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Enzootic abortion of ewes
Last month, the Welsh National Public Health Service published a warning titled, “Advice to pregnant women during the lambing season.” It underlined the risk of pregnant women becoming infected with pathogens that cause abortion in pregnant sheep.
The warning mentioned Chlamydophila abortus, Toxoplasma gondii and listeria as the most important abortifacient sheep pathogens in Wales. The warning also indicated that if a pregnant woman becomes infected with these organisms, they may harm her unborn child.
The warning included a detailed list of precautionary measures to be applied by pregnant women who may be exposed to lambing or aborting sheep. This may happen either on commercial farms or in other facilities where sheep are kept, such as children’s farms and petting zoos. Pregnant women were encouraged to seek medical advice if they experience fever or influenza-like symptoms or if they are concerned that they may have acquired infection from a farm environment.
Abortion in sheep flocks is estimated to have an annual incidence of 2% to 3% in most countries, although some flocks may experience abortion storms involving large numbers of ewes. In addition to three pathogens mentioned in the Welsh warning, sheep abortions may be caused in certain countries by other pathogens which are hazardous to human health. These include Brucella melitensis, Coxiella burnetti (the pathogen of Q fever), Salmonella spp (eg S. typhimurium) and Rift Valley fever virus.
The most prevalent abortifacient sheep pathogen in developed countries is Chlamydophila abortus. In 2007, it was identified as the cause of abortion in 529 (41%) of 1,296 submissions to the national investigation centers/veterinary laboratories in Great Britain, continuing to be, as in many previous years, the most commonly diagnosed infectious cause of abortion in sheep.
Some abortus-related pathogens of sheep, such as Campylobacter fetus and Salmonella abortus-ovis, are not zoonotic.
Enzootic abortion of ewes
C. abortus (formerly Chlamydia psittaci serotype 1) infects the placenta, causing a disease in sheep known as enzootic abortion of ewes (EAE) or ovine enzootic abortion (OEA). The organism causes a similar disease in goats, but infections of cattle, pigs and horses are thought to be less common.
The main routes of transmission of C. abortus are through ingestion of organisms shed in vaginal fluids and placental membranes at the time of abortion or lambing or through inhalation of aerosols from the environment. EBs can remain viable in the environment for several days, depending on environmental conditions.
Another potential route of transmission is through direct infection of the fetus via the placenta.
An initial outbreak within a flock that may give rise to only a few abortions can lead to more than 30% of the ewes aborting or giving birth to stillborn or weak offspring in the following year. In subsequent lambing seasons, the incidence of abortion is likely to remain at 5% to 10% if affected animals are left untreated.
Infection of pregnant ewes with C. abortus up to five to six weeks prior to parturition leads to clinical disease and results in either abortion in the final two to three weeks of gestation or the birth of stillborn or weak lambs that frequently die in the first few days of their life. The duration of pregnancy in sheep is 142 to 150 days (breed-dependent).
Infection of ewes within the last five to six weeks of pregnancy often leads to the development of a latent infection, where ewes appear to be uninfected until the next lambing season. Surviving lambs born to infected mothers may be affected by EAE in their first pregnancy.
Persistent, subclinical or latent states can be maintained for several months following infection of non-pregnant ewes, before the onset of pregnancy triggers bacterial multiplication.
C. abortus is undetectable by any means in latently infected ewes, and it is not until around day 90 of gestation that the organisms are reactivated in some way, resulting in clinical expression in the placenta and ultimately abortion.
Although there are generally no premonitory indications of the impending abortions, vaginal discharges can be observed for up to 48 hours prior to lamb loss. These discharges can continue for two to three weeks, adding to the environmental spread of infection. Following abortion, ewes develop protective immunity. However, immunity is not necessarily sterile, as some ewes are reported to continue to shed infectious organisms seasonally for up to three years, increasing the spread of disease. Most ewes remain reproductively sound and can continue in subsequent breeding programs.
Currently available vaccines against OEA are based on inactivated whole organisms and a temperature-sensitive live attenuated mutant strain.
Human infection
In humans, infection may be asymptomatic but where symptoms occur, they are commonly of an influenza-like nature with headache, chills, fever, joint pains and non-productive cough. Photophobia, vomiting, sore throat and myocarditis may also occur. In pregnancy, a more severe form of the disease may occur, the majority of reported cases occurring between 24 and 36 weeks. This is characterized by systemic illness with disseminated intravascular coagulation (a hemorrhagic disorder), renal and hepatic complications. It is these cases that are most commonly associated with stillbirth or abortion, which generally occurs three to eight days after the onset of the symptoms
The main effects of chlamydiosis in pregnancy are severe, sometimes life-threatening, disease in the mother and stillbirth or abortion. If the pregnancy survives the acute infection, there appears to be no risk of long-term problems. There is no evidence that this infection can result in abnormalities in the baby when it is born.
The route of transmission to humans is not known with certainty. Inhalation of aerosols and dusts heavily contaminated with C. abortus appears to be the likely route of infection. The organism is concentrated in the uterus of pregnant sheep. Contact with aborting sheep, sheep at risk of abortion, dead lambs and placenta are thus considered to represent a risk for humans. Where lambing and handling of pregnant and post-partum ewes takes place indoors, the risk of human exposure to the organism may be greater than in open pastures.
By its nature the risk is limited mainly to those actively working with sheep, including veterinary surgeons, and their immediate families. Women are at risk if they have close contact with ewes at the time of aborting or giving birth, with new-born lambs or with placenta or products of conception. Infection has also been associated with handling of clothing and boots contaminated by contact with infected animals.
Prevention of human infection
To avoid the possible risk of infection, pregnant women are advised that they should:
- Not help to lamb or milk ewes.
- Avoid contact with aborted or new-born lambs.
- Avoid contact with the afterbirth.
- Avoid handling (including washing) clothing, boots, or any materials that have come into contact with ewes, lambs or afterbirth.
- Should ensure that full hygiene precautions are followed by all household members.
- Ensure partners attending lambing ewes observe full hygiene procedures.
- Ensure that if they or their partners do come into contact, they should shower, bath or wash thoroughly (including hair) as soon as practicable after handling a lambing ewe.
- Scrub hands, and keep finger nails short and clean.
- Wash clothes used in lambing separately, pregnant women should not handle dirty clothes worn during the lambing season - if it is not possible to clean up thoroughly during the night, then sleep in separate bedrooms.
- Should not handle any vaccines, and should avoid contact with recently vaccinated sheep.
For more information:
- K. Kerr, G. Entrican, D. McKeever, D. Longbottom (2005). Immunopathology of Chlamydophila abortus infection in sheep and mice. Research in Veterinary Science 78 (2005) 1–7.
Arnon Shimshony, DVM, is Associate Professor at the Koret School of Veterinary Medicine Hebrew University of Jerusalem, Rehovot, and is the ProMED-mail Animal Diseases Zoonoses Moderator. Dr. Shimshony was Chief Veterinary Officer, State of Israel, from 1974 to 1999.