Equine Herpesvirus - 1    

Significance in Canada
Clinical Signs
Prevention and Control


Link to image source: http://www.tulane.edu/~dmsander/Big_Virology/Special/EHV1/EHVpage.html

Link to image source: http://www.tulane.edu/~dmsander/Big_Virology/Special/EHV1/EHVpage.html


o       EHV-1 is ubiquitous worldwide and the horse population is the reservoir of the virus

o       Clinical signs for the virus vary and abortions may result


o       Transmission of the disease between individual horses or herds of horses is affected by age, season, and geographical conditions.  Furthermore, immune status of individuals and populations may affect transmission of EHV-1

o       Virus spread is either by direct or indirect contact with infectious nasal secretions, aborted fetuses and placenta, or by placental fluids

o       Can have latent infections and carrier states for EHV-1

o       The incubation period (time from exposure to virus and development of clinical signs) is approximately 2 to 10 days

o       Mares may abort several weeks to months after a clinical or subclinical infection with EHV-1

o       The neurological disease associated with EHV-1 occurs more commonly in mares after abortion storms, but also occurs in barren mares, stallions, geldings, and foals after and outbreaks of EHV-1 respiratory infections.

Link to image source: http://www.ca.uky.edu/gluck/BiblioEHV1.asp


 o      Horses will show signs of malaise (depression), pharyngitis, cough, have a serous nasal discharge, and show inappetance

o       Submandibular or retropharyngeal lymphadenopathy may be visible

o       Fever of 38.9 - 41.7 C (102-107 F)

o       Often the fever is diphasic (has two peaks) and a cell-associated viremia coincides with the second temperature peak, meaning the virus has entered the bloodstream (circulation)

o       Neutropenia (abnormally low neutrophils in the blood) and lymphopenia (abnormally low lymphocytes in the blood) may be observed with a CBC (complete blood count)

o       EHV-1 is also described as Equine Viral Rhinopneumonitis because secondary bacterial infections are common and frequently manifest with mucopurulent nasal discharge and pulmonary disease (sporadic cause of interstitial pneumonia)

o       Neurological Disease (EHV-1 encephalomyelopathy)

§         May be the primary disease, or can occur following rhinopneumonitis or abortion

§         Neurological deficits will have an acute (rapid) onset and usually do not progress after 24 hours

§         Signs vary from mild in-coordination and posterior paresis to sever posterior paralysis with recumbency, loss of bladder (urine dribbling) and tail function, and loss of sensation to the skin in the perineal and inguinal areas.  

§         Paralysis can advance to quadriplegia and death

§         Important to realize that neurological disease can occur in all ages of horses


o       EHV-1 strains have a preference for vascular endothelium, especially in the nasal mucosa, lungs, adrenal, thyroid and CNS tissues

o       Viral access to peripheral tissues occurs by cell-associated viremia, and this is evident as abortion or neurological disease occur

o       Lesions associated with EHV-1 abortion include: interlobular lung edema and pleural fluid; multifocal areas of hepatic necrosis, petechiation of the myocardium, adrenal gland, spleen, and thymic necrosis.  Intranuclear inclusion are found in the lung, liver, adrenal and lymphoreticular tissues

o       Mares that abort seldom show premonitory signs and abortions often occur 2-12 weeks after infection, usually between months 7 to 11 of gestation.  Mare exposure in late gestation can result in foals born with an explosive viral pneumonitis.

o       There does not seem to be any damage to the mare's reproductive tract and subsequent conception does not seem to be impaired

o       These foals are vulnerable to secondary bacterial infections and have a propensity to die within hours

o       Neurological Disease (EHV-1 encephalomyelopathy)

§         No gross lesions are associated with EHV-1 neurologic disease in a live horse.  However, mild hemorrhage in the meninges, brain and spinal cord parenchyma may be noted on necropsy

§         Histologically, there is discrete vasculitis with endothelial cell damage and perivascular cuffing, thrombus formation and hemorrhage.  Advanced cases may exhibit areas of malacia.  Neurologic lesions can occur anywhere in the brain and spinal cord.

Aborted fetus and placenta

Image Source: http://www.thehorse.com/pdf/ehv/bythenumbers.pdf


o       Equine viral rhinopneumonitis (EHV-1) cannot be differentiated from equine influenza, equine viral arteritis, or other equine respiratory infections based on clinical signs.

o       Definitive diagnosis is determined by isolating the virus from nasopharyngeal swab and citrated blood samples taken early in the infection, and by serologic testing of acute and convalescent sera

o       For EHV-1 suspicious abortions, diagnosis is based on characteristic gross antigen in fetal tissues.

o       Lung, liver, adrenal and lymphoreticular tissues are productive sources of virus and serologic testing of mares after the abortion has little diagnostic value

o       Neurological Disease (EHV-1 encephalomyelopathy)

§         Diagnosis based on observation of characteristic vascular lesions in histological sections of CNS tissue (brain and spinal cord) of dead or destroyed horses.

§         Cerebro-spinal fluid (CSF) is often xanthochromic (yellowish discolouration) with increased protein content and normal numbers of cells

§         Diagnosis is based on clinical findings and an increase in antibody concentration in paired serum samples


o       no specific treatment

o       anti-pyretics (compounds that reduce fevers) are recommended if fever is greater than 40 C (104 F)

o       rest and nurse to minimize the secondary bacterial complications (or treat with antibiotics if they occur)

o       prenatally infected foals with EHV-1 often succumb regardless of antibiotic treatment

o       Neurological Disease (EHV-1 encephalomyelopathy)

§         Infected horses showing neurological disease can recover and have a good prognosis if they are recumbent for only 2-3 days. 

§         However, varying degrees of supportive care may be needed to avoid pulmonary congestion, pneumonia, ruptured bladder, or bowel atony (lack of tone). 

§         Prognosis of the neurological form depends on severity of signs and length of recumbency.  A small percentage of cases will have neurologic sequela even after recovery


o       Immunity occurs after a natural infection of EHV-1 and involves aspects from both arms of the immune system (humoral and cellular immunity)

o       Little cross-protection occurs between virus types after a primary infection of immunologically naive foals

o       Significant cross-protection develops in horses after repeated infections with a particular virus type.

o       Most horses are latently infected with EHV-1 and the infection remains dormant until stress and immunosupression may result in recrudescence of disease and shedding of infectious virus

o       Immunity to reinfection of the respiratory tract may persist for up to 3 months, but multiple infections result in a level of immunity that prevents clinical signs of respiratory disease.

o       Management practices that reduce viral spread are recommended for EHV-1

o       Horses arriving on a premises should be isolated for 3 to 4 weeks before co-mingling with resident horses (especially pregnant mares)

o       Minimizing stress and stressful procedures will reduce the likelihood of viral shedding from horses with latent infections

o       House and pasture mares away from weanlings and yearlings if possible

o       Isolate horses with they are ill, especially if an outbreak of respiratory disease or abortion occurs, then disinfect contaminated areas if possible

o       Animals should not leave the premises for 3 weeks after recovery of the last clinical case

o       Parenterally administered modified live vaccines (MLV) are licensed in some countries and inactivated vaccines are the only vaccine produced that is currently recommended by the manufacturer as an aid  in preventing EHV-1 abortions

o       Administer vaccines during months 3,5,7 and 9 of pregnancy.

o       Humoral immunity induced by vaccination against EHV-1 persists for only 2 to 4 months

o       Virus vaccines do not cover all strains that horses may be exposed to (may not be cross-protective for all subtypes of the vaccine)

o       Foal vaccination should begin at 3 to 4 months old and a second dose given 4 to 8 weeks later.  Booster vaccinations may be indicated as often as every 3-6 months through maturity

o       All horses on the premises should be included in the EHV-1 vaccination program

o       Vaccination does not protect from the neurological form of this disease

o       For exotic equidae:

§         only killed virus vaccines are recommended because of uncertainty of whether MLV are adequately attenuated

§         single vaccination given to foals at 3-4 months of age and at 4 month intervals up to 1 year

§         mares should be immunized every 4 months to maintain sufficient protection against abortion because even after recovery from natural infection protective immunity only lasts for approximately 4 months