Respiratory Disease in Horses, Part 2

By Dave Sauter, DVM, Kulshan Veterinary Hospital

In last month’s issue, we presented Part One of a two part series on respiratory disease. The first half of the article described the anatomy and physiology of the equine respiratory system. The second half focused on the examination of the equine respiratory system, including a description of special procedures utilized in making a diagnosis. Today, in Part Two, the focus will shift to common diseases of the lower respiratory tract, including discussion on their treatment and prevention.

VIRAL DISEASES

Influenza

Equine influenza is extremely contagious with a short incubation period, only 1 to 5 days. Consequently it can spread rapidly through a barn, infecting nearly 100% of the horses. The virus releases enzymes that alter the mucocilliary apparatus, which enables the virus to attach to the lining of the respiratory tract. The virus causes cell damage, which leads to inflammation, massive white blood cell infiltration, and edema. This inflammation occurs in the pharynx, larynx, trachea, bronchi, bronchioles, and into the lungs themselves. Signs of disease include high fever (103ºF to 106ºF), persistent dry cough, loss of appetite, watery nasal discharge, depression, sore muscles, swollen limbs, and enlarged lymph nodes. Opportunistic bacteria commonly take advantage of the compromised conditions and invade, penetrating deeper into the lungs causing bacterial bronchopneumonia.

There are no specific anti-viral drugs approved for equine influenza. Treatment consists of supportive measures to help cope with the effects of the infection as the horse develops an immune response to eliminate the virus. Non-steroidal anti-inflammatory drugs such as Banamine help control the fever, improving attitude and appetite. Antibiotics often are prescribed to help prevent secondary bacterial infections. In very sick horses, intravenous fluids are necessary to treat dehydration. In some situations when horses are not eating and dehydrated, mineral oil is administered through a stomach tube to help prevent impaction. Leg wraps are helpful to treat limb edema. Once the infection is over, it is EXTREMELY IMPORTANT to allow adequate rest. The basic rule of thumb is a minimum of three weeks off from training and competition; with an additional week for each day the horse had a fever. Returning to work prematurely can lead to other chronic respiratory problems.

Prevention of equine influenza is largely through regular vaccination. Challenge studies have demonstrated good protective effects with two vaccines in particular. Calvenza is a killed influenza-rhinopneumonitis combination vaccine that can be administered intra-nasally or intramuscularly. Flu-Avert is a modified live vaccine for influenza that is administered only by the intra-nasal route. It is especially usefully when immunity needs to be boosted in a hurry since it stimulates an immune response quickly and it avoids potential side effects that are seen with intramuscular injections.

In addition to effective vaccination, good management practices are important. Proper nutrition and parasite control are mandatory in promoting overall good health. Good barn ventilation reduces the burden of dust, mold, urea, and pathogens floating around in the air. Transporting horses long distances and moving them into new facitilties causes stress and reduces defenses to infection. Vaccination prior to such anticipated stress will boost immunity to provide extra protection. New arrivals should be isolated from the rest of the herd for three weeks. Monitoring and recording their body temperature daily helps detect early infections. Here are a few valuable online resources on the subject of equine transport:

• Website: www.thehorse.com
• “Guidelines for Hose Transport by Road & Air”, booklet edited by Catherine W. Kohn, published in 2000, available through the American Horse Show Association (www.ahsa.org). 
• For equine accident rescue information: www.mspca.org/ambulance.htm. 

Rhinopneumonitis

Rhinopneumonitis is more commonly referred to as “Rhino.” It is caused by a herpes virus. There are several types of equine herpes virus (EHV). EHV-4 almost exclusively causes respiratory disease. EHV-1 can cause respiratory disease, abortion in pregnant mares, and serious neurological disease. The respiratory form of infection has a short incubation period, only 1 to 3 days. Signs of respiratory disease include fever (103º to 106ºF), loss of appetite, watery nasal discharge, dry cough, swollen limbs, and enlarged lymph nodes under the jaw and throat. Virus invades the lining of the throat and tonsils, resulting in inflammation. As the virus multiplies, it invades the blood stream and spreads to the lower respiratory tract, causing inflammation there as well. Many of the signs are similar to influenza, but Rhino tends to spread through the barn more slowly. Secondary bacterial infections are common. Like influenza, treatment is largely supportive. Recent research in the use of the antiviral drug acylovir has shown promise in treating equine herpes virus infection but has not yet been approved Prevention is largely through effective vaccination, good management practices, and containment when cases develop.

BACTERIAL INFECTIONS

The mucocilliary apparatus normally plays a critical role in the removal of bacteria normally present in inhaled air. Viral infection results in damage to the mucocilliary apparatus and to other defenses. Opportunistic bacteria normally present in the environment, such as Streptococcus zooepidemicus, are able to gain a foothold and to penetrate deep into the respiratory tract. Infection results in inflammation, edema, and mucus accumulation in the airways. Signs of bacterial infection include gurgling respiratory sounds, productive cough, increased respiratory rate, increased respiratory effort, and poor stamina. A thick, creamy nasal discharge may be present. Exhaled air has a foul odor in some cases. There may be an intermittent fever but often the temperature is normal. Examination by the veterinarian will reveal abnormal lung sounds, such as crackles as mucus plugs rattle in the airways, wheezes as air is forced through narrowed airways, and dull areas where air simply fails to flow at all.

Treatment of bacterial infections includes supportive measures as described for viral infections. Antibiotics are often prescribed. Culture samples collected from the trans tracheal aspirates or Broncho-Alveolar-Lavage (BAL) procedures provide useful information on the specific bacteria and which antibiotics are effective treatment. Bronchodilators such as clenbuterol are very effective and helpful in the treatment of pneumonia. Clenbuterol causes relaxation of the smooth muscles that line the small airways of the lungs, causing them to open wider and allow better airflow. Clenbuterol also thins respiratory secretions and improves mucocilliary transport. Often overall treatment duration and ultimate success are greatly enhanced by these supportive effects of clenbuterol.

RECURRENT AIRWAY OBSTUCTION (HEAVES)

Recurrent airway obstruction is also known as heaves or COPD, an acronym for chronic obstructive pulmonary disease. Experts advise us not to use the term COPD anymore because there is already a human condition with this name and it is a much different disease. Recurrent airway obstruction of horses is more similar to human asthma. Horses with heaves have a hypersensitivity to various airborne irritants, such as molds, urea, insect body parts, etc. This hypersentivity results in spasmodic contractions of the small airways (bronchioles), referred to as bronchospasm. Mucus accumulates, airways become plugged, and there is difficulty breathing, especially exhaling air trapped in the plugged small airways. Because there is markedly increased resistance to air flow, these horses need to employ their abdominal muscles to generate more force to move the trapped air. Eventually they develop noticeably larger, stronger abdominal muscles, creating the classically recognized heave line.

As the disease progresses, emphysema can develop as air continues to build behind the mucus plugs. Heaves is generally seen in older horses and there is no breed or sex predilection. Affected horses generally do not have a fever but do have a cough, especially when eating or exercising. The cough gets worse as the disease progresses, sometimes with fits of coughing. Breathing while at rest becomes more and more labored. Exercise becomes increasingly more difficult.

There is a separate but similar disease called Inflammatory Lower Airway Disease that has similarities to heaves. Unlike heaves, this disease affects younger horses and the symptoms are not as severe.

Treatment can be divided roughly into two categories, medical and management. Medical treatment includes bronchodilators such as clenbuterol, described above. Bronchodilators play a critical role by relaxing the bronchospasms, allowing the small airways to open and pass air, much like they do for human asthmatics. Corticosteroids are powerful anti-inflammatories and they also temper the immune system. They reduce the hypersensitivity the airborne irritants and therefore reduce the trigger mechanisms that lead to bronchospasms. Management changes are critical in the treatment of this disease. Many of the worst respiratory irritants are found inside the barn and in baled hay. Therefore, keeping these horses outdoors as much as possible is very helpful. When in the barn, good ventilation is imperative. Another extremely helpful change is to discontinue feeding baled hay. Instead, feed some combination of complete feed, beet pulp, alfalfa pellets, or hay cubes. This is perhaps the most important factor in controlling the disease and reducing the need for medications.

Special thanks again to Boehringer-Ingelhem for illustrations in this article and to Ken West and Dr. Robert Stenbom from technical services.