How flies spread parasites and disease

By Tania Millen, BSc, MJ

Flies are the bane of many horses’ existence from spring through autumn. In Canada, horse-biting flies include gnats (no-see-ums), black flies, stable flies, deer flies, horse flies, and mosquitoes, all of which create painful sores. Non-biting flies include house flies, eye gnats, face flies, bot flies, and warble flies, and can be just as irritating. It’s not just buzzing and biting that’s worrisome; flies may transmit parasites and diseases, can cause a horse’s immune system to overreact, and some use horses as hosts, all of which impact horse health. Preventing horses from being bitten is key. Before diving into solutions, here’s a summary of the damage that flies can inflict on Canada’s horses.

Biting insects such as horse flies (left, above) and deer flies (right, above) can make summer miserable for horses with their persistent, aggressive pursuit and painful bites. Both horse and deer fly bites make a scissor-like incision, after which they lap up the blood. Their saliva contains an anti-coagulant which can cause a severe reaction in the victim. Photos (left, above): iStock/Mirceax; (right, above): Canstock/BRM1949. 

Flies as Vectors

When a fly bites a horse or ingests mucus from eyes, nose, and lesions, the blood or fluids it ingests may contain diseases that the horse already carries. That fly can then transmit those diseases to the next horse it bites, thereby spreading disease from ill horses to healthy horses.

Equine Infectious Anemia (identified by a Coggins blood test) is commonly transmitted by biting horse flies, stable flies, and deer flies. West Nile virus and Eastern and Western Encephalomyelitis (EEE/WEE) are all transmitted by mosquitoes. Flies also transmit anthrax and the virus responsible for equine sarcoids.

Zebra-striped horse blankets help to ward off biting flies by disrupting the ability of flies to land. Studies have shown that flies are just as attracted to zebras as they are to horses of other colours and do not deter flies at a distance, but once the winged pests get close they tend to fly past or bump into the animal. One researcher explained that stripes may dazzle flies in some way once they are close enough to see them with their low-resolution eyes. Photo: Shutterstock/Anakul

Related: How Zebra Stripes Repel Flies

Flies as Transport

Flies can also transport the larvae of equine stomach worms (habronema). The larvae are inadvertently picked up when flies feed on manure containing the habronema larvae. Then the hitchhiking larvae leave their fly transporters at fly bite sites or wounds on horses, creating infection, irritation, and itch. The resulting “summer sores” — raised, itchy, weeping bumps — are slow to heal and may recur due to an allergic reaction by the horse. 

Biting stable flies and non-biting flies attracted to wounds on horses may also carry fungi that cause local infection or equine granular dermatitis (rain scald).

Flies can also carry strangles bacteria from one horse to another, along with the viral particles of equine influenza, rhinopneumonitis, and vesicular stomatitis.

Related: Intestinal Parasites in Horses

The barn swallow is a medium-sized songbird at 15-19 cm in length, weighing 17-20 grams. Although these insectivores can be messy, if given access to a ledge or a beam to build their mud nests, they will happily earn their keep by eating a variety of insects at a rate of up to 60 an hour or 850 a day. They most often catch insects in flight, including mosquitoes and flies such as houseflies and horse files. Unfortunately, the barn swallow population in Canada has declined to about 25 percent compared to what it was 40 years ago. Photo: Shutterstock/Bob Hilscher

Fly Irritation

For some horses, fly bites are so irritating that the bite area becomes inflamed, raw, itchy, bare, and/or infected. The result is dermatitis and sweet itch, along with wounds that attract more flies. Excessive fly populations can also cause anemia and weight loss. In areas with overwhelming numbers of flies or particularly aggressive biting flies, horses can be so disturbed that they panic.

Fly-Parasite Relationships

Horses host numerous parasites, but warble larvae (screwworm), bot larvae, and stomach worms are intertwined with the external fly population.

Warble flies lay eggs on the horse and when the larvae hatch, they bore into the skin and migrate to the backbone and esophagus. When the weather warms up, the larvae migrate to the skin of the horse’s back where lumps form (often, inconveniently in the saddle area). The lumps have a minute breathing hole for the larvae, which they eventually escape through, then fall to the ground and eventually hatch into flies.

The bot fly cycle is a bit different. There are two types of bot flies that bother horses, both of which have an external fly stage and multiple internal larvae stages. The better-known bot fly looks like a bee and lays cream-coloured eggs on horses’ legs. When the horse bites or licks their legs where the eggs are attached, the eggs are ingested, hatch in the mouth and implant there for three to four weeks where they may cause swelling and ulceration. They then pass into the stomach and intestines as larvae, where they attach and grow into large oval larvae. When mature, they’re pooped out in manure and burrow into the soil for a few months, after which they hatch into flies and the cycle repeats. The lesser-known but highly irritating nose bot has a similar life cycle, except that the fly lays eggs in or near the horse’s nostrils. The larvae then hatch and wriggle into the horse’s mouth and gums. One female bot fly can lay as many as 1,000 eggs in her seven-to-ten day life.

Related: Bugs Be Gone: Fly Control for Your Horse Barn

Tiny, cream-coloured parasitic botfly eggs attached to the hair on a horse’s leg. Photo: Shutterstock/Sari ONeal

The stomach worm (habronema) life cycle is similar to the bot cycle except that there’s no “stomach worm fly.” Instead, the worm relies on house, face, or stable flies to pick up worm larvae from manure or old bedding and deposit them near the horse’s mouth. The horse ingests the larvae, which then mature into adult worms in about two months, causing minimal damage to the horse. The adult worms lay eggs that are then pooped out in the horse’s manure, hatch as larvae, and are picked up by flies to start the cycle again. Stomach worm larvae only cause the horse grief when they’re deposited into bites or wounds, as explained above.

There are many good reasons to control the fly population around horses. Let’s consider solutions.

Related: What's Biting Your Horse?

Control and Prevent

There are two main ways to control flies and the parasites or diseases they carry — reduce the fly population and prevent flies from biting horses. Here’s a summary of options. 

1. Control manure

Manure piles are fly larvae breeding areas and “salad bars” for mature flies, so reducing the amount of manure prevalent where horses are living will help reduce the fly population. Collecting manure from stalls, pens, and fields, and storing it correctly is the first step. Manure piles located near stalls and pens mean flies can go back and forth between the pile and the horses, so it’s preferable to place the manure pile away from the horses. Building the pile correctly, turning it, and allowing it to heat up and kill fly larvae also helps reduce the fly population.

When manure piles are located near pens and stalls, as shown, flies can easily move from the manure pile to the horses causing irritation and spreading parasites and disease. Manure should be removed and stored away horse living and housing areas. Photo: Tania Millen

2. Utilize predatory flies

Seeding the manure pile with predatory flies, which feed on the biting fly larvae, interrupts the biting fly life cycle, thereby reducing the horse-biting fly population. Predatory flies look like tiny wasps but do not bite horses or sting humans. They must be “planted” in the manure pile before and throughout fly season to maintain control of the biting fly population. After discussing the size of the manure pile(s) onsite and distance to the horses, suppliers will mail flies and larvae to customers in a large envelope. Although it sounds counterintuitive to intentionally introduce more flies to the farm, predatory flies are an effective, simple, and eco-friendly method of reducing horse-biting flies.

Tiny parasitic wasps kill filth flies, such as house flies and stable flies, by piercing the fly pupae and depositing their eggs inside. Once hatched, the tiny parasite larvae consume the inside of the pest fly pupae, killing it. Parasitic wasps are an effective and environmentally-friendly way to reduce biting fly populations. Photo: Dreamstime/Henk Wallays 

3. Feed smelly supplements

Feeding horse supplements that contain garlic, apple cider vinegar, and brewer’s yeast can deter flies from biting. However, it can take up to six weeks of feeding the supplements for them to fully permeate the horse. Effectiveness depends on individual horses. 

4. Use insect growth regulators

Insect growth regulators can be added to the horse’s grain ration to “seed” the horse’s manure. They contain chemicals such as diflurobenzuron and cyromazine that pass through the horse’s digestive tract without harming the animal and are pooped out in the horse’s manure. At that point, they act as larvicides, killing the fly larvae of house and stable flies. Since biting fly larvae are prevented from developing into flies, the overall fly population is reduced.

Related: When Horses Get Allergies

5. Strategically deworm

Horse deworming products contain different chemicals that affect different parasites. Ivermectin and moxidectin are effective against bot larvae and habronema in horses. It’s general practice to deworm in autumn once bot flies are no longer laying their eggs and other flies are no longer prevalent. Autumn deworming also prevents larvae from taking nutrients from the horse during the winter months and inducing weight loss.

6. Prevent bites

Regular application of chemical sprays and creams are often the easiest way to prevent fly bites. Many products and home remedies are available, all providing varying degrees of effectiveness. However, physical barriers including fly sheets, face masks, and leggings can all prevent flies from biting horses. 

Solid material and mesh fly sheets usually cover the neck, body, and sometimes the belly, and extend lower on the legs than typical blankets. Some have a softer drape while others are made from a stiffer mesh. Like any horse blanket, different brands and styles fit different horse shapes better. When deciding on a sheet, consider the expected weather during fly season and required durability to horse antics. Many sheets can be too hot on some days and not all products will stand up to roughhousing.

Physical barriers include fly sheets to cover the horse’s body and neck, fly masks, and leggings. Photo: Tania Millen

Face masks may cover just the eyes or extend over the ears and nose as well. The types of flies and your horse’s tolerance will dictate how much of the horse’s head to cover. In areas with nose bots, fly masks with an extended nose piece which covers the nostrils are worthwhile.

Related: Sweet Itch in Horses: Not so Sweet

Foot stomping to avoid being bitten by flies is exhausting and can cause injury. Leggings are a good solution and provide significant relief. They’re easy to fit and have simple Velcro® closures which are snug around the pastern and looser below the knee. A stiff batten holds them upright to prevent “slouching.”

An easy and effective way to keep flies at bay is regular application of chemical sprays and creams. Photo: Clix Photography

All told, flies are more than just a nuisance; they cause extensive health issues to individual horses and the larger horse community. As climate changes, flies may expand their range, carrying parasites and diseases with them to new areas. Therefore, limiting horse exposure to flies can significantly reduce horse illness and the spread of disease — something all horse owners and riders can support.

Related: Biosecurity on the Horse Farm

Related: Horse Care Through the Seasons

More by Tania Millen

Main photo: iStock/Deepblue4you