Oestrus ovis

• Also known as the sheep nasal bot fly

• Larvae parasitise the nasal chambers of sheep and goats

• Found in most sheep rearing areas of the world

Recognition

• 13-15mm long

• Grey colouring

• Black spots on abdomen

• Clear wings

• Larvae have distinct black bands on each body segment

Life cycle

• Larvae are squirted into the nostrils of sheep in a jet of liquid by the adult fly

• The larvae crawl caudally into the nasal cavity and feed on the nasal mucosa and mature before returning to the nostrils

• Larval development takes up to 2 months
  • Larvae can overwinter in the nasal cavity if deposited late in the summer

• Once the larvae have developed they are sneezed out and pupate on the ground

• The adult fly emerges 1 months later

• Adult flies only live for 2-3 weeks

Pathogenesis

• Adult flies cause annoyance
  • Interrupts feeding
  • Leads to a decreased weight gain

• Larvae cause nasal irritation, nasal discharge and sneezing
  • Irritate the nasal mucosa with oral hooks and spines causing a viscous exudate to be produced from which they feed
  • Heavy infestations lead to erosion of the bones in the sinuses (turbinate bones)
  • Penetration of the brain leads to false gid (high stepping gait and incoordination)

• Pathology in the nasal cavity

Control

• Systemic insecticides can be used in heavy infestations

• In warmer countries, strategic prophylactic treatment can be used