The use of a delayed hypersensitivity test and long-acting oxytetracycline in a flock affected with ovine enzootic abortion

In a flock of 283 Clun Forest ewes affected with ovine enzootic abortion (OEA) 272 were tested for an allergic skin reaction to demonstrate a delayed-type cellular immunity to Chlamydia psittaci (ovis), the cause of OEA, between 62 and 87 days gestation. One hundred and eighty-six ewes were negative and 86 were positive to the test. Nine days after the test 65 ewes, randomly selected from the negative group, were each injected with 1·2 g of a long-acting preparation of oxytetracycline. The injection was repeated 13 days later. Twenty-nine (10·7%) of the 272 ewes aborted or produced stillborn lambs, and OEA was confirmed in 14 of them. Twenty-seven of the 29 were negative to the skin test and two were positive. Only five of 65 ewes negative to the skin test and treated with two injections of oxytetracycline aborted or had stillborn lambs compared with 22 of 116 untreated ewes negative to the skin test. Three months after lambing ended 20 of 29 ewes that aborted or had stillborn lambs were retested. The two that reacted previously remained positive and 13 that were negative to the first test were now positive. Five still had no reaction. More abortions and stillbirths occurred among ewes that were negative to the skin test than in those that were positive ( P < 0·01) despite the fact that a proportion of negative ewes had been treated with oxytetracyline. This indicated that the cell-mediated immune response to C. psittaci (ovis) demonstrated by the skin test was protective. There were fewer abortions and stillbirths among ewes negative to the skin test treated with oxytetracycline than in untreated negative ewes ( P < 0·05), indicating that this treatment of susceptible animals reduced the incidence of OEA. After experiencing clinical disease most ewes developed a cell-mediated immune response to C. psittaci (ovis).