Be on guard for Fog Fever.

Byline: By Iain Carrington

The weather has been variable again this year but some very good hay and silage has been made and aftermaths have been growing fast. This means we should all be on our guard for Fog Fever, a disease generally seen in cattle grazing aftermath.

Classically this is an acute pneumonia affecting mainly adult beef cattle, which is seen at grass in the late summer and autumn. It is normally associated with a change to more lush pasture, especially regrowth or silage aftermath, though I have seen cases in cattle grazing on permanent pasture after warm, wet weather has caused a growth spurt in the grass.

The cause is thought to be an intoxication with a compound called L-tryptophan, which is found in large quantities in regrowth. This is then metabolised in to a compound called 3-Methyl indole, which has a toxic effect on the lungs, causing the symptoms of pneumonia.

The disease is normally seen within two weeks of the animals being introduced to new pasture. The symptoms range from mild to severe depending on the degree of intoxication, the animals presenting with normal temperature, dullness, tranquil behaviour, varying degrees of breathing difficulties and very little coughing. In younger animals the symptoms can be confused with those of lungworm, but the frequent cough associated with this parasite helps to avoid this.

When this condition is suspected the animals should be removed from the pasture as soon as possible. This may be difficult with severely affected cases, which may have to be treated where they are and only moved slowly to avoid severe breathing difficulties.

Treatment consists of anti-inflammatory drugs and I usually give antibiotic cover. Even after treatment mortality among the more severely affected cases can still be high, the speed at which they are identified and removed from the pasture being a major factor in survival rates. Once removed from the pasture the cattle should be offered conserved forage such as hay to dilute the grass in the rumen and so reduce the level of L-tryptophan. Return to grass should only come after full recovery and then in a controlled fashion so as to allow the cow's rumen microbes to adapt, taking up to three weeks.

This disease can be markedly reduced or prevented in most cases by grazing management. Restricting access to the fresh pasture either by time or by strip grazing will allow time for the cows to adapt to the change to a more lush diet. Feeding hay or silage daily before turning cattle on to the pasture may also help. If the cattle are grazing permanent pasture extra vigilance is required to watch for a sudden flush of grass, which might precipitate the disease.