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By John Byrd, DVM
 
Problem #3:  Chronic cough, Runny nose
 
Round worms can also be responsible for coughs and runny noses in young horses most often under one year of age, but may even occur in horses slightly older.  One stage of the life cycle of the roundworm is larval migration to and through the lungs.  During the migration through the lungs the larvae may cause considerable inflammation that may be considered a type of pneumonia.  Pneumonia is frequently complicated with a viral infection and occasionally with bacterial infections.  Often I am asked how much permanent damage is done to the lungs during the time the larvae are in the lungs especially when horses are to be used as performance horses later in life.  I have seen horse that had fairly severe roundworm infection with severe respiratory signs that went on to be outstanding racehorses and other performance horses.  I have discussed this with pathologists that perform necropsies on horses and they have told me they seldom see much residual damage to the lungs even though the respiratory signs were severe when the horse was young.
 
Chronic coughs can be caused by lungworms in any age horses that are kept with donkeys.  It seems that the lungworm causes little if any problems for donkeys, but when a horse gets them they cause a dry chronic cough.  The larvae are coughed up and passed in stool, but are seldom found.  In donkeys the larva develops into adults in the lungs and passes eggs that can infect horses that are grazing with the donkeys.  However, in the horse the larvae do not develop into adults and pass infective eggs, therefore; the horse is considered a dead end host.  In order to make a diagnosis of lungworms a sample should be taken from the donkeys that are grazing with the horses.