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Equine Respiratory Virus

Robert J. Webster
Dr. Susan C. Eades


Similar to the experience of people with cold viruses, infection with equine respiratory viruses cause upper airway disease, even though the infection may extend to the lower respiratory tract. The equine viral diseases are difficult to differentiate with certainty due to the similarity of clinical signs. A common presentation for an animal that is infected with a respiratory virus will be an elevated temperature, a cough, and nasal discharge. The severity of these clinical signs will depend on the type of virus the horse has contracted, and the individual horse’s immunologic response to the infecting agent.

The main route of exposure for horses infected with respiratory viruses is through aerosilization with subsequent inhalation of the viral particles, and through direct fomite contact (e.g. infected feed buckets, water troughs, equipment, etc). This makes the contact rate very high in situations where horses are congregated in a barn (e.g. racetrack and show barn environments). The high contact rate and short incubation period and an unvaccinated population can result in an explosive epidemic, especially when the population is not protected by immunity from recent vaccination or previous exposure.

Both the antibodies (humoral) and white blood cells (cellular) play a role in protecting the horse in viral disease. When the viral agent infects the individual, it initially invades the upper respiratory system. Once in the nasal passages, the dominant antibody produced is immmunoglobin A (IgA). This antibody acts to neutralize and immobilize the infectious agent and keep the virus from crossing the cell’s lining of the respiratory passage. Once the virus penetrates the lower respiratory tract, the virus comes in contact with immunoglobin M (IgM) and immunoglobin G (IgG). IgM and IgG are capable of enhancing phagocytic activity by neutrophils and resident macrophages. Previous infections or administration of vaccinations will stimulate production of these antibodies.

Diagnosis of viral infections in the equine is usually done at presentation by observing the clinical signs. Viral isolation techniques are available but are of limited benefit to the veterinarian due to time period required for identification of the infectious agent. The patient has usually cleared the infection by the time laboratory results are obtained. Even successful viral isolation does not alter the therapeutic protocol for treating viral infections. Treatment is palliative in nature and designed to treat the clinical symptoms, because specific therapy is not available. Viral isolation may be beneficial in herd outbreak situations. A positive identification of a specific organism will benefit the veterinarian by allowing for specific viral vaccination recommendations to the owner. This may slow or halt the spread of the disease within the population.

As mentioned earlier, treatment for equine viral respiratory disease is supportive and designed to keep the patient comfortable, prevent dehydration, and to minimize occurrence of secondary infections. The animal should be placed in a well-ventilated stall or shed and protected from the elements. Nonsteriodal antiinflammatory drugs should be used to lower fevers, to improve appetite, and to provide analgesia for possible myalgia. Rest should be enforced for a minimum of one week after signs have diminished. If coughing and/or fever persist longer than one week, more rest is indicated. A convalescent period of three to four weeks is generally needed for complete resolution of most uncomplicated viral infections. If the fever persists for more than five consecutive days and the nasal discharge changes from serous to mucopurulent, a secondary bacterial infection should be suspected. A transtracheal wash should be performed and the exudate submitted for culture and sensitivity testing. Antimicrobial therapy should be initiated based on the results.

Equine Influenza Equine influenza has worldwide distribution and is frequently seen in mobile populations of animals. It has a short incubation period (one to three days) and sudden onset of clinical signs. The disease is most commonly found in animals one to three years of age. The disease can infect older individuals but the signs are usually milder and subclinical. The winter and spring seasons appear to have more frequent infections because the lower ambient temperatures and humidity enhance viral survivability.

Clinical Signs: The first clinical sign in an animal infected with equine influenza is sudden onset of a high biphasic temperature, possibly 106˚F. This is usually followed by cough and serous nasal discharge. The cough may persist as long as three weeks and is easily induced by tracheal palpation and the animal may be reluctant to move due to myalgia. A complete blood count may reveal transient changes in circulating leukocyte numbers.

Prevention: Isolation of diseased animals and new introductions is the key to control. Although vaccination does not guarantee protection, it will decrease the duration and intensity of the illness, thereby reducing viral shedding into the environment. The vaccines that are available contain modified live virus and are recommended for vaccine followed by a booster two to three weeks later. The animal is then revaccinated on a yearly basis. It is recommended that younger animals, and those at higher risks for contracting influenza, be revaccinated at two to six month intervals.

Equine Viral Arteritis Equine viral arteritis has vast geographical distribution but is not as prevalent as equine influenza. It is believed that the virus spreads slowly and the incidence of seropositive individuals increases with age. Incidence of outbreaks has been isolated to areas where animals are congregated.

Clinical Signs: Clinical signs can vary from subclinical to severe disease and death. The animal typically presents with fever as high as 105˚F for one to five in duration. Anorexia, depression, serous nasal discharge, lacrimation, and coughing may soon follow. Edema may occur in the limbs, palbebra, and scrotum in advanced cases. Infections with equine viral arteritis will also cause abortions in pregnant mares.

Prevention: A modified live vaccine has been recently developed. It has been shown to provide complete or partial protection against the clinical signs associated with equine viral arteritis, but it does not prevent viral replication. The duration of protection associated with the vaccine is as long as two years. Foals gain passive protection through the ingestion of maternal antibodies present in colostrum.

Equine Herpes Virus Equine herpes virus 1 (EHV-1) and equine herpes virus 4 (EHV-4) are the most important alphaherpes virus affecting the equine respiratory tract. Both viruses have demonstrated a latent phase with possibility of recrudescence at later times.

Clinical Signs: The clinical signs of EHV-1 and EHV-4 are indistinguishable. They usually affect animals within their first year of life of when they enter training situations. The acute form of the disease usually produces a high, biphasic fever with serous nasal discharge that rapidly turns mucopurulent. Coughing is not a dependable clinical sign for this virus. EHV-1 infections have also been associated with abortions and foal deaths. The disease can cause myeloencephalopathy, tachycardia, hyperemic mucous membranes, and intractable diarrhea in infected foals.

Prevention: Vaccines for EHV-1 and EHV-4 are available separately, together, or combined with equine influenza virus. The increased titer from the vaccine is short-lived and may not prevent infection, but it usually lessens the severity of clinical signs associated with the disease. There is no protection for the neurologic form of EHV-1 infection. Revaccination is recommended every two to six months. Newborns are protected from antibodies present in maternal colostrum.