Experimental studies of the transmission of enzootic abortion in ewes

Abstract

There have been reports recently of an increase in the incidence of EAE causing a major loss in sheep production despite the available control measures and this has stimulated further work into all aspects of the disease.

A flock of 37 non-vaccinated blackface ewes at mid-gestation were used in the experiment. Eight ewes were inoculated with 1 ml of Chlamydia suspension containing 6 x 10⁷ organisms. The inoculated ewes and 29 controls were run in the same paddock and allowed to lamb together. All the 37 ewes were sampled weekly for serum and any abortion material was collected for post-mortem examination. The sera were tested by CF test and placental and foetal smears were made and stained by modified Ziehl-Neelsen. Foetal lung and placental tissues were taken for tissue culture isolation. Serum samples were also fractionated by Sephadex gel filtration as described by Bryson (1980).

All eight inoculated ewes aborted at average mean incubation period of 40 days and seven were positive for EAE and all eight had a good sero-conversion before and after abortion. Of the in contact controls, seven out of 29 ewes aborted with positive EAE diagnosis by picking up the infection naturally. One had twins with one twin lost by dystokia and was negative for EAE, and the two others were undiagnosed. The rest lambed normally, although foetal membranes were collected from two of them and both were positive for EAE. The majority of the controls had a good sero-conversion with titres equal to or greater than 1/64. The losses among the controls occurred on average 77 days after the start of the experiment and a mean interval between EAE abortions in the inoculated and control ewes was 37 days.

There was a change in specific IgM/IgG ratios, although it occurred over a short period and the CF test which was used was found to be unsuitable for this type of analysis. It was concluded that the inoculated ewes produced a typical EAE infection with transmission to in contact controls occurring at lambing leading to subsequent abortion in the same season. The fact that a large proportion of the controls 'were infected and developed a high post-lambing titres yet lambed normally, suggests that great care is necessary in the interpretation of titres in sero-diagnosis. It was also clear that control measures should be directed to replacements for flock.