viral infection in horses - horses running

Respiratory Viral Infections in Horses

Last Updated on


Gayle Leith MS, DVM, MA

Diplomate of the American Board of Veterinary Practitioners

Certified in Equine Practice

Emily Thometz DVM

Respiratory viral infections in horses can lead to poor performance and may require time off for recovery. Nasal congestion and lethargy are common among youngsters when they are weaned or begin training but can be found in any age. A wide variety of pathogens can cause these symptoms and testing may be helpful to determine the cause, treatment, and prevention of future outbreaks.

Common causes of viral respiratory tract infections in horses include: equine influenza (EIV), equine herpes (EHV-1 and -4), and Equine Rhinitis (ERAV).  These viral infections are easily spread through fomite (buckets, manure forks, stall walls, and nose-to-nose contact. Young horses are especially prone due to additional stress from weaning, training, or changing locations and/or pasture mates.

Equine influenza A type 2 (A/equine 2)

Equine influenza A type 2 (A/equine 2), is one of the most common infectious diseases of the respiratory tract of horses. It is endemic in the equine population of the United States. Equine influenza virus does not constantly circulate, but is sporadically introduced by an infected horse. The influenza virus infection can be avoided by the quarantine of newly arriving horses for at least 14 days, and by appropriate vaccination before exposure. All horses should be vaccinated against equine influenza unless they live in a closed and isolated facility.

EIV has two main ‘strains:’ clade 1 and clade 2. Currently, only clade 1 has been spread from horse-to-horse in the United States. Clade 2 has been found in Europe and Asia, as well as in a few imported horses in quarantine in the U.S. due to modern horse travel. It is not a question of whether it will spread throughout the U.S. but rather a question of when it will come; therefore, it is important to be protected against both.


Historically, equine influenza vaccines have been administered at intervals as short as every 3 months to horses considered at high risk of infection. All currently marketed equine influenza vaccines are likely to provide protection of at least six months duration. This is true for both of the modified live vaccines on the market today, and for inactivated vaccines.  There are three types of equine influenza virus vaccine currently marketed:

Inactivated vaccines

Each of these has been shown to be efficacious in providing protection against clinical disease and viral shedding when used appropriately. These vaccines frequently include multiple strains of equine influenza virus A2 representing the major circulating strains.

Modified-live (MLV) cold-adapted equine influenza /A2 vaccine

This product is administered intranasally. The vaccine has proven to be very safe and a single administration to naïve horses is protective for 12 months, although only a 6-month claim is made on the product data sheet.

Canary pox vector vaccine

This product is to be administered by intra-muscular injection and has been shown to provide protection of at least six months duration. A two-dose priming regimen is recommended, with boosters at a six-month interval. The vaccine is safe to use in foals as young as four months of age, and there is some evidence of efficacy in the face of maternal immunity. Because this vaccine generates high levels of antibody response, it is likely to be suitable for pre-foaling boosters.

Equine Herpesvirus (EHV-1 and 4)

Equine herpes has received a lot of publicity over the last couple years because of the neurologic condition that it can cause. However, it is important to remember that it may also lead to reproductive and respiratory diseases. Herpes can become latent in an animal and may recrudesce later in life.  Equine herpesvirus type 1 (EHV-1) and equine herpesvirus type 4 (EHV-4) can each infect the respiratory tract, causing disease that varies in severity from sub-clinical to severe and is characterized by fever, lethargy, anorexia, nasal discharge, and cough. Infection of the respiratory tract with EHV-1 and EHV-4 typically first occurs in foals in the first weeks or months of life, but recurrent or recrudescent clinically apparent infections are seen in weanlings, yearlings, and young horses entering training, especially when horses from different sources are commingled. Equine herpesvirus type 1 causes epidemic abortion in mares, the birth of weak nonviable foals, or a sporadic paralytic neurologic disease (equine herpesvirus myeloencephalopathy-EHM) secondary to vasculitis of the spinal cord and brain.

Risk factors for outbreaks amongst equine populations at a show (reference Ogden – Utah, EHM outbreak) include: number of classes competed in and vaccination status. Biosecurity practices can certainly be increased both while at home and on the road. On the farm, house horses should be separated into small groups by age, usage, and gestation.  Minimizing shared equipment and reducing movement between groups can also help. These preventative measures may be much harder to achieve in boarding facilities. Regular monitoring with physical evaluations of temperature, attitude, and appetite can help catch outbreaks earlier and stay ahead of them. Hand hygiene is always important, particularly in the barn! While on the road, make sure stalls are disinfected before use, which is fairly easy to do on your own.  Diluted disinfectants, such as bleach, applied to a striped stall can help reduce risk. An immunization program, especially a more intensive one for high risk horses (such as young stock and show horses), should be implemented to protect the entire population. Although the vaccine may not prevent the disease completely, it can decrease severity and shedding of the disease. Finally, it is important to be prepared if there is an outbreak by having a plan on how to reduce contamination between horses and isolate sick animals.

Vaccination for Equine Herpesviruses (Rhinopnuemonitis)

Inactivated vaccines

A variety of inactivated vaccines are available, including those licensed only for protection against respiratory disease. Performance of the inactivated low antigen load respiratory vaccines is variable, with some vaccines outperforming others. Performance of the inactivated high antigen load respiratory/abortion vaccines is superior, resulting in higher antibody responses and some evidence of cellular responses to vaccination. This factor may provide good reason to choose the high antigen load respiratory/abortion vaccines when the slightly higher cost is not a decision factor.

Modified live vaccine

A single manufacturer provides a licensed modified live EHV-1 vaccine.  It is indicated for the vaccination of healthy horses 3 months of age or older as an aid in preventing respiratory disease caused by equine herpesvirus type 1 (EHV-1).


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).

Vaccination against either EHV-1 or EHV-4 can provide partial protection against the heterologous strain; vaccines containing EHV-1 may be superior in this regard.

Equine Rhinitis

In horses, only two rhinitis viruses have been identified: equine rhinitis A virus (ERAV), formerly known as equine rhinovirus 1, and equine rhi­nitis B virus (ERBV), formerly known as equine rhinovirus 2.

Recent studies have shown that ERAV has remained stable, but ERBV has evolved into three different serotypes: ERBV1, ERBV2, and ERBV3. (Black, Hartley 2005).

However, transmission of equine rhinitis virus is believed to be similar to that of other equine respiratory viruses (equine influenza virus, equine herpesvi­rus). Equine respiratory viruses spread through groups of horses in aerosol­ized secretions dispersed by cough­ing. Both direct contact and indirect (fomite) contact with nasal secretions are also likely routes of infection.

Clinically, horses affected with the rhinitis virus demonstrate fever, swollen submandibular lymph nodes, nasal and eye discharge, cough, and occasionally lower leg swelling.  This viral infection is diagnosed by virus isolation, RT-PCR testing and serology.


Historically, no commercial vaccines were available to vaccinate for ERVs. More recently, a conditional license has been issued for Equine Rhinitis A Vaccine, killed virus.  Basic quarantine procedures along with client education, which may help minimize the development and spread of viral respiratory infections and the sequelae.

Overall, these viruses are fairly common and usually present themselves with high morbidity and low mortality. Preventative measures, such as vaccines and biosecurity plans, should be considered for all horse populations.



Black WD, Hartley CA, Ficorilli NP, et al. Sequence variation divides equine rhinitis B virus into three dis­tinct phylogenetic groups that correlate with serotype and acid stability. J Gen Virol 2005; 86(Pt 8):2323-2332.

Pusterla, N. (2016). “Relevance and Impact of Common and Lesser Characterized Respiratory Viruses Associated with Upper Airway Disease”.


About the authors

Dr. Gayle Leith is a 1988 Wisconsin graduate, has a M.S. in Reproductive Physiology and is interested in medicine, lameness, and theriogenology. She has been with Arizona Equine since 1988. Arizona Equine Medical & Surgical Centre offers a range of equine diagnostic and treatment services at this facility in Gilbert, Arizona. .As the largest center of its kind in the desert southwest region we offer ultrasonography, nuclear scintigraphy, MRI, endoscopy and computed and digital radiology. The surgical facilities are equally extensive and include arthroscopy, colic surgery, laser surgery, laparoscopy and many types of soft tissue surgery. Dr. Gayle Leith is a boarded member of the American Board of Veterinary Practitioners – Certified in Equine Practice. Dr. Leith first became boarded in 1995, she successfully recertified in 2005 and 2015. She is certified until 2025. The American Board of Veterinary Practitioners members must re-certify every 10 years to maintain their diplomate status. ABVP certifies veterinary practitioners with exceptional knowledge, skill, and competency in the care of the total patient.  The ABVP board certified veterinarian has demonstrated they are capable of providing a high level of clinical practice.  Dr. Leith’s hobbies include spending time with her family, showing horses she has raised and trained in the amateur hunter divisions, oil painting, traveling, and target shooting.

Dr. Emily Thometz was born and raised in Minnesota, where she became actively involved with 4-H, FFA, and showing horses. Her love of the West drew her to Colorado State University, where she earned Bachelor’s Degrees in Animal Science and Agricultural Business in 2012. In the spring of 2011 she completed a six month internship in Kentucky at Hagyard Equine Medical Center, where she focused on neonatal intensive care. She moved back to Minnesota to attend the University of Minnesota College of Veterinary Medicine, earning her DVM in 2016. She has an off the track thoroughbred, and in her free time she loves riding, running and skiing. Her veterinary interests include sports medicine, surgery and reproduction.


Images used under creative commons license – commercial use (10/5/2016) hlseffigy (Flickr)

No Comments

Sorry, the comment form is closed at this time.