Dr. David Marlin and Dr. Kristie Pickles teamed up to talk about the horse with the runny nose. What lower and upper respiratory tract conditions can cause nasal discharge?
N.B. This article contains descriptive anatomical images of respiratory conditions.
Dr. David Marlin & Dr. Kristie Pickles
Horses can have nasal discharge and mucus for many reasons. The first challenge is to figure out where the nasal discharge is coming from; this helps to narrow down the field of causes. Broadly speaking, this can be divided into the upper and lower airways; the upper airways include the air conduction passages in the head and throat, while the lower airways consist of the trachea and lungs. Table 1 provides guidelines for determining whether the nasal discharge is from the upper or lower respiratory tract. Your veterinarian will be able to confirm the origin of the nasal discharge by performing an endoscopy.
| Upper airway | Lower airway |
| Nasal secretions from a single nostril | Nasal secretions from both nostrils |
| Rare cough | Frequent cough |
| Noisy inspirations | Noisy exhalations |
| Poor performance / Work intolerance | Poor performance / Work intolerance |
| Noises during work (gurgling, whistling) | Shortness of breath at rest or during work |
| Swelling of the muzzle or under the jaw or in the neck area | Occasional pain while walking |
Runny nose: upper respiratory tract causes
Sinusitis
Sinusitis can be divided into primary and secondary sinusitis. Primary sinusitis is caused by viruses, bacteria and occasionally fungi. Secondary sinusitis is caused by a mass (usually a cyst or tumor) within the sinus, or advanced dental disease, such as an infected tooth. Horses generally present with a smelly, purulent nasal discharge and facial swelling above the upper cheek teeth on the side of the diseased sinus. Diagnosis is made by oral examination, endoscopy, and radiography of the head. Primary sinusitis is usually treatable with antibiotics only in the early stages, while more chronic disease often requires sinus lavage to remove thick purulent material due to poor drainage from the equine sinuses.
Guttural pouch disease
Bacterial infection of the guttural pouch is usually caused by Streptococcus bacteria, the most serious and important manifestation of which is equine adenitis caused by Streptococcus equi var. equi. It is a very contagious infection that spreads rapidly among horses. The disease circulates continuously in the equine population due to a small percentage of horses who become chronic carriers and, despite appearing clinically healthy, intermittently shed the bacteria found in their nasal secretions. Horses with equine adenitis present with fever, inappetence, swollen lymph nodes in the throat area and a purulent nasal discharge (Fig. 1). Both guttural pouches are usually infected, and thus nasal discharge comes from both nostrils. Diagnosis is by culture and PCR (DNA test) of swabs taken in the throat or guttural pouch wash. Despite being a bacterial infection, equine adenitis is not usually treated with antibiotics because it is believed to increase the risk of horses becoming carriers.
Infectious viruses
Equine influenza virus, equine rhinovirus, andequine herpesvirus types 1 and 4 can all cause infectious diseases of the upper respiratory tract. These viruses tend to cause a watery or white nasal discharge, fever, inappetence, and lethargy. In addition, a severe cough is often present with equine influenza. If a secondary bacterial infection occurs, the nasal discharge becomes purulent. These viruses can be spread even by apparently healthy-looking horses. When major changes occur in the structure of the virus, or if the percentage of vaccinated horses falls below the required quota to provide so-called “herd immunity,” major disease outbreaks may occur, such as the equine influenza epidemic in 2019. These infections can be diagnosed by swabbing the back of the throat. Treatment is symptomatic (fever control with anti-inflammatories) and rest. Adequate rest is very important to allow the airway to heal properly before resuming exercise, otherwise nonspecific airway disease may persist for many weeks.
Runny nose: lower respiratory tract causes
Equine Asthma Syndrome
Equine asthma syndrome is the name used nowadays for the disease formerly called recurrent airway obstruction (RAO) and chronic obstructive pulmonary disease (COPD). The name equine asthma better reflects the similarity to human asthma and the breadth of clinical signs-from mild (no clinical signs, visible only through extensive diagnostic testing) to severe, where clinical signs such as increased respiratory effort, coughing, and nasal discharge are present even when the horse is at rest. Inflammatory airway disease, a common subclinical disease that causes poor performance, is now called mild equine asthma.
Equine asthma is believed to be caused by an allergy to airborne particles (allergens) in the environment. Generally, clinical signs are related to the stall environment (and thus to winter), but some horses show clinical signs at pasture: the famous “early summer equine asthma.” The respiratory tract of the boxed horse is exposed to a myriad of airborne allergens and dust particles. Even a completely healthy horse can develop airway inflammation if exposed to a very dusty environment. Most scientific evidence indicates that fungal spores, found in hay and straw, are the main allergens. When a sensitive horse breathes in the allergen, the respiratory system overreacts and the small airways go into spasm and narrow. The airways also become inflamed and produce more mucus and inflammatory cells. Once this happens, the horse’s lungs become hyper-reactive to other irritants such as dust, ammonia from urine, and even cold air. However, the disease is reversible, and if the allergen(s) are removed, the airway returns to normal and clinical signs disappear (“disease remission”).
Equine asthma occurs in adult horses, usually older than 7 years. Traditional symptoms include coughing, a white, mucopurulent nasal discharge (Fig. 3), and possibly increased respiratory effort. In severe cases, obvious abdominal strain is visible when the horse breathes, hence the old name “Bolsaggy.” Mild asthma without obvious clinical signs is seen in younger horses (from 2-3 years of age) and may present only in the form of poor performance. A tentative diagnosis of asthma can usually be made using historical information (such as exposure to dry hay) and clinical signs. Reduction in clinical signs following improvement in the horse’s environment confirms the diagnosis. Endoscopy is useful to allow direct visualization of the trachea and the first part of the lower airway (Fig. 4).
Excess mucus and inflammatory cells produced by the lungs can be observed in the lower trachea and in the sample collected (by tracheal lavage) for analysis. Respiratory secretions can also be collected from the small airways using a technique called ‘lung lavage’ or ‘bronchoalveolar lavage.’ During disease remission, these secretions and their cellular contents return to normal.
If the causative allergens are known, asthma can be prevented: for example, if there is a strong presence of allergens in the stall environment, a horse can often be kept in remission by avoiding hay and straw. If this is not possible and you are feeding hay or hay-silo, then purification by high-temperature steam is an effective way to reduce dust and respirable allergens in the air before feeding. For horses with “field” allergies, this is much more difficult, as allergens are difficult to determine and especially to avoid. Once diagnosed, the horse will always be susceptible to asthma; at this point, the disease will be kept under control, not treated. Preventing asthma is the best solution because it avoids having to treat it with drugs. If good environmental management cannot be ensured, treatment with oral or inhaled steroids and/or bronchodilators may be necessary.
Pneumonia / Pleurisy
Bacterial infection of the lungs (pneumonia) is most commonly found in foals, and Rhodococcus equi is the most common cause. In adult horses, pneumonia can occur following an episode of esophageal obstruction (“choking”), in which food material and saliva are accidentally aspirated, sometimes following a long journey. Travel longer than 6 hours is a known risk factor for pleuropneumonia, a serious condition in which bacterial infection affects both the lungs and the chest cavity. Horses present with fever, cough, nasal discharge, increased respiratory rate and, when affected by pleuropneumonia, may also show reluctance to move. Diagnosis is by chest ultrasonography to identify lung abscesses, collapsed lung lobes, and the presence of fluid in the chest cavity. Treatment requires intensive care, intravenous antibiotics with additional drainage of any infected chest fluid present (Fig. 5), and is not always successful.
Equine mulitinodular pulmonary fibrosis
Recently, equine herpesvirus 5 has been identified as the cause of fibrotic lung disease caused by equine multinodular pulmonary fibrosis (EMPF). Horses present with severe respiratory distress, intermittent fever, cough, nasal discharge, decreased appetite, weight loss, thin body condition, and lethargy. This disease is rare but can be identified by PCR (Polymerase Chain Reaction) and lung biopsy. Responses to antiviral drugs have been found, but most horses unfortunately require euthanasia.
Foreign body
Inhalation of a foreign body, such as a stick, can be a rare cause of a runny nose (Fig. 6). Usually endoscopic removal can be done with antibiotic treatment of any secondary bacterial infection.
Neoplasia
Neoplasia (cancer) of the lower respiratory tract is a rare cause of purulent nasal discharge. Horses may present with fever or depression, or they may be fine. Cancerous masses are usually localized by a combination of endoscopy, radiography, and ultrasonography. Unfortunately, this disease is not curable in horses.
Pests
The larva of the intestinal roundworm (Parascaris equorum) migrates through the equine lung, and is a common cause of nasal discharge and parasitism in unborn and foals. Horses develop a natural immunity to this parasite around two years of age. Currently, widespread resistance of Parascaris to ivermectin is being found; this poses a risk of parasite development in stallions taking only this vermifuge.
The equine lungworm Dictyocaulus arnfieldi can cause nasal discharge and coughing in adult horses. The donkey is the preferred host, and it is rare for an infection in horses to cause egg-producing adults to develop. Therefore, horses must graze next to donkey to contract the infection. Worm treatment with ivermectin is successful.
