Enzootic abortion of ewes (ovine chlamydiosis) (infection with Chlamydophila abortus)
Description and importance of the disease: Ovine chlamydiosis, also known as enzootic abortion of ewes (EAE) or ovine enzootic abortion (OEA), is caused by the bacterium Chlamydia abortus. Chlamydial abortion typically occurs in the last 2–3 weeks of pregnancy with the appearance of stillborn lambs and inflamed placentas. However, infection can also result in the delivery of full-term stillborn lambs or weak lambs that do not survive longer than 48 hours. Infected ewes can also give birth to healthy lambs. There are rarely any predictive signs that abortion is going to occur, although behavioural changes and a vulval discharge can be observed in the last 48 hours of pregnancy. Diagnosis of enzootic abortion depends on the detection of antigen or nucleic acid of the causative agent in the products of abortion or vaginal excretions of freshly aborted females. A humoral antibody response may be detected following abortion. Goats as well as sheep and, less commonly, cattle, pigs, horses and wild ruminants, can be affected. Chlamydiosis of small ruminants caused by C. abortus is zoonotic and the organism must be handled with appropriate biosafety precautions. Pregnant women are particularly at risk. Identification of the agent: The basis for a positive diagnosis of infection with C. abortus depends on a history of abortion in sheep or goats (often in late pregnancy), evidence of purulent to necrotising placentitis with vasculitis, and the demonstration of large numbers of the organism in affected placentae by quantitative polymerase chain reaction (PCR) or antigen tests or in stained smears. The still moist fleece of fetuses or their abomasal content or vaginal swabs of females that have freshly aborted are also useful. It is important to distinguish cotyledonary damage caused by Toxoplasma gondii and, in stained smears, to be aware of the morphological similarities between C. abortus and Coxiella burnetii, the agent of Q fever. Chlamydial organisms in tissues and smears can be detected by staining, or antigen-detection methods (immunohistochemistry or immunofluorescence), whereas chlamydial DNA can be detected by PCR-based methods including real-time PCR and DNA microarray. Some of these methods are available in commercial kit form. Chlamydia abortus can be isolated only in living cells; thus facilities for growth in cell cultures or chicken embryos, with appropriate biohazard containment, are required. Serological tests: A rise in antibody titre to C. abortus, which can be detected by enzyme-linked immunosorbent assay (ELISA), is common after abortion or stillbirth, but this does not occur in every case. Chlamydia abortus shares common antigens with other Chlamydia species and some Gram-negative bacteria, so that the complement fixation (CF) test or crude ELISAs are not specific and no longer recommended. Serological screening during the period after parturition helps to identify infected flocks, to which control measures can then be applied. Serological tests to differentiate between vaccinated and naturally infected sheep or goats (DIVA tests) are not currently available.