Diagnostic Laboratory Services- EHV Clearance for Equine Abortion
EQUINE ABORTION AND NEONATAL DEATH INVESTIGATION, WITH CLEARANCE PROCEDURES FOR EHV INFECTION
Introduction
We recommend that clearance for animal movement on breeding farms following an abortion should be based on a three-stage testing procedure to avoid the risk of false positive or a false negative results.
- gross postmortem examination
- histological examination of fixed foetal and placental tissue sections
- specific Equine Herpesvirus (EHV) DNA polymerase chain reaction (PCR) test.
If there remains any doubt, immunoperoxidase (IP) staining for EHV antigen may be required (approximately 5% of cases). Clearance tests can be carried out on selected tissues sent to AHT or BCL following gross post mortem examination elsewhere (Option 1) or as part of a complete post mortem investigation carried out at AHT or BCL (Option 2).
Between 2002 and 2006 AHT have diagnosed a small number of atypical EHV-1 abortions where infection was confined to the placenta, without detectable transmission to the fetus. These abortions usually presented clinically as premature placental separations, and generally occurred as single incidents. Although the significance of these apparently uncommon cases of placental infection remains to be fully established, in one case an atypical abortion of this type preceded a large abortion epizootic in unvaccinated animals, so the potential for viral transmission to in-contact pregnant mares cannot be ruled out. We have performed EHV PCR analysis of placentae as a routine part of our abortion investigations since 2002. We have now accumulated sufficient data on the continuing importance of this type of abortion to justify routine inclusion of placental PCR within our abortion clearance procedures.
Option 1: Postal Samples
Following a preliminary gross post mortem examination performed by the attending veterinary surgeon or at a pathology laboratory experienced in equine abortion investigations, other than AHT or BCL, appropriate tissues for herpesvirus diagnosis or clearance are: fetal liver, lung, spleen, adrenal gland and thymus. Pooled fresh tissue preferably in transport medium (or otherwise in a labelled sterile container) should be submitted for PCR and tissues fixed in 10% neutral buffered formalin (NBF) should be submitted for histopathological examinations. A second paired set of samples labelled from the allantochorion (cervical star, body and both horns) should also be submitted in transport medium for PCR and in NBF for histopathological examinations. These samples from the placenta are valuable for diagnosis of placentitis as well as atypical EHV abortion. The cost for the combined PCR and histological testing is £70.00, and the usual turnaround time for both tests is up to 48 hours for samples received by 9am Monday to Thursday, and up to 96 hours for samples received between Friday and Sunday. Where the two tests give discrepant results (or only fixed tissue is received) the IP stain will be performed within the total £70 charge. Results for IP stain take up to one week.
Option 2: Post Mortem Examination Performed at AHT or BCL
The post mortem examination involves consideration of equine Herpesvirus status, as well as other potential causes of abortion or neonatal death. The placenta should always be submitted with the fetus, for optimal diagnostic assessment. The cost of the post mortem examination is £260.00, including laboratory and disposal charges. Turnaround time would be as in option 1 above.
Note: The advantage of Option 2 is the involvement of a pathologist experienced in equine abortion and neonatal death investigations in all phases. A full post mortem examination should, in most cases, provide a final diagnosis when sufficient foetal and placental tissues are available for examination, even when Herpesvirus infection is not involved. Other diagnoses may not be determinable if only selected tissues, i.e. those specifically required for herpesvirus diagnosis, are submitted.
All EHV-1 diagnoses are copied to the AHT epidemiology team, who are able to offer advice on further testing, isolation and movement of horses, as are veterinary surgeons at BCL. Further information can be found in the Horserace Betting Levy Board Code of Practice on Equine herpesvirus-1, which is available on request to Diagnostic Laboratory Services.
EQUINE ABORTION AND NEONATAL FOAL DEATH
Prompt diagnosis of any abortion or neonatal foal death is important in safeguarding individual mare health and fertility and, in the case of infectious abortions (such as EHV-1 or ‘virus abortion’), in reducing risk of further abortions or foal losses in in-contact mares. Because the major source of infection in cases of virus abortion is the aborted fetus, and to a lesser extent the fetal membranes and fluids, the aborting mare must be isolated from any other mares in late pregnancy until a diagnosis has been made. Meanwhile, movement of horses on and off the premises should be stopped.
Diagnosis of the cause of abortion is made by examination of the fetus and membranes, which should be sent to a veterinary pathology laboratory experienced in equine abortion and neonatal death investigations, carefully contained in a leak-proof bag, with the help and advice of the attending veterinary surgeon. The mare should be examined by a veterinary surgeon to check for any complications related to the abortion, such as retained placenta.
If it is not possible to send the whole fetus and membranes to the laboratory, samples of the following tissues should be collected to achieve rapid herpesvirus abortion clearance: pooled liver, lung, spleen, adrenal gland, thymus and placenta (in a separate tube).
Using microscopic examination of tissues collected from the fetus and placenta, in combination with the PCR test, which is sensitive enough to detect even minute quantities of viral DNA, it is usually possible to achieve a diagnosis of herpesvirus abortion within 48 hours.
If herpesvirus infection is confirmed, isolation, movement restrictions and strict hygiene procedures should be followed. These procedures are detailed in the Horserace Betting Levy Board (HBLB) Code of Practice for Equine herpesvirus (EHV). Copies of the code may be requested from the Board (telephone 020 7333 0043), and are available on the Board’s website: www.hblb.org.uk
The Animal Health Trust and Beaufort Cottage Laboratories each operate a 7 day-a-week abortion screening service using the systems outlined above, and can be contacted by the stud veterinary surgeon in the event of an abortion.
