When and how do veterinarians evaluate horses’ lower airways?

The widespread application of endoscopy in the early 1980s revolutionized veterinarians’ ability to comprehensively evaluate horses’ upper airways. Even the mighty 1-meter scope, however, cannot extend much beyond the ­carina—where the trachea divides into the right and left mainstem ­bronchi—leaving the lower airways out of reach and sight.

Even a long, thin scope wouldn’t allow direct visualization of the entire lower airway. This is because the diameter of the horse’s airways rapidly becomes smaller and smaller beyond the bifurcation, extensively dividing and subdividing into the bronchioles and, ultimately, the alveoli.

Sheer volume is also a tremendous barrier, as the horse’s lungs contain approximately 10 million alveoli. The airways are so voluminous, in fact, stretched out they could pave a 1,500-mile road from Chicago to Las Vegas and beyond.

Without visualizing the lower airways—the bronchi, bronchioles, and alveoli that make up the lung fields distal to (below) the carina—­directly, how can veterinarians best evaluate a horse’s airways? What are they looking for, and what tools do they have at their disposal?

Reasons for a Lower Airway Examination

“The major indications for a lower airway evaluation in an adult horse include coughing, bilateral nasal discharge, exercise intolerance, and/or increased respiratory rate either at rest or during work,” says Kate Hepworth, DVM, Dipl. ACVIM, assistant clinical professor of equine medicine at North Carolina State University’s School of Veterinary Medicine, in Raleigh.

More specifically, “coughing, regardless of whether it is something more chronic or a more recent development, should prompt a lower airway evaluation,” adds Kara Lascola, DVM, MS, Dipl. ACVIM-LA, CVA, associate professor of equine internal medicine at Auburn University, in Alabama. “Especially if that cough is accompanied by a fever, increased respiratory effort at rest, and/or reduced athletic performance or ­exercise intolerance.”

Both infectious and noninfectious conditions can result in lower airway disease requiring an evaluation to achieve a definitive diagnosis.

“Depending on the time of year and the horse’s history, equine asthma and pneumonia tend to be the two most common causes of cough in an adult horse,” says Hepworth.

For infectious disease Lascola says bacterial pneumonia is an important differential, especially in a horse with fever and risk factors such as recent transport, viral respiratory disease, and esophageal obstruction (choke).

“Other conditions that need to be considered in horses with reduced athletic performance include cardiac and musculoskeletal disease,” she says. “Additionally, increased respiratory rates can be a response to fever or pain. A horse with an increased respiratory rate may occasionally be misinterpreted as painful and present for evaluation of colic when they actually have underlying respiratory disease.”

Less common differentials include fungal (rather than bacterial) pneumonia, equine multinodular pulmonary fibrosis (typically associated with equine herpesvirus), and possibly congestive heart failure. Obscure differentials include trauma such as blunt force injuries that damage the airways, as well as neoplasia (tumors). However, primary lung tumors are exceedingly rare in horses.

Step 1: Look at the Horse’s History

Veterinarians must consider certain factors when performing diagnostics to identify the source of respiratory disease.

“The first is to determine whether the cause is most likely an infectious agent, like a bacterium, virus, or fungus, or noninfectious like asthma,” says Hepworth. “History is often the most important part of that equation. For example, a horse with a chronic cough that eats well and has no history of travel or exposure to new horses and who has not had any fevers is far more likely to have chronic asthma.”

On the other hand, she says, a horse that travels frequently and has only been coughing since it returned from a show a week ago is more likely to have either a viral upper respiratory tract infection or bacterial pneumonia.

Step 2: Select Appropriate ­Diagnostics

Lascola’s diagnostic regimen depends on whether she suspects, based on history and physical exam, the cough is due to an infectious or noninfectious cause.

In cases of possible infectious airway disease She starts by performing a thoracic examination and potentially a rebreathing test (which causes the horse to take deeper, slower breaths, making it easier to hear lung sounds) followed by thoracic ultrasonography.

“Radiographs may also be performed to complement ultrasound if more extensive pulmonary disease is suspected, as radiographs allow for better evaluation of deeper lung structures/abnormalities,” Lascola explains, as opposed to the lung surface.

Next, she performs bloodwork, including a complete blood cell (CBC) count to look at the white blood cells (which can provide information on the severity of inflammation) as well as fibrinogen levels. Fibrinogen is an acute phase protein that identifies inflammation often due to systemic infection.

Serum amyloid A (SAA) is another acute phase protein. “My institution does not perform serum amyloid A testing, but it is definitely used by many practitioners in addition to or instead of fibrinogen,” says Lascola.

She prefers to perform a transtracheal wash (TTW) when she suspects bacterial pneumonia. “Some of the horses I see have already had an empirical course of antibiotics to which they did not respond,” she says. “A TTW allows me to attempt to culture the fluid to identify the specific bacteria involved in the pneumonia and identify antibiotics that might be effective for treatment. Thoracocentesis (collection of fluid from the thoracic cavity outside the lungs) may be performed in horses with … pleural pneumonia, which is a more severe manifestation of bacterial pneumonia. This fluid may also be submitted for culture to select an appropriate antibiotic.”

In cases of suspected noninfectious airway disease Beginning again with a thoracic auscultation and potentially a rebreathing examination, Lascola then jumps to a bronchoalveolar lavage (BAL) instead of diagnostic imaging. The BAL provides information on the white blood cells present in the lower airways, which can help rule equine asthma in or out.

“Thoracic ultrasound or radiographs do not yield any findings specific to asthma and are therefore not always routine,” she says. “These may be performed to rule out concurrent pneumonia or other less common chronic respiratory conditions that can present with clinical signs similar to asthma, such as neoplasia or equine multinodular pulmonary fibrosis.”

In the case of equine asthma, Lascola often recommends performing a CBC and blood serum biochemistry, if possible. This is because asthmatic horses will most likely be treated with ­corticosteroids.

“I prefer to have a baseline assessment of overall animal health given the ­potential side effects of corticosteroid drugs,” she says.

Basic hematology such as a CBC can potentially indicate an infection based on a high white blood cell count. Hepworth warns, however, that while bloodwork can sometimes help differentiate infectious lower airway disease from noninfectious, the two are not always mutually exclusive, and it can’t completely rule either in or out. Even SAA can’t always provide a clear answer (Viner et al., 2017).

“SAA does tend to be higher in bacterial pneumonia than equine asthma,” says Hepworth. “But, depending on the stage of the disease process, it still may not be significantly elevated in cases of bacterial pneumonia.”

While it might seem remiss not to perform a CBC or measure SAA levels, Hepworth notes, “if we are working with a limited budget, I nearly always prioritize airway sampling and imaging over bloodwork in cases of respiratory disease.”

Step 3: Sample the Airways

Lascola says she uses TTW when working up cases she suspects are due to bacterial pneumonia, while BAL is more appropriate for equine asthma.

Because horses can have multiple problems, Hepworth says she sometimes performs both procedures.

Veterinarians can collect samples from the upper airways, the nasal passages, and pharynx to test for specific viruses and bacteria. However, infectious agents identified in the upper airway are often not the same ones causing disease in the lower airways.

“In lieu of nasal, pharyngeal, and in some cases even tracheal sampling, I instead recommend performing a BAL with subsequent evaluation of the fluid via cytology,” notes Hepworth, something she says commercial laboratories readily complete.

“In some instances, a viral upper respiratory infection such as equine herpesvirus or equine influenza virus can ­predispose a horse to a secondary bacterial pneumonia,” Hepworth explains. “However, these viruses are less likely to be the primary cause of lower airway disease. Taking cultures from nasal discharge does not generally reflect the bacteria that are present in the lungs because there are so many ‘normal’ or commensal bacteria present in the nasal passages.”

She adds that when she suspects neoplasia or interstitial (causing lung inflammation and damage) disease, lung biopsy might be the only way to get an accurate diagnosis once other methods have failed.

Step 4: Perform Diagnostic Imaging

As noted, vets can use radiography or ultrasonography to image the lungs.

“Many portable radiograph units can take high-quality images of limbs and head, but they often lack the power to be able to obtain good images of the chest,” says Hepworth. “This is why ultrasound is my go-to when I am trying to rule out pneumonia as the cause of a cough or a fever.”

Kate Hodson, DVM, owner of Hodson Veterinary Services LLC, in Hebron, Indiana, also acknowledges the limitations of this imaging modality. “In the field, radiographs are harder for me to obtain with my setup just due to the power and the plate size I have,” she says. “If it’s a smaller pony I can usually obtain adequate images, but with a larger horse it’s a little more difficult. In an outdoor setting, even an ultrasound can pose a challenge depending on what time of year it is because I don’t always have the luxury of a heated facility during the winter. Some equipment will not function properly if the patient isn’t able to be transported to a more appropriate facility.”

For horses that are coughing and have normal bloodwork, no history of fever, and no change in appetite, Hepworth says she would likely go straight to BAL rather than perform an ultrasound.

“In this case asthma would be a far more likely diagnosis,” says Hepworth. “On ultrasound the lungs of a horse with asthma generally look completely normal, whereas there are often significant changes visible with pneumonia.”

Hepworth reminds practitioners that ultrasound only shows the very surface of the lung, so to have a truly complete picture of the lungs, both radiographs and ultrasound are required.

Step 5: Test Airway Function

In a perfect world lung function testing would complement a BAL to assess asthma severity. Unfortunately, very few facilities offer lung function testing due to the equipment necessary to perform it.

Step 6: Work With a Specialist

Hodson admits to readily consulting with a specialist or referring cases when treatment appears to be refractory.

“For consultation, I’ll have the case history and whatever diagnostics I have readily available for the specialist to review and make recommendations for next steps,” says Hodson. “If the horse is not responding, there is always a chance there is a cardiac component or maybe even cancer if there is fluid in the chest cavity visible on ultrasound or ­radiographs.”

As a referral specialist, Lascola recommends the specialist and referring veterinarian work as a team. Like Hodson, the primary practitioner in the field can typically perform an initial respiratory assessment with a detailed physical examination and (if needed) basic bloodwork.

“Primary practitioners may also have portable ultrasound units that allow them to perform a basic thoracic ultrasound to give them some additional information,” she says. “However, diagnostics like ultrasound, TTW/BAL, endoscopy/­bronchoscopy, and radiography are less commonly performed because of equipment and personnel needs.”

Other reasons to refer pertain to management. Complicated cases or very sick patients might need round-the-clock monitoring or supportive care, in which case referral is warranted, says Lascola.

“Ultimately, decisions of referral depend on owner willingness and how sick the patient is,” she says. “For example, an uncomplicated pneumonia can frequently be diagnosed and successfully treated with first-line empirical antibiotics. A horse that has signs of more severe systemic illness or more aggressive pneumonia or who worsens or does not respond to initial treatment within 24-48 hours should be referred if possible.”

Take-Home Message

No tool can completely rule every potential diagnosis in or out. However, the more pieces of information your veterinarian gathers, the lower the chances of missing something.

“Plenty of times, bloodwork may be completely normal in a horse with asthma, but in rare cases bloodwork in an asthma suspect may show significant inflammation that could point to a different, rarer cause of disease,” says Hepworth. “Similarly, while most horses with pneumonia have visible changes on the surface of the lung that can be seen by ultrasound, some horses will only have deep lesions that might only be seen with radiographs.”

When in doubt, take Hodson’s advice and consult or refer.