Rhinopneumonitis (Equine Herpes Virus)

The start of spring show season also brings with it the start of Flu/Rhino season.  In recent years, we have seen a huge increase in the prevalence of wild type (neurologic) EHV-1 cases across the country.  

Equine Herpes Virus types 1 and 4 are what are commonly vaccinated against in horses as “rhino or rhinopneumonitis.”   Foals are typically infected by the virus in the first few months of life, and develop a variable immunity to the virus, assisted by the antibodies they receive from their dam’s colostrum, and by vaccinations.  This immunity keeps the virus from causing disease (normally a respiratory disease that varies in severity from sub-clinical to severe and is characterized by fever, lethargy, anorexia, nasal discharge, and cough), but does not prevent infection.  The virus becomes a part of the horse’s respiratory tract, and may be shed normally and in increasing levels during stress. 

This can make EHV difficult to prevent even in closed herds and farms as a stressful event such as severe weather changes, shipping, introducing new animals, or showing can increase shedding of the virus and cause disease.  Vaccination for Equine Herpes Virus at regular intervals will help booster the horse’s immune system and keep the virus in check during periods of stress. 

Equine Herpes Virus type 1 can also mutate and become “wild type EHV-1” or “neuroEHV-1” which can cause Equine Herpes Myelitis.  This is a disease that causes an inflammation of the nervous system and spinal cord secondary to inflammation of the blood vessels. This strain of the virus is what we are currently seeing outbreaks of across the country.  Unfortunately there are no vaccines specific for wild type EHV-1, as there a number of different mutations that may occur.  Regular vaccination for EHV can help decrease the amount of virus that is being shed by the body, and will booster the animals overall immune response to the herpes viruses.

The American Association of Equine Practitioners states:

“All available vaccines make no label claim to prevent the myeloencephalitic form of EHV-1 (EHM) infection. Vaccines may assist in limiting the spread of outbreaks of EHM by limiting nasal shedding EHV-1 and dissemination of infection. For this reason some experts hold the opinion that there may be an advantage to vaccinating in the face of an outbreak, but in advance of EHV-1 infection occurring in the group of horses to be vaccinated. The vaccines with the greatest ability to limit nasal shedding include the 2 high-antigen load, inactivated vaccines licensed for control of abortion (Pneumabort-K®: Pfizer; & Prodigy® Merck), a MLV vaccine (Rhinomune®, Boehringer Ingelheim Vetmedica) and an inactivated vaccine, (Calvenza®, Boehringer Ingelheim Vetmedica).”

We highly recommend vaccinating all animals who will be shipping, showing, or having regular contact with new animals.  This includes trail riding, fox hunting, and any stabling situation where contact with new horses may occur.

Vaccination and instituting basic bio-security procedures such as quarantining if possible and monitoring temperatures on any new arrivals, or post shipping/showing are the best possible methods of prevention and early identification of Equine Herpes Virus (neurologic or respiratory form).  Obviously avoiding contact with horses which have come from areas with known outbreaks decreases the risk of infection as well.

Any horses which are showing potential neurologic signs (dragging a toe, stumbling, difficulty getting up or down, apparent difficulty urinating, or dribbling urine) or which are found to have a temperature of 103 or greater, should be brought to the attention of your veterinarian.  Early identification of this potentially fatal and reportable disease is key in increasing the success of supportive care and treatment.