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Know the signs of fog fever

Vet Advice with Dr. Ron Clarke

Fog fever is not an uncommon condition in adult beef cattle this time of year. It is often sudden in onset and can be a cause of sudden death affecting a significant number of mature cows. It has nothing to do with “fog,” it is linked to nutrition not infection, and body temperature in affected cattle is typically normal unless associated with exertion and breathing difficulties. Fog fever is one of the common names for a specific pneumonia of adult cattle, scientifically known as acute bovine pulmonary emphysema and edema (ABPEE). A related condition called atypical interstitial pneumonia (AIP) occurs in feedlot cattle.

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Fog fever was first described in Europe as a disease of grazing cattle more than 200 years ago. It appeared in cattle grazing on “fog lands,” pastures supporting lush new growth after being cut for haylage or silage.

The main factors are hungry cattle that have been on dry feed for some time, and then allowed free access to rapidly growing, lush green feed.

The syndrome has been seen in cattle that are moved from mountain grazing to valleys late in the fall or in cows returning home to graze after being at a community pasture. In some cases, regrowth on a pasture late in the fall triggers outbreaks without moving cows from one pasture to another.

The typical situation predisposing to fog fever involves hungry, adult cattle on dry summer pastures suddenly moved to lush, rapidly growing green pasture like hayland used for fall grazing, or fields of lush grazing corn.

Fog fever is caused by an abrupt change in diet and subsequent biochemical changes in the rumen. Rumen fermentation patterns adapt to levels of protein in feed. With the change to lush green pasture, dietary protein concentration increases dramatically. One of the amino acids in enriched plant protein, tryptophan, is converted by rumen bacteria to a substance called 3-methylindole (3-MI), which is absorbed through the rumen wall and circulated. Three-MI is toxic to primary cells lining the lungs. Destruction of respiratory tissue progresses as 3-MI moves from the rumen to the lungs.

Clinical signs

  • Multiple adult cattle showing respiratory signs within two weeks of moving from dry feed to lusher grazing.
  • Nursing calves are seldom affected. Yearlings are less susceptible than mature cows.
  • Sheep and horses are much less susceptible than cattle.
  • Pastures of any sort (grass or legume) as well as annual crops (corn, Sudan grass, etc.) can predispose to ABPEE (fog fever). Fog fever can also occur when animals preferentially graze lush growth in lowland meadows or water discharge areas, while the remainder of a pasture is dry.
  • Outbreaks usually develop within five to10 days of a change to better grazing and rarely occur after animals have been on a pasture field for three weeks or more.
  • The disease normally appears during the late summer and fall as pastures change.
  • Outbreaks can involve from 10 per cent to 50 per cent of mature cows.
  • Difficulty breathing, coughing and frothing at the mouth are primary signs.
  • Sudden death is a common feature.
  • Cattle do not normally have a fever.
  • Anxiety and attempting to separate from the herd is common.
  • Collapse and death.

About the author


Dr. Ron Clarke

Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen ([email protected]) or WCABP ([email protected]).



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