Fog Fever

HAF RESEARCH data show that fog fever is a disease of the autumn. This is because it is almost always associated with cattle who have moved from a dry summer pasture to a lush autumn one

What is Fog Fever?

Fog fever is an acute pneumonia of adult cattle which occurs within 4 to 10 days of moving from an over grazed pasture to a fresh one. A wide range of forages have been associated with fog fever, including, rape, kale, turnip tops, alfalfa and many grasses. This is a disease caused by a toxin, 3-methylindole. This is a toxin produced as a result of breakdown in the rumen of D,L,tryptophan, present in high levels in lush forage. Once absorbed from the rumen the toxin damages the cells lining the lungs resulting in pneumonia.

Clinical Signs

  • Severe difficulty breathing, often noisy and accompanied by open-mouthed breathing and frothing
  • Anxiety, often separate themselves from the rest of the herd
  • Collapse and death
  • Coughing infrequent, temperature usually not very high


  • On the clinical signs described above
  • Only occurs in animals over 1 year in age
  • Time after change in pasture very characteristic (4 to 10 days)


  • There is no specific treatment for fog fever. Indeed mild cases will commonly recover without treatment. Seek veterinary advice for moderately or severely affected cows
  • Removal from pasture is controversial, it may not actually prevent further cases, while moving very sick cows can make the disease worse and should be done with extreme care. Furthermore, leaving cows on pasture will often not result in new clinical cases. Seek veterinary advice


  1. Limit total grazing time in the first 10 days when cattle are moved to a new pasture. The cattle should graze the pasture for 1 or 2 hours the first day increasing gradually by 10 days.
  2. Feeding monensin or chlortetracycline for the first 10 days has been shown to be effective, if grazing management is not possible.


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