Melophagus ovinus

Melophagus ovinus
Melophagus ovinus illustration from British Entomology
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Diptera
Family:	Hippoboscidae
Genus:	Melophagus
Species:	M. ovinus
Binomial name
Melophagus ovinus (Linnaeus, 1758)
Subspecies
M. ovinus ovinus (Linnaeus, 1758) M. ovinus himalayae Maa, 1969
Synonyms
Hippobosca ovina Linnaeus, 1758 Melophagus montanus Ferris & Cole, 1922

Melophagus ovinus, or the sheep ked, is a brown, hairy fly that resembles a tick. This wingless fly is about 4 to 6 mm long and has a small head, is a fly from the family Hippoboscidae. They are blood-feeding parasites of sheep.^[1] The sheep ked feeds on the blood of its host by inserting their sharp mouthparts into capillaries beneath the skin. ^[2] The legs of the sheep ked are very strong and are tipped with claws. Sheep ked lives their whole lives in the wool of sheep. Sheep ked are most commonly found on the neck, shoulders and underbelly of the host animal.^[3] Although they are often referred to as the “sheep tick”, sheep keds spend their entire life cycle on their host which is distinguishable from the characteristics of a true tick. Additionally, sheep keds have six legs whereas true ticks have eight legs. ^[4]

Distribution

Native to most of Europe including Iceland, and the Faroe Islands, as well as North West Africa, Mongolia, and North India. Introduced and established in Kenya, South Africa, Japan, Australia, New Zealand, most of North America, and many parts of South America^[5] including Tristan da Cunha and the Falkland Islands.^[1]

Hosts

The primary host of M. ovinus is the domestic sheep. There are also doubtful records on Argali (Ovis ammon), Bighorn Sheep (Ovis canadensis) and Dall Sheep (Ovis dalli).^[1]

Life Cycle

Sheep ked lives for typically four to six months, in this time they may produce from 10 to 20 larvae. The female fly will produce a single larva at a time, retaining the larva internally until it is ready to pupate. The larva feeds on the secretions of a milk gland in the uterus of the female. After three larval instars, a white pre-pupa which immediately forms a hard dark puparium. This is deposited on the wool of the sheep and are attached with a glue-like material. The larva immediately hardens and becomes a darker color forming a puparium, which will contain the pupa. This pupal stage lasts for 19 to 23 days in the summer and 20 to 36 days in the winter. Pupal stages are not susceptible to insecticides. If removed from the host, the adult lives for 7–10 days.^[3]

Immune Response and Effects

In lambs the sheep ked may cause anemia and reduce weight gain.^[3] The sheep ked feeds on the blood of its host and therefore causes irritation to the sheep, leading it to rub, producing both loss and damage of the wool. It also makes firm, hard nodules that develops on the skin called a cockle, this will reduce the value of the hide. The ked feces also stains the sheep's wool reducing its value.^[3] They also transmit Trypanosoma melophagium nonpathogenic protozoan parasite of sheep.^[6] A sheep’s immune response to keds reduces capillary flow to the skin. Although this response is trying to combat the ked infestation, it also results in a less abundant and lower quality fleece. ^[7]

Treatment

Adult keds can be killed using treatment dips and sprays most commonly containing ivermectin or pyrethrin. ^[8] However, the pupae stage is resistant to treatment and therefore, treatment must be repeated at 24 to 28 day intervals. However, using insecticides with a 3 to 4 week residual activity would also eliminate emerging adult keds from the pupae. ^[9] Additionally, because both adult keds and pupae stages live in the wool of the sheep, shearing can dramatically reduce the ked population. It would be most beneficial to shear before insecticide treatment. Most modern treatments used to control sheep lice will also control sheep ked infestations.

Disease vector

It has been indicated by experiments that the sheep ked is capable of transmitting bluetongue virus in sheep, though there is little evidence that they are bluetongue disease vectors in nature.^[10]

Sheep Resistance to Keds

Some sheep have been shown to be resistant to the harmful effects of sheep keds. Resistant sheep’s skin was histologically examined and showed arteriolar vasoconstriction in addition to fibrinoid degeneration of the tunica media in the lower dermis. This subepidermal region showed an increased infiltration of eosinophils and lymphocytes. The upper dermis of resistant sheep showed an increase of empty capillaries, whereas the upper dermis of susceptible sheep showed capillaries filled with red cells. It was determined that the resistance of some sheep to keds was due to prolonged cutaneous arteriolar vasoconstriction. In resistant sheep, keds were unable to obtain enough blood and eventually died from starvation. ^[11]

Subspecies

The subspecies Melophagus ovinus himalayae Maa, 1969 is from Nepal and Tibet. Its host is the yak (Bos grunniens), and domestic cattle.^[1]

See also

• Ked itch

References