With the weather beginning to warm the issues of surrounding fly control return. At best, flies are an irritance. At worst, they cause stress to the horse and they transmit bacteria, parasites and viruses. Fly control has its varying challenges depending on fly size, habitat and feeding patterns.

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The two categories of flies

Biting Flies - This type of fly feeds on animal blood and can cause allergic reactions, pain and disease spread. Within this group are horse flies, midges, mosquitoes, black flies and stable flies.

Non-biting flies - This type of fly does not feed on blood. Instead they feed on bodily secretions such as mucus. It is important to note that non-biting flies can also transmit diseases to the horse and other animals.

To add to the issues that can be caused by biting and non-biting flies. Flies can lay larvae in the skin. This is called myiasis but more commonly known as fly strike. This can have harmful affects if not treated early as the larvae feed on the flesh of the animal.

For the most part, flies in the UK are just an annoyance as there is little threat of disease spread compared to other countries. That being said, Equine Infectious Anaemia (EIA) is seen worldwide and is prevalent in Europe.

Equine Infectious Anaemia

EIA is a viral disease that causes intermittent fever, anaemia, weight loss and in some cases loss of life. The signs of EIA can vary dependent on the stage of the disease. Early signs show bleeding, inflammation and jaundice.

EIA, as a notifiable disease has strict control measures for the reduction of spread. Animals found to be infected are to be euthanised and any exposed horses must be quarantined for a minimum of 60 days with regular testing.

Contaminated blood is one the causes of EIA spread and the disease can also be passed from mare to foal in the utero. However, the most common route of EIA transission is from biting flies, particulary horse and stable flies.

Luckily in the UK horse flies do not transmit EIA, however, rising climates may eventually lead to the disease entering the country.

Horse Flies

Persistent, agressive and hard to control, horse flies are one of the most well know flies for horse owners. As part of the Tabanidae family, horse flies are atracted to mammal species as they require the protein in mammalian blood for reproduction.

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Horse flies are attracted to warmth, smell and bright light which is why they are more attracted to horses with darker coats as they are polarotactic (attracted to polarised light reflected by dark skin/coats). It has been found that stripes, spots and white coats reflect depolarised light. This has lead to a lot of fly rugs being stripy/spotty look as horse flies are not as attracted.

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Horse flies are also strongly attracted to the odur from sweat and urine therefore, washing horses rugs and keeping stables clean helps deter flies.

The reason for the notoriously painful bite of a horse fly is due to the six stylets in their mouth parts. They slice into the skin and their saliva contains compounds that promote bleeding and prevent clotting. The discomfort of the horse fly bite trigger the horse to knock them off. This then results in multiple bites and further pain.

Midges

In respects to flies causing horses irritaion, perhaps the most annoying of them all is the midge (Cullicoide). These tiny flies swarm and bite horses, specifically in the mane, tail and belly region. This is an unpleasant experience but can also cause more damaging effects in horses who are allergic to their bites. Some horses have a hypersensitivity to the saliva of midges – the allergens in the midge’s saliva bind with Ig-E, causing an allergic reaction. This hypersensitivity is known as Ig-E mediated allergic dermatitis. 

In layman’s terms, this condition is referred to as ‘sweet itch’. This is most commonly shown when horses intensely rubbing the affected areas (mane and tail) on solid structures such as fencing. Sweet itch can range in severity. Some horses suffer broken hair or patchy hair loss. Others continue to itch until the skin is broken and raw. This can result in oozing lesions.

Culicoides are also accountable for transmitting the parasite that causes onchocerciasis in horses. This is a condition that causes itching and skin irritation. 

If possible, horses should be stabled when midges are most active. Most active at dawn and dusk, midges prefer wet, marshy areas. As they are poor fliers, having a fan outside the stable helps prevent them from entering. A well-fitted fly rug is also necessary for horses who suffer from sweet itch.

The Stable Fly

As the name suggests, the Stable Fly is often located around stables and livestock. Its Latin name, Stamoxys calcitrans, means ‘sharp mouth’, also suggesting its biting tendencies. Unlike many other blood-feeding flies, both sexes feed on hosts, which if often early in the morning or late afternoon. 

Stable flies often bite horses on the legs, stomach and teat area. As well as being an annoyance,they also transfer disease. They are the intermediate host for the stomach worm, Habronema muscase. Currently with our temperate climate, the occurence of these nematodes is rare, but with rising temperatures, they may become more familiar. Stable flies also carry the eye worm, Thelazia. Their larvae can cause lesions in the conjunctiva of the eye. 

The Black Fly

Flies can also spread viruses from one location to another on the same horse. Culpable for this is the Black Fly. Small and annoying these flies target the horse’s face- particularly the ears. It is thought that Black flies are involved in the development of aural plaques and potentially have a role to play in spreading sarcoids on a horse’s body. 

Fly Prevention

It is difficult to shied horses thoroughly with many different species of flies, who all have their own preferences for environment and feeding times. However, the following precautions should be observed to help control the burden of flies: 

  • Clean environment. Regularly removing dirty bedding removes the attractive smell of urine and clearing faeces removes the breeding site for many flies. Good stable and yard hygiene should include stables being mucked out and paddocks poo-picked daily. It is important to keep the muck heap some distance from the stable yard to decrease contact of flies and horses. 
  • Physical barriers. Fly rugs and fly masks create a physical barrier to inhibit flies from biting. It is crucial that these items fit properly to prevent gaps that flies can enter from or sores and rubbing from inappropriate fit. Striped, spotted, or white rugs deter horseflies. 
  • Chemical deterrents. There is a vast amount of fly sprays available. Products containing highly effective active ingredient D.E.E.T. will be beneficial to control flies. 
  • Avoid particular times of day if possible. Midges are active at dawn and dusk, so stabling horses at these times will decrease contact. Similarly, horseflies like bright sunlight – supplying shade will give horses somewhere to go to avoid them. 
  • Avoid damp areas. Midges and many flies swarm over water, be that water troughs, rivers or damp woodlands. Horses should be kept away from these areas as much as possible.
  • Midges struggle to fly in winds above 7mph so turning horses out on high ground with optimum airflow or increasing ventilation in the stable helps to minimise midge activity.