Research

Clinical Case Review

Atypical Interstitial Pneumonia is Becoming Typical

By: Dr. Dan Thomson Production Animal Consultation

Atypical Interstitial Pneumonia, commonly known as AIP, continues to be a considerable problem in the beef industry.

We don’t know a lot about AIP, but we do know the following:

1. The highest incidence rate is in July and August.

2. It occurs more frequently in heifers than in steers.

3. Most cattle will have been on feed at least 90 days.

4. There is no effective treatment for AIP.

Atypical interstitial pneumonia was commonly confused with Bovine Respiratory Syncytial Virus (BRSV) because lungs from cattle that die from both look very similar. It was once thought that BRSV was the cause of AIP, but that has been ruled out because diagnostics on cattle dying from AIP or non-AIP causes had the same incidence of BRSV at the time of death. Atypical interstitial pneumonia has been called dust pneumonia primarily due to seasonal conditions in the summer when AIP cases occur.

We have two types of pneumonia in cattle: bronchopneumonia and interstitial pneumonia. Bacteria and contamination cause bronchopneumonia. It is confined to the ventral (bottom) and cranial (front) areas of the lung. The infected areas will be hard and dark. Bronchopneumonia usually results from bacteria sliding down the trachea into the lungs. If the animal’s immune system is suppressed, the bacteria are allowed to colonize in the lungs causing disease. Viruses, such as IBR or BVD, cause immunosuppression leasing to a secondary bacterial infection. Bacteria, such as Mannheimia or Pasteurella, are normally found in the sinuses of cattle but if the cattle are stressed, these bacteria become the secondary invaders causing pneumonia. Interstitial means within the spaces or areas of tissues or organs. Interstitial pneumonia generally occurs from something in the bloodstream being carried to the lungs or fine particles being inhaled reaching all areas of the lungs. Traditional AIP used to be called fog fever in cows. The most common situation was when cows were turned out on lush forages in damp conditions. They over-consume the amino acid tryptophan in the grass which is converted to 3-methylindole (3-MI) in the rumen by bacteria. The 3-MI is transported in the blood to the lungs. In the lungs there are enzymes that convert 3-MI to 3-MEIN. 3-MEIN then is toxic to the pneumocytes (cells in the lungs). This toxic insult causes the pneumocytes to swell.

Acute respiratory distress syndrome (ARDS) is an “AIPlike” syndrome that occurs in humans. There are 60 known causes of ARDS in humans. The most common causes of ARDS in humans are septicemia (blood bacterial infection); inhalation of smoke, water or vomit; endotoxin inhalation; and trauma to the chest area. Thinking of feeder cattle being on a constant diet with consistent tryptophan intake, it is possible that cattle suffer from ARDS in the feedlot rather than traditional AIP like fog fever cows. The combination of heat stress, dust inhalation (endotoxins in manure), sepsis, and other factors could be additive resulting in clinical disease.

Clinical signs of AIP come on very quickly. We will generally see AIP in larger cattle. The cattle will present themselves in grave respiratory failure. Their backs will be swayed, and they will be breathing out of their mouths (Figures 1, 2). Most of these animals will stand around a water tank. They are thirsty but can’t stop panting long enough to get a drink. The lungs at necropsy for AIP/ARDS cattle that die are severely swollen, large, and heavy (Figures 3, 4). It is no wonder that the cattle have difficulty breathing. The increased cellular mass in the lungs decreases the areas for air. Also, the animal has trouble exhaling due to the fact that the lungs are too big for the chest cavity and don’t want to decompress.

Effective treatment for AIP is non-existent. The use of anti-inflammatory agents such as dexamethasone and flunixin meglumine has not been rewarding. Long-term low dose dexamethasone and ventilation are treatments in hu- man medicine. Case fatality rates in cattle or humans with AIP/ARDS are around 50 percent. Good animal husbandry and low stress cattle handling are imperative. Some animals will not make the trip all the way to the hospital due to the stress and decreased lung capacity. Using a trailer to move these animals may save their life. Another way to relieve the animal’s stress is to have a shaded area in the hospital. These types of practices should not be limited to AIP pulls.

Documented prevention practices that work have been lacking in the area of AIP research in cattle. Researchers tried feeding vitamin E and aspirin on arrival to prevent AIP but neither had an effect. The jury is still out on the

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