Causes of ovine abortion, vaccination protocols and uptake: an overview

In 2021, Ceva Animal Health estimated the cost of a lamb abortion to be £85 – for an outbreak of Chlamydia abortus, this is roughly £2550 lost per 100 ewes (Mearns, 2007). C. abortus, Toxoplasma gondii, Campylobacter spp., Listeria monocytogenes and Salmonella spp. are the most common causes of infectious ovine abortion (Figure 1), with other notable pathogens including Schmallenberg virus and Border disease. Occasionally, abortion storms will occur due to these infections, causing more than 20% of ewes in a flock to abort or give birth to weak lambs (Sheep Health and Welfare Group, 2020). Several of these pathogens have been proven zoonotic, causing miscarriage in pregnant women. This, in addition to the previously mentioned economic consequences, highlights an important need to investigate preventative therapies.

Despite introduction of the associated C. abortus vaccines, it is not uncommon to treat sheep with tetracyclines during an abortion storm (Sheep Health and Welfare Group, 2020) and, similarly, antibiotic treatments are still used in flocks with T. gondii – although where the causative agent is known to be T. gondii, antibiotics are completely contraindicated as T. gondii is a parasitic protozoan. However, due to the growing concern around antimicrobial resistance and research demonstrating the benefits of vaccines, it is now increasingly common to inoculate ewes prior to mating, to the extent that the National Office of Animal Health (NOAH) lists vaccination against T. gondii and C. abortus as category one – ‘highest priority’ vaccination – in the UK flock (Statham et al, 2022). This article will review the vaccination protocols for the most common pathogens, their efficacy and the uptake of these vaccines.