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Ectoparasites - Oestrus

Oestrus ovis: Sheep nasal fly, nose bot, sheep gad-fly.

General Description: Adult flies are about 12 mm. long. The body is greyish-brown, with many small black spots on the thorax, which is covered with fine, light brown hair. Young larvae are white or light yellow. As the larvae grow, they develop dark transverse bands on their upper surface. When full grown, they are up to 3 cm. long, tapered in front with a flat surface.

Life Cycle: Adult female flies deposit larvae around the nostrils of the host. The larvae crawl into the nasal and frontal sinuses. Growth to the pupal stage requires 2 weeks to 9 months, longer in cold weather. Full-grown larvae crawl out of the nose and pupate on the ground for 3 to 9 weeks before becoming adults. The mature flies live only about 2 weeks.

Location: Larvae live in nasal passages and frontal and nasal sinuses.

Geographical Distribution: Worldwide.

Significance: Fly worry causes sheep to become unthrifty. Oestrus are well-known parasites but economic effects are not well known.

Effect on Host: Fly strikes result in decreased appetite, restlessness, and poor gains. Nasal myiasis causes irritation, nasal discharge, and sneezing. A rare complication occurs when a larva migrates through the skull and injures the brain, causing incoordination and nervous disorders.

Diagnostic Information: Clinical signs are helpful in identification. Flies and larvae are difficult to find unless the sheep dies.

Control: Fly repellents are not very effective. Oral dosing with systemic insecticides is used to kill the larvae. Fumigation of sheep is done occasionally, as is injection of insecticides into the nostrils. Ivermectin is effective against all stages of Oestrus ovis.

 
Typical attitude of sheep to avoid fly attack – crowding and holding noses close to ground   Early larval instars on nasal mucosa
     
 
Section through nasal and frontal sinus (pointer at a mature instar)   Mucopurulent discharge (snotty nose) is a typical sign
     
Larvae and pupa

           

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