Atypical interstitial pneumonia (Fog Fever)


Atypical interstitial pneumonia, also commonly known as fog fever, is a syndrome farmers see in their cattle after placing the animals on lush green grazing. Animals that are fed a dry ration prior to grazing lush pastures are more likely to be affected by this syndrome. Fog fever is also known as acute bovine pulmonary oedema and emphysema (ABPEE).


Disease progression:

Cattle affected by this syndrome have been on dry feed for extended periods of time and the rumen fermentation pattern has adapted to this situation. With the change to lush green pasture the dietary protein concentration increases dramatically. One of the amino acids in this plant protein, tryptophan, is the culprit. The tryptophan in the feed is converted by rumen bacteria to a substance called 3-methylindole (3-MI) at a very high rate.

This 3-MI is absorbed through the rumen wall and circulated around the body. The 3-MI is toxic to the primary cells (Type 2 pneumocytes) that line the interior surface of the lungs at the alveolar-blood barrier. Thus, as the high levels of 3-MI move from the rumen to the lungs, more and more lung tissue is destroyed.


Clinical Signs:

  • Difficulty breathing
  • Coughing
  • Frothing at mouth
  • Cattle do not normally run a fever- This is an extremely important point as the clinical signs seen with fog fever are very similar to those seen with Bovine respiratory disease.
  • Anxiety (separation from group)
  • Collapse
  • Death



Unfortunately, there is no specific treatment for fog fever. Mild cases may recover without treatment, should they be caught early enough. Most severely affected cattle will not respond to any treatment and will die within a day or two.
Removing cows from pasture has not been scientifically proven to reduce cases. Some believe it prevents further cases, whilst others believe that moving sick animals can make the disease worse as the animals will be stressed and their respiratory effort will increase. Should animals be severely affected, remove them from the pastures in a calm and quiet manner to prevent them running, as this may result in death.

Some literature suggests the use of systemic diuretics. However, the cost involved of doing this along with the poor success rate following treatment does not make the treatment beneficial.


  • Limiting time grazing in the first 10 days when cattle are moved to a new pasture will reduce the chances of cattle developing AIP.
  • Allowing cattle to graze a couple of hours a day and building the length of grazing up is recommended.
  • Feeding monensin or chlortetracycline for the first 10 days can help control the situation. Always be aware the dangers of feeding too much monensin! It will result in necrosis of the cardiac muscle resulting in acute death.

Dr Jarred Morris

082-559 1941

BVSc MSc (Agric)