Horse louse flies are Dipteran (two wings) insects of the genus Hippobosca that suck blood and are important for horses and other equids (donkeys, mules, etc.), but can also affect other mammals such as dogs, cattle and even humans. Birds may also be affected. Together with sheep keds and other insect genera they belong to the family of Hippoboscidae.
They are obligate parasites, i.e. they can neither survive nor complete their life cycle without parasitizing their hosts.
There are seven Hippobosca species worldwide. The main species of veterinary importance are:
- Hippobosca equina, the horse louse fly. Affects horses and other equids, occasionally cattle, dogs and humans. It is found mainly in Europe, Asia and parts of Africa.
- Hippobosca variegata, the cattle louse fly. Affects mainly cattle but also horses. Found mainly in Africa.
- Hippobosca longipennis, the dog louse fly. Affects mainly carnivores, including dogs. It is found mainly in Africa, Asia, the Middle East, occasionally in Eastern Europe. A few outbreaks have been reported in the USA caused flies carried by carnivores imported from Africa for zoos. However they have not become established.
Louse flies are obligate parasites, i.e. they cannot complete their life cycle without parasitizing their hosts.
The disease caused by Hippobosca louse flies is called hippoboscidosis.
Are horses or other animals infected with louse flies contagious for humans?
- YES. Hippobosca equina may very occasionally infect humans. But the risk is rather low in Europe: these flies are very uncommon and usually they do not leave their hosts unless disturbed.
Adult louse flies are 7 to 9 mm long, have a dark brown to black color, and their body is flattened and scarcely haired. Wings have a dark reddish to grayish color and reach beyond de abdomen. Hippobosca equina keeps its wings once on a host, thus it can also fly away. Other species shed their wings upon finding a host. Their legs are short and end in strong claws that they use for firmly attaching to the host. Both males and females have piercing mouthparts and feed on blood of their hosts. A blood meal last about 15 minutes. Since they cannot store blood, they bite their hosts rather frequently. Adult flies live for 4 to 5 months. Off the host without a blood meal they do not survive more than 2 weeks.
Louse flies live almost permanently on their host, often in clusters in body parts with little to no hair (e.g. perineal region, between the hind legs, udders, etc.). Species that keep their wings may fly away if disturbed, but will return very quickly to their host. Adult females do not lay eggs. Instead, the eggs develop inside the uterus and are deposited at a pre-pupal stage. Deposited larvae complete pupation within hours and usually fall to the ground. Adults emerge 3 to 4 weeks later to find a host. Each female deposits very few larvae, 5 to 12 larva during her life.
Harm to horses and livestock and economic loss due to louse flies
Horse louse flies are seldom a problem in Europe, and are not found in the Americas. Where they occur, affected animals suffer considerable itching and react by scratching, rubbing and biting the infected areas. This can cause self-injuries that may become infected or attract screwworm flies.
Diagnosis is confirmed by identification of the flies in the affected body parts, mainly under the tail and between the hind legs.
Prevention and control of infestations with louse flies
Regular grooming of horses helps diminishing the number of flies. Frequently changing the bedding in stables will eliminate deposited pupae that otherwise will re-infect the animals.
There is very little information on chemical control of louse flies. The reason is that being a minor parasite, almost no insecticide approved for use on horses or livestock includes a claim for louse fly control. It can be assumed that concentrates for spraying or dipping horses and/or cattle will provide some control. They contain mainly organophosphates (e.g. coumaphos, chlorfenvinphos, etc.), synthetic pyrethroids (e.g. cypermethrin, deltamethrin, permethrin, etc.) or mixtures. Based on the preference of louse flies for unhaired body parts it is unlikely that ready-to-use pour-ons will provide sufficient control. However, it is unclear whether the dose recommended for these products to control other external parasites (ticks, lice, etc,) will control louse flies as well.
To our knowledge no macrocyclic lactones (mainly ivermectin and moxidectin) is approved for louse fly control and there are no reports that they would be effective against louse flies at the usual therapeutic dose of 200 mcg/kg.
So far biological control of louse flies (i.e. using its natural enemies) is currently not possible. Learn more about biological control of flies and other insects.
Click here if you are interested in medicinal plants for controlling external parasites of livestock, horses and pets.
Resistance of Hippobosca spp to insecticides
So far there are no reports on resistance of Hippobosca spp to insecticides.
This means that if an antiparasitic fails to achieve the expected efficacy, chance is very high that either the product was unsuited for the control of louse flies or it was used incorrectly.
Ask your veterinary doctor! If available, follow more specific national or regional recommendations for louse fly control