Evidence of Chlamydophila abortus vaccine strain 1B as a possible cause of ovine enzootic abortion

Chlamydophila abortus, the agent of ovine enzootic abortion (OEA), is a major cause of lamb mortality worldwide. Disease can be controlled through the use of vaccines based on the 1B temperature-sensitive mutant strain of C. abortus. This study investigated suspected OEA cases across Scotland for the presence of the 1B strain by analysis of recently identified unique point mutations (9). Thirty-five cases were C. abortus-positive and 14 came from vaccinated flocks. Analysis of single nucleotide polymorphisms by PCR-RFLP and sequence analysis revealed the presence of point mutations consistent with the presence of the 1B vaccine strain in 5 of these 14 samples. Quantitative real-time PCR revealed comparable numbers of genome copies of the 1B strain in infected placentas to those present following wild-type infection. This study is the first to demonstrate the presence of the 1B vaccine strain in the placentas of OEA cases and suggests a probable causal role in the disease.