Skip to main content

Respiratory Diseases in Horses: What You Can Do to Prevent Them

Respiratory diseases can be costly for the horse owner/manager in many ways. Respiratory disease causes lost training time. Damage to the respiratory tract can take weeks for full repair and ability to return to work. Competition horses may miss valuable point shows or competitions. The expense of treatment includes both medications and time spent by the owner. Concern about spread of disease through a facility can impact boarding and training revenues.


The ideal management plan focuses on:

  • prevention of disease through vaccination
  • managing exposure risks
  • early detection
  • prevention of spreading disease


Vaccination can eliminate or reduce the incidence of disease. Horse owners can manage exposure risks by isolating new introductions to the herd. Maintaining overall health of the animals with good diet and exercise can strengthen the immune response. When traveling and attending events with large numbers of horses, owners should be aware of precautions to prevent transmission of disease. Early detection of respiratory disease is generally associated with better outcomes. Early detection also may help farm managers minimize the spread of the disease through the facility. While elimination of respiratory disease in all horses cannot be accomplished, the incidence and severity of these diseases can be decreased, thereby decreasing the associated costs.


The major categories of respiratory diseases affecting horses are:

  • infectious ( those caused by viruses or bacteria)
  • allergic
  • parasitic


This fact sheet will focus on the infectious and parasitic causes of respiratory disease. For this discussion, these diseases will be considered individually. Be aware horses may contact more than one disease. Often, one organism may produce respiratory tract damage that paves the way for another organism to cause a more serious condition.


Viral Respiratory Disease

The most important viral respiratory diseases affecting horses are equine herpes virus, equine influenza and equine viral arteritis. Equine herpes virus and influenza probably cause the majority of viral respiratory diseases seen in our area.


Equine Herpesvirus (EHV)

Equine herpes virus is one of the most common respiratory diseases affecting horses. There are multiple types of equine herpesvirus (EHV) and types EHV-1 and EHV-4 are the most common. Equine herpesvirus infections can cause respiratory disease, neurologic disease and abortion. Clinical signs of the respiratory form of EHV-1 and EHV-4 are typically seen in young horses (less than two years old), however, horses of any age can develop the disease. Equine herpes virus often is associated with mild coughing, clear drainage from the nose and/or eyes and possibly a fever and decreased appetite. It is highly contagious from horse to horse and often will affect all horses in a barn in a matter of days. Both EHV-1 and EHV-4 are spread via aerosolized secretions from infected coughing horses. In addition, contact with nasal secretions through handlers, buckets, walls and cleaning and grooming equipment can spread the disease. This virus can survive in the environment from seven to 30 days, depending on the environmental conditions.


Secondary bacterial infection commonly appears as a consequence to the viral infections. Nasal discharge usually is clear at first, but often changes to yellow, then green after a few days. Horses typically will develop a mild cough and some will have minor lymph node enlargement under their jaws. In uncomplicated cases, most horses will recover in seven to 14 days.


As with other herpes virus diseases, the immunity after natural infection or vaccination often is short-lived. Horses may become reinfected within four to six months of having the disease. Repetitive infections are thought to boost a horse’s immunity, so reinfected horses often are asymptomatic. Horses can become carriers of herpes virus and may shed the virus during stressful events.


A veterinarian is best suited to recommend the type and frequency of vaccination for herpes to prevent respiratory disease.


Equine Influenza Virus

Equine influenza is caused by an RNA virus and is widespread in the equine population of the U. S. and throughout much of the world. Cases of influenza occur most commonly in the spring and fall. Influenza has a short incubation period of only one to three days, and can therefore spread rapidly through a group of susceptible horses. Coughing horses disperse the virus into the air, and other horses are infected when they inhale the virus particles. The virus attacks and damages the lining of the respiratory tract.


It takes three weeks for the lining of the respiratory tract to regenerate, which predisposes the tract to bacterial infections. Some of the lining may actually slough and result in multiple areas of denuded respiratory tract lining. Most horses will be infectious for at least five days after onset of clinical illness. The length of time the virus is excreted will vary among horses, depending on their immunity.


The clinical signs seen with influenza are similar to those associated with other viral respiratory diseases. Fever usually is present and ranges from 103 F to 106 F. Horses have varied degrees of depression and anorexia. In uncomplicated influenza cases, most clinical signs last from two to 10 days, but affected horses often develop a dry, nonproductive cough that can last two to three weeks or longer. The nasal discharge is usually clear at first, but can become thick and pus-like. Intermandibular lymph nodes may become slightly enlarged, and some horses appear to have muscle soreness. Horses can develop a generalized vasculitis, which is manifested as swelling of the limbs, inflammation of the heart resulting in a high heart rate and inflammation of the eye resulting in tearing and squinting of the eye.


A veterinarian is best suited to make recommendations on the type and frequency of vaccination for influenza to prevent respiratory disease. Horses considered at high risk of infection will require more frequent vaccinations.


Equine Viral Arteritis (EVA)

Equine viral arteritis is a viral disease of horses that can cause respiratory disease and abortions. Even though the virus has a worldwide distribution, incidence of the disease is uncommon, but has occurred in Oklahoma. EVA has an incubation period longer than the other viral respiratory diseases, ranging from three to 14 days. The virus is shed in nasal secretions. Stallions affected with EVA may shed the virus in their semen for years and are capable of transmitting the disease to mares in live cover, cooled and frozen semen. As with other viral respiratory infections, EVA is acquired by inhalation of the virus, exposure to virus contaminated objects or environment.


The clinical signs associated with EVA infection are similar to other viral respiratory diseases, but can vary from severe to subclinical. However, this disease should be suspected if the following are present:

  • More discharge from the eyes than usually seen with other respiratory diseases
  • Depression and loss of appetite
  • Red and inflamed lining in the nasal passages
  • Swelling in the limbs and ventral abdomen
  • Abortions during or within a few days of the illness


The Oklahoma Department of Agriculture, Animal Industry Services works with horse owners throughout the state to protect the equine population (there are an estimated 278,000 horses in our state) from the introduction of disease and to ensure that they are able to move interstate and internationally. EVA is a reportable disease in Oklahoma. While EVA typically is not life-threatening to otherwise healthy adult horses, EVA is of special concern to horse breeders because it can cause abortion in pregnant mares, death in young foals and breeding stallions may become permanent carriers of the virus. The increase in international trade of horses and semen for artificial insemination has increased the risk of passing equine arteritis virus from one equine population to another. In 1984, there was an EVA epidemic in America’s thoroughbred industry, which led major restrictions by the international community on the movement of horses from the U.S.


Treatment of Viral Respiratory Diseases

Viral respiratory diseases can mimic more serious respiratory diseases. Your attending veterinarian will examine affected horses to determine the seriousness of the illness. The most important component of therapy for uncomplicated viral respiratory diseases is rest. Stress should be kept to a minimum. If possible, the horse should not be hauled long distances and forced exercise of any kind should be discontinued. Affected horses should be housed in an area with good ventilation, and dust should be kept to a minimum. Unless the weather is inclement, sunshine and fresh air may be the best environment for recovery.


It is important to remember antibiotics are not effective against viruses. However, for the horse with severe viral respiratory disease or one that is stressed for another reason, appropriate antibiotics may be administered prophylactically to decrease the chances of opportunistic bacteria causing a more severe disease. It also is important to remember it can take a prolonged period of time for the respiratory tract to heal from a viral infection. The typical recommendation for rest of the respiratory system after a viral infection is one week of rest for every day of fever.


Complications of Viral Respiratory Diseases

Uncomplicated viral respiratory diseases usually are self-limiting, and the affected horses recover with no long-lasting ill effects. However, horses with viral respiratory diseases that are also stressed by shipping, training, racing, surgery, etc., may develop severe, and sometimes fatal, pneumonia. Frequent examination of the affected horse’s respiratory tract is indicated to monitor for secondary complications like pneumonia, pleuropneumonia and myocarditis (inflammation of the heart).


Bacterial Respiratory Disease

Many bacteria are capable of causing respiratory disease in horses. Some bacteria only cause disease when viruses or other agents have first damaged the respiratory tract. Other bacteria are capable of producing disease independently. For this discussion, we will consider the two most important bacterial diseases for Oklahoma horses—Strangles and Rhodoccus equi.



Strangles is a bacterial disease caused by Streptococcus equi. It is most commonly seen in one- to five-year-old horses. However, any age horse can get the disease, and older horses that have waning immunity may develop the disease even if they had it at a younger age. There usually is a two- to six-day incubation period, and 30 to 100 percent of the horses on the farm may develop the disease. Horses affected with this disease have a copious thick white to yellow nasal discharge containing high numbers of the bacteria.


They contaminate the environment, including feed tubs, water buckets and walls with the bacteria. Anything in the horses’ environment contaminated with the bacteria (such as handlers’ boots and clothes, flies, brushes or halters) can be a source of infection for other horses. Horses acquire the disease by direct contact with the bacteria, either by inhalation or ingestion. The bacteria itself may survive in the environment for a few weeks. This disease is difficult to eradicate because some horses shed the bacteria for long periods. Affected horses will typically shed the bacteria for at least four to six weeks. One survey reported that one mare shed the bacteria for ten months, and long-term asymptomatic carriers may be more common than previously thought. Natural immunity to the disease after infection usually is long-lasting. However, some individuals may acquire the disease more than once.


Clinical signs associated with strangles include fever (103 F to 106 F); serious nasal drainage evolving to a thick, yellow to white and copious nasal discharge; moist cough; difficulty swallowing; depression and decreased or nonexistent appetite. Lymph node swelling between the mandibles and in the throat latch area is usually present. The lymph node swelling with strangles is frequently severe and results in abscesses that often rupture and drain pus. Most horses recover without complications.


Complications include becoming a chronic carrier of the disease, asphyxia due to enlarged lymph nodes compressing the larynx or windpipe, bastard strangles (spreading to other areas of the body), pneumonia, guttural pouches filled with pus, purpura hemorrhagica and heart disease. Horses also may develop severe muscle wasting secondary to strangles. The average length for the course of this disease is 23 days. It may take up to two weeks after exposure to display clinical signs.


Rhodococcus equi Pneumonia

Rhodococcus equi pneumonia is a sporadic disease that usually occurs in one- to six-month-old foals. It is endemic on some farms, and the disease will appear in 1 to 10 percent of each foal crop. Most infections will be in foals two to three months of age, which is when maternal antibody concentration is waning. Foals can be infected at birth, but will not show signs of illness until they are one to six months of age. The organism is commonly found in the soil and often can be cultured from the feces of dams that had affected foals. Foals probably become infected by inhalation of the bacteria from the soil. The disease does not appear to be transmitted by direct contact between horses.


One of the major problems in dealing with Rhodococcus pneumonia is it has an insidious onset. Foals usually start with a low-grade fever, but may have temperatures up to 106 F. They may or may not cough in the initial stages of the disease. Often, there is no abnormal nasal discharge early in the disease process. As the disease progresses, the foals become depressed, nursing is diminished, difficult breathing becomes apparent and weight loss or failure to grow may be evident. In addition to respiratory disease, Rhodococcus equi can cause diarrhea, septic and nonseptic synovitis (joint inflammation), bone infection and intra-abdominal abscessation.


The fundamental components of Rhodococcus pneumonia treatment are early detection, rapid and accurate diagnosis, and specific antibiotic therapy. It often is necessary to continue the antibiotic therapy for several weeks.


Control of Rhodococcus equi pneumonia is very difficult, even with current knowledge and available products. There is no commercial vaccine available. Prophylactic hyperimmune serum administered intravenously has been reported to be effective. Other prophylactic protocols have been published and may be worth discussing with a veterinarian. Since it is thought that the natural disease is acquired by inhalation, and the bacteria is known to be in the soil, it may be beneficial to reduce dust and properly ventilate housing. As with all respiratory diseases, reduce stress by not overcrowding and by keeping the handling of foals to a minimum. Frequent removal of manure will help minimize environmental contamination and may reduce the incidence of infection. It also is important to verify the foal has had proper colostrum intake shortly after birth.


Respiratory Diseases Caused by Internal Parasites

Two equine internal parasites migrate through the lungs as a portion of their life cycle. The equine roundworm is one of the most common internal parasites of young horses. It is probably present on every horse operation in Oklahoma. After this parasite is ingested by the horse, it then migrates from the intestine to the liver and then to the lung, where it is coughed up and swallowed to become an adult in the intestine. Foals ingesting large numbers of the parasite experimentally develop a cough and nasal discharge similar to other respiratory diseases. In uncomplicated cases, the horse’s temperature remains near normal. It is unknown if under natural conditions this parasite causes clinically detectable respiratory disease. However, it is quite possible the damage caused by this parasite predisposes young horses to other respiratory diseases. For this reason, a good deworming program is recommended for all foals. Most horses develop a resistance to roundworms by two to three years of age.


The equine lungworm, even though not commonly seen in Oklahoma, can cause chronic coughing in affected horses. Horses are not the favorite host of this parasite and do not usually transmit the disease to other horses. In most instances, horses acquire this

disease when pastured with donkeys.


Prevention Program

Each horse owner, in consultation with an attending veterinarian, should develop a written vaccination program appropriate for their situation. Accurate vaccination records should then be maintained on all horses. All horses in Oklahoma should be vaccinated in the spring for eastern and western encephalomyelitis, tetanus, West Nile Virus (WNV) and rabies. Additional vaccinations may be recommended for your horses to prevent respiratory diseases. Your local veterinarian can prescribe the best vaccination program for your particular situation. Additional information regarding Equine Vaccination Programs is available in Fact Sheet VTMD-9119.


The newborn foal obtains protection from respiratory diseases from their dams’ colostrum. This colostral protection provides passive immunity. Colostral protection or passive immunity lasts a varied amount of time, which is related to the amount of protective antibodies in the colostrum and how fast the antibodies are used up by the foal. To further boost the passive immunity of the colostral transfer to the foal, it also is advantageous to keep the mare at the facility where she is to foal at least six to eight weeks prior to foaling.


Colostral antibodies against some diseases last longer than others. In general, the protective levels of antibodies acquired from the dam are too low to be protective by two to three months of age. Foals vaccinated at this age have a problem arise because the residual maternal antibodies will cause the foal’s immune system to be unresponsive to vaccinations. This effect can last for weeks or months.


Vaccination is not a substitute for good management practices. Respiratory infections spread through groups of horses by direct contact between animals, handlers and contaminated surfaces or through inhalation.Horse owners can minimize the spread of infectious organisms by

  • Maintaining overall health of animal which allows for ideal function of immune system, including:
    • Parasite control program
    • Sound nutrition program
    • Regular exercise program
    • Adequate ventilation
    • Dust control
    • Shelter from rain and sun
  • When traveling with horses, use your own water buckets. Some viruses and bacteria can survive on communal troughs. Do not swap tack or tools with other at shows or events.
  • Consider quarantine for all animals brought onto the farm, especially those returning from events such as shows, racetracks or sales. It may be necessary to quarantine these animals for up to four weeks before they are allowed to commingle with resident horses.
  • If possible, pregnant mares should be kept in a group away from other horses, especially weanlings and yearlings.
  • When cases of respiratory disease or abortion occur, all affected horses should be isolated and not allowed to commingle or leave the premises — a veterinarian will advise the proper timing.
  • Handlers caring for the sick horses should limit their contact with healthy horses. Stalls, trailers and equipment that have come in contact with sick horses should be disinfected with products containing phenols, chloride or quaternary ammonium. It also is best for healthy horses to be housed in a separate airspace from sick horses.
  • Make a habit of washing or sanitizing hands when moving from horse to horse. If there are visitors to your farm, post signs asking people to avoid going down the aisle touching every horse.
  • Take the temperatures of all traveling horses twice daily. Know your horse’s normal temperature and follow the daily temperatures after returning home from a trip.

Consult a veterinarian concerning vaccination protocols. A veterinarian can evaluate each horse as an individual considering age, health and environment to recommend vaccination and management programs that will provide maximum protection for each horse.


Elisabeth J. Giedt, D.V.M.
Director of Continuing Education, Extension and Community Engagement
Center for Veterinary Health Sciences
Oklahoma State University

Was this information helpful?
Fact Sheet
Weaning and Management of Weanling Horses

A guide on pre-and-post weaning foals and recommendations for proper care and management through weaning season.

Fact Sheet
Minimizing Impacts to Wildlife from Livestock Infrastructure

A guide to the best management practices to protect livestock from outside wildlife in relation to their fencing and water quality.

BirdsHabitat ManagementLivestockLivestock Health & DiseaseLivestock Health, Disease & NutritionLivestock ManagementRaptorsWildlife
Fact Sheet
Reproductive Management of the Mare

How to understand the physiology, estrous period and recordkeeping practices for proper management of a mare’s reproductive cycle.

HorsesLivestockLivestock Genetics & Reproduction
Back To Top