The control and treatment of venereal disease

Contagious Equine Metritis (CEM)

CEM caused by Tayorella equigenitalis is a notifiable disease in the UK. Any occurrence must be reported to the Divisional Veterinary Manager of DEFRA.

Clinical signs

Most stallions carrying the disease show no outward signs. Mares may have a grey, mucoid vulval discharge or they can also be symptomless carriers of the disease.


The disease is transmitted:

  • during mating, including artificial insemination
  • during teasing,
  • by poor hygiene standards when handling mare

Diagnosis is confirmed by culture or PCR when a swab is taken from:

The Mare

  • the clitoris – the fossa and sinuses
  • a specific endometrial swab during oestrus (NB some stallions studs request this as routine)

The Stallion

  • from the urethra, urethral fossa, penile sheath and pre-ejaculatory fluid if possible.

The swabs must be sent to an approved laboratory.


Stallions are treated by thorough cleaning of the penis with an approved antiseptic solution and application of an antibiotic ointment for 5-7 days following removal of all accumulated smegma.

Stallions require three sets of negative swabs taken at intervals of at least seven days before they can be confirmed free of infection. In addition, the first three mares mated or inseminated by the stallion should have clitoral swabs taken three times at intervals of at least seven days, starting two days after mating or insemination. These must be negative.

Mares are treated with intrauterine infusions of antibiotics in addition to thorough cleaning of the clitoral region and topical antibiotic treatment. With stubborn infections that are difficult to clear, surgical removal of the clitoris may be performed.

Freedom from infection in mares is confirmed by three negative clitoral swabs taken at intervals of at least seven days and three negative endometrial swabs taken during successive oestrous periods.


If a case is confirmed, there is a Code of Practice published by the Horserace Betting Levy Board (HBLB) that must be strictly adhered to. This includes:

  • Stopping all breeding activity immediately
  • Isolation and swabbing of infected horses
  • Swabbing of at risk contacts
  • Notification of the relevant breeders association
  • Notification of owners of mares who have left the premises, are booked to the stallion or have been inseminated with semen
  • Testing of stored semen
  • Foaling of pregnant mares that have been exposed to the infection in isolation. The placenta must be burned and both filly and colt foals swabbed

Breeding should not be resumed until the premises are confirmed as free from the disease.


The disease can be prevented by strict adherence to the recommendations for swabbing in the HBLB Code of Practice. The swabs should be taken from mares and stallions after 1st January of the year in which breeding activity is planned. Additional swabs are taken from stallions in the middle of the breeding season.

Equine Viral Arteritis (EVA)

EVA is also a notifiable disease in the UK. The highly contagious virus is common worldwide including mainland Europe.

Clinical signs

Clinical signs:

  • Fever, depression, lethargy
  • Stiff movement
  • Nasal discharge
  • Conjunctivitis (‘pink eye’)
  • Swelling around the eye, lower legs and reproductive tract
  • Abortions
  • Aerosol spread – direct contact with droplets from the respiratory tract e.g. coughing or snorting
  • Infected semen – during natural covering and Artificial Insemination
  • Contact with aborted fetus and placenta of infected mares
  • Serology – blood samples to look for antibodies to the virus
  • Virus detection tests – nasopharyngeal swab, urine, semen and aborted material. In the case of abortion, the foetus and placenta are sent to the lab

There is no effective treatment for EVA.


Establish freedom of infection before breeding – routine blood test taken at pre-breeding exam.

Equine Infection Anaemia (EIA)

EIA is sometimes referred to as ‘swamp fever’ and is notifiable. It is common worldwide, including Europe. There were two confirmed cases in January 2010 in Wiltshire UK after importation from Romania and Belgium.

  • Insect vectors such as biting flies (horse flies and stable flies)
  • Bodily fluids – saliva, nasal secretions, faeces, semen, blood and milk
  • Contamination of equipment with blood or other bodily secretions
  • From mare to foal via the placenta, or colostrum or milk in new-born foals
Clinical signs
  • Recurring fever
  • Anaemia
  • Oedema
  • Emaciation
  • Death

Infected horses are euthanised on welfare grounds and also to protect the rest of the horse population from infection under government notifiable disease regulations.


No vaccine is available therefore control is based on establishment of freedom of infection. Serology is performed on blood samples taken at pre-breeding exams.

Equine Herpesvirus (EHV)

EHV is a common virus that affects horse populations worldwide. Two forms of the disease exist; EHV-1, which causes respiratory disease, abortions and paralysis; and EHV-4, which usually causes respiratory disease only but can cause abortions.

  • Aerosol droplets from the respiratory tract e.g. coughing and snorting
  • Contact with aborted foetus, foetal fluids and membranes
  • Foals born from infected mares are highly contagious and shed the virus in the environment
  • Indirect transmission from the environment as the virus can survive for several weeks
  • Horses commonly act as carriers and shed the infection without showing any clinical signs
Clinical signs
  • Develop 2 weeks to months after infection
  • Mild fever, coughing and nasal discharge
  • Foals that are infected in the uterus from the mare are usually born with weakness, jaundice, difficulty breathing and neurological signs. They usually die within 3 weeks
  • Abortions
  • Paralysis – incoordination of the hind limbs, urinary incontinence and in severe cases lying down and unable to stand (recumbancy)
  • Virus isolation is performed on nasopharyngeal swabs and blood samples
  • Aborted foetus – detailed samples are collected by the vet from the aborted foetus and placenta

No specific treatment for EHV. Rest, intensive care and nursing are necessary to minimize secondary bacterial infections.


Management practices that reduce viral spread:

  • New horses should be isolated for 3-4 weeks before mixing with resident horses
  • Pregnant mares should be kept together in small fixed groups according to their stage of gestation. No mixing of pregnant mares with weanlings, yearlings or any horses in training that are high risk carriers
  • Stressing mares in late pregnancy should be avoided, to minimize the activation of latent carriers
  • Isolate affected horses and seek veterinary advice

Vaccinations are available and widely used in the UK. Non-pregnant horses can be given a primary course of 2 injections 4 – 6 weeks apart with boosters every 6 months. Pregnant mares are vaccinated with an EHV vaccine at 5, 7 and 9th month of gestation.

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