Fog fever is one of the common names for a specific pneumonia of adult cattle, which is scientifically known as acute bovine pulmonary emphysema.
A related condition occurs in feedlot cattle and is usually referred to as acute interstitial pneumonia.
Fog fever was first described as a disease of grazing cattle more than 200 years ago in Europe. Its occurrence was associated with the grazing of “fog lands,” which are pastures that have lush new growth after being cut for haylage or silage. There is actually no link between the occurrence of this particular pneumonia and any atmospheric condition, such as fog or smog.
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In North America, the syndrome is seen primarily in beef cows older than two. Veterinarians and producers find it a frustrating disease to treat, and it often occurs in an outbreak where 10 to 50 percent of the herd may be affected.
The outbreak usually begins four to 10 days after moving cattle abruptly from dry and overgrazed pasture to a more lush pasture.
Lush is a difficult term to define precisely, and we don’t fully understand all of the risk factors associated with these outbreaks. 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 can trigger outbreaks without even moving cows from one pasture to another.
The pneumonia that develops is different from many of the other pneumonias with which ranchers are familiar. Most pneumonic conditions are caused by bacteria and/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 typically affects only adult cows. The protein is converted by the bacteria in the rumen and produces a toxin known as 3-methyl-indole when the cattle are exposed to a new pasture with higher levels of tryptophan and their rumen bacteria are not gradually adapted to it. The toxin is circulated in the cow’s blood stream to the lung, where it causes severe cellular damage. The result is a severe, untreatable respiratory syndrome.
Clinical signs consist of a sudden onset of severe respiratory distress. Affected cows have laboured breathing and often breathe with an open mouth. They will grunt forcibly on expiration, and froth will develop around their mouths. They often appear to be anxious and have difficulty breathing but are not depressed in the way that cattle are with bacterial pneumonias. The affected animals are reluctant to walk and don’t graze. In some cases, they may go down and not be able to get up. The severe respiratory distress can cause pockets of air to accumulate under the skin of the animal. The animal’s temperature is often only slightly elevated.
Affected animals usually die within two to three days of the onset of symptoms, and the mortality rate is 30 to 100 percent.
The lungs are large, heavy, swollen and full of fluid. They often have a marbled appearance and a rubbery or meaty texture. A veterinarian can often make a diagnosis based on the clinical signs of the disease and on the post-mortem findings.
The disease is especially frustrating because little can be done to prevent or treat further cases once it is diagnosed. Producers should consider emergency slaughter or salvage for severe cases if possible. However, slaughter is an option only in animals that have not received recent antibiotics and have no possibility of drug residues in their system.
Affected cattle should be moved with extreme caution. Moving them off pasture can cause so much stress that they often die. Moving them after being on the pasture for four days or more is of little value and is unlikely to prevent any further cases from developing.
Veterinarians often prescribe an anti-inflammatory medication to try to limit the inflammation and swelling in the lungs, but the prognosis is often poor and treatment success is limited at best.
Fog fever is difficult to prevent because there are probably risk factors that we haven’t yet identified. It would be wise to not move cattle abruptly from dry overgrazed pasture to lush pasture and try to make the transition more gradual.
Another form of feed supplementation may help with the transition. The feed additive monensin has been shown to reduce the production of the 3-methyl-indole toxin, but we cannot predict all the circumstances that will cause this disease to occur.
