The macroscopic appearance and associated histological changes in the enamel organ of hypoplastic lesions of sheep incisor teeth resulting from induced parasitism.

Twenty-one penned female sheep were infected with a high dose of nematodes (200,000 Trichostrongylus vitrinus and 25,000 Ostertagia circumcincta) during first incisor development when aged 7-8 months. Three sheep were infected with 200,000 T. vitrinus only; 10 sheep were untreated. Anorexia, lassitude and severe diarrhoea were seen in 14 of the infected sheep after 21-26 days. Anthelmintic was given to sheep showing undue distress to end the infection and to all remaining sheep on day 33. Animals were killed at 3 stages: during the infection, at intervals during later tooth development, and at the time of eruption of the incisor teeth, 7-8 months after the parasites were given. Hypoplasia of the labial enamel of 15 out of 19 teeth from sheep killed after recovery from the infection was classified according to the extent and depth as pits, grooves or larger areas of missing enamel with ledge-formation cervically. The amount of missing enamel related to the severity of the systemic disturbance which affected the secretory ameloblasts. During the period of active infection, some secretory ameloblasts showed vacuolization or were shorter than in controls; some cells were no longer adjacent to the organic matrix; some continued to function but abnormally and later became separated from the retained organic matrix by connective tissue. Cervically-placed ledges resulted either from the recovery and resumption of activity of some ameloblasts or from differentiation of new secretory cells after the infection ended.