Abortion vaccines in sheep

Abortion vaccines are available for the three main infectious causes of abortion. It is possible to control enzootic abortion without antibiotics by using effective vaccination strategies. 

The control of enzootic abortion was identified as one of three hotspot areas for the reduction of antibiotics in the sheep industry, in the RUMA Targets Task Force Report 2020. The use of antibiotics to control enzootic abortion is not considered best practice responsible use.

The three main infectious causes of abortion, which have been consistent over the last five years, are:

• Enzootic abortion of ewes (EAE) caused by Chlamydia abortus
• Toxoplasmosis
• Campylobacter spp.

Although the epidemiology of these three infections is very different, any one of them can cause outbreaks of abortion in flocks. In an outbreak, up to 40% of ewes may either abort or produce weak, poorly viable lambs.

The cost: £11–48m per year in 2005 (Bennett R. 2005) for enzootic abortion to the national flock.

Effective vaccines against EAE and toxoplasmosis are available:

• Enzovax is a live vaccine for the active immunisation of breeding female sheep against C. abortus infection
• Cevac^® Chlamydia is a live vaccine for the active immunisation of breeding female sheep against C. abortus infection
• Inmeva Suspension For Injection inactivated vaccine for the active immunisation of breeding female sheep against C. abortus infection and Salmonella enterica subsp. enterica serovar Abortusovis
• Toxovax is a live, concentrated vaccine containing tachyzoites of the S48 strain of Toxoplasma gondii

Assumptions

Numerator: The number of doses of vaccine administered has been calculated by multiplying the number of packs sold by the number of doses per pack.

Denominator: The common industry recommendation is to vaccinate all breeding females before the first joining with the ram. Although revaccination may be advisable after three to four years, it is not thought that a considerable amount of vaccine is used in this way. The total number of vaccine doses that would be required to protect the national UK flock from EAE and toxoplasmosis has been estimated based on the assumption that all ewes intended for first-time breeding in June should receive a dose of vaccine. It is accepted that this denominator is probably an underestimation of the actual number of ewes that should receive vaccination, but it was chosen for both simplicity and repeatability.

Vaccination uptake

From 2013 to 2021 there was a steady increase in uptake, going from one in three to half of ewes vaccinated against EAE. This dropped slightly in 2022, to 44.1%. Uptake appears to have recovered in 2023 with 46.4% of all breeding ewes, including ewes intended for first-time breeding, were vaccinated for EAE.

In 2023, 29.1% of breeding ewes and ewes intended for first-time breeding were vaccinated for toxoplasmosis. This is an increase back to similar levels seen in 2020 and 2021 after a dip to 20.3 % observed in 2022.

The manufacture of these live abortion vaccines is known to be complicated and the timing for administration is generally concentrated over a short period, just before the breeding season. In recent years, there have been several occasions when whole batches of vaccines have failed quality control and supply has been limited at key times of the year. These manufacturing issues are likely to affect both supply and uptake.