Eggs under the jaw are harder to see and remove; they are deep in the hair. Smearing petroleum jelly along the bottom of the jaw tends to kill the eggs by sealing off their air supply and suffocating the developing larvae.
“I am not a veterinarian. I am a veterinary entomologist. But most veterinarians also recommend chemical control in the form of
a dewormer. Even though these larvae are
not worms, they can be killed by a deworm- ing drug when they are inside the horse,” says Machtinger. Some dewormers like ivermectin can kill the first stage larvae while they are still in the mouth tissues, even before they migrate down into the GI tract, but you definitely want to kill these parasites before they settle in for the winter in the horse’s stomach.
“The usual recommendation is to treat twice a year—once after the first killing frosts (when there will be no more adult flies laying eggs on the horse and you are killing any bots that have gotten into the horse), and once in the spring,” she says. A deworming in the spring will kill any that might have got- ten missed before they are passed out in the manure to start the cycle all over again.
“The dewormings targeting bots can usually be scheduled at the same time you’d be doing treatments for the other internal parasites—the nematodes (worms) that commonly live in the GI tract. The treatment for bots is pretty easy and straightforward and mainly depends on what part of the country you are in and whether bot flies are fairly common or not. When I lived in the mid- Atlantic area, I spent a lot of time with my horses and never saw a bot fly or bot fly eggs, but when
I moved to Florida there were many bot flies. I actually caught one with my bare hands out in the field with my horses, because they are poor fliers (compared to a bee or a fly),” Machtinger says. When a bot fly is bothering the horses, you can sometimes just grab it.
Ivermectin is an effective dewormer for bots. It paralyzes them wherever they are in
the body, not just in the stomach. Unable to move, they cannot eat, and soon die. The usual recommendation for treatment, especially if you are unable to pick the eggs off the hairs as they are laid, is to treat with ivermectin about one month after the first eggs are laid and at two-month intervals throughout the fly season.
In northern climates with a short season, bots can be effectively controlled with just one treatment after killing frosts in the fall when there are no more flies to lay eggs, and another treatment in early spring to kill any in the
SPEEDHORSE May 2021 75