22/09/2025
Fog fever is a condition most commonly seen within a 4-14 days of cattle moving to lush grazing, from sparse pasture: a scenario many will be facing after this latest, most welcome rainfall.
Whilst grazing the poorer forage, rumen flora will have adapted to this diet. When faced with lush green pasture, the protein ingested dramatically increases with little time for the rumen bugs to adapt. Proteins are broken down to amino acids in the rumen. One of these amino acids, L-tryptophan is further broken down, very effectively, to a substance called 3-methylindole (3-MI). 3-MI is toxic to cells that line the surface of the lungs. As more 3-MI is converted in the rumen, more and more lung tissue is destroyed.
Clinical signs:
Difficulty breathing/open mouthed breathing
Not always coughing
Frothing at mouth
No raised temperature
Separation from group, not grazing
Collapse
Death
There is no specific treatment for fog fever. Mild cases may recover without treatment. 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. The stress associated with movement may hasten death in severely affected animals. Supplementary feeding to lessen the intake of grass may be more appropriate.
Prevention of fog fever is through pasture management, gradually increasing grazing time over a period of 10-12 days to give rumen flora time to adapt to the increased protein content of the diet.
