Flies and Fly Control - By Dr. Lesely Lunn

There are several types of fly that are of importance to horse owners.

MUSCA DOMESTICA -THE HOUSE FLY:

These are annoying to the horse and can carry disease. They feed on moist organic matter such as manure, garbage, moist hay etc. they have soft mouth parts and suck up their meals by feeding on faeces or ocular secretions that are infected with pathogens. They can transfer these pathogens to other animals when crawling on their lips or eyes. They are also capable of directly transmitting bacteria on their bristly legs.

Females lay 100-150eggs on various substrates such as manure, rotting feed etc. The life cycle from egg to adult takes 7 to 14 days depending on the environmental conditions.

STOMOXYS CALCITRANS -THE STABLE FLY:

This is the one with the sharp, biting mouthparts. Both males and females feed on blood. They usually attack the lower parts of the horse where the painful bites lead to irritation and foot stamping. In extreme situations the continual stamping may jar the limbs leading to joint problems.

The female lays 25 to 50 eggs in rotting organic matter. A favourite laying site is decomposing grass cuttings and it is important that after mowing all cuttings are picked up and removed or correctly composted.

This is the fly that is responsible for biting the tips of dogs’ ears. Dogs may develop non-healing sores on the edges of the ear flap (pinna). These become painful and it is advisable to treat the dogs daily with a fly repellent.

BLOW FLIES:

These are only a problem where wounds are left unattended. They are classified as primary or secondary blowflies (sometimes called blue bottles). They have a bright metallic colour.The primary fly lays eggs in carcasses or where tissue is contaminated by urine, faeces or bacterial infection. Proteolytic enzymes produced by the feeding larvae of the primary fly create the liquefied tissue which is the meal of choice for the secondary larvae.Blowflies are attracted by smell thus the importance of preventing wounds from becoming septic. Severe blowfly “strike” can lead to toxaemia and even septicaemia and death.

HABRONEMIAISIS

Habronema is a stomach worm of horses, not a fly. It is included here because it is transmitted by flies –both the house fly and the stable fly. Fly larvae ingest the worm larvae which are passed out in the manure of the infected horse. The habronema reaches the infective stage in the maggot and remains in the fly`s body at maturity. Worm larvae emerge from the fly when it is feeding on moisture e.g from the eyes, genitalia, around the lips or a moist wound. The larvae burrow into the tissue and cause a granulomatous reaction, sometimes called summer sores. Non healing lesions in which calcified granular material can be felt result.

TREATMENT IS WITH IVERMECTIN OR MOXIDECTIN.

Please note-these are worms and not “bots” which are the larvae of the gastrophilus fly. Gastrophilus lays eggs which resemble grass seeds. Those laid near the mouth will hatch and penetrate the mucous membranes while those laid on the legs or trunks are licked orrubbed off by the horse. They eventually migrate to the stomach via the pharynx. These larvae are what we commonly refer to as bots. When selecting a “dewormer’ it is important to know if it also kills bots. The eggs can be removed with a bot egg removing “knife” or inactivated by washing the horse in a mild acidic solution such as vinegar or soda water.

Control of flies depends on the elimination of the breeding site/s. Manure must be picked up regularly and removed from the premises or correctly composted.

Care must be taken to keep areas where moisture might accumulate, dry -i.e. around water troughs; where stables are sluiced.

There are biological methods of control such as the release of parasitic wasps which prey on the fly larvae.

The presence of flies can be minimised by the use of screens, fans, traps and sticky papers.

Insecticides can be used on ceilings, walls etc but great care must be taken to avoid licking by horses or contamination of feed and water.

The horse can be made more comfortable by the use of fly sheets, masks or fringes but these do not reduce the number of flies.

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