A horse can carry the influenza virus without being symptomatic.
Influenza is highly contagious and considered the major respiratory disease affecting horses. It is a severe, acute respiratory disease with a short incubation period of one to four days. The virus gains entry through the nasal mucosa and attacks the lining of the respiratory tract, causing damage and eventually death of large areas of mucosa. It is transmitted by inhaling respiratory secretions of infected horses and young, unvaccinated horses that are crowded into poorly ventilated facilities and older horses with less resistance are most susceptible to influenza.
Coughing
A characteristic feature of influenza is the loud, dry, hacking cough that later becomes moist and productive. The cough typically lasts for several weeks, even though the actual illness lasts three to seven days. Coughs are self-perpetuating. Coughing irritates the airways, dries out the mucous lining and lowers resistance to infection. This leads to further coughing.
Causes
Influenza is a contagious illness caused by two species of myxovirus, which can be subdivided into several strains, each capable of causing rapidly developing outbreaks of respiratory disease.
Symptoms
Symptoms other than a cough include a high temperature (ranging up to 106 degrees), clear nasal discharge, depression, sneezing, anorexia--loss of appetite and failure to eat--enlarged lymph nodes and muscle soreness.
Complications
The most common secondary complication is bacterial rhinitis. The nasal discharge becomes thick and puslike and the horse's breathing is noisy. A persistent fever and labored respiration may be signs of the onset of pneumonia.
Treatment
Isolate the horse to prevent spreading the disease to other horses in the barn. Stall rest is important. The stall should be dry and well ventilated--do not close the barn up to keep the horse warm. Doing so will only increase the respiratory symptoms. Instead, to keep the horse warm, use a blanket and leg wraps. Be sure to provide clean water and free choice feedings. Do not exercise the horse until the cough is gone. The horse should receive rest for at least three weeks; returning to training too quickly can cause a relapse, chronic bronchitis and damage to the heart muscles and cells of the liver. Mycolytic agents--drugs that liquify tacky mucous--and antibiotics are used to help control secondary infections.
Prevention
The recommended flu immunizations schedule for foals whose dam has been vaccinated, protecting the foal for the first five months through the colostrum--the first milk of the mare containing vital maternal antibodies--is to give the first dose at nine months, second dose at 10 months, third dose at 11 months, fourth dose at 12 months and doses thereafter every three months. With adult performance horses the recommended flu immunizations schedule is every three to four months. Pleasure horses, however, can receive annual boosters, with an added booster prior to likely exposure. Influenza epidemics may be caused by new strains of the virus. Accordingly, vaccines may have to be modified from time to time to provide the best coverage. Current vaccines immunize against myxovirus A-1 and A-2. Vaccinations are not 100 percent effective in preventing influenza, but they do reduce the severity of the disease.
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