Several veterinarians have diagnosed outbreaks of pneumonia in adult cows that were recently moved to a new pasture.
Pneumonia in adult cows is uncommon and these specific cases were attributed to a condition known as fog fever. The unique name is somewhat confusing.
Fog fever is a common name for Acute Bovine Pulmonary Emphysema (ABPE). It was first described as a disease of grazing cattle more than 200 years ago in Europe and was associated with grazing fog lands, which are pastures that have lush new growth after being cut for haylage or silage.
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There is no link between fog and the occurrence of this pneumonia.
In North America, the syndrome is seen primarily in beef cows older than two and most commonly associated with movement in fall from dry summer grazing to lush green pastures. It often occurs in an outbreak where 10 to 50 percent of the herd is affected.
The outbreak usually begins five to 10 days after moving cattle abruptly from dry and overgrazed pasture to lush pasture.
Lush is a difficult term to define and we don’t fully understand all the risk factors associated with outbreaks.
The syndrome has been seen in cattle being moved from mountain grazing down to valleys late in the fall or in cows returning home to graze after being at a community pasture. In some cases, we may see regrowth on a pasture late in fall and outbreaks occur without even moving cows from one pasture to another.
The pneumonia that develops is unique and different from more familiar forms.
Most pneumonic conditions are caused by bacteria or viruses and are more common in young and growing cattle. This respiratory syndrome is caused by a protein in the lush pasture called tryptophan and it typically only affects adult cows.
When cattle are exposed to a pasture with higher levels of tryptophan and their rumen bacteria are not adapted to this protein gradually, it is converted and produces a toxin known as 3-methyl-indole.
This circulates in the cow’s bloodstream and once the toxin reaches the lung, it causes severe cellular damage. The result is an untreatable respiratory syndrome.
Clinical signs consist of a sudden onset of severe respiratory distress. Affected cows will have laboured breathing and often breathe with an open mouth and extended neck.
They will grunt forcibly and have froth around their mouths. These cattle often appear anxious and have obvious difficulty breathing, but they are not depressed in the way that cattle with bacterial pneumonias are.
Affected animals are reluctant to walk, don’t graze and in some cases may go down and not be able to get up.
The severe respiratory distress can cause pockets of air to accumulate under the skin.
Affected animals often die within two to three days of the onset of symptoms and the mortality rate ranges from 30 to 100 percent.
On autopsy, the lungs are large, heavy, swollen and full of fluid and often have a marbled appearance and rubbery texture.
Fog fever is especially frustrating because once diagnosed, little can be done to prevent or treat further cases. Antibiotics are of no benefit and their use will eliminate the option of emergency slaughter.
Affected cattle should be moved with extreme caution. Moving them off pasture often causes so much stress that they die.
Moving cattle after being on the pasture for four days or more is of little value and unlikely to prevent further cases.
Veterinarians often prescribe an anti-inflammatory to limit swelling in the lung of affected animals, but the prognosis is poor.
Fog fever or ABPE is relatively rare but there are certain years where environmental conditions are more conducive to outbreaks.
Try to make the transition from dry overgrazed pasture to lush pasture gradually. Some form of feed supplement may help with that transition, and the feed additive monensin has been shown to reduce production of the toxin responsible for lung damage.
