NADIS disease bulletins are written specifically for farmers, to increase awareness of prevalent conditions and promote disease prevention and control, in order to benefit animal health and welfare.

Farmers are advised to discuss their individual farm circumstances with their veterinary surgeon.

Chlamydial (Enzootic) Abortion of Ewes

Chlamydial (enzootic) abortion is one of the two most commonly diagnosed causes of ovine abortion in the UK, which costs the sheep industry about £15 million per year. Ewes become infected by ingestion or inhalation of the bacterium Chlamydophila abortus from an environment contaminated by infected placentae or vaginal discharge. The outcome following infection is dependant on the ewe's immune status and stage of pregnancy.

· ewes which have previously aborted due to chlamydial abortion are immune to further chlamydial abortions, but may continue to shed C. abortus in vaginal discharges following subsequent lambings

· infection of susceptible ewes during the first half of pregnancy sometimes results in abortion during the final three weeks of the same pregnancy (in practice this is uncommon, other than in flocks with both early and late lambing ewes)

· infection of susceptible ewes during late pregnancy or when not pregnant usually results in abortion during the final three weeks of the subsequent pregnancy

· surviving ewe lambs born to aborted ewes, or normal ewe lambs born to ewes which aborted during a previous season, become infected through licking/inhaling placental fluids/vaginal discharges on their coats and abort during their first pregnancy

· rams and wethers are not involved in transmission of infection

Chlamydial abortion typically occurs in the final 3 weeks of pregnancy. Ewes appear healthy until about three days before aborting, when they may stop eating and a red vaginal discharge is sometimes noted. Lambs may be born dead and decomposed, freshly-dead or alive. Live-born lambs are often weak, pot-bellied and unable to suckle, and are susceptible to starvation, hypothermia and neonatal diseases. Varying amounts of discoloured uterine discharge are present for several days after abortion, but are ewes are otherwise healthy. Retained placenta and consequent metritis sometimes follow.

The diagnosis of chlamydial abortion is based on the laboratory demonstration of C. abortus on aborted placentae. Your vet can advise you on the most appropriate samples to submit.

Prevention of chlamydial abortion in clean flocks

The disease is usually introduced with purchased sheep; orphan lambs and breeding replacements are the most common sources. Occasionally foxes or seagulls are blamed for carrying aborted placentae onto neighbouring farms.

· maintain a closed ewe flock (not an option for most crossbred ewe flocks)

· never buy in orphaned lambs

· only purchase replacement ewe hoggs and gimmers from accredited-free sources (Premium Health Scheme)

· isolate any aborted ewes

· investigate abortions if they occur in more than 1% of the flock (submit lambs and placentae)

Control in known infected flocks

The primary source of infection is the aborted lamb and placenta. Ewes may have a vaginal discharge for about three weeks following abortion, which is also a source of infection and aborted live lambs carry the infection on their coats for several days. Other sheep become infected when they lick or inhale any of the above.

· immediately isolate all aborted ewes and their lambs

· remove contaminated bedding from lambing pens

· 20 mg/kg of long-acting oxytetracycline administered to the ewes approximately 3 - 6 weeks before lambing may increase the proportion of viable lambs born, but will not eliminate the abortion problem

· vaccination.

Elimination of chlamydial abortion from an infected flock is very difficult, however the level of infection can be significantly reduced using a combination of purchasing accredited-free replacement ewes, running replacements as a separate flock at lambing, hygiene at lambing and vaccination.

Vaccination

Modern vaccines have been shown to very effective for the prevention of chlamydial abortion in previously unexposed sheep. Experimental data also indicate that vaccination may reduce the risk of abortion in infected ewes in the face of an outbreak. Ideally all animals are vaccinated once in the first year and in subsequent years breeding replacements are vaccinated once at any time up to 4 weeks before mating. (The inactivated vaccine can also be given from 4 weeks after ram removal.)

Premium Health Scheme

The Premium Health Scheme is a national, voluntary, farmer-led programme with the aim of supplying buyers with breeding replacement ewes free of chlamydial abortion. The scheme has about 2000 members and represents about 7% of the national breeding flock. Your vet can provide you with more information about the Premium Health Scheme.

Neil Sargison BA VetMB DSHP FRCVS