8 Feb 2016
Louise Silk
Job Title
Image: Freeimages/Linda van Klaveren.
Enzootic abortion (EAE) is the most common cause of infectious abortion in sheep in the UK (Longbottom and Coulter, 2003) and one of the most significant causes of lamb mortality worldwide (Longbottom et al, 2013).
Veterinary Investigation Diagnosis Analysis data in 2005 (Mearns, 2007) showed EAE as the cause of 37% of diagnosed ovine abortions, where a diagnosis had been made in 53% of submissions. In 2005 the cost of controlling EAE to the sheep industry of Great Britain was estimated, using disease models, at approximately £16.5 million per year (Bennet et al, 2005). This article will briefly discuss EAE the disease, before considering the economic consequences of new and ongoing infections in a sheep flock.
EAE is caused by the bacterium Chlamydophila abortus. Infection of a naïve ewe occurs following exposure to the bacteria – usually as a result of direct contact with abortive material. Other sources of infection include the vaginal discharges of a ewe that has aborted – bacteria are shed in the vaginal discharges of such ewes for up to three weeks after the abortive event.
In some cases carrion may be responsible for transporting infected abortive materials between farms. The bacteria can remain viable in the environment for several days, making contamination a major issue during a disease outbreak.
The infectious organism enters the body via ingestion or inhalation (Jones and Anderson, 1988) and targets the placenta of the infected ewe during the subsequent pregnancy, at around 90 days of gestation, resulting in a suppurative necrotising placentitis.
In most cases, during the first year of infection in a naïve flock, there will be only a very small number of abortions, which may go unnoticed. This will be followed by an abortion storm during the subsequent lambing period, where up to 20% to 30% of the flock may be affected. On farms where there are multiple lambing periods, or the lambing period is prolonged, infection may become apparent in the same pregnancy if it occurs at least six weeks prior to lambing. If a ewe lamb is born to an infected ewe and survives, this ewe lamb may have been infected at birth and go on to abort in its first pregnancy.
In intensively managed flocks an abortion storm can result in abortions in up to 30% of the ewes. In endemically infected flocks, however, an abortion rate of approximately 5% per year is common. There are generally few clinical signs associated with disease. Ewes may be seen to be slightly off colour for 24 hours prior to the abortion and may produce a reddish vaginal discharge. Abortions typically occur in the final three weeks of gestation. Any lambs from infected ewes surviving birth tend to be weak and unable to suckle.
Diagnosis is based on examination of abortive materials. In infected ewes gross examination of the placenta reveals thickened, oedematous intercotyledonary areas and necrotic cotyledons. The placenta can be covered with a brownish exudate. Smears of the intercotyledonary areas stained with modified Ziehl-Neelsen stain will identify the organism as red-stained elementary bodies. Histopathology reveals intracytoplasmic inclusions in chorionic epithelial cells and an inflammatory infiltrate. Immunohistochemistry, ELISA or the fluorescent antibody test can be used to identify chlamydial antigens. PCR or microarray can be used to identify chlamydial DNA.
The Scottish Agricultural College, in conjunction with MSD Animal Health, run a free testing service to check for the presence of EAE (as well as toxoplasmosis) in flocks that are not currently vaccinating. This involves testing six to eight blood samples from ewes that have aborted and looking for raised antibody levels. The standard serological test for EAE is a complement fixation test, with titres of at least 4/32 considered positive and suggestive of recent infection. The interpretation of the results, however, is not straightforward and false positives can occur with cross reactions occurring with other Chlamydophila species. Vaccination titres tend to be low unless animals have also experienced natural challenge with C abortus.
In the face of an outbreak it is recommended to mark and isolate all aborting ewes, dispose of all contaminated bedding and abortive material and to disinfect thoroughly any contaminated pens. Whole flock treatment with antibiotics (Aitken and Longbottom, 2007) – oxytetracycline at a dose of 20mg/kg – can be used three to six weeks pre-lambing to increase the number of viable lambs born to ewes known to be infected with C abortus (Mearns, 2007).
Antibiotic treatment is likely to be most effective in flocks with an extended lambing period. It is important to note ewes that have aborted should not be used to foster ewe lamb replacements. Unfortunately, once a flock is infected with EAE, elimination is extremely difficult. This is due to the establishment of a latent carrier status in some previously infected ewes. Unfortunately, current diagnostic techniques are unable to identify latently infected animals until parturition (Sachse et al, 2009), by which time the disease has already been transmitted to numerous, unidentifiable, naïve individuals in the flock.
Prevention of the disease in a naïve flock comes from buying disease-free accredited ewes, provided the home flock has been confirmed as disease free. If ewes are bought from unaccredited sources, or if disease is known to be endemic on the home farm, vaccination of replacements would be recommended.
Two live attenuated vaccines are available for vaccination of non-pregnant ewes: Enzovax (MSD) and Cevac (Ceva Santé Animale). These vaccines should be administered at least three to four weeks prior to mating. Once the whole flock has been vaccinated, it is generally recommended only replacements need vaccinating in subsequent years.
There is also one inactivated vaccine, Mydiavac (Benchmark Animal Health), which can be used both pre-tupping for prevention and during pregnancy to aid in the control of EAE in infected flocks. Check vaccination data sheets for exact specifications for the use of these vaccines. It is important to note antibiotics should not be administered at the same time as the vaccines as this will reduce vaccine efficacy (Longbottom et al, 2013).
There is evidence indicating vaccinal strains of C abortus have been responsible for causing late-term abortions in animals vaccinated with commercial live attenuated vaccines (Wheelhouse et al, 2010; Livingstone et al, 2014). In studies carried out over a three-year period, PCR restriction fragment length polymorphism tests were used to differentiate between wild and vaccinated strains of C abortus in affected animals.
Approximately 30% of samples submitted from affected animals showed vaccinal strains of C abortus. Despite this the advice remains to continue using the vaccines until next generation safer subunit/subcellular vaccines are developed (Livingstone et al, 2014).
Finally, it is important to note C abortus is a zoonotic disease and can infect the human placenta resulting in abortion, still birth and serious maternal illness (Longbottom and Coulter, 2003). Pregnant women should avoid contact with pregnant, lambing and aborting ewes in flocks where disease cannot be ruled out.
EAE causes considerable losses to our sheep industry. Despite the availability of effective vaccines and advice on disease prevention there are still large numbers of flocks where endemic disease goes unnoticed. An abortion or barren rate of greater than 2% should always provoke further investigation, as should a cluster of several abortions within a short period of time. Identification of the presence of disease will allow preventive strategies to be put in place to reduce future losses and improve the overall economic performance of the flock.
Tables 1 to 4 provide example figures for the possible economic consequences of EAE in a flock and highlight the importance of not only preventing, but also controlling, this disease.
It can be seen from the aforementioned figures an outbreak of EAE and ongoing endemic infection has potential to have severe economic consequences for sheep farmers. In some cases it may well be the difference between making a profit or loss. It is vital we encourage investigation of poor flock performance to allow control measures to be put in place where infection is identified, and to encourage strict biosecurity protocols to prevent the introduction of disease into naïve flocks.
