Enzootic abortion of ewes (ovine chlamydiosis)

Ovine chlamydiosis, also known as enzootic abortion of ewes (EAE) or ovine enzootic abortion (OEA), is caused by the bacterium Chlamydophila 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 and it is not uncommon to observe delivery of a dead and a weak or healthy lamb. 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 isolation and identification of the causative agent or detection of the agent or its nucleic acid 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 deer, can be affected. Chlamydiosis of small ruminants is a zoonosis and the organism must be handled with 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 necrotic placentitis, and the demonstration of large numbers of the organism in stained smears of affected placentae. The still moist fleece of fetuses or vaginal swabs of females that have freshly aborted are also useful. Care is needed 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 antigen can be detected by enzyme-linked immunosorbent assay, immunohistochemistry or the fluorescent antibody test, whereas chlamydial DNA can be detected by the polymerase chain reaction or by microarray. Some of these methods are available in commercial kit form. Chlamydophila abortuscan be isolated only in living cells; thus facilities for growth in chicken embryos or cell cultures, with appropriate biohazard containment, are required. Serological tests: A rise in antibody titre to C. abortus, detected by the complement fixation (CF) test, is common after abortion or stillbirth, but this does not occur in every case. Chlamydophila abortus shares common antigens with C. pecorum and some Gram-negative bacteria, so that the CF test is not wholly specific, nor does it distinguish between responses to vaccination and to infection. Low CF titres need to be interpreted with caution, particularly if these are encountered in individual animals or in flocks with no history of abortion. Alternative serological tests have been developed and some commercialised, but none has been sufficiently appraised so far for field use. A delayed hypersensitivity reaction to chlamydial antigen can be elicited in infected sheep, but the procedure is not amenable to routine use. Requirements for vaccines: Inactivated and live vaccines are available that have been reported to prevent abortion and to reduce excretion. They assist in control of the disease but will not eradicate it. Serological screening during the period after parturition helps to identify infected flocks, to which control measures can then be applied.