A two-year old Limousin bull died of interstitial pneumonia. The bull was running with a group of 26 cows and calves near Bathurst in late April 2007. The owner noticed that the bull was dull and lethargic but that the remainder of the mob appeared healthy. He fed the cattle cabbages for two weeks and then put them in a paddock after cabbage harvest for two weeks before the incident.
On clinical examination the bull was alert, reluctant to walk, dyspnoeic, its heart rate and respiratory rate were increased and its mucous membranes were pale. No abnormal lung sounds were noticed at this stage. Blood was taken for haematology. Two days later the bull was re-examined. It had deteriorated, was severely dyspnoeic and showed subcutaneous emphysema. It died following examination, and autopsy revealed extensive pulmonary emphysema and pneumonia.
On histopathological examination of the lung, a severe diffuse subacute interstitial pneumonia and bullous pulmonary emphysema were seen. The lung pathology had features of more acute lesions including protein rich oedema fluid and hyaline membranes in alveolar lumina, as well as subacute lesions including proliferation of type 2 alveolar epithelial cells, marked smooth muscle hypertrophy and fibroplasia.
This case is consistent with Acute Respiratory Distress Syndrome or ‘fog fever,’ sometimes reported 7-10 days after cattle graze brassicas. Intoxication is caused by a sudden increase of tryptophan in the rumen. Tryptophan is metabolised in the rumen to 3-methylindole which enters the lung via the blood stream. In the lung, it is further metabolised to the ultimate pulmonary toxin.
In Europe and America, ‘fog fever’ occurs in autumn, 4-10 days after cattle are moved from a dry to a lush green pasture. ‘Fog fever’ is reported when cattle graze several pastures species and after grazing brassicas post-harvest. It almost always occurs in adult cattle. Mature beef cows are regarded as most susceptible with a lower incidence in dairy cows, bulls and younger animals. ‘Fog fever’ is rare in Australia.
Treatment is of limited value once animals are sick, and handling animals in respiratory distress for examination and treatment can hasten death.
In the northern hemisphere, when the disease is expected, producers are advised to gradually introduce stock to risky forages and limit their grazing. Ionophors may have a role in prevention as they inhibit 3-methylindol production.
The owner moved the remainder of the mob from the paddock and no further cases were reported.