Influenza (Flu, Two-Year-Old Cough) is caused by two types of
viruses: myxovirus A, subtypes 1 and 2. The short incubation
period of one to three days produces explosive outbreaks and a
rapid spread. Symptoms include fever, a dry, hacking and
persistent cough with muscle sore-ness, depression and loss of
appetite. Lymph nodes become swollen and legs have edema.

The highly contagious virus is spread by the inhalation of
contaminated water droplets moving through the air or from
contaminated facilities or objects. A horse can shed the virus
for up to eight days. The myxovirus damages respiratory cells,
making the horse susceptible to secondary bacterial infections
such as bronchitis, pleurisy or pneumonia. Mild cases become
severe with stress or adverse condi-tions. Two weeks of rest
before resuming train-ing is required for adequate recovery.
Failure to rest completely produces a persistent cough.

Antibiotics are necessary for secondary infections. Vaccinations
should begin at three months of age and be repeated a month
later. Horses with minimal stress or exposure need only a yearly
vaccination. Horses that are shown regularly or are constantly in
contact with other horses should be vaccinated every three
months. Protective immunity is adequate for only six months.
Local reactions to the vaccines have occurred in the neck
muscles. Hygienic precautions can be taken to reduce the easy
spread of the virus by disinfecting vans and by quarantining new
arrivals.