A survey of thelazia worms in the eyelids of British cattle.

Thelazia skrjabini and T gulosa were found in the eyes of 237 of 566 bovine heads (41.9 per cent) examined in a Surrey abattoir during 1976. The prevalence of infection increased during late June, July and August, when a new generation of worms emerged in eyes. The burden in eyes ranged from one to 170 worms, with a mean count of 10.44 +/- 3.23. Worms were found behind and in third eyelids and in the lacrimal ducts of lower eyelids, which makes their detection very difficult by clinical examination. They were significantly more common in cattle 21 to 38 months old than in those less than 21 months old. Lesions occurred in 14 of 327 (4.3 per cent) infected eyes. No worms were seen in eye washings from cattle in nine herds experiencing outbreaks of keratoconjunctivitis.