Control of Infection

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No horse known or suspected to have disease caused by EHV should be sent to a stallion stud or to premises where there are brood mares, particularly pregnant mares.

Where abortion, stillbirth, foal death or illness in a foal within 14 days of birth may be EHV related, the following actions should be taken: 

  1. Seek veterinary advice immediately;
  2. For abortions, stillborn foals and newborn foal deaths:
    •  Where it was found, immediately place the aborted fetus and its placental membranes or the dead newborn foal in double wrapped strong leak-proof bags and/or containers, taking care to avoid further contamination of the stud farm environment and/or personnel during transportation;
    • Place the mare in strict isolation;
    • I mmediately cordon the area where the aborted fetus and its placental membranes were found to prevent other pregnant mares (including those that the aborted mare has been in contact with prior to abortion) accessing the area and once the material has been safely removed apply liberal amounts of virucidal disinfectant to the area;
    • In conjunction with the attending veterinary surgeon, arrange for appropriate samples (preferably the entire aborted fetus with its placental membranes or the dead newborn foal, carefully doublewrapped in strong leak-proof plastic bags and containers) to be sent to a suitable laboratory for specific examination for EHV. These materials must be handled under strict hygienic conditions;
    • Ensure that the attendant dealing with the aborted material and area has no contact with other horses, especially pregnant mares.
  3. For sick live foals:
    •   Place the mare and foal in strict isolation;
    • In conjunction with the attending veterinary surgeon, arrange for samples (usually nasopharyngeal swabs and heparinised or EDTA blood) to be sent in leak-proof containers to a laboratory for specific examination for EHV;
    • Ensure that the attendant has no contact with other horses, especially pregnant mares.
  4.    Stop horse movements off the premises and do not allow any pregnant mare onto the premises until EHV is excluded as the cause of the abortion, stillbirth, foal death or foal illness;
  5.    Disinfect and destroy contaminated bedding; clean and disinfect the premises, equipment and vehicles used for horse transport under the direction of the attending veterinary surgeon;
  6.    If preliminary laboratory results indicate EHV, divide pregnant mares with which the infected mare had contact into smaller groups of similar foaling dates to minimise the spread of any infection and turn them out into isolated paddocks on the same studfarm as the abortion occurred. If the infected mare was already in a small group of pregnant mares, divide the group into even smaller groups, as some may still abort and this may minimise further spread of infection. Any non-pregnant mares with which the infected mare had contact should be maintained as a ‘closed’ group until EHV infection is ruled out. 

If EHV is confirmed:

  1.   Maintain isolation, movement restrictions and hygiene measures for at least 28 days from the date of the last EHV abortion, stillbirth or newborn foal death.
  2.   Barren mares, maiden mares and mares with healthy foals at foot, can be admitted onto the premises (providing there is no sign of infection at their home premises) but must be kept separate from pregnant mares.
  3.   Barren mares, maiden mares and mares with healthy foals at foot on the affected premises can be moved 28 days after the last EHV abortion, providing they can be placed in quarantine for 14 days following arrival at their new premises. Serological monitoring at a 10-14 day interval to look for signs of seroconversion during this period is advised.

It may be possible, under the direction of the attending veterinary surgeon and in consultation with stud owners/managers of where they may move, to move non-pregnant mares earlier than 28 days e.g. for mating if:

  • The geography and management of the studfarm (separate staff and utilities, e.g. tractors, feed deliveries and muck disposals) allows for strict isolation of the aborted mare(s). This should include separate access roads, stables and paddocks, with adequate separation between the isolated area and the other mares (see Appendix 6);
  •  The non-pregnant mares for movement, including for mating as a walking-in mare, have been isolated from pregnant mares and handled by separate staff (see Appendix 6)at least from the time of the abortion, stillbirth or newborn foal death;
  • testing of blood samples taken immediately and again 14 days later (in the same laboratory as a paired serological assay) indicates that they have not been infected;
  • there is no other clinical or laboratory evidence of spread or infection;
  • the owner/manager of the premises (or stallion unit) to which the mare(s) is(are) to be moved understands full details of the EHV infection and, following his/her own veterinary advice, agrees to the move or to allow the mare to walk in. 

        4. Pregnant mares due to foal in the current season must stay on the premises until they foal a healthy foal;

        5. Mares that have aborted must be isolated from other horses for 28 days after abortion and from pregnant mares due to foal that season and mares in early pregnancy for the remainder of that season;

        6. Present evidence indicates a low risk of spread of infection if mares are mated on the second (30 days) heat cycle after their EHV abortion. Following veterinary advice further testing may be requested by the stallion owner before mating is allowed.

         7.Mares that return home pregnant from premises where abortion occurred the previous season should foal in isolation at home. If this is not possible, the stud to which the mare is to be sent in the current season must be informed so that they can seek veterinary advice and take appropriate managerial and biosecurity precautions. 

Walking-in mares

If the stallion unit is separated geographically from the pregnant mares, and is attended by separate staff, walking-in for covering by the stallions can continue unhindered (except for pregnant mares who have aborted or are in contact with an abortion, for at least 28 days following the last abortion. Following mating, the mare(s) involved should be kept isolated from any pregnant mares who are still due to foal that season.

If neurological EHV is suspected in any horse:

  1. Seek veterinary advice immediately;
  2. Stop all breeding activities unless (where the affected horse(s) is(are) not at the stallion unit) the stallion unit is separated geographically from the pregnant mares, and is attended by separate staff; 
  3. Stop all movement on and off the premises until neurological EHV has been ruled out or, if it is confirmed, for at least 28 days after resolution of the last case;
  4. Keep the affected horse in isolation with strict barrier nursing and biosecurity;
  5. Arrange for separate staff to attend to the affected horse(s), using appropriate protective clothing and biosecurity protocols to reduce the risk of spread of infection;
  6. In conjunction with the attending veterinary surgeon, arrange for appropriate samples, including the carcasses of dead animals (see ‘Diagnosis’ on page 35) or appropriate samples in leak-proof containers to be sent to a laboratory for examination;
  7. Divide horses into small groups in order to minimise exposure in the event that there is EHV infection active among the affected group of animals, keeping pregnant mares separate from all others;
  8. Do not allow any pregnant mare onto the premises until EHV has been excluded as the cause of the neurological disease;
  9. Disinfect and destroy bedding; clean and disinfect premises, equipment and vehicles used for horse transport, under the direction of the attending veterinary surgeon), using appropriate protective clothing and biosecurity protocols. 

If nerological EHV is confirmed, a policy should be decided with the attending veterinary surgeon. This should include screening and clearance of each group before individuals in the group return home. Individuals should then be isolated at home, especially pregnant mares until after foaling. Detailed advice on specific cases can be obtained from equine infectious dosease experts, or specialist equine veterinary practices. An outline control protocol for neurological EHV is provided below: 

  •  Implement high standard biosecurity and biocontainment procedures as advised by the attending veterinary surgeon;
  • Wherever possible attending veterinary surgeons should liaise closely with equine infectious disease experts and/or specialist equine veterinary practices to discuss the implementation of the control protocol and in particular on decisions when it is appropriate to resume normal operations;
  • In the early stages of many neurological EHV outbreaks it is necessary for an entire premises to be quarantined and tested in order to establish the likely extent of the infection, that may be entirely subclinical (no obvious clinical signs) in some horses. These animals may act as an important source of new infection in susceptible horses;
  • The most effective sampling strategy for neurological EHV involves:   
    • Two clotted blood samples taken at a 10-14 day interval from onset of clinical signs for serological testing (antibody levels in the blood).
    • Blood sample taken in heparin or EDTA anticoagulant tubes for PCR testing or virus isolation (during viraemia, when the virus is circulating in the bloodstream),
    • Nasopharyngeal swabs for PCR testing (when the virus is being shed from tissues in the nose and throat);
  •  It is recommended that a second clotted blood sample is taken to detect fourfold or greater rises in antibody levels (seroconversion) that would indicate infection occurring at about the time of the first sample (a technique called ‘paired serology’);
  • Initial laboratory testing may quickly establish that the infection is geographically restricted to isolated parts of the premises. In these situations it may be possible, following review of laboratory data and with the approval of the attending veterinary surgeon and the testing laboratory, to resume normal operations in the non-affected parts of the premises, usually though with heightened disease awareness and biosecurity measures in place.
  • Approval to resume normal operations on the entire premises is made by the attending veterinary surgeon and the testing laboratory in the light of accruing clinical and laboratory information. 

In all the situations above, communication of and about the EHV infection is extremely important. Failure to communicate can contribute to spread of infection to the detriment of all owners and their horses, particularly mare owners. The owner/manager of the affected horse(s) or premises should inform: 

  • The national breeders' association;
  • Owners (or those authorised to act on their behalf) of:
    • mares at the premises;
    • mares due to be sent to the premises;
  • Others:
    • Those responsible for the management of premises to which any horses from the stud are to be sent;
    • Those responsible for the management of premises to which any horses have been sent in the previous 28 days, with the condition that owners of those horses (or those authorised to act on their behalf) must be informed immediately;
    • Those responsible for the management of premises to which any pregnant mares (that have been in-contact after the first three months of pregnancy) have been sent, with the condition that owners of those mares (or those authorised to act on their behalf) must be informed immediately.