The advent of autumn combined with the increase in rainfall means that the risk of fog fever is high on many farms this year as cattle move from dry summer pastures to lush autumn ones.
What is Fog Fever?
Fog fever is an acute pneumonia of adult cattle, which is associated with movement from poor quality pasture to rapidly growing lush forage. The disease has been recognised in the UK for a long time, which is why despite its name it is not associated with foggy weather nor do affected cattle always have a fever. ‘Fog’ is the old term for a new growth of grass appearing on a field that has been mown or grazed. There does appear to be a difference between breeds in how susceptible they are to fog fever with Hereford and Hereford-cross cattle being thought to be particularly susceptible. There is also a very strong age effect as young growing cattle (up to 1 year of age) that are grazing the same pasture as affected adults usually remain unaffected.
Fog fever usually occurs within 4 to 10 days of moving from an over grazed pasture to a fresh one. Although most outbreaks of fog fever occur in cattle on grass, it is important to stress that it is not just grass that can cause fog fever; the disease has also been seen in cattle on rape, kale, turnip tops and lucerne. In some cases simply changing pasture fields in the autumn has led to outbreaks of fog fever. In such cases outbreaks tend to occur on the same pasture in successive years. Once the first frosts occur the risk of disease falls rapidly.
Fog fever is not caused by an infectious organism, instead it is caused by a toxin 3-methylindole. This toxin is produced as a result of breakdown in the rumen of D,L,tryptophan, which is 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
- Many of the adults in an affected group will have clinical signs. Usually <5% will have severe signs
- Apparent sudden death can occur, particularly if the cattle are not checked regularly. However death is rare in fog fever (occurring in <3% of affected cattle)
- Severely affected cattle have severe difficulty breathing. In such cases breathing is usually noisy and accompanied by frothing and an open-mouth.
- Less severely affected cattle tend to have rapid shallow breathing (without obvious signs of difficulty). As these clinical signs can be difficult to pick up in the field without getting close to the animal, early cases of fog fever are often missed.
- Even in severely affected cases coughing is only infrequently heard
- Body temperature is usually not very high
- Affected animals often separate themselves from the rest of the herd, but they tend to be more approachable than normal
Diagnosis
- 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)
Treatment
- There is no specific treatment for fog fever. Mild cases will commonly recover without treatment; however in the early stages of an outbreak it may be difficult to predict whether mild cases will stay mild.
- For more severely affected cattle supportive treatment is essential. Non-steroidal anti-inflammatories have been shown to be helpful; other potential treatments include corticosteroids and diuretics. Get advice from your vet.
- Antibiotics are of no benefit in most cases but in severe cases their use may prevent secondary infection.
- Removal from pasture is controversial, as 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.
Prevention
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 so that the cattle are on the pasture full-time after 10 days.
