Equine sinusitis is the most common disease affecting the paranasal sinuses in horses.
Sinusitis is an inflammatory condition that often involves excessive nasal discharge. Horses with sinus diseases can also experience swelling in the face or reduced appetite.
The causes of sinusitis vary but can include dental disease or bacterial infection of the upper respiratory tract.
Treatment for this condition often presents a challenge to veterinarians because of the large size of the horse’s sinus structures, their complex anatomy, difficulties accessing the area, and the advanced state of the disease before a diagnosis is made. 
What is Sinusitis in Horses?
There are two general types of equine sinusitis: primary and secondary.
Primary sinusitis involves a viral or bacterial infection and is usually caused by an upper respiratory tract infection of the paranasal sinuses. It can occur in horses of any age.
Secondary sinusitis is more common and is typically seen in horses over four years of age. It is the result of another problem that occurred first which then led to the development of sinusitis. 
Secondary sinusitis is usually related to dental disease in the horse.
Cases of sinusitis can be acute (severe but having a fairly short duration) or they can be chronic (recurring or lasting a long period of time).
Unfortunately, horses with chronic sinusitis tend to experience worsening symptoms over time.
Symptoms of Sinusitis in Horses
Chronic or recurring nasal discharge from one nostril that isn’t resolved with normal antibiotic treatment is the most common symptom of equine sinusitis. The discharge is usually thick and may be green or yellow.
Clinical signs of secondary sinusitis are similar to those of primary sinusitis, but nasal discharge may be foul-smelling with secondary sinusitis. 
The second most common sign of equine sinusitis is facial distortion, more common with secondary than primary sinusitis. Facial distortion worsens in chronic stages of the disease, especially in young horses. 
Horses with sinusitis can develop excessive tearing of the eyes, known as epiphora. 
Sinus disease may also cause abnormal respiratory noise. If a fungal infection or tumors within the sinuses are present, a condition known as exophthalmos in which one or both eyes bulge may occur. 
Causes of Sinusitis
Dental disease is the most common cause of all sinusitis cases, usually involving the first molar, fourth premolar, and third premolar teeth. Occasionally, more than one tooth is involved. 
The second most common cause of sinusitis is a bacterial or viral infection, often involding Streptococcus equi and Streptococcus zooepidemicus pathogens.
Secondary sinusitis is often the result of infection from mixed bacteria populations, including anaerobes. Anaerobic infections are more difficult to treat than many other types of infections due to the slow growth of anaerobic organisms. 
- Tumors in the sinuses (neoplasia);
- Sinus cysts;
- Progressive ethmoidal hematoma (PEH);
- Trauma to the sinuses;
- Open wounds on the face;
- Post-operative complications from tooth repulsion surgery;
- Destructive diseases of the sinuses;
- Blockage of the passage between the front and back sinuses;
- Adrenal tumors in horses with Cushing’s disease (PPID);
- Fungal infection (mycosis); and
- Tissue death in the conchal sinuses.
In humans, sinusitis can result from long-term nasogastric intubation, which can lead to nasal inflammation. This inflammation restricts nasal drainage and also allows bacteria to colonize.
There have been several documented cases of sinusitis caused by long-term nasogastric intubation in horses as well, but it is uncommon. 
How is Sinusitis Diagnosed?
Diagnosing equine sinusitis can be challenging for veterinarians, especially in horses with chronic cases. Distinguishing between primary and secondary sinusitis can also be difficult.
However, it’s important to determine the exact cause of the disease to determine the best treatment protocol.
Primary sinusitis is only diagnosed if all secondary causes have been ruled out. If a case is resolved after treating the symptoms, it is assumed to be primary sinusitis. 
Many cases of secondary sinusitis require a combination of imaging techniques to determine what is impacting the horse’s sinuses. Plain radiographs (x-rays) can be used but often don’t show the cause of disease.
Computed tomography (CT) is a diagnostic imaging procedure that uses a combination of X-rays and computer technology to produce images. CT scans can provide a more thorough assessment of bony changes. 
Another advantage of CT over radiographs is that CT scans provide clear, unobstructed images of the horse’s teeth. The downside is that CT typically requires general anesthesia, specialized equipment, and special tables that position the horse for imaging. 
A third imaging technique which is helpful in diagnosing sinusitis is scintigraphy. This technique uses a scintillation counter or similar detector in combination with a radioactive tracer to obtain images of the nasal cavity.
Scintigraphy can be performed without anesthesia and tends to be less expensive and more widely available than CT. However, it does pose a potential radiation hazard for veterinarians and technicians and must be used with care. 
A technique known as percussion, where the veterinarian taps sharply against the overlying bones of the sinuses, can also be used to detect material within the sinuses. However, this method isn’t always reliable.
An oral examination should also be performed to detect possible dental problems. 
Nasal endoscopy, in which the veterinarian inserts a thin, flexible tube equipped with a light and camera into the nasal passages, can be used to detect sinus abnormalities. This procedure can also be used to rule out other upper respiratory diseases or guttural pouch infection. 
Direct endoscopic examination, also known as sinoscopy is often used when results of radiography, other imaging techniques, and nasal endoscopy are inconclusive. This procedure requires sedation and local anesthesia in order to insert an arthroscope or endoscope directly into the horse’s sinuses for exploration. 
Sinoscopy can also be used to treat underlying conditions. It is often reported as the most helpful diagnostic tool when compared with radiography and endoscopy. 
In older horses with signs of sinusitis, the area where the hard palate and the maxillary cheek teeth meet should also be carefully examined for the presence of abnormal tissue. This type of tissue is often squamous cell carcinoma that has invaded the nasal cavity or sinuses. 
Collecting nasal secretions through a process known as centesis can be helpful in distinguishing primary and secondary sinusitis. Centesis can be performed with the horse standing and mildly sedated. 
Examining paranasal sinus secretions should not be the only diagnostic method relied upon, but it can be a helpful tool when oral and radiographic examinations don’t provide clear answers. 
Treating Primary Sinusitis
Horses with primary sinusitis are easily misdiagnosed and may be treated with several doses of antibiotics as the first line of attack.
However, this doesn’t always resolve symptoms and the horse will continue to have sinus drainage, usually from one nostril. This leads to frustration for the owner and the veterinarian. 
Primary sinusitis is better treated with sinus lavage to flush out the sinuses and clear any infected material.
Lavage involves flushing large volumes of warm sterile saline or a highly diluted antiseptic solution through the sinuses. To be effective, it typically needs to be repeated several times. 
Broad-spectrum antibiotics may be added to the lavage solution. Because the lining of the sinuses is extremely sensitive, the sinuses should never be flushed with solutions containing soap or surgical scrubs. 
Systemic antibiotics may be prescribed based on culture and testing of nasal secretions collected through centesis. While waiting for culture results, treatment with antimicrobials such as penicillin, TMS, and/or metronidazole is often recommended. 
Delaying treatment can allow primary sinusitis to progress into chronic osteitis (infection of the bone), death of nasal soft tissue and bone, or deep abscesses in the sinus cavities.
Advanced cases require surgical debridement and removal of dead tissue through a bone flap. 
Treating Secondary Sinusitis
Secondary and chronic sinusitis are notoriously difficult to treat. The causes of the disease and the type of bacteria involved both need to be considered and the inciting cause must be addressed before other treatments begin. 
The first step is usually the removal of diseased teeth or other primary problem, along with removing abnormal mucosa. The sinus cavity is usually flushed and systemic antibiotics are prescribed.
If symptoms continue, this may be a sign of persistent osteitis, abscesses, failure to remove all of the infected tooth root or infected bone, or using the wrong antibiotic. 
Equine Sinus Surgery
If lavage and antibiotic treatment do not improve symptoms, surgery is the next option. However, not every horse is a good candidate for sinus surgery. 
Surgery can usually be performed with sedation while the horse is standing. There are two types of standing sinus surgery: sinus trephination and sinus flap surgery. 
Sinus trephination can be performed by most veterinarians. Sinus flap surgery, on the other hand, requires detailed anatomical knowledge and can lead to several complications. This surgery should only be performed by experienced surgeons trained with the technique. 
Horses may experience a recurrence of symptoms after sinus flap surgery. One study reported that as many as 13-28% of cases relapsed after surgery, requiring further investigation and/or surgical intervention. 
Surgical site infection occurs in about 10% of horses after sinus surgery. Establishing drainage and removing suture material usually results in quick resolution of infection. 
An alternative to surgery is injecting antibiotics directly into the maxillary sinus. Though not common, this method may be warranted in non-responsive chronic cases of sinusitis or in cases where surgery is not an option. 
Chronic sinusitis has a poor response rate and