What is Fog Fever?

Acute bovine pulmonary emphysema and edema (ABPEE) is one of the more common causes of acute respiratory distress in cattle, particularly adult beef cattle, and is characterized by sudden onset, minimal coughing, and a course that ends fatally or improves dramatically within a few days. It is a disease involving groups of cattle; morbidity may be >50%, although usually only a small minority develops severe respiratory distress. Typically, ABPEE occurs in the fall, 5–10 days after change to a better, often lush, pasture. A similar condition has been reported when cattle are fed on a wide variety of grasses, alfalfa, rape, kale, and turnip tops.


The affected cattle have been on dry feed for an extended period of time and the rumen fermentation pattern has adapted to this situation. With the change to lush green pasture the dietary protein concentration increases dramatically. One of the amino acids in this plant protein, tryptophan, is the culprit. The tryptophan in the feed is converted by rumen bacteria to a substance called 3-methylindole (3-MI) at a very high rate.

This 3-MI is absorbed through the rumen wall and circulated around the body. The 3-MI is toxic to the primary cells that line the interior surface of the lungs. Thus, as the high levels of 3-MI move from the rumen to the lungs more and more lung tissue is destroyed.


  • Difficulty breathing
  • Coughing
  • Frothing at mouth
  • Cattle do not normally run a fever
  • Anxiety (separation from group)
  • Collapse
  • Death


Limiting time grazing in the first 10 days when cattle are moved to a new pasture will reduce the chances of cattle falling ill.  Allowing cattle to graze a couple of hours a day and building this up is advisable.
It is also thought that feeding monensin or chlortetracycline for the first 10 days can help control the situation.Prevention is important as it is common for 50 per cent of a herd to become ill, and up to 30 per cent may die.


Severely affected animals have so little pulmonary reserve that any driving or handling must be done with caution to prevent immediate death. Removal of cattle from the offending pastures may not prevent the development of new cases for the next 4–7 days. No treatment has been identified that will reverse the fully developed lesions of ABPEE.


One approach to control is dietary management, including the following options: 1) avoiding pastures likely to induce ABPEE, 2) feeding hay before turn out on pasture and limiting exposure time on suspect pastures, 3) limiting grazing time and gradually increasing exposure to the pasture over time, 4) using pastures before they become lush, 5) delaying use of lush pastures until after a hard frost, 6) initially grazing pastures with less susceptible stock (cattle <15 mo old or sheep), or 7) using strip grazing.

A medical approach to control involves feeding monensin or lasalocid, which inhibit the bacteria that convert l-tryptophan to 3-methylindole. Treatment with monensin can be started 1 day before introduction to pasture, whereas lasalocid requires a 6-day pretreatment period. These drugs are of no benefit after onset of clinical signs.

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