Respiratory Distress in Young Foals
Dr. Melissa Mazan
Bronchointerstitial pneumonia is one of the most common causes of respiratory distress in foals. It is a sporadic, rapidly progressive disease characterized by acute respiratory distress and high mortality in foals between one week and eight months of age. Other causes of respiratory distress are viral or bacterial pneumonia, chest trauma, heart defects, and allergic reactions to vaccines. Many cases of bronchointerstitial pneumonia appear during hot weather, especially if the foal has been transported in hot weather.
Bronchointerstitial tells us that the disease affects the airways (bronchi) and the interconnecting tissue (interstitial) of the lungs. The fact that both areas are damaged is the reason that foals go into distress.
Bronchointerstitial pneumonia causes respiratory distress by filling air sacs that normally exchange oxygen and carbon dioxide with fluid and damaged cells. The air sacs are rendered useless. The lung goes from spongy and air-filled to a rubbery and fluid filled. The damaged lungs try desperately to inhale and exhale, but the heavy lungs are too stiff, and the airsacs no longer allow oxygen to diffuse into the blood stream. Consequently, the entire body becomes deprived of oxygen.
Foals present with a history of sudden onset of respiratory distress, high fever, coughing, and nasal discharge.
No one cause of bronchointerstitial pneumonia has been identified. It is most likely that bronchointerstitial pneumonia is actually a rogue inflammatory response of the lung to varying insults. Researchers suspect that diseases such as Rhodococcus equi, a serious cause of bacterial pneumonia in foals of this age group, or Pneumocystis carinii, a more rare fungal cause of pneumonia in people and horses whose immune systems are depressed, may be the underlying triggers of bronchointerstitial pneumonia.
A common factor in many cases of bronchointerstitial pneumonia is the use of erythromycin, an antibiotic that is invaluable in the treatment of Rhodococcus equi. Erythromycin might suppress the lung's ability to mount an immune response, thus allowing 'oddball' infections, such as Pneumocystis carinii, to become established in the lung. Also, erythromycin has been shown to cause primary hyperthermia (overheating) in a small subset of foals.
Researchers speculate that bronchointerstitial pneumonia may be complicated by hyperthermia in some cases – in essence, the signs of distress in bronchointerstitial pneumonia may be due to thermal injury to the lung, or hyperthermic effects on the body.
This type of pneumonia is generally seen on farms where there are multiple mares and foals, so there may be a stress or infectious cause that is more likely to appear on breeding farms. Yet another potential cause of bronchointerstitial pneumonia is plant toxins, although none have been conclusively identified as yet.
Death rates from bronchointerstitial pneumonia are high. It progresses rapidly, and is often refractory to treatment.
Watch to Watch For
Foals will suddenly appear ill, or there may have been a mild cough or nasal discharge, prior to the onset of respiratory distress.
Foals are depressed and stop nursing the mare.
Foals will have a dramatic elevation of respiratory rate (some over 100 breaths per minute).
The nostrils will be flaring and the foal will often extend his head and neck in an effort to get more air.
The foal will often appear to be 'pumping' with his abdomen, and he may show paradoxical breathing. Paradoxical breathing refers to a pattern in which the abdomen springs out during inhalation, but the chest goes in -- the chest normally goes out with inhalation, so this is 'paradoxical.'
The foal may even have a visible 'heave line' due to his effort to breath out. The 'heave line', or visible extra effort and delineation of the muscles of the flank, is usually only seen in older horses with clinical signs of heaves, or recurrent airway obstruction.
The foal may or may not have a cough and nasal discharge. Often, the effort to breath is so great that the foal doesn't have the time or the energy to cough.
The foal will often be dehydrated.
You will notice that the even the slightest effort exhausts the foal.
If you can examine the foal's mucous membranes, you will usually see that they are dusky, or cyanotic, indicating that the foal is not able to get enough oxygen into the body.
In some foals the clinical course of disease is so rapid, that the foal is found dead. Typical signs of bronchointerstitial pneumonia are found on autopsy.