The Collaborative International Dictionary
OEstrus \[OE]s"trus\, n. [L., a gadfly; also, frenzy, fr. Gr. ? gadfly; hence, sting, fury, insane desire, frenzy.]
(Zo["o]l.) A genus of gadflies. The species which deposits its larv[ae] in the nasal cavities of sheep is [oe]strus ovis.
A vehement desire.
Specifically: (Physiol.) The periodical sexual impulse of animals; the period during which female animals are most receptive to males; heat; rut.
Wikipedia
Oestrus ovis (Sheep Bot Fly) is a widespread species of fly of the genus Oestrus. It is known for its parasitic predation and damage to sheep, deer, goats and sometimes cattle. There have also been many records of horse, dog and human infestation. In some areas of the world it is a significant pest which affects the agricultural economy.