Biological and pathological characterisation of novel plasmid-carrying avian Chlamydia abortus strain 84/2334

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Many infectious diseases affect livestock, impacting not only on the health and welfare of the animals but also on the economic sustainability of the agricultural sector and future food security. Ovine enzootic abortion (OEA) is an economically important endemic disease of worldwide significance in small ruminant species, caused by the Gram-negative bacterium Chlamydia abortus. In the UK it is the most common cause of infectious abortion in sheep, resulting in foetal death in the last 2-3 weeks of pregnancy or the delivery of weak offspring and causing losses in a flock of up to around 30%. Losses in the UK have been estimated to be around £25M per annum. The organism can also infect humans causing spontaneous abortion, and in whom infections can be life-threatening. Vaccination is currently considered the most effective way of controlling disease and vaccines based on inactivated or live whole-organisms are commercially available for use in small ruminants. However, live vaccines have been reported to cause infection and disease in some animals and inactivated vaccines have been found to have much lower effectiveness in protecting animals from infections. Thus, there is a need to develop safer, more effective vaccines for use in livestock. Reducing infection and disease burden in animals will also reduce the risk of infections in humans. In recent years, research has progressed into developing methods for manipulating the genomes of human chlamydial species, making use of an endogenous plasmid (DNA that is extra to that present on the chromosome) that many of the strains possess. These plasmids are also present in avian chlamydial species (Chlamydia psittaci) from which C. abortus evolved. However, no plasmid has been found to date in C. abortus strains. Recently, we have characterised the genome of a novel strain, designated 84/2334, which has been reclassified from C. psittaci to C. abortus and carries a plasmid. Therefore, this strain could potentially be used to develop a new C. abortus vaccine for controlling OEA. However, before doing this, we need to know how biologically similar the strain is to C. abortus and determine whether the strain causes infection and disease similar to that caused by C. abortus. In this project we will compare the growth dynamics of the 84/2334 strain in relation to both C. abortus and C. psittaci, investigate its growth cycle in the target host cell, identify the key molecules involved in causing pathogenesis, and determine its potential to cause infection and abortion. These studies will identify whether the strain is more similar to classical C. abortus than to C. psittaci and determine its suitability for use in future vaccine development studies.