Etiology

• Equine herpesvirus-3   Equine herpesvirus  .
• Classification of equine herpesvirus types:
  • EHV-1 - abortion, respiratory tract disease   Respiratory: EHV infection  and neurological disease   CNS: myeloencephalopathy - EHV  .
  • EHV-2 - upper respiratory disease   Respiratory: EHV infection  , pharyngeal lymphoid hyperplasia   Pharynx: lymphoid hyperplasia  , pharyngitis and pneumonia in foals   Lung: pneumonia - interstitial (foals)  .
  • EHV-3 - equine coital exanthema.
  • EHV-4 (formally EHV-1 subtype 2) - respiratory tract disease   Respiratory: EHV infection  .
  • EHV-5 - equine multinodular pulmonary fibrosis (EMPF)   Lung: multinodular fibrosis  .

Predisposing factors

• Mare or stallion involved in breeding.
• Venereal contact at coitus.

Specific

• Other transmitters include insects, veterinary surgeons/grooms, instruments, ultrasound probe.
• Damage to the vulva from foaling injuries or surgery may encourage infection.

Pathophysiology

• Equine herpesvirus-3 infection   →   venereal transmission   →   lesions on external genitalia.
• Viral replication in stratified epithelium of epidermal tissue within skin or at mucocutaneous margins.
• Highly contagious.
• Non-venereally transmitted form recognized in maiden fillies and colts.
• Hypothesized that latent infection is established in sciatic and/or sacral ganglion.

• Venereal infection with EHV-3   →   stallions may rarely show systemic signs, eg dullness, anorexia, pyrexia.
• Lesions (papules or nodules) up to 2 mm diameter develop first on penis, then prepuce 2-5 days later   →   vesicles up to 1.5 cm diameter   →   circumscribed pustules with raised border   →   slough and ulcerate (3-10 mm in diameter)   →   heal over 10-14 days by granulation leaving depigmented spots. Penis and prepuce may be painful and edematous.
• Some affected stallions refuse to breed.
• Recurrent cases can occur; recrudescence can also be seen after foaling.
• Secondary bacterial infection delays healing and increases inflammation/pain.

Mares

• Rarely show systemic signs.
• Vulvar mucosal and perineal skin lesions   →   multiple discrete circular nodules (2 mm diameter) progressing to vesicles/pustules which rupture to scabrous erosions   →   hyperemic erosions with sharply defined margins   →   heal over 10-14 days unless complicated by secondary bacterial infection.
• Secondary bacterial infection withStreptococcus zooepidemicus  Streptococcus spp  is common.
• Signs include vulval discharge, tail switching, increased frequency of urination and arching of back.
• Lesions around the anus can be associated with anorectal lymphadenopathy, constipation and tenesmus.
• Delayed foaling or reduced pregnancy rates may occur due to missed breeding opportunities.
• Rarely may cause reduced conception rates for that particular mating.
• Some mares show recurrence of lesions, usually around the time of parturition.

Maiden fillies and colts

• Non-venereal spread from infectious in-contacts.
• Pyrexia and painful coalescing skin lesions over the anus and vulva in fillies, and perineum, scrotum and inside hindlimbs in colts.

Immunity

• Reinfection without clinical signs is common.
• Virus may remain latent in genitalia.
• Use of corticosteroids has caused recrudescence experimentally.

Timecourse

• Incubation period 5-10 days.
• Unless complicated the lesions granulate and heal within 2-3 weeks leaving depigmented circular lesions.
• Covering can begin during the next estrus.

Epidemiology

• EHV-3 usually passed from subclinically infected mare   →   stallion at breeding   →   transmitted to susceptible mares prior to developing clinical signs.