Chlamydial Infection in sheep: Immune Control versus Fetal Pathology

Chlamydial abortion was first described by Greig1 in 1936 and named enzootic abortion of ewes (EAE). At that time he suggested that it was the result of environmental factors such as dietary deficiency. It was not until 1950 that Stamp and colleagues2 demonstrated that it was an infectious condition caused by an organism of the ‘psittacosis-lymphogranuloma venereum group’. In the mid 1960s specific phenotypic traits of the chlamydial strains were identified (sulphadiazine sensitivity and glycogen accumulation), which together with inclusion morphology became the basis by which the strains were differentiated into the two species Chlamydia trachomatis and C. psittaci3. The C. psittaci group consisted of strains from a wide variety of animal sources, including that responsible for EAE, while the C. trachomatis group consisted of strains from human sources. The development of DNA-based classification methods, particularly DNA—DNA reassociation studies4,5, in the 1980s led to the designation of two additional species, C. pneumoniae6 and C. pecorum7. These studies also supported the classification of chlamydial strains into eight species groups, with a ninth identified in 19938, highlighting the need for a revision of chlamydial taxonomy. In 1999 Everett and colleagues9 proposed a reclassification of the order Chlamydiales and its taxa based mainly on phylogenetic analyses of the 16S and 23S rRNA genes, but also on corroborating genetic and phenotypic information. A summary of this reclassification is shown in Table 1. The family Chlamydiaceae, which previously had only one genus Chlamydia, has been divided into two genera, Chlamydia and Chlamydophila. Within these genera five new species, in addition to the existing four, have been proposed. The organism responsible for ovine abortion, which was previously classified as serotype 1 C. psittaci, has been given species status and named Chlamydophila abortus.