Page 78 - May 2021

EQUINE HEALTH
“Ever since the macrolides became available in deworming products, it has been easy to control bots. Ivermectin, especially, is extremely effective against bots, and though moxidectin is not as good, it is still quite effective.”
You can use clippers or the specially designed tools that scrape off the eggs.
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.
stomach that might have been missed before they pass out of the horse to pupate in the ground. Unless you have nearby neighbors who do not control bots in their horses, you can completely eradicate these pests with diligent egg removal and/or treatment at the proper time of year.
“Unfortunately, fly repellents do not deter bots that are laying eggs on a horse because these flies don’t really come into contact with the horse that much,” Machtinger says. They are not landing on the horse to suck blood, and not spending much time around horses except to briefly hover and lay eggs.
Thus, the best control is simply to remove the eggs and treat the horses with an appropri- ate deworming drug at the times of year to kill the larvae--in your particular region and seasons. “Consult your equine veterinarian
to help you make the right decisions—with recommendations for treatment and use of the proper drugs. Even though there haven’t been any drug resistance issues with bots yet, we commonly see resistance problems with other internal parasites,” says Machtinger.
“Even if you are using a product that hasn’t had any resistance problems in bots, it may promote resistance in other parasites, so you want to work with your veterinarian to make sure you are using the appropriate products for your region, your horse and the parasites you are targeting,” she says. Control of parasites is a broad issue; you should not just look at one specific species and ignore the others.
In terms of treating horses for internal parasites, including bots, Roger Moon,
PhD, Professor of Entomology, University of Minnesota (Saint Paul), says one problem is that some equine veterinarians tend to pre- scribe dewormers based on the calendar, rather than the severity of measured problems.
“The actual need for dewormers aimed at worms in the GI tract can be diagnosed through fecal egg counts, but it is easier to just schedule deworming and be done with it. A calendar program is simple and cheap, but it is also a prescription for resistance
to commonly used deworming products. Fortunately, stomach bots and their kin have not yet been diagnosed with resis- tance, probably because they only have one
generation per year, and bot treatments are not widespread,” says Moon.
“Bots inside an individual living horse cannot be diagnosed. Their damage to horse health depends on numbers per animal and the horse’s age. One way to monitor numbers is to check ‘road apple’ piles in early summer when the larvae are being passed with the manure,
or after a deworming that kills them in the stomach, and they all pass out (appearing as little pink maggots in the manure). But, seeing them go out in feces in early summer or after a deworming when they are all in the stomach is like watching the fire engine pass by — dam- age to that individual horse has already been done,” he says.
“A few years ago, a student of mine who loved grooming horses monitored the number of eggs of the common horse bot fly on horses’ leg hairs during one summer on five premises in southwestern Minnesota. Overall, half of the 15 horses on three of five premises became infested. Infestations occurred only in mid-August to mid-September. We were surprised at how wide- spread the bots actually were. I found an earlier study, from Delaware back in the 1960s, where fly eggs were found through the entire summer, so the timing of egg-laying will likely vary geo- graphically and from year to year,” he says.
In the fall, horse owners traditionally treat for bots about 30 days after the first hard frost—whenever it happens in their region. Once those flies are gone, there are no more eggs laid on the horse. Some of the eggs already on the hair coat may continue to infect the horse for several weeks, however, so there may be some transmission continuing into winter
in some regions—and this is why some horse owners give a second treatment in the spring. If you have to choose a single time of year to get the most for your money from one treatment, however, it would be early winter.
“Ever since the macrolides became avail-
able in deworming products, it has been easy to control bots. Ivermectin, especially, is extremely effective against bots, and though moxidectin is not as good, it is still quite effective,” says Moon. If horses were not treated in the fall, they can be treated for bots any time during winter or early spring as long as it’s ahead of when the larvae will be passed out with the manure, to burrow
into the ground and pupate—which can be as late as April or May, in northern climates. This will keep any new adult flies from hatching out and reduce the incidence of bot flies pestering horses that next summer.
“Horse health at an individual premise or horse farm may best be protected by dosing the whole herd with ivermectin or moxidectin after the first hard frost in fall, which ends egg lay- ing, but before the little bots have worked their way through horses’ mouths and moved down to stomach and reached damaging sizes,” says Moon. “This should eradicate all the bots on your farm. But if neighboring premises are not treating their horses effectively, you may need to repeat treatments annually because of flies coming in from the neighbors.”
76 SPEEDHORSE May 2021