External Parasites

Bitey, Itchy, Crawly Things!

Fly masks are indispensible during fly season.

Fly masks make for happier horses during fly season.

External parasites can be just yucky, annoying, or downright dangerous. 

These include house flies, “friendly” flies, stable flies, horse flies, deer flies, bot flies, mosquitoes, culicoides, lice and ticks. 

Controlling these parasites leads to greater comfort and health for your horse. Methods vary, according to the individual parasite.

Non-biting Flies

House flies and “friendly” flies (those large, striped flies seen the past few years; their larvae are parasites of Tent caterpillars) have lapping mouth parts and so don’t bite, but annoy horses by landing on them. Horses may continually stomp their feet which can lead to lameness issues. The flies can also cause conjunctivitis by landing and feeding around their eyes. House flies lay their eggs on manure and other decomposing organic matter.

Biting Flies

Horse fly.

A horse fly.

Stable flies, horse flies and deer flies are biting flies. In addition to annoying horses by their mere presence, their bites can be very painful and irritating, sometimes causing large, oozing lumps at the site of the bite. Stable flies lay their eggs on manure and decomposing organic matter, horse and deer flies lay their eggs in masses on plants near water.


Bot flies are the adult phase of the internal bot parasite. The bot has no feeding mouthparts and thus is unable to bite. It annoys horses by following horses around, laying many tiny yellow eggs on the horse’s legs, face and body. The horses chew on the eggs, causing them to hatch, starting the internal part of the bot’s life cycle.

Mosquitoes and Culicoides


The all-too-familiar mosquito.

It’s not just about the itch that accompanies a mosquito’s bite. Mosquitoes can carry West Nile Virus, and Eastern or Western Encephalomyelitis, all of which can be fatal. People can also become infected with these diseases. Mosquitoes lay their eggs in still or standing water.

Some horses are extremely allergic to Culicoides bites. They have a systemic reaction in addition to reaction at the site of the bite. Their manes, tails, and faces are usually involved, with rubbing and hair loss the primary signs. Some are so allergic that their entire bodies become involved. Culicoides lay their eggs in still water, similar to mosquitoes.


There are two types of lice that affect horses; biting and sucking. Both types complete their entire life cycles on the horse. The biting lice are more common, and they can cause intense itching as they feed on the horse’s skin. They attach their tiny eggs to the horse's hair, most commonly near the mane.

As their name implies, sucking lice suck blood, and in large numbers, can kill a horse.



Ticks are becoming more common.

Ticks are becoming an increasing problem in some areas of Vermont and New Hampshire. They have a long, multi-stage life cycle, during several stages of which may attach themselves to horses. Several species over winter on mice. Ticks are important external parasites in that they can carry and transmit both Lyme Disease and Anaplasmosis.

Be sure to read our FAQ about ticks, their prevention, and Lyme disease treatments.

Strategies for Control

Parasitic Wasp.

Parasitic wasp.

Primary control of all external parasites includes limiting or eliminating breeding sites. This means eliminating standing water such as unused water tanks, old tires, low and swampy spots where mosquitoes, culicoides, horse and deer flies may breed on or near.

The use of tiny parasitic wasps to kill the flies in the pupal stage has proven to be a very effective control method, especially if started before fly season. Trapping of adults prior to egg laying is another method of control. Fly traps, such as the Red Top, are quite useful in attracting adult house, stable and some horseflies.

Fly Sheet

Fly sheets offer great protection.

Fly sheets and leg coverings, including entire body-covering sheets for those allergic to culicoides, provides additional relief. Keeping horses stabled during high fly season also is helpful. Stable, horse and deer flies feed primarily during the day. Culicoides and mosquitoes feed primarily at dusk, dawn and during the night. They are also weak fliers, so keeping a horse in a stall with a fan on creates enough of a breeze to significantly reduce these tiny insects.

Finally, we come to insecticides. We all know from experience that most spray products don’t last as long as labeled for. We’ve become fond of the spot-on type of product for long-term fly control. They last pretty well for 10-14 days. Fly sprays can be used with them to improve fly control while riding.

Several products say that they also control ticks. These include Buzz Off and the Repel-X. DEET-containing products marketed for humans also control ticks, and while not labeled for use on horses, they have been reported to be safe and effective at controlling ticks.

LYME Disease

Tick season will be here before we know it. We will be starting our spring Lyme clinics in late February—stay tuned.

Winter Education Talk

Don't miss our educational event!

February is Dental Awareness Month!

We offer a 10% discount on all equine dental procedures performed in our heated clinic January 29-March 2. Get a jump on Spring by getting your horse’s teeth in pristine shape! Read more...

Evergreen Equine
of Vermont
Dr. Heather Hoyns

Mailing Address:
P.O. Box 126
Reading, Vermont 05062

(802) 484-9100
(802) 484-9104


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