Undifferentiated distal respiratory tract infection, or snotty nose syndrome, refers to a mild to moderate bacterial infection of the lower airways. Foals develop snotty nose syndrome between the ages of two and eight months. Many different bacteria can contribute to snotty nose syndrome, rather than a specific pathogen.
The bacteria that are most commonly involved are all bacteria that are NORMALLY found in the foal's nasopharynx (the passageway leading to the mouth) and larynx (the voice box, which leads to the trachea, or windpipe). The most common bacteria are Streptococcus zooepidemicus, Bordetella bronchiseptica, Klebsiella pneumoniae, and various Staphylococcal species. Over eighty percent of all cases are caused by Streptococcus zooepidemicus.
Although snotty nose syndrome is characterized by bacterial infection, the foal probably gets this infection because of intrinsic changes that are taking place within his or her own body. Ordinarily, the bacteria may make their way down into the trachea, and even into the larger bronchi (breathing tubes), but the body clears the airways and lungs of these bacteria very easily. In the healthy animal, especially the healthy older animal, no infection occurs.
Between the ages of two and eight months, several things are happening. First, the foal receives important antibodies from his dam shortly after birth. This is called passive immunity, and the foal gets it through the first milk, or colostrum. As the name implies, this is a passive situation, and the antibodies that the foal has received this way start to decline at 2 to 4 months. The foal will have started to mount his own active immune system, but the immune system simply isn't up to speed yet.
Secondly, the respiratory system has its own special immune system, called respiratory mucosal immunity. This mucosal immunity helps to repel invaders such as the bacteria that normally inhabit the upper airways. In foals, this mucosal immunity is very poorly developed.
For these reasons, there is a high likelihood that any foal between the ages of 2 and 8 months can develop snotty nose syndrome, simply because of factors intrinsic to the development of the foal.
What to Look For
If you have a large farm, you will notice that the snotty nose syndrome spreads like a very slow wave - and it is most common in the summer. This is not because it is warm, or humid, but because most foals are born in early spring and are the right age for passive immunity to be waning in the summer.
Although most cases of snotty nose syndrome can be diagnosed based on history and physical examination findings, your veterinarian may choose to examine your foal using the endoscope. Other possible tests include radiographs and a transtracheal aspirate.
Make sure that you know how to give the antibiotics that your veterinarian prescribes. Make sure that you give the antibiotics for the appropriate amount of time. Don't stop giving the antibiotic just because your foal isn't coughing any more. Antibiotic treatment tends to get rid of signs long before it actually eliminates the bacteria from the body.
Monitor your foal carefully for any sign of relapse.