Ovine chlamydiosis - the most common infectious cause of prenatal lamb death in the UK

Published on 22 October 2012 in Sustainability and Communities , Food, health and wellbeing

Introduction

Ovine Enzootic Abortion (OEA) (also known as Enzootic Abortion of Ewes (EAE) or ovine chlamydiosis) is caused by the bacterium Chlamydia abortus (formerly known as Chlamydophila abortus) and is one of the major infectious causes of abortion in sheep and goats worldwide, and the most common cause in the UK. The disease is considered to be ‘endemic’ in the UK as it is constantly present in the livestock population, but also ‘enzootic’ as it only affects a limited number of animals at any one time. The disease is a major health and welfare issue for the livestock industry with large economic implications amounting to tens of millions of pounds annually. Chlamydia abortus can also infect cattle, pigs and horses but disease outbreaks are much more sporadic and less frequent. Infection can also be passed from animal to human (zoonotic) in whom it can cause spontaneous abortion (miscarriage). It can also be potentially life-threatening for pregnant women, although reported cases of transmission are rare.

It is difficult to identify infection in individual animals within a flock, making the disease difficult to control. The first sign of infection is often the appearance of dead or live weak lambs in the last few weeks of pregnancy with visibly diseased placentas and a vaginal discharge of infective fluid, which can last for several days. The infected placentas, coats of dead lambs, contaminated bedding and vaginal discharges are the major sources of infection for other animals. In an extended lambing season uninfected ewes can pick up infection from another ewe and abort in the same lambing season. There is little evidence that the disease can be transmitted mechanically by rams.

The spread of disease is difficult to control as infection of a non-pregnant ewe only becomes apparent during pregnancy and it is currently impossible to identify infection in such animals through diagnostic testing. For this reason care must be taken when sourcing and introducing replacement stock. The introduction of only a few infected animals may be sufficient to lead to an abortion storm affecting up to 30% of ewes within two years. Once an animal has aborted it is considered immune to further disease, although it can still infect other animals. Although antibiotics can be used to reduce losses at the first signs of a problem they do not eradicate the disease. Thus, currently the most effective means of controlling infection is through the use of vaccination.

Key Points

Ovine chlamydiosis is a common disease of sheep (and goats) worldwide that results in abortion and stillbirth.
The organism responsible, Chlamydia abortus, is principally transmitted through ingestion or inhalation of the contaminated products of birth and contaminated bedding/pasture.
Infection is difficult to detect in infected animals prior to abortion occurring.
Antibiotics can be used to limit lamb loss but this may not eliminate the shedding of infectious organisms at birth and thus possible infection of other animals, leading to further losses in subsequent lambing seasons.
Disease can be controlled through the use of the commercially available ‘live’ vaccines, but this does not guarantee that some ewes will not still abort or other animals become infected.
Once an animal has aborted as a result of EAE it will not abort again as a result of this pathogen.
Care should be taken when sourcing replacement animals to avoid introducing the disease. Animals should be sourced from accredited or closed-flocks, or they should be vaccinated upon entry into a ‘clean’ flock.

Research Undertaken

Research at Moredun has focussed on using cutting edge molecular and immunological technologies to increase our understanding of the pathogen, specifically identifying and characterising the components of the organism that interact with the host animal. These coupled with studies to further our understanding of disease pathogenesis and studies on the host immune response have allowed the identification of targets for the development of novel diagnostic tests and new vaccines.

Diagnosis of infection is usually confirmed in regional veterinary laboratories by the rise in antibody titre that occurs at the time of abortion or lambing using the complement fixation text or CFT. This test lacks sensitivity in that it does not identify all positively infected animals. It also lacks specificity because the test is based on a protein that is present in all chlamydial species, and sheep are commonly infected with another chlamydial species known as Chlamydia pecorum, thus the test detects antibodies to this organism as well. Moredun has identified a group of proteins present on the surface of Chlamydia abortus, specifically a fragment of one of these proteins, which has much greater sensitivity and specificity than the CFT or other diagnostic tests on the market. This diagnostic test, which has also been shown to be effective in detecting infection prior to abortion or lambing, is being developed further for future commercialisation.

Although there are currently two commercially-available vaccines, based on a ‘live’ strain of the pathogen, EAE still remains a problem in the UK. This could be due to lack of uptake but also because while we know vaccination reduces the incidence of abortion, it does not completely stop it. It had been assumed that this was because animals were vaccinated on top of an existing infection and thus not effective in stopping the existing infection from causing abortion. However, studies at Moredun using novel molecular techniques that differentiate infected animals from those that have been vaccinated (so-called ‘DIVA’ test) have identified the presence of the vaccine strain only in the products of abortion of some vaccinated animals, implicating the vaccines as a probable cause of lamb death. Thus, for this reason, as well as other safety implications for human health, continuing issues surrounding the production of the vaccine by one of the manufacturers, there is a need for the development of next generation safer vaccines. Until that time though the recommendation is to continue using the vaccines, as they are currently the most effective means of controlling infection.

Policy Implications

Chlamydia abortus is the most important infectious cause of abortion in sheep in the UK, and one of the most common causes worldwide. It is also a significant safety concern for immuno-compromised individuals and pregnant women. Reproductive failure is a major issue for livestock producers, both in terms of economic losses and animal health and welfare, reducing the efficiency and sustainability of sheep production. Current strategies to prevent and control disease aim to improve early identification of infection and prevention of infection through the development of improved diagnostic tools and vaccines. This in turn will result in the increased efficiency of livestock production, improve animal health and welfare, reduce the potential for zoonotic transmission to humans and reduce the impact on the environment.

Author

Dr David Longbottom david.longbottom@moredun.ac.uk

Topics

Sustainability and Communities , Food, health and wellbeing