Melophagus ovinus

The ked Melophagus ovinus is a Dipteran (two-winged fly) without wings, and occurs on sheep around the world, including in Canada.


Summary

The ked Melophagus ovinus is a Dipteran (two-winged fly) without wings, and occurs on sheep around the world, including in Canada.  The life cycle is entirely on the sheep.  In the female keds the eggs released from the ovary develop into larvae in the body cavity.  The larva then emerges on to the fleece and pupates immediately, then releases an adult ked.  The major effects of keds on sheep are irritation and the attempts at relief.  If present in large numbers their blood-feeding may lead to anaemia, and ked faeces can stain the fleece.

Melophagus ovinus is not known to be zoonotic.

Taxonomy

Phylum: Arthropoda
Subphylum: Uniramia
Class: Hexapoda
Order: Diptera
Family: Hippoboscidae

Melophagus ovinus are wingless, blood-feeding flies. The Family Hippoboscidae includes M. ovinus and approximately three hundred species of winged forest flies (also known as louse flies) that infest either mammals or birds.

Note: Our understanding of the taxonomy of helminth, arthropod, and particularly protozoan parasites is constantly evolving. The taxonomy described in wcvmlearnaboutparasites is based on that in the seventh edition of Foundations of Parasitology by Larry S Roberts and John Janovy Jr., McGraw Hill Higher Education, Boston, 2005.

Morphology

Adult M. ovinus are up to approximately 5 mm in length, and are leathery, brownish, hairy, dorso-ventrally flattened with six strong legs with obvious terminal claws, but no wings.

Host range and geographic distribution

Melophagus ovinus is a parasite of sheep around the world, although there are very occasional records from other domestic as well as free-ranging animals.

Life cycle - direct

The entire life cycle of keds occurs on the sheep. Adult female M. ovinus mate within hours of emergence from the pupa, and after fourteen days (required for development of the ovary, production and hatching of the egg to release the larva within the female’s abdomen) a larva is produced and deposited into the fleece. Subsequently, a female M. ovinus will produce approximately one larva a week for up to fifteen weeks. The larvae are glued to the wool and pupate within a few hours. The adult keds hatch after a further three to four weeks and begin to blood-feed. The development rate within the pupa is influenced by environmental temperatures, which are optimal at 20 C to 30 C. The life cycle can be completed in four to five weeks.

Life Cycle: Melophagus sp.

Epidemiology

The transmission of M. ovinus is either by direct contact with an infested sheep, most often from ewes to their lambs – possibly resulting in a gradual transfer of the ked population to the lambs, or with an environment containing adults that have dropped from the fleece. Thus crowding enhances transmission. Ked numbers on ewes often peak during pregnancy, but the causes for this are unknown.

In general, the prevalence and intensity of keds on sheep is highest in the winter and lowest during the summer, and highest on lambs and lowest on adult sheep. Fleece type has an influence on ked populations. Also, in general lambs are infested with young keds, and adult sheep with older ones.

Summer shearing causes the loss of most or all of the ked population on a sheep. Some keds may fall from the fleece and die, and grooming by the sheep may also reduce ked numbers.

Many keds are infected with the trypanosome protozoan Trypanosoma melophagum, which they transmit among sheep. Although T. melophagum is non-pathogenic for sheep, it can cause significant mortality among the keds.

Pathology and clinical signs

The major significance of keds is the damage they cause to the fleece and the resulting down-grading of fleece value. The activities of the adult keds may cause irritation, and efforts by the sheep to relieve this may also lead to damage to the fleece, and to reduced production. In addition, very heavy burdens of M. ovinus may lead to blood loss anemia.

Diagnosis

History and clinical signs may be helpful. Recovery and identification of the adult keds from the fleece will confirm the diagnosis.

Treatment and control

There are few products approved in Canada for the treatment of Melophagus ovinus in sheep. These include the permethrin based products ECTIBAN 25 FLY-KILLER and DOKTOR DOOM RESIDUAL SURFACE INSECTICIDE SPRAY.

Additional information on the products mentioned is available from the Compendium of Veterinary Products (Twelfth Edition, 2011), or from the manufacturers.

Control of sheep keds is sometimes incidental to the control of other ectoparasites, and vice versa. For effective control, it is essential to treat all in-contact sheep. Although sheep can develop immunity to keds, its significance in the field is unclear.

Public health significance

Melophagus ovinus is not known to be zoonotic, but that does not mean that very rare human infestations do not occur.
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