About 17% of equine operations have had a horse develop the equine equivalent of the common cold—-infectious upper respiratory disease. Upper respiratory tract (URT) infections can be caused by viruses and bacteria.

Age (in this case, 1 to 5 years), stress, frequent contact with many horses, and lack of vaccination are all risk factors. Affected horses often have a history of contact with an unfamiliar horse—-usually a newcomer to the barn or a competitor at a horse show. However, direct contact is not required for transmission. For instance, equine herpesvirus (EHV) can spread through aerosolized secretions from clearing nostrils or coughing. Fomites (contaminated objects such as water buckets and feed troughs) and vectors (organisms such as people and insects) can also be responsible for transmission. Incubation, or time of infection before clinical signs appear, ranges from 36 hours for flu to three to seven days for EHV and three to 14 days for Streptococcus equi (strangles). 

Clinical Signs

Most of us associate unrelenting cough, copious nasal discharge, exercise intolerance, swollen lymph nodes, fever, edema (fluid swelling), and depression/anorexia with a URT infection. In reality, especially during the early stages of infection, many horses manifest a single clinical sign: fever. It may cause inappetence, anorexia, and dehydration if it gets high enough. But many horses experience a short-lived fever that goes unnoticed.

A “silent carrier” with no clinical signs of disease can spread the infection to the index case in the barn, or the first animal with apparent signs. For example, 80% of the horse population has EHV in latent form, shedding only during times of stress. This is how respiratory infections occur within closed herds. Carriers also can pass the infection to less-immune-competent individuals, such as young, old, and pregnant animals.  

Diagnostics

Testing can help you determine if a febrile horse has a URT infection; results will dictate what biosecurity measures to take in your barn.

The advent of quantitative polymerase chain reaction (PCR) testing, an advanced molecular diagnostic technique, has allowed efficient screening for most common respiratory diseases via a single nasal/nasopharyngeal swab or wash. The most common viral equine URT infections include influenza, equine rhinitis virus, EHV-1, -2, -4, and -5, equine arteritis virus, and adenovirus. Bacterial URT infections include strangles in adults and Rhodococcus in foals. Veterinarians can also use blood and nasal swab tests, such as virus isolation and serology, to screen for particular diseases.

Tests do have some limitations, depending on the stage of infection and the volume of the horse’s secretions. For example, veterinarians can usually only detect rhinitis virus during the infection’s acute phase (the first three to four days). 

Treatment

The wisdom of using rest and fluids in cases of the common cold also applies to horses. To encourage water consumption you might need to supplement the horse’s food or offer an additional water source with salt (1-2 tbsp/day of loose table salt or a salt lick) or electrolytes. A general rule of thumb is to rest a horse one week for every day of fever, for a minimum of two to three weeks after infection.  

Because respiratory tract epithelium (lining) takes approximately 21 days to regenerate, horses with viral infections might be susceptible to complications such as pneumonia for three weeks following initial infection. Insufficient rest following viral infections might also be a predisposing factor for developing persistent airway inflammation and could play a role in inflammatory airway disease development. 

To control fevers veterinarians treat horses with antipyretics. Horses with EHV infections might require antivirals, and veterinarians might recommend antibiotics for secondary bacterial infections or severe cases of strangles.

Prevention

Through vaccination, quarantining incoming horses for 21 days, separating horses by age group and pregnancy status, disinfecting equipment, and practicing appropriate biosecurity protocols, you might prevent infection spread throughout the barn.

There is also evidence that immunomodulation can help prevent latent carriers from shedding infection during times of stress. Administering immunostimulants prior to travel to a horse show or other stressful event might help prevent and/or ameliorate respiratory disease.

If you discover a horse has a fever and possible respiratory signs, isolate him and call your veterinarian. With swift treatment and careful biosecurity techniques, you have a good chance of preventing the infection from affecting the entire barn.